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Article

Facile Redox Synthesis of Novel Bimetallic Crn+/Pd0 Nanoparticles Supported on SiO2 and TiO2 for Catalytic Selective Hydrogenation with Molecular Hydrogen

by
Olga A. Kirichenko
1,2,*,
Elena A. Redina
1,*,
Gennady I. Kapustin
1,
Marina S. Chernova
1,
Anastasiya A. Shesterkina
1,3 and
Leonid M. Kustov
1,4
1
N.D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, 47 Leninsky Prospect, 119991 Moscow, Russia
2
M.V. Lomonosov Institute of Fine Chemical Technologies, Russian Technological University MIREA, 86 Vernadsky Prospect, 119571 Moscow, Russia
3
Nanochemistry and Ecology Department, National University of Science and Technology(MISiS), 2 Leninsky Prospect, 119991 Moscow, Russia
4
Chemistry Department, Moscow State University, Leninskie Gory 1, Bldg. 3, 119992 Moscow, Russia
*
Authors to whom correspondence should be addressed.
Catalysts 2021, 11(5), 583; https://doi.org/10.3390/catal11050583
Submission received: 8 April 2021 / Revised: 26 April 2021 / Accepted: 29 April 2021 / Published: 30 April 2021
(This article belongs to the Special Issue Mono- and Bimetallic Nanoparticles in Catalysis)
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Abstract

:
The bimetallic Crn+/Pd0 nanoparticles have been synthesized for the first time by a two-step redox method. The method includes the deposition of Pd0 nanoparticles on the surface of SiO2 and TiO2 carriers followed by the deposition of Crn+ on the surface of Pd0 nanoparticles using the redox procedures, which are based on the catalytic reduction of Crn+ with H2 in aqueous suspensions at ambient conditions. Transmission (TEM) and scanning (SEM) electron microscopy, X-ray photoelectron spectroscopy (XPS), Fourie-transformed infrared spectroscopy of adsorbed CO (FTIR-CO), and CO chemisorption studies were performed to characterize the morphology, nanoparticle size, element, and particle distribution, as well as the electronic state of deposited metals in the obtained catalysts. A decrease in nanoparticle size from 22 nm (Pd/SiO2) to 2–6 nm (Pd/TiO2) makes possible deposition of up to 1.1 wt.% Cr most likely as Cr3+. The deposition of CrOx species on the surface of Pd nanoparticles was confirmed using FTIR of adsorbed CO and the method of temperature-programmed reduction with hydrogen (TPR-H2). The intensive hydrogen consumption in the temperature ranges from −50 °C to 40 °C (Cr/Pd/SiO2) and from −90 °C to −40 °C (Cr/Pd/TiO2) was first observed for the supported Pd catalysts. The decrease in the temperature of β-PdHx decomposition indicates the strong interaction between the deposited Crn+ species and Pd0 nanoparticle after reduction with H2 at 500 °C. The novel Crn+/Pd/TiO2 catalysts demonstrated a considerably higher activity in selective hydrogenation of phenylacetylene than the Pd/TiO2 catalyst at ambient conditions.

1. Introduction

Palladium has unique properties in the catalysis of diverse reactions [1,2,3,4,5]. Modification of Pd nanoparticles in the catalytic materials is currently considered as a promising way to improve the catalytic properties of Pd-based materials, especially in selective hydrogenation of organic compounds [6,7,8,9]. The promotion of Pd with Ga, Zn, In, Pb, Cu, Bi, Te, Ce, Zr [7,8,10,11], Sn, Ni, Pb, Ag, Au, Pt, Rh, Ru, Cu, Co, Ge, [9], Fe [12,13,14,15,16] has been successfully used. There is no single interpretation to explain the effect of second metal on the performance of Pd catalysts. Depending on the nature of both the co-metal and the reaction, the beneficial presence of a co-metal was interpreted in terms of geometric effects, electronic effects, and mixed sites. In the case of partly reduced second metal Pd–Mn+, the promoting effect was attributed to a positively charged cationic species Mn+ activating the functional groups of a substrate, which becomes easily hydrogenated. Electronic modifications upon alloying Pd were used to interpret the better selectivity observed in the selective hydrogenation of alkadienes and alkynes to alkenes. The presence of the second metal at the surface or in the bulk was shown to change the relative adsorption strength of the alkynes, alkadienes, and alkenes, which resulted in an increased reactivity for alkynes and a decreased reactivity for alkenes.
The influence of Cr additives on the catalytic behavior of Pd catalysts in selective hydrogenation is poorly studied, and contradictory results were obtained for Cr-Pd catalysts. When chromium is incorporated in the network of palladium in the charcoal supported Pd-Cr catalysts, the catalysts became more active in the hydrogenation of 1,3-butadiene and fully selective for the formation of butenes [17]. Introduction of both Cr6+ by impregnation with an aqueous solution of Pd-Cr mixed salts and Cr0 by vapor deposition onto the Pd/SiO2 catalysts was shown to decrease the activity in hydrogenation of double C=C bonds thus improving the selectivity in the partial hydrogenation of butadiene [18]. In these catalysts, Pd0 facilitates the reduction of CrOx nanoparticles to Cr0 at 600 °C. The Cr deposition using impregnation with ammonium chromate followed by calcination at 550 °C strongly enhanced the sulfur resistance of supported Pd-based catalysts, as well as the activity in toluene hydrogenation that increased with the Cr/Pd atomic ratio and reached its maximum when the Cr/Pd was equal to 8 [19]. Methylcyclohexane and dimethylcyclopentane were the main products. Recent studies revealed that the deposition-precipitation of Pd0 nanoparticles on TiO2 support in the presence of CrO42− ions resulted in changes of the redox behavior of the materials and improving the catalytic properties in dehydrogenation and hydrogenation of aromatics [20].
The goal of the present work is to reveal the conditions of redox-deposition of CrOx species from H2CrO4 aqua solution on the surface of supported Pd nanoparticles, their further reduction with H2, as well as to evaluate the influence of thus deposited CrOx species on the catalytic activity of Pd catalysts in hydrogenation of unsaturated bonds on an example of phenylacetylene (PhA) hydrogenation. Two catalysts consisting from Pd nanoparticles on a support were selected as initial samples: (i) 3%Pd/SiO2, whose redox-modifying with iron oxide resulted in enhanced PhA selective hydrogenation [21]; it was taken as a reference sample; (ii) 1%Pd/TiO2, whose redox-modifying with chromium oxide resulted in improving the catalytic properties in dehydrogenation and hydrogenation of aromatics [20]. It should be also mentioned, that hexavalent chromium (Cr6+) possesses high toxicity [22]. However, chromium acid is actively used in many industries, in which a large amount of waste chromic acid solution is generated [23]. The involvement of such waste chromic acid in the production of a new type of Cr-Pd catalytic systems, where CrO42− ions are reduced to nontoxic Cr2O3 or Cr(OH)3 species, seems to be an attractive way of waste chromic acid utilization.

2. Results and Discussion

The preparation methods of bimetallic Pd catalysts influence the chemical state and the spatial distribution of both components thus affecting the catalytic behavior [8]. To control the Crn+ effect, the modifier should be deposited on the Pd nanoparticle surface in a proper way, and low reaction temperatures are preferable to avoid the formation of alloys. The deposition of a second metal on the surface of a noble metal as a three-dimensional deposit is possible by the method of a redox reaction with adsorbed species (RRA) [24]. A reductant that is preadsorbed selectively on the primary metal reduces the ions of the second metal, causing its deposition on the surface of primary metals, and two metals interact. Hydrogen, which is pre-adsorbed on the noble metals, is commonly used to prepare bimetallic catalysts by RRA. For instance, by using pre-adsorbed hydrogen and RRA, Pt [25] and Au [26] were deposited on reduced Pd0 particles. Recently it was shown that a transition metal oxide (iron oxide) can be deposited on the Pd nanoparticle surface by the modified RRA techniques that supposed the primary RRA deposition of a water-insoluble inorganic salt of transition metal [21]. Here we propose the redox method based on the following facts. The Pd-adsorbed H2 is shown [20] can be effective for supporting the metal oxide species onto the surface of precious metal nanoparticles via reduction of a soluble anion, which contained a transition metal in the highest oxidation state, to the insoluble hydroxide of metal in the lower oxidation state. The direct redox reaction between CrO42− and H2 is impossible in an aqueous solution at room temperature. The adsorption of CrO42− anions on the SiO2 and TiO2 surface cannot proceed at pH higher than the point of zero charge varying in the range 1–4.2 for the different silica materials and equal to 6.3 for the commercial TiO2 Degussa P-25 [27]. Therefore, it is conceivable to perform RRA precipitation only on the surface of Pd nanoparticles maintaining a high enough pH value. Strategies for the CrOx deposition on the surface of Pd nanoparticles via the RRA method can be presented as following schemes (“Sup” means support):
1. Reduction by preadsorbed hydrogen (RA procedure):
  • Preliminary H2 adsorption from a gas flow on a supported Pd nanoparticle
H2 + Pd/Sup → H2aдc/Pd/Sup
  • Subsequent redox reaction in aqueous slurry
CrO42− + H2aдc/Pd/Sup → H2O + CrOx/Pd/Sup
2. Catalytic reduction by gaseous hydrogen (RC procedure):
  • Permanent H2 adsorption in an aqueous slurry on a supported Pd nanoparticle
H2 + Pd/Sup → H2aдc/Pd/Sup
  • Simultaneous redox reaction in slurry
CrO42− + H2aдc/Pd/Sup → H2O + CrOx/Pd/Sup
3. Co-reduction with the supported polyhydroxo complexes (PHC) of Pd (CoR procedure):
  • Preliminary CrO42− chemisorption from solution in the slurry
CrO42− + 2H+ + PHC/Sup → 2H2O + CrO4/PHC/Sup
  • Permanent H2 adsorption in an aqueous slurry on supported Pd nanoparticle
H2 + Pd/S → H2aдc/Pd/Sup
  • Simultaneous reduction in aqueous slurry
H2 + CrO4/PHC/Sup → H2O + CrOx/Pd/Sup
H2 + PHC/Sup → H2O + Pd/Sup
CrO42− + H2aдc/Pd/Sup → H2O + CrOx/Pd/Sup

2.1. Elemental Composition, Nanoparticle Size and Morphology

The deposited amount of Crn+ for the samples on 3%Pd/SiO2 (Pd/S) was found to be limited both for the RA and RC procedures and was slightly larger for the RC procedure. According to calculations based on Cr concentration in solutions, Cr loading in the sample was supposed to be 0.50 wt.%, but it was only 0.26 wt.% for Cr/Pd/S-RA and 0.36 wt.% for Cr/Pd/S-RC samples because of incomplete Cr6+ removal from solutions.
For 1%Pd/TiO2 (Pd/T) sample and its precursor PHC/TiO2, complete removal of Cr6+ from solutions were obtained at the conditions presented in Table 1, and the higher Cr loading values have been reached by both RC and CoR procedures. The possibility of wider variation in Cr loading opened the prospects for studies; therefore, more intensive characterization of the Cr/Pd/T samples has been done.
The electron microscopy studies of the obtained samples showed the influence of the preparation method on the morphology, the particle size distribution, and the element spreading over the sample. On the SEM images of selected samples, one can see the secondary structures consisted of the agglomerates of round-shaped particles (Figure 1, Figure S1).
According to SEM-EDX analysis data, in all samples, Cr was uniformly distributed throughout the sample with random areas of increased concentration. The local areas of high and low concentration of Pd were also observed, especially in the sample Cr/Pd/T-CoR (Figure 2A). It seems that during the CoR preparation a weakly acidic environment is created in the Pd-PHC/TiO2 suspension after the addition of a chromic acid solution which leads to the partial dissolution of PHC (especially, at 60 °C) and the transfer of palladium ions into the solution. Further, two main processes are possible: (1) transfer of Pd ions in the solution, leading to the growth of PHC particles and their aggregates and resulting in the appearance of local regions with high and low Pd concentrations at a normal total Pd content; (2) adsorption of Pd cations on the support surface followed by their reduction and the surface diffusion of Pd atoms with the formation of clusters that serve as the new centers of redox deposition of Cr particles, and the result is a uniform Cr distribution. The deposition of Cr species via RC-method, i.e., after supporting and reduction of Pd-PHC to metallic Pd0 nanoparticles, resulted in more uniform Pd and Cr distribution in the sample with the same Pd loading (Figure 2B). It is most likely due to the weak dissolution of metallic Pd and the less possible effect of Pd redispersion on the surface of TiO2. However, the areas with the increased local concentration of both Pd and Cr appeared with further increase of Cr loading in the samples prepared by RC procedure (Figure 2C), and the slightly stronger acidity of the initial solution might be a reason for this redistribution (Table 1).
The TEM study (Figure 3, Figure S2) revealed a narrow particle size distribution with a mean particle size of 3.7 nm for the Cr/Pd/T-CoR sample, while the sample of the same composition prepared by the RC method is characterized by the shift of particle size distribution to the higher values with a mean particle size of 4.1 nm. Therefore, the assumption of surface diffusion of palladium atoms with the formation of clusters that serve as new centers of redox deposition of chromium particles during the CoR method is more plausible. Redispersion of Pd in acidic slurry seems to occur also during the RC method under deposition of a higher amount of Cr species from more concentrated chromium acid, and smaller particles are observed in Cr/Pd/T-RC-3 sample than in the catalyst Cr/Pd/T-RC.
The XPS survey spectra show that the surface of samples is composed mainly of oxygen, titanium, carbon, palladium. The Cr peaks have been reliably recorded in the survey spectrum only for the sample Cr/Pd/T-RC-3 with the highest Cr loading of 1.1 wt.% (Figure 4a). Overlapping of Cr 2p region with Ti 2s region hindered revealing the lower Cr loadings. No peaks Cl 2p3/2 were found at 198.8 eV or 198.2 eV, which exhibits the absence of chloride ion in the palladium environment [28] and Ti-Cl bonds [29], suggesting good washing of the samples during their preparations.
Analysis of the wide-scan XPS spectra of the surface for the samples Cr/Pd/T prepared by RC and CoR procedure shows no significant difference in the atomic percentages of the elements (Table 2). The presence of carbon and increased O/Ti atomic ratio is the feature of commercial TiO2 aerooxide P-25. The obtained atomic ratios O/Ti and C/Ti (Table 2) are similar to those found previously for P-25 produced by Germanic Degussa Co (Frankfurt, Germany) [27].
XPS analysis has revealed Pd enrichment over the TiO2 surface to 2.2 wt.%, as compared with a bulk Pd loading of 1.0 wt.%, which can be due to the deposition of small Pd nanoparticles of the size of 2–6 nm on the surface of nonporous TiO2 particles with considerably larger size of more than 20 nm. The atomic ratio Cr:Pd was 2.8 that is considerably higher than the value calculated for the bulk chemical composition of the sample Cr/Pd/T-RC-3 (Table 1). The calculated surface Cr concentration of 3.1 wt.% exceeds the Cr loading in the sample, which indicates high dispersion of the deposited Cr species.
The high-resolution spectra of Ti 2p are identical for all samples and correlate to TiO2. Figure 4b represents the high-resolution spectrum of the Pd 3d region recorded for the Cr/Pd/T-RC-3 sample. As shown in Figure 4b, the Pd state can be reproduced as a sum of two doublets 3d5/2 and 3d3/2: 1-1′ main peaks and one satellite (asymmetry like in metallic Pd0) and 2-2′ symmetric peaks corresponding to Pdn+. The most important spectral parameter values obtained as a result of the fitting are presented in Table 3. The obtained BE (binding energy) values for Pdn+ are between reported for PdO 336.3 eV and for PdO2 [30], and Pdn+ content is low. The variations in the calculated values are within the experimental errors (BE ± 0.2 eV, HW ± 0.02 eV, I ± 5%) excepting electrostatic charge that is lower in the samples prepared by RC procedure, which allowed us to conclude that neither the preparation procedure nor Cr loading affected Pd electronic state.
The spectrum of the Cr 2p region was clearly observed only for the sample with the highest Cr content of 1.1 wt.% (Figure 4c). Its shape is distorted due to overlapping with Ti 2s, nevertheless, BE can be estimated as 576.9–577.4 eV that can be assigned to Cr3+ in Cr2O3 (576,5 eV) [31,32] and in surface species CrOx (577.0–577.6 eV) [33] or more likely in Cr(OH)3 (577.3 eV) [34,35]. It should be pointed out that Cr(OH)3 is the major Cr component of passive layers electrochemically formed on alloys in aqueous solutions. It seems that Cr6+ content is very small. Would chromium be present in the two main oxidation states: Cr3+ and Cr6+ in comparable amounts (at least 21.3 relative %), the doublet should be observed in the spectrum due to the chemical shift (binding energy separation) between the Cr3+ and Cr6+ [31]. However, light asymmetry of Cr2p peak at 578–583 eV suppose that presence of smaller portions of Cr6+ cannot be excluded. BE of Cr6+ in 13%Cr2O3/Al2O3 sample, which was partly oxidized at 600 °C, was found to be 579.3 eV [29]. For Cr/ZrO2 samples after redox cycles BE of Cr6+ varied 578.8–579.5 eV [36].
Figure 5 shows FTIR spectra of CO adsorbed on the Pd/T and Cr/Pd/T samples. The IR spectra of CO absorbed on Pd can be divided into two regions: linear CO (2000–2100 cm−1) and multi-coordinated (two or three-coordinated) CO (1995–1800 cm−1). The part of the spectra due to linearly adsorbed CO is represented by a sharp band about 2085–2090 cm−1 due to CO residing on corner Pd atoms with a low-frequency shoulder at about 2050 cm−1 due to, most probably, CO bound to (111/111) and (111/100) particle edges [37]. The intensity of these bands slightly increased for the sample Cr/Pd/T-CoR which can be due to the decrease in the particle size that was observed with TEM (Figure 3). The opposite changes were observed after Cr deposition on the Pd nanoparticles surface by RC method at the same Cr loading, which may be due to the partial blocking of the Pd sites on the corner Pd atoms and the Pd particle edges with CrOx species. For all samples, the part of the spectra related to multi coordinated CO contains a band at 1980 cm−1 that can be assigned to two-coordinated (bridged) CO on Pd(100) facets [37]. The significantly decreased intensity of this band in the spectrum of the sample Cr/Pd/T-RC indicates the possible redox deposition of CrOx species on Pd(100) facets. In addition, the spectrum of CO adsorbed on the Pd/T sample contains a broad band at 1920 cm−1 while the spectra of CO on Cr/Pd/T-CoR and Cr/Pd/T-RC samples contain an intense broad band at about 1900 cm−1. The band at 1920 cm−1 is due to either bridged CO on particle edges or three-coordinated CO on Pd(111) facets [37,38]. The band at about 1900 cm−1 can be assigned to three-coordinated CO on Pd(111) or bridged CO on (100) planes. The tailing of the spectra below 1850 cm−1 can be explained by the presence of additional broad and poorly-resolved absorption bands around 1800 cm−1. The absorption around 1800 cm−1 was previously assigned to the adsorption of CO at the metal/support interface (C bonded to the metal and O bonded to the support) [39], which makes evidence for a wide perimeter of the boundary between Pd atoms and the highly dispersed CrOx particles located whether on the surface of Pd nanoparticle or nearby its contact area with TiO2 support.
The obtained values of irreversible CO chemisorption (nCO,), calculated Pd dispersity (γPd), and crystallite size (hemisphere) are listed in Table 4. The overall value of CO chemisorption measured per gram of Pd decreased significantly after Cr deposition on the Pd nanoparticles, which can be due to the partial blocking of the sites of CO chemisorption by the deposited CrOx species. The applied calculations give a slight decrease in Pd dispersion and the increased nanoparticle size. It should be mentioned that TEM studies show even larger average particle size, the difference in the particle size exhibits a partial character of covering the Pd nanoparticles with CrOx solids. The Pd particle size and dispersity differed in an order of magnitude for the monometallic Pd samples prepared on SiO2 and TiO2 supports. Therefore, for the SiO2-supported samples, the low Cr loadings can be due to the small number of sites for H2 adsorption on the surface of the big Pd nanoparticles. Calculating the ratio of Cr ions in the solution to the surface Pd atoms, the value of 12 was obtained for Cr loading of 0.5 wt.%, which means the lack of adsorption sites for the achievement of such Cr loading. In the synthesized Pd/T sample, the Pd dispersity was considerably higher, and the mentioned ratio Cr:Pd drops to 2.7, opening the possibility of the higher Cr loading. A complete Cr withdrawal from solution was really found even during the preparation of the sample containing 1.1 wt.% Cr.

2.2. TPR-H2 Experiments

The XPS and FTIR of adsorbed CO techniques concern mostly with the surface composition, whereas a comprehensive picture of all the reducible species can be obtained with a TPR technique, whose results are depicted in Figure 6, Figure 7 and Figure 8. The TPR curves for the initial Pd/support samples (Figure 6a and Figure 7a) exhibited the negative peak of hydrogen release at 0–80 °C with the peak minimum at 55–60 °C, which is characteristic for the decomposition of the palladium β-hydride phase [40], indicating the presence of the Pd0 nanoparticles. The negative peak of hydrogen release was observed previously for the mono- and bimetallic Pd samples synthesized via different redox procedures [20,21]. The β-PdHx phase can be formed during the preliminary low-temperature treatment of a sample with a hydrogen-containing flow at the beginning of the TPR experiment [12,13,14]. Besides this peak of hydrogen release, the weak overlapping peaks of hydrogen consumption were observed at temperature region from 200 °C to 500 °C for the Pd samples on both supports, and intensive hydrogen consumption from −100 °C to −50 °C was found only for the samples on TiO2. Comprising these curves with the curves for Pd-PHC supported on γ-alumina and for the Pd0 nanoparticles resulted from their reduction with hydrogen at room temperature (Figure 7d), one may conclude that (i) residual amount of Pd-PHC is negligible in the Pd/TiO2 samples reduced with hydrogen at room temperature in the slurry; (ii) H2 consumption below 0 °C is feature of very small (2.8 ± 0.3 nm) Pd0 nanoparticles; (iii) the weak hydrogen consumption at a temperature higher than 250 °C may be due to reduction of some adsorbed species with product deletion from a sample, and, for the Pd/TiO2 sample, a Pd-catalyzed reduction of support is possible as well.
The redox deposition of CrOx species resulted in considerably decreased values of hydrogen release (Figure 8) for the samples both on Pd/S and Pd/T. These data indicate depositing the CrOx species on the Pd surface and their interaction. The curves and positions of the minimum hydrogen release correspond to the decomposition of palladium hydride in the initial palladium samples (Figure 6a and Figure 7a). After reduction at 500 °C, two new intensive overlapping peaks of hydrogen release arise at lower temperatures instead of the previous one for SiO2-supported bimetallic samples (Figure 6b), while the effect is weaker for TiO2-supported samples (Figure 7). No similar changes in the TPR curves were observed earlier for bimetallic Pd catalysts. It should be mentioned that interaction between the metallic Pd particles and the transition metal ions was shown previously to result in the complete disappearance of hydrogen release [9,12,13,14], in the case of the catalyst preparation via other than redox procedures. The bimetallic samples prepared by the redox procedures, which were developed in the present work, retain the ability to release H2 similar to the β-PdHx phase.
Changes in the shape of TPR curves after redox CrOx deposition as compared with the curve of the monometallic Pd sample also evidenced the presence of CrOx species on the Pd surface. The curve shape depends on the nature of the support used and Cr loading. The deposition of CrOx on Pd/SiO2 by RC procedure gave rise to a broad peak of H2 consumption from −87 to 27 °C with a sharp maximum at −4 °C (Figure 6a). The opposite effect of the disappearance of H2 consumption peak was revealed in this region for the similar sample on the TiO2 support (Figure 7a), whereas the triple increase of Cr loading resulted in a highly intensive sharp peak with a maximum at a considerably lower temperature both for initial and reduced at 500 °C samples (Figure 7b). The peak in the same region was observed on the TPR curve of the initial Cr/Pd/T-CoR sample with a lower Cr loading, yet it disappeared after sample reduction (Figure 7c). Therefore, this consumption most likely has dissimilar origins for different samples. For Cr/Pd/S samples it may be attributed to an irreversible reduction of some CrOx species deposited on the surface of large Pd nanoparticles, while for Cr/Pd/T samples the most probable explanation is Pd-catalyzed reversible reduction of CrOx clusters located on the surface of Pd nanoparticles. The H2 consumption in this region was reported earlier for Pd-Fe-O/SiO2 samples, which were prepared by calcining the metal salt precursors co-precipitated on a silica gel support, and was attributed to the reduction of bimetallic Fe2O3–PdO nanoparticles [41,42], as well as for FeOx/Pd/SiO2 nanocatalysts obtained by redox deposition of iron oxide on Pd nanoparticles [21,43].
Weak H2 consumption that was detected in the range of 300 to 500 °C even for the initial Pd/S sample can be due to the Pd-catalyzed reduction of impurities in the commercial SiO2 support, whereas the H2 uptake values for the samples on TiO2 correspond to the well-known reduction Ti4+→Ti3+ catalyzed by Pd0 in P-25 materials [44]. It should be mentioned that significant H2 consumption peaks in the range of reversible reduction of CrOxn− weakly anchored to oxide support (330–500 °C) [36,45] were revealed only for the Cr/Pd/T sample prepared by CoR procedure at Cr loading 0.43 mass % (Figure 7c).
The shape of the TPR curve for the sample on TiO2 was changed as well after reduction-reoxidation treatment of the samples (Figure 7). The intensity of the negative peak at 55 °C strongly decreased, and the minimum shifted to a lower temperature of 44 °C. Nevertheless, broad overlapping peaks of H2 release were observed in the range −33 to 75 °C, and the total value of H2 evolution decreased only slightly as compared with the initial Cr/Pd/T-RC sample (Figure 8). Such variation in the TPR curve may be due to the enlargement of Pd particles via the sintering of smaller particles. Yet the most surprising result was revealed in the range of negative temperatures: an intensive peak of hydrogen uptake from −90 to −40 °C. The phenomenon is supposed to be caused by the incorporation of Crn+ in the surface or subsurface layers of metallic Pd nanoparticles during a reduction in the TPR system at 500 °C.
Therefore, the procedure of sample synthesis seems to be the major reason for so strong a difference in the reduction behavior of bimetallic nanoparticles. The conditions of RA and RC procedures suppose deposition of CrOx species only on the surface of preliminary formed Pd0 nanocrystallites with no incorporation of Crn+ ions into Pd0 lattice. RC procedure makes possible deposition of larger Cr loading than RA procedure. During synthesis via the CoR procedure, the Pd0 crystal growth proceeds via reduction of Pd-PHC nanoparticles modified with CrO42− ions that are commonly accompanied by the formation of dislocations and grain boundaries making possible diffusion of ions from the surrounded liquid into the growing particles. Moreover, the possible dissolution of Pd-PHC may result in additional mass-transport of Pd2+ ions in solution resulting in the particle size decrease or growth. The reduced Pd moieties can adsorb H2 causing a reduction of admixed ions and their encapsulation in the forming Pd nanoparticles.

2.3. Catalytic Activity

The results of experiments on testing the catalytic activity in the liquid-phase hydrogenation of phenylacetylene (PhA) revealed a strong influence of the CrOx redox deposition on the catalytic properties of the catalysts prepared both on SiO2 and TiO2 supports. The catalytic activity of the SiO2-supported Cr/Pd/S samples strongly depended on Cr loading and conditions of thermal treatment. For the samples with the maximum possible Cr content prepared via both RA and RC procedures, the activity was drastically lower than one of the initial Pd/S catalysts. If the complete conversion on the Pd/S sample was reached at 20 °C in 30 min, the Cr/Pd/S samples conversion was only 7% both on as-prepared samples and on the reduced at 500 °C samples. This fall in the activity may be due to the coverage of large Pd nanoparticles with CrOx nanoparticles and possible blocking of the sites for PhA adsorption. It was recently shown that Pd terraces enable the high rate of acetylene bond hydrogenation to C=C bond on large Pd NPs of 20 nm in Pd/Al2O3 catalyst [46].
However, when the reaction temperature was increased to 45 °C, the activity of catalysts enhanced considerably, especially for the Cr/Pd/S sample with the ratio Cr:Pdv = 0.018 (Table 5). Both calcination in air and reduction in the hydrogen flow slightly enhanced its activity. Nevertheless, the activity and selectivity of the bimetallic samples were significantly lower than that of the initial Pd/S catalyst. It should be mentioned that inhibiting effect of CrOx deposition on hydrogenation of double C=C bonds over a Pd catalyst was confirmed in the present work as well by the fact of more prolonged time required for complete hydrogenation of styrene (St), which appeared as a product of PhA hydrogenation, to ethylbenzene (EtB) over the Cr-modified catalysts. Hence, CrOx deposition by RRA procedures on the large Pd nanoparticles, by any way, resulted in a considerable decrease in the catalytic activity of the Cr/Pd/S samples in selective PhA hydrogenation.
The influence of CrOx deposition on the smaller Pd nanoparticles in Cr/Pd/T samples was revealed to be more complicated and dissimilar, depending on the procedure used and Cr loading. Comparing the catalytic activity and selectivity for the Cr/Pd/TiO2 samples with almost equal Cr contents and Cr/Pdv ratios (Figure 9, Table 6) allow us to conclude that their catalytic properties strongly depend on the reduction behavior governed by the preparation procedure. The sample Cr/Pd/T-RC, which showed no H2 consumption below 20 °C in the TPR run, exhibited higher catalytic activity and selectivity than the parent Pd catalyst in the hydrogenation of triple to double carbon bonds in PhA. Whereas there was a significant decrease in the catalytic activity and selectivity for the sample Cr/Pd/T-CoR, the TPR curve profile showed the large H2 uptake at temperatures below 0 °C. The chemical composition, the calculated sizes of Pd nanoparticles, Cr/Pds ratios, the profiles and values of H2 evolution due to β-PdHx decomposition for these samples, Pd electronic state are close, yet FTIR spectra of adsorbed CO exhibit a significant difference in the adsorption properties of the Pd surface sites. Therefore, the influence of the sample synthesis procedure seems the major reason for so strong a difference in the reduction and catalytic behavior.
During synthesis via the CoR procedure, the formation of Pd0 crystallites initially proceeds via reduction of Pd-PHC nanoparticles modified with CrO42− ions, and as a result, the Crn+ ions may be blocked. The conditions of the RC procedure suppose deposition of CrOx species only on the surface of preliminary formed Pd0 crystallites with no incorporation of Crn+ ions into Pd0 lattice. The boundaries of CrOx species with the surface of Pd nanoparticles, as well as with TiO2 support—the presence of which partly confirmed by poorly resolved bands below 1850 cm−1 observed as tailing in DRIFT-CO spectra of Cr/Pd/T samples—may serve as additional sites for PhA adsorption thus enhancing the catalytic activity, and time of complete PhA conversion decreased with Cr loading up to 0.72% (Figure 10a). At the same time, the initial rate of H2 consumption considerably increased as compared with the Pd/T sample (Table 6) that indicated the enhanced activation of H2 molecules on the surface of bimetallic nanoparticles. It should be mentioned, that selectivity to St at complete PhA conversion was the highest for the sample with Cr loading 0.36% (Figure 10b).
To sum up, the highly dispersed bimetallic nanoparticles on TiO2 support that were synthesized via the catalytic reduction by gaseous hydrogen (RC procedure) are considerably more active than the initial Pd catalyst in the reaction of the selective liquid-phase hydrogenation of phenylacetylene. It was also shown that the same procedure of parent Pd modification with CrO42− ions resulted in irreversible catalytic effect for SiO2 and TiO2 supported Pd NPs characterized by different mean Pd NPs sizes.

3. Experimental

3.1. Catalyst Synthesis

The deposition of Pd0 nanoparticles on the surface of carriers was performed via deposition of Pd precursor nanoparticles on the surface followed by a reduction to Pd0 nanoparticles with hydrogen. The starting 3%Pd/SiO2 material was synthesized by incipient wetness impregnation of commercial silica gel (ChimMed, Russia, KSKG, SBET = 98 m2 g−1, Dpore = 26 nm) with an aqueous solution of [Pd(NH3)4]Cl2, followed by drying and reduction in a hydrogen flow at 400 °C. This temperature is known to be sufficient for the reduction of PdCl2 to Pd0 [47]. The starting 1%Pd/TiO2 material was synthesized by the method of deposition-precipitation of Pd polyhydroxo-complexes (PHC) on the titania support followed by reduction of PHC to metallic nanoparticles with hydrogen in a slurry as described previously [20]. The method includes (i) the preparation of an aqueous slurry of TiO2 powder in a preliminary prepared solution that contained H2PdCl4 and Na2CO3 in a ratio 1:2.0 ± 0.1 at 4 °C; (ii) deposition of Pd-PHC by keeping the stirred slurry at 4°C and then at 23 °C for 1 h, heating the slurry to 60°C and maintaining this temperature for 4 h; (iii) reduction of thus supported Pd-PHC with H2 for 2 h in the slurry cooled to 23 °C; (iv) separation, washing, and drying the sample under vacuum of 40 mbar at 40 °C using a rotary evaporator. The commercial TiO2 powder (P-25 AEROXIDE, Evonic, Germany, SBET = 57 m2 g−1, nonporous, dNPs = 21 nm) was used as a support.
The Cr/3%Pd/SiO2 materials were prepared using the RA and RC procedures of reduction with adsorbed hydrogen. According to the RA procedure, a 3%Pd/SiO2 sample just after reduction was cooled to room temperature in a hydrogen flow, and then a chromic acid solution of the required concentration (H2CrO4 aq, Reachim, Russia) was poured into a quartz reactor to the sample with no exposition to air. The ratio of the added solution volume to the water capacity of the Pd sample was 1.5:1. The obtained suspension was periodically shaken and kept at room temperature for a certain time. The sample was separated by centrifuging, and a solid was dried on a rotary evaporator at 40 mbar and 60 °C. The residual CrO42− content in the separated solution was determined by volumetric titration with KI solution. Preparation by the RC procedure was performed as follows. The preliminary reduced 3%Pd/SiO2 sample (0.22 g) was placed in a two-neck round-bottom flask (250 mL), and 30 mL of a chromic acid solution was added forming a slurry with pH = 4.4. After purging the flask with H2, the slurry was stirring for the required time under a weak H2 flow at 23 °C. The completeness of Cr precipitation was checked by probing with KJ and starch solutions. The complete Pd and Cr3+ deposition was also confirmed by EDS elemental analysis (Table S1). The optimal solution concentrations and time of reduction required for complete precipitation were found in the preliminary experiments, and their values are listed in Table 1. The sample was separated by centrifuging, and the pH of the separated solution was 8.3. Then the sample was triple washed with double deionized water. The obtained solid was dried using a rotary evaporator at 40 mbar and 60 °C. The samples were denoted as “Cr/Pd/S-RA” and “Cr/Pd/S-RC”, correspondingly.
The Cr/1%Pd/TiO2 materials were prepared according to the RC procedure similar to the one used in this work for the preparation of Cr/Pd/S-RC samples, as well as by the CoR procedure similar to the one described in [20]. According to the RC procedure, the washed sample 1%Pd/TiO2 was introduced in a flask poured with a proper amount of a chromic acid solution of required concentration (H2CrO4 aq). After purging the flask with H2, the slurry was stirred for 3–5 h under a weak H2 flow at 23 °C until the absence of Cr6+ in a probe of solution. After separation from the solution, the sample was washed with double distilled water (pH = 6.3). The CoR procedure supposed modification of 1%Pd/TiO2 synthesis by the introduction of chromic acid in the slurry before the step of reduction of supported Pd-PHC. The detailed scheme of preparation of Pd-PHC/TiO2 as well as 1%Pd/TiO2 samples is depicted in [20]. The samples were marked as “Cr/Pd/T-RC” and “Cr/Pd/T-CoR”, correspondingly. The Cr mass content in the samples is presented in Table 1.
When working with chromic acid, contact with eyes and skin was avoid. A self-suction respirator, acid-proofing rubber gloves, security coat, chemical-resistant protective glasses were used.

3.2. Catalyst Characterization

The SEM observations were carried out using Hitachi SU8000 (Hitachi, Tokyo, Japan) field-emission scanning electron microscope (FE-SEM). Images were acquired in secondary electron mode at 2 kV accelerating voltage. To gain SEM images, before measurements the samples were mounted on a 25 mm aluminum specimen stub, fixed by conductive plasticine-like adhesives, and coated with a thin film (15 nm) of carbon. The morphology of the samples was studied considering the possible influence of carbon coating on the surface. EDS-SEM studies were carried out using Oxford Instruments X-max 80 EDS (High Wycombe, UK) system at 20 kV accelerating voltage.
Morphology of the samples and the Pd particle size distribution were studied using Hitachi HT7700 (Tokyo, Japan) transmission electron microscope. Images were acquired in bright-field TEM mode at 100 kV accelerating voltage. A target-oriented approach was utilized for the optimization of the analytic measurements [48]. Before measurements, the samples were deposited on the 3 mm carbon-coated copper grids from suspension in isopropanol. The average particle size (dav) of Pd NPs was calculated as dav = Σnidi/n, where ni represents the number of particles with the diameter di, n is the total number of calculated particles. The total number of calculated particles for each sample was 300, and at least 7 TEM images were used to obtain the statistics of the particle size distribution. Calculations were made using Digimizer Image Analysis Software.
X-ray photoelectron spectra of the CrOx/Pd/TiO2 samples were recorded on a PHI5000 VersaProbeII (ULVAC-PHI, Inc. 2500 Hagisono, Chigasaki, Kanagawa, Japan) spectrometer. The spectrometer was preliminarily calibrated by the BE of the Au 4f level = 83.96 eV and Cu 2p3/2 = 932.62 eV. Correction of BE scale was performed based on a peak position for Pd 3d5/2 doublet 1-1′ shifted to set 335,1 eV, BE of Ti 2p3 being 459,0 eV. The monochromatic Kα radiation of the Al anode (1486.6 eV) at a power of 50 W was used as an X-ray source. The high-resolution spectra were recorded with a pass energy of 23.5 eV, spot size of 200 μm, step size of 0.2 eV. Non-linear least-squares fitting was performed using the Gauss-Lorentz algorithm including asymmetry for doublet 1-1′ Pd 3d.
Infrared spectra were recorded on a Nicolet Protege 360 (Nicolet Instrument Corporation, Madison, WI, USA) spectrophotometer in transmittance mode (spectral resolution 4 cm−1). Powdered materials were pressed into self-supporting wafers (15–25 mg cm−2) and placed in a vacuum infrared quartz cell equipped with CaF2 windows. The IR spectra of carbon monoxide adsorbed on the samples (20 Torr) were measured after (i) evacuation of the samples for 1 h at 40 °C, (ii) subsequent reduction in hydrogen at 22 °C at static conditions for 1h (two cycles of exposure of a specimen to 100 Torr of H2 for 30 min with evacuation to 10−4 Torr in-between the H2 exposures) (iii) final evacuation at 40 °C for 1 h. Carbon monoxide was adsorbed on the reduced samples at the equilibrium pressure of 20 Torr at ambient temperature. The spectra given herein are difference spectra with the spectra of the samples before CO adsorption as a background.
Since X-ray phase analysis of 1%Pd/TiO2 samples on P-25 is not informative [44], the size (d) and the dispersion (γ) of supported metallic Pd nanoparticles were calculated based on Sinfelt method [49]—the value of irreversible CO chemisorption measured at 35 °C using a Micromeritics ASAP 2020 Plus instrument (Micromeritics, Norcross, GA, USA). Before chemisorption measurement, the CrOx/Pd/TiO2 sample was evacuated at 40 °C for 60 min, reduced in a hydrogen flow at 25 °C for 40 min, and evacuated at 35 °C for 90 min. to a pressure of 10−3 mm Hg. The chemisorption on Pd was obtained by subtracting the support contribution from the total. After evacuating the sample, the CO adsorption isotherm was measured (8 points in the range of 100–450 mm Hg.) Then CO is evacuated for 30 min and the repeated isotherm was measured. The intersection of the linear region of the difference isotherm with the ordinate axis is taken as the amount of chemisorption. The dispersion was calculated taking into account the stoichiometric factor (SF) calculated from the data of the IR spectra of adsorbed CO as a ratio of signal areas for linear absorbed CO and bridge CO bands.
The materials were studied by the TPR-H2 method using the setup described in [21,42,50] and the similar two-stage procedure The TPR-H2 studies included (1) TPR-H2 run from −100 °C up to 500 °C followed by reduction at this temperature until H2 consumption ceased; (2) in situ exposition to oxygen at room temperature; and (3) TPR-H2 run of the reoxidized sample. The highest reduction temperature of 500 °C was selected based on the results of our previous studies on the reducibility of the systems CrOx/TiO2 [51] and CrOx/Pd/TiO2 [20], as well as on the data published for CrO3/SiO2 [36]. Before TPR runs the samples (0.10 ± 0.01 g of fraction 0.25–0.35 mm) were kept in an Ar flow at 40 °C for 1.5 h, then they were cooled to −100 °C. At this temperature, the flow was switched to the reducing gas, and the sample was maintained at these conditions until the baseline became stable (about 30 min). Then the samples were heated from −100 °C to 500 °C with a heating rate of 10 °C min−1 in a flow (30 mL min−1) of reducing mixture (5% H2 in Ar) and reduced at 500 °C until H2 consumption became negligible. The reduced sample was cooled down to room temperature in the Ar flow and then was kept in a (5% O2 in He) flow (40 mL min−1) in the TPR system for 2 h and further in this oxidizing medium overnight. Thus treated sample was then reduced again in the TPR run. A thermal conductivity detector (TCD) was used in order to record changes in H2 concentration and to measure H2 uptake and release. Before TCD, the reduction products were passing through a cold trap at −100 °C to remove H2O. Hydrogen consumption (release) was determined based on the measured values of area for peaks on the time-dependence TCD curves and the preliminary calibration with CuO (Aldrich-Chemie GmbH, 99%, Darmstadt, Germany) pretreated in an Ar flow at 300 °C. The presented TPR profiles were normalized per 1 g of material.

3.3. Catalytic Activity Test

The catalytic behavior of the prepared bimetallic and monometallic Pd nanoparticles was characterized in the model reaction of the liquid-phase hydrogenation of phenylacetylene (PhA) with molecular hydrogen. The catalytic activity was studied at 20 °C and atmospheric hydrogen pressure in a glass reactor in a batch mode according to the procedure reported previously in [21,41,52] as follows. A three-neck glass reactor was poured with ethanol (19 mL) and the catalyst sample (30 mg) was added. The reactor was connected with an H2 supplying system, and after purging with an H2 flow, was vigorously shaken for 30 min (until H2 consumption stopped). Then undecane (internal standard) and PhA were introduced in the reactor using syringes and shaking continued. The molar ratio PhA:Pd was 290 and 480 for SiO2- and TiO2-supported samples correspondingly, PhA concentration in solution was 0.13 mol L−1. The time dependences of the PhA and product concentrations, as well as H2 consumption, were determined. Samples of the reaction mixture were analyzed by GLC with an internal standard method. Only styrene (St) and ethylbenzene (EtB) were detected as reaction products, and the carbon balance was better than 99%. The results were shown in a common way to take into account the time dependences of the PhA conversion and selectivity to the products.

4. Conclusions

The preparation procedures based on reduction with H2 preadsorbed on Pd0 nanoparticles make possible deposition of CrOx species from aqueous solutions of chromic acid at ambient conditions on the surface of Pd0 nanoparticles supported on the commercial oxide materials. During further reduction in a hydrogen-contained gas flow, Crn+ cations seem to diffuse in bulk, and their oxidation with O2 from gas is retarded at room temperature. SEM, EDS-SEM, TEM, XPS, FTIR of adsorbed CO, and CO chemisorption studies were performed to characterize the morphology, nanoparticle size, element, and particle distribution, as well as electronic state of deposited metals in the obtained catalysts. The redox behavior of novel materials prepared via the novel procedures of the redox deposition of CrOx species on the metallic Pd nanoparticles has been studied using the method of temperature-programmed reduction with hydrogen (TPR-H2). The TPR curve profiles obtained in this work indicate a strong interaction between the deposited CrOx species and Pd0, as well as confirming the deposition of CrOx species on the surface of Pd nanoparticles, thus testifying to the availability of the surface of the modified Pd nanoparticles for H2 adsorption with the formation of β-PdHx both in the initial and reduced samples. The decrease in the temperature of β-PdH decomposition from −11°C–+80 °C to −60 °C–+40 °C, as well as the intensive hydrogen consumption in the temperature range of −50 to +40 °C (Cr/Pd/SiO2) and −90 to −40 °C (Cr/Pd/TiO2), have been shown for the first time for the oxide-supported bimetallic Pd materials. Redox-deposition of CrOx species on Pd0 nanoparticles influences the catalytic properties of palladium catalysts in hydrogenation of unsaturated C-C bonds, and the effects depend on the Cr/Pd ratio, the preparation procedure, and the size of Pd NPs, as well as support in the initial Pd catalyst. The highly dispersed bimetallic nanoparticles on TiO2 support that were synthesized via the catalytic reduction by gaseous hydrogen (RC procedure) were considerably more active than the initial Pd catalyst in the reaction of selective liquid-phase hydrogenation of phenylacetylene to styrene at room temperature and atmospheric hydrogen pressure, and consecutive hydrogenation of triple C≡C to double C=C and C-C bond proceeds.

Supplementary Materials

The following are available online at https://www.mdpi.com/article/10.3390/catal11050583/s1, Figure S1: SEM images of the samples: (a) Cr/Pd/T-CoR; (b) Cr/Pd/T-RC; (c) Cr/Pd/T-RC-3, Figure S2. TEM images and particle size distribution of the samples: (a) Cr/Pd/T-CoR; (b) Cr/Pd/T-RC; (c) Cr/Pd/T-RC-3, Table S1: Elemental analysis of the samples obtained by SEM-EDS.

Author Contributions

Conceptualization, L.M.K. and O.A.K.; methodology, O.A.K.; validation, G.I.K. and M.S.C.; formal analysis, E.A.R. and A.A.S.; investigation, O.A.K., G.I.K., and E.A.R.; resources, E.A.R. and A.A.S.; data curation, O.A.K. and E.A.R.; writing—original draft preparation, O.A.K. and E.A.R.; writing—review and editing, O.A.K. and E.A.R.; supervision, L.M.K.; project administration, E.A.R.; funding acquisition, E.A.R. and A.A.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Russian Science Foundation, grant no 17-73-20282, the part concerns activity of SiO2 supported catalysts was supported by RFBR, project number 19-33-60001.

Data Availability Statement

Data is contained within the article or Supplementary Material.

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Arnold, H.; Döbert, F.; Gaube, J. Hydrogenation reactions. In Handbook of Heterogeneous Catalysis, 2nd ed.; Ertl, G., Knözinger, H., Schüth, F., Weitkamp, J., Eds.; Wiley-VCH Verlag GmbH&Co. KGaA: Weinheim, Germany, 2008; Volume 7, pp. 3266–3359. [Google Scholar]
  2. Nasrollahzadeh, M.; Sajjadi, M.; Shokouhimehr, M.; Varma, R.S. Recent Developments in Palladium (Nano)Catalysts Supported on Polymers for Selective and Sustainable Oxidation Processes. Coord. Chem. Rev. 2019, 397, 54–75. [Google Scholar] [CrossRef]
  3. Trzeciak, A.M.; Augustyniak, A.W. The Role of Palladium Nanoparticles in Catalytic C–C Cross-Coupling Reactions. Coord. Chem. Rev. 2019, 384, 1–20. [Google Scholar] [CrossRef]
  4. Alonso, D.; Baeza, A.; Chinchilla, R.; Gómez, C.; Guillena, G.; Pastor, I.; Ramón, D. Solid-Supported Palladium Catalysts in Sonogashira Reactions: Recent Developments. Catalysts 2018, 8, 202. [Google Scholar] [CrossRef] [Green Version]
  5. Xu, X.; Shuai, K.; Xu, B. Review on Copper and Palladium Based Catalysts for Methanol Steam Reforming to Produce Hydrogen. Catalysts 2017, 7, 183. [Google Scholar] [CrossRef]
  6. Louis, C.; Delannoy, L. Selective hydrogenation of polyunsaturated hydrocarbons and unsaturated aldehydes over bimetallic catalysts. In Advances in Catalysis; Elsevier: Amsterdam, The Netherlands, 2019; Volume 64, pp. 1–88. ISBN 978-0-12-817099-1. [Google Scholar]
  7. Dasgupta, A.; Rioux, R.M. Intermetallics in Catalysis: An Exciting Subset of Multimetallic Catalysts. Catal. Today 2019, 330, 2–15. [Google Scholar] [CrossRef]
  8. Marakatti, V.S.; Peter, S.C. Synthetically Tuned Electronic and Geometrical Properties of Intermetallic Compounds as Effective Heterogeneous Catalysts. Prog. Solid State Chem. 2018, 52, 1–30. [Google Scholar] [CrossRef]
  9. Coq, B.; Figueras, F. Bimetallic Palladium Catalysts: Influence of the Co-Metal on the Catalyst Performance. J. Mol. Catal. Chem. 2001, 173, 117–134. [Google Scholar] [CrossRef]
  10. Mashkovsky, I.S.; Markov, P.V.; Bragina, G.O.; Baeva, G.N.; Rassolov, A.V.; Bukhtiyarov, A.V.; Prosvirin, I.P.; Bukhtiyarov, V.I.; Stakheev, A.Y. PdZn/α-Al2O3 Catalyst for Liquid-Phase Alkyne Hydrogenation: Effect of the Solid-State Alloy Transformation into Intermetallics. Mendeleev Commun. 2018, 28, 152–154. [Google Scholar] [CrossRef]
  11. Mashkovsky, I.S.; Smirnova, N.S.; Markov, P.V.; Baeva, G.N.; Bragina, G.O.; Bukhtiyarov, A.V.; Prosvirin, I.P.; Stakheev, A.Y. Tuning the Surface Structure and Catalytic Performance of PdIn/Al2O3 in Selective Liquid-Phase Hydrogenation by Mild Oxidative-Reductive Treatments. Mendeleev Commun. 2018, 28, 603–605. [Google Scholar] [CrossRef]
  12. Pinna, F.; Selva, M.; Signoretto, M.; Strukul, G.; Boccuzzi, F.; Benedetti, A.; Canton, P.; Fagherazzi, G. Pd-Fe/SiO2 Catalysts in the Hydrogenation of 2,4-Dinitrotoluene. J. Catal. 1994, 150, 356–367. [Google Scholar] [CrossRef]
  13. Juszczyk, W.; Pielaszek, J.; Karpiński, Z.; Pinna, F. Reaction of 2,2-Dimethylpropane with Dihydrogen over Silica-Supported PdFe Catalysts. Appl. Catal. Gen. 1996, 144, 281–291. [Google Scholar] [CrossRef]
  14. Yang, J.; Li, S.; Zhang, L.; Liu, X.; Wang, J.; Pan, X.; Li, N.; Wang, A.; Cong, Y.; Wang, X.; et al. Hydrodeoxygenation of Furans over Pd-FeOx/SiO2 Catalyst under Atmospheric Pressure. Appl. Catal. B Environ. 2017, 201, 266–277. [Google Scholar] [CrossRef]
  15. Redina, E.A.; Kirichenko, O.A.; Shesterkina, A.A.; Kustov, L.M. Unusual Behavior of Bimetallic Nanoparticles in Catalytic Processes of Hydrogenation and Selective Oxidation. Pure Appl. Chem. 2020, 92, 989–1006. [Google Scholar] [CrossRef]
  16. Shesterkina, A.A.; Kustov, L.M.; Strekalova, A.A.; Kazansky, V.B. Heterogeneous Iron-Containing Nanocatalysts—Promising Systems for Selective Hydrogenation and Hydrogenolysis. Catal. Sci. Technol. 2020, 10, 3160–3174. [Google Scholar] [CrossRef]
  17. Ouchaib, T.; Moraweck, B.; Massardier, J.; Renouprez, A. Charcoal Supported Palladium and Palladium-Chromium Catalysts: A Comparison in the Hydrogenation of Dienes with Silica Supported Metals. Catal. Today 1990, 7, 191–198. [Google Scholar] [CrossRef]
  18. Borgna, A. New Supported Palladium-Chromium Catalysts: Characterization and Catalytic Properties. J. Catal. 1991, 128, 99–112. [Google Scholar] [CrossRef]
  19. Hu, L.; Xia, G.; Qu, L.; Li, M.; Li, C.; Xin, Q.; Li, D. The Effect of Chromium on Sulfur Resistance of Pd/HY–Al2O3 Catalysts for Aromatic Hydrogenation. J. Catal. 2001, 202, 220–228. [Google Scholar] [CrossRef]
  20. Kustov, L.M.; Tarasov, A.L.; Kirichenko, O.A. Microwave-Activated Dehydrogenation of Perhydro-N-Ethylcarbazol over Bimetallic Pd-M/TiO2 Catalysts as the Second Stage of Hydrogen Storage in Liquid Substrates. Int. J. Hydrog. Energy 2017, 42, 26723–26729. [Google Scholar] [CrossRef]
  21. Kirichenko, O.; Strekalova, A.; Kapustin, G.; Shesterkina, A.; Redina, E.; Kustov, L. Redox Behavior of Novel FeOx/Pd/SiO2 Catalytic Nanomaterials. J. Therm. Anal. Calorim. 2019, 138, 1913–1922. [Google Scholar] [CrossRef]
  22. Richard, F.C.; Bourg, A.C.M. Aqueous Geochemistry of Chromium: A Review. Water Res. 1991, 25, 807–816. [Google Scholar] [CrossRef]
  23. Pfennig, A. Kirk-Othmer Encyclopedia of Chemical Technology, Vol. 1. Herausgeg. von J. I. Kroschwitz und M. Howe-Grant. John Wiley & Sons, New York—Chichester—Toronto 1991.4. Aufl., XXXII, 1087 S., zahlr. Abb. u. Tab., geb., Subskriptionspreis US-$ 225,-. Chem. Ing. Tech. 1992, 64, 1134. [Google Scholar] [CrossRef]
  24. Barbier, J. Handbook of Heterogeneous Catalysis; Ertl, G., Knözinger, H., Weitkamp, J., Eds.; Wiley-VCH: Weinheim, Germany, 1997; p. 257. [Google Scholar]
  25. Barbier, J. Redox methods for preparation of bimetallic catalysts. In Preparation of Solid Catalysts; Ertl, G., Knozinger, H., Weitkamp, J., Eds.; WILEY-VCH Verlag GmbH: Weinheim, Germany, 1999; pp. 526–540. [Google Scholar]
  26. Redina, E.A.; Kirichenko, O.A.; Greish, A.A.; Kucherov, A.V.; Tkachenko, O.P.; Kapustin, G.I.; Mishin, I.V.; Kustov, L.M. Preparation of Bimetallic Gold Catalysts by Redox Reaction on Oxide-Supported Metals for Green Chemistry Applications. Catal. Today 2015, 246, 216–231. [Google Scholar] [CrossRef]
  27. Kosmulski, M. PH-Dependent Surface Charging and Points of Zero Charge. IV. Update and New Approach. J. Colloid Interface Sci. 2009, 337, 439–448. [Google Scholar] [CrossRef] [PubMed]
  28. Huang, S.; Wang, J.; Li, Y.; Tang, J.; Zhang, X. Microstructure Characterization and Formation Mechanism of Colloid Palladium for Activation Treatment on the Surface of PPTA Fibers. Appl. Surf. Sci. 2020, 516, 146134. [Google Scholar] [CrossRef]
  29. Guo, J.; Mao, L.; Zhang, J.; Feng, C. Role of Cl− Ions in Photooxidation of Propylene on TiO2 Surface. Appl. Surf. Sci. 2010, 256, 2132–2137. [Google Scholar] [CrossRef]
  30. John, F.; Moulder William, F.; Stickle Peter, E.; Sobol Kenneth, D. Handbook of X-ray Photoelectron Spectroscopy; Chastain, J., Ed.; Perkin-Elmer: Eden Prairie, MN, USA, 1992; Volume 118. [Google Scholar]
  31. Aronniemi, M.; Sainio, J.; Lahtinen, J. Chemical State Quantification of Iron and Chromium Oxides Using XPS: The Effect of the Background Subtraction Method. Surf. Sci. 2005, 578, 108–123. [Google Scholar] [CrossRef]
  32. Vasconcelos Borges Pinho, P.; Chartier, A.; Moussy, J.-B.; Menut, D.; Miserque, F. Crystal Field Effects on the Photoemission Spectra in Cr2O3 Thin Films: From Multiplet Splitting Features to the Local Structure. Materialia 2020, 12, 100753. [Google Scholar] [CrossRef]
  33. Liu, B.; Šindelář, P.; Fang, Y.; Hasebe, K.; Terano, M. Correlation of Oxidation States of Surface Chromium Species with Ethylene Polymerization Activity for Phillips CrOx/SiO2 Catalysts Modified by Al-Alkyl Cocatalyst. J. Mol. Catal. Chem. 2005, 238, 142–150. [Google Scholar] [CrossRef]
  34. Xu, L.; Zuo, Y.; Tang, J.; Tang, Y.; Ju, P. Chromium–Palladium Films on 316L Stainless Steel by Pulse Electrodeposition and Their Corrosion Resistance in Hot Sulfuric Acid Solutions. Corros. Sci. 2011, 53, 3788–3795. [Google Scholar] [CrossRef]
  35. Keller, P.; Strehblow, H.-H. XPS Investigations of Electrochemically Formed Passive Layers on Fe/Cr-Alloys in 0.5 M H2SO4. Corros. Sci. 2004, 46, 1939–1952. [Google Scholar] [CrossRef]
  36. Liotta, L.F.; Venezia, A.M.; Pantaleo, G.; Deganello, G.; Gruttadauria, M.; Noto, R. Chromia on Silica and Zirconia Oxides as Recyclable Oxidizing System: Structural and Surface Characterization of the Active Chromium Species for Oxidation Reaction. Catal. Today 2004, 91–92, 231–236. [Google Scholar] [CrossRef]
  37. Lear, T.; Marshall, R.; Antonio Lopez-Sanchez, J.; Jackson, S.D.; Klapötke, T.M.; Bäumer, M.; Rupprechter, G.; Freund, H.-J.; Lennon, D. The Application of Infrared Spectroscopy to Probe the Surface Morphology of Alumina-Supported Palladium Catalysts. J. Chem. Phys. 2005, 123, 174706. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  38. Garbowski, E.; Feumi-Jantou, C.; Mouaddib, N.; Primet, M. Catalytic Combustion of Methane over Palladium Supported on Alumina Catalysts: Evidence for Reconstruction of Particles. Appl. Catal. Gen. 1994, 109, 277–291. [Google Scholar] [CrossRef]
  39. Boujana, S.; Demri, D.; Cressely, J.; Kiennemann, A.; Hindermann, J.P. FT-IR and TPD Studies on Support-Metal and Promoter-Metal Interaction. Pt and Pd Catalysts. Catal. Lett. 1991, 7, 359–366. [Google Scholar] [CrossRef]
  40. Pinna, F.; Signoretto, M.; Strukul, G.; Polizzi, S.; Pernicone, N. Pd-SiO2 Catalysts. Stability of β-PdHx as a Function of Pd Dispersion. React. Kinet. Catal. Lett. 1997, 60, 9–13. [Google Scholar] [CrossRef]
  41. Shesterkina, A.A.; Kirichenko, O.A.; Kozlova, L.M.; Kapustin, G.I.; Mishin, I.V.; Strelkova, A.A.; Kustov, L.M. Liquid-Phase Hydrogenation of Phenylacetylene to Styrene on Silica-Supported Pd–Fe Nanoparticles. Mendeleev Commun. 2016, 26, 228–230. [Google Scholar] [CrossRef]
  42. Kirichenko, O.; Kapustin, G.; Nissenbaum, V.; Mishin, I.; Kustov, L. Evaluation of Stability of Silica-Supported Fe–Pd and Fe–Pt Nanoparticles in Aerobic Conditions Using Thermal Analysis. J. Therm. Anal. Calorim. 2014, 118, 749–758. [Google Scholar] [CrossRef]
  43. Kirichenko, O.A.; Strekalova, A.A.; Kapustin, G.I.; Shesterkina, A.A. A New Redox Method for Depositing FeOx on the Surface of Pd(0)/SiO2 Nanoparticles—Catalysts for Selective Phenylacetylene Hydrogenation. Russ. J. Phys. Chem. A 2018, 92, 2396–2398. [Google Scholar] [CrossRef]
  44. Ouyang, L.; Tian, P.; Da, G.; Xu, X.-C.; Ao, C.; Chen, T.; Si, R.; Xu, J.; Han, Y.-F. The Origin of Active Sites for Direct Synthesis of H2O2 on Pd/TiO2 Catalysts: Interfaces of Pd and PdO Domains. J. Catal. 2015, 321, 70–80. [Google Scholar] [CrossRef]
  45. Zaki, M.I.; Fouad, N.E.; Bond, G.C.; Tahir, S.F. Temperature-Programmed Reduction of Calcined Chromia-Coated Alumina and Silica Catalysts: Probing Chromium (VI)-Oxygen Species. Thermochim. Acta 1996, 285, 167–179. [Google Scholar] [CrossRef]
  46. Markov, P.V.; Mashkovsky, I.S.; Bragina, G.O.; Wärnå, J.; Bukhtiyarov, V.I.; Stakheev, A.Y.; Murzin, D.Y. Experimental and Theoretical Analysis of Particle Size Effect in Liquid-Phase Hydrogenation of Diphenylacetylene. Chem. Eng. J. 2021, 404, 126409. [Google Scholar] [CrossRef]
  47. Jóźwiak, W.K.; Maniecki, T.P. Influence of Atmosphere Kind on Temperature Programmed Decomposition of Noble Metal Chlorides. Thermochim. Acta 2005, 435, 151–161. [Google Scholar] [CrossRef]
  48. Kachala, V.V.; Khemchyan, L.L.; Kashin, A.S.; Orlov, N.V.; Grachev, A.A.; Zalesskiy, S.S.; Ananikov, V.P. Target-Oriented Analysis of Gaseous, Liquid and Solid Chemical Systems by Mass Spectrometry, Nuclear Magnetic Resonance Spectroscopy and Electron Microscopy. Russ. Chem. Rev. 2013, 82, 648–685. [Google Scholar] [CrossRef]
  49. Yates, D. The Catalytic Activity of Rhodium in Relation to Its State of Dispersion. J. Catal. 1967, 8, 348–358. [Google Scholar] [CrossRef]
  50. Kirichenko, O.; Kapustin, G.; Nissenbaum, V.; Strelkova, A.; Shuvalova, E.; Shesterkina, A.; Kustov, L. Thermal Decomposition and Reducibility of Silica-Supported Precursors of Cu, Fe and Cu–Fe Nanoparticles. J. Therm. Anal. Calorim. 2018, 134, 233–251. [Google Scholar] [CrossRef]
  51. Kirichenko, O.A.; Nissenbaum, V.D.; Kapustin, G.I.; Kustov, L.M. Thermal Analysis of Ammonium Trioxalatometallate Complexes Supported on Titania and Reducibility of Their Decomposition Products. Thermochim. Acta 2009, 494, 35–39. [Google Scholar] [CrossRef]
  52. Shesterkina, A.A.; Kozlova, L.M.; Kirichenko, O.A.; Kapustin, G.I.; Mishin, I.V.; Kustov, L.M. Influence of the Thermal Treatment Conditions and Composition of Bimetallic Catalysts Fe—Pd/SiO2 on the Catalytic Properties in Phenylacetylene Hydrogenation. Russ. Chem. Bull. 2016, 65, 432–439. [Google Scholar] [CrossRef]
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Figure 1. SEM images of the samples: (A) Cr/Pd/T-CoR; (B) Cr/Pd/T-RC; (C) Cr/Pd/T-RC-3.
Figure 1. SEM images of the samples: (A) Cr/Pd/T-CoR; (B) Cr/Pd/T-RC; (C) Cr/Pd/T-RC-3.
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Catalysts 11 00583 g002 550
Figure 2. SEM-EDX maps for the samples: (A) Cr/Pd/T-CoR; (B) Cr/Pd/T-RC; (C) Cr/Pd/T-RC-3.
Figure 2. SEM-EDX maps for the samples: (A) Cr/Pd/T-CoR; (B) Cr/Pd/T-RC; (C) Cr/Pd/T-RC-3.
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Catalysts 11 00583 g003 550
Figure 3. TEM images and particle size distribution of the samples: (A) Cr/Pd/T-CoR; (B) Cr/Pd/T-RC; (C) Cr/Pd/T-RC-3.
Figure 3. TEM images and particle size distribution of the samples: (A) Cr/Pd/T-CoR; (B) Cr/Pd/T-RC; (C) Cr/Pd/T-RC-3.
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Figure 4. The XPS spectra for the sample Cr/Pd/T-RC-3: (a) the survey spectrum (b) high-resolution spectrum and curve fits to the Pd 3d region; (c) high-resolution Cr 2p spectrum.
Figure 4. The XPS spectra for the sample Cr/Pd/T-RC-3: (a) the survey spectrum (b) high-resolution spectrum and curve fits to the Pd 3d region; (c) high-resolution Cr 2p spectrum.
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Figure 5. FTIR spectra of CO adsorbed at the pressure of 20 Torr on Pd/T, Cr/Pd/T-CoR, and Cr/Pd/T-RC samples.
Figure 5. FTIR spectra of CO adsorbed at the pressure of 20 Torr on Pd/T, Cr/Pd/T-CoR, and Cr/Pd/T-RC samples.
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Figure 6. TPR curves of the samples: (a) initial Pd/S and Cr/Pd/S- RC samples and (b) the TPR-reduced samples after exposition to O2 in the TPR system.
Figure 6. TPR curves of the samples: (a) initial Pd/S and Cr/Pd/S- RC samples and (b) the TPR-reduced samples after exposition to O2 in the TPR system.
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Figure 7. TPR curves of the initial samples and the TPR-reduced samples after exposition to O2 in the TPR system (-ro): (a) Pd/T and Cr/Pd/T-RC samples; (b) Cr/Pd/T-RC-3 sample; (c) Cr/Pd/T-CoR samples; (d) Pd-PHC supported on γ-alumina and 1.5%Pd/Al2O3 sample reduced with H2 at room temperature.
Figure 7. TPR curves of the initial samples and the TPR-reduced samples after exposition to O2 in the TPR system (-ro): (a) Pd/T and Cr/Pd/T-RC samples; (b) Cr/Pd/T-RC-3 sample; (c) Cr/Pd/T-CoR samples; (d) Pd-PHC supported on γ-alumina and 1.5%Pd/Al2O3 sample reduced with H2 at room temperature.
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Figure 8. Hydrogen release due to β-PdHx decomposition for different samples before (solid fill) and after reduction at 500 °C (point fill).
Figure 8. Hydrogen release due to β-PdHx decomposition for different samples before (solid fill) and after reduction at 500 °C (point fill).
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Figure 9. Time dependence of the PhA conversion (solid lines) and selectivity to St (dashed lines) for the initial Pd/TiO2 and redox prepared Cr/Pd/TiO2 catalysts with Cr loading of 0.36%.
Figure 9. Time dependence of the PhA conversion (solid lines) and selectivity to St (dashed lines) for the initial Pd/TiO2 and redox prepared Cr/Pd/TiO2 catalysts with Cr loading of 0.36%.
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Figure 10. Time dependence of (a) PhA conversion and (b) selectivity to St for the initial Pd/TiO2 and redox prepared Cr/Pd/T-RC catalysts with different Cr loading.
Figure 10. Time dependence of (a) PhA conversion and (b) selectivity to St for the initial Pd/TiO2 and redox prepared Cr/Pd/T-RC catalysts with different Cr loading.
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Table
Table 1. Catalyst composition and conditions of synthesis.
Table 1. Catalyst composition and conditions of synthesis.
SampleCr, wt.%Cr:Pdv[Cr6+],
mmol L−1		τRR,
h		pHipHms
Cr/Pd/S-RA0.0260.0182.62--
Cr/Pd/S-RA0.2600.18026.0184.4-
Cr/Pd/S-RC0.3600.7401.5254.48.3
Cr/Pd/T-RC0.3600.7405.952.88.3
Cr/Pd/T-RC-20.7201.5012.032.710.3
Cr/Pd/T-RC-31.102.2018.032.610.7
Cr/Pd/T-CoR0.3600.7402.965.79.7
Cr/Pd/T-CoR-20.4300.8703.565.39.5
Cr:Pdv—the element atomic ratio in a sample; [Cr6+]—initial Cr concentration in a solution of a slurry; τRR—time of redox reaction in a slurry; pHi and pHms—pH of solutions before and after reduction in a slurry. S—SiO2 support, T—TiO2 support. RA—reduction by preadsorbed hydrogen; RC—catalytic reduction by gaseous hydrogen; CoR—co-reduction with the supported polyhydroxo complexes of Pd (PHC).
Table
Table 2. The element atomic percentages (at. %) and their relative concentrations on the surface.
Table 2. The element atomic percentages (at. %) and their relative concentrations on the surface.
SampleO 1sTi 2pC 1sPd 3dCr 2p1/2O/TiC/Ti
Cr/Pd/T-CoR 60.523.514.50.6-2.60.6
Cr/Pd/T-RC 60.423.415.40.6-2.60.7
Cr/Pd/T-RC-361.023.113.00.51.42.60.6
RC—catalytic reduction by gaseous hydrogen; CoR—co-reduction with the supported polyhydroxo complexes of Pd (PHC).
Table
Table 3. Results of the Pd 3d spectra approximation for Cr/Pd/TiO2 samples.
Table 3. Results of the Pd 3d spectra approximation for Cr/Pd/TiO2 samples.
CatalystSpectral ParametersPd 3d
1-1′
Pd0		2-2′
Pd2+		Satellite
Pd0
Cr/Pd/T-CoRBE, eV335.1337.1347.4
Half width, eV1.33.03.0
Intensity, %761410
Cr/Pd/T-RC-2BE, eV335.1337.2347.2
Half width, eV1.33.03.0
Intensity, %77167
Cr/Pd/T-RC-3BE, eV335.1337.2347.2
Half width, eV1.33.03.0
Intensity, %79147
RC—catalytic reduction by gaseous hydrogen; CoR—co-reduction with the supported polyhydroxo complexes of Pd (PHC).
Table
Table 4. Characterization of Pd nanoparticles.
Table 4. Characterization of Pd nanoparticles.
SampleCr,
%		Cr:PdvnCO,
mmol g−1Pd		SFSPd, m2 g−1γPd, %Cr:PdsdPd, nmdTEM, nm
Pd/S0-0.261.80245.3-21 22 (XRD)
Pd/T0-2.11.80180 40-2.8 ± 0.3-
Cr/Pd/T-RC 0.360.741.91.8216037 1.93.1 ± 0.34.1 ± 0.9
Cr/Pd/T-CoR 0.360.741.91.81160 36 1.93.1 ± 0.33.7 ± 0.9
Cr:Pdv—the element atomic ratio in a sample; dPd—size of Pd nanoparticle; γ Pd—dispersion of Pd; SF—stoichiometric factor. RC—catalytic reduction by gaseous hydrogen; CoR—co-reduction with the supported polyhydroxo complexes of Pd (PHC).
Table
Table 5. The major results on the catalytic activity tests of Cr/Pd/S catalysts at 45 °C.
Table 5. The major results on the catalytic activity tests of Cr/Pd/S catalysts at 45 °C.
SampleCr, wt.%Cr:PdvT, °Cr0(H2)τPhA, minS99St, %τSt, min
Pd/S--400 (H2)0.583.5913
Cr/Pd/S-RA0.0260.01860 (air)0.24117810
Cr/Pd/S-RA0.0260.018400 (air)0.405736
Cr/Pd/S-RA0.0260.018400 (H2)0.336818
Cr/Pd/S-RA0.260.18500 (H2)0.0517273180
T—the temperature of catalyst thermal treatment; r0(H2)—initial rate of hydrogen consumption (molH2 molPd−1 s−1) during reaction; τ- time of complete conversion of PhA and St respectively; S99St—selectivity to styrene at almost complete PhA conversion. RA–reduction by preadsorbed hydrogen
Table
Table 6. The major results on the catalytic activity tests for the Cr/Pd/T catalysts at 20 °C.
Table 6. The major results on the catalytic activity tests for the Cr/Pd/T catalysts at 20 °C.
SampleCr, % wt.Cr:Pdvr0(H2)cτPhAa, minS99St b, %τSta, min
Pd/T--0.1432922.5
Cr/Pd/T-RC0.360.740.45249510
Cr/Pd/T-RC-20.721.500.709837
Cr/Pd/T-RC-31.102.200.5698511
Cr/Pd/T-CoR 0.360.740.1435816
r0(H2)—initial rate of hydrogen consumption (molH2 molPd−1 s−1) during reaction; τ—time of complete conversion of PhA and St respectively; S99St—selectivity to styrene at almost complete PhA conversion. RC—catalytic reduction by gaseous hydrogen; CoR—co-reduction with the supported polyhydroxo complexes of Pd (PHC).
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Kirichenko, O.A.; Redina, E.A.; Kapustin, G.I.; Chernova, M.S.; Shesterkina, A.A.; Kustov, L.M. Facile Redox Synthesis of Novel Bimetallic Crn+/Pd0 Nanoparticles Supported on SiO2 and TiO2 for Catalytic Selective Hydrogenation with Molecular Hydrogen. Catalysts 2021, 11, 583. https://doi.org/10.3390/catal11050583

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Kirichenko OA, Redina EA, Kapustin GI, Chernova MS, Shesterkina AA, Kustov LM. Facile Redox Synthesis of Novel Bimetallic Crn+/Pd0 Nanoparticles Supported on SiO2 and TiO2 for Catalytic Selective Hydrogenation with Molecular Hydrogen. Catalysts. 2021; 11(5):583. https://doi.org/10.3390/catal11050583

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Kirichenko, Olga A., Elena A. Redina, Gennady I. Kapustin, Marina S. Chernova, Anastasiya A. Shesterkina, and Leonid M. Kustov. 2021. "Facile Redox Synthesis of Novel Bimetallic Crn+/Pd0 Nanoparticles Supported on SiO2 and TiO2 for Catalytic Selective Hydrogenation with Molecular Hydrogen" Catalysts 11, no. 5: 583. https://doi.org/10.3390/catal11050583

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