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Article

Impact of Oxygen-Containing Groups on Pd/C in the Catalytic Hydrogenation of Acetophenone and Phenylacetylene

by
Pengyao You
1,2,
Liming Wu
1,
Lu Zhou
1,
Yong Xu
3,* and
Ruixuan Qin
1,3,*
1
College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China
2
College of Chemistry and Materials Science, Sichuan Normal University, Chengdu 610068, China
3
Innovation Laboratory for Sciences and Technologies of Energy Materials of Fujian Province (IKKEM), Xiamen 361102, China
*
Authors to whom correspondence should be addressed.
Catalysts 2024, 14(8), 545; https://doi.org/10.3390/catal14080545
Submission received: 15 July 2024 / Revised: 8 August 2024 / Accepted: 15 August 2024 / Published: 21 August 2024
(This article belongs to the Special Issue Heterogeneous Catalysis for Selective Hydrogenation)
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Abstract

:
Pd/C catalysts play a pivotal role in contemporary chemical industries due to their exceptional performance in diverse hydrogenation processes and organic reactions. Over the past century, researchers have extensively explored the factors influencing Pd/C catalyst performance, particularly emphasizing the impact of oxygen-containing groups through oxidation or reduction modifications. However, most studies use respective Pd/C catalysts to analyze the catalytic reactions of one or a class of chemical bonds (polar or non-polar). This study investigates alterations in Pd/C catalysts during temperature-programmed reduction (TPR) and evaluates the hydrogenation activity of unsaturated polar bonds (C=O, acetophenone) and non-polar bonds (C≡C, phenylacetylene) in Pd/C catalysts. The experimental results indicate that the reduction of Pd/C decreases the content of oxygen-containing groups, reducing hydrogenation activity for acetophenone but increasing it for phenylacetylene. This research highlights the preference of regular Pd surfaces for non-polar bond reactions and the role of Pd/oxide sites in facilitating polar bond hydrogenation. These discoveries offer essential insights into how oxygen-containing groups influence catalytic performance and allow us to propose potential avenues for enhancing the design and production of Pd/C catalysis.

1. Introduction

The Pd/C catalyst is one of the most extensively utilized catalysts in contemporary chemical industries. Its exceptional catalytic activity allows for widespread application across various catalytic hydrogenation processes and organic reactions. Research on Pd/C catalysts continues to thrive due to the ongoing development of carbon materials such as carbon powder, activated carbon [1], graphene [2,3], carbon nanotubes [1,4], and fullerenes [5], as well as the incorporation of additional additives [6,7,8]. Recently, there has been a substantial amount of ongoing research in this area [9,10,11,12,13,14,15], including studies on the hydrogenation of acetophenone [16,17,18,19,20] and phenylacetylene [21,22,23,24,25], as well as reviews [26,27,28,29] focusing on Pd/C catalysts. For almost a century, researchers have diligently explored the factors (e.g., carbon support, palladium precursors, preparation conditions, and additives) influencing Pd/C catalytic performance using diverse methodologies [29]. Among these, investigating the impact of oxygen-containing groups in Pd/C catalysts through oxidation or reduction has been extensively explored. Commonly employed oxidizing agents such as sodium hypochlorite [30], hydrogen peroxide [31], nitric acid, ozone, and oxygen [32] are used for oxidative modifications of carbon. This modification not only boosts the hydrophilicity of the activated carbon surface, but also provides robust adsorption sites for palladium precursor attachment. Consequently, this enhances the dispersion of Pd across the activated carbon surface, thereby facilitating the catalytic efficacy of Pd/C catalysts.
Numerous studies validate the impact of oxygen-containing groups by employing oxidized–modified catalysts in diverse reactions (e.g., rosin disproportionation [33], 4-chlorophenol hydrogenation [34,35], cinnamaldehyde hydrogenation [31], and nitrobenzene hydrogenation [36]) and observing enhanced catalytic activity. However, research has noted that nitric acid modification might lead to a decline in furfural hydrogenation catalytic performance [37]. Another approach involves diminishing the oxygen-containing groups present in Pd/C while monitoring alterations in its structure [38]. Weihong Xing and colleagues observed that subjecting the carbon support to high-temperature calcination in an argon atmosphere led to a reduction in oxygen-functional groups on the carbon surface [39]. This reduction amplified the hydrophobicity of Pd/C while introducing defects that resulted in smaller Pd particles. This combined effect significantly boosted the catalytic activity for transforming phenol into cyclohexanone. Additionally, Siping Pang observed an enlargement in Pd particle size within Pd/C catalysts post-hydrogen reduction at 200 °C, subsequently diminishing the effectiveness of the hydrogenolytic debenzylation of tetraacetyldibenzylhexaazaisowurtzitane [40]. Moreover, Xiaonian Li’s research demonstrated that, after hydrogen reduction at 600 °C, the Pd particle size within Pd/C catalysts increases, enhancing the lipophilicity and improving selectivity in the nitro group hydrogenation of ortho-chloronitrobenzene [41].
In previous studies, divergent views have been expressed regarding the impact of oxygen-containing groups, with some advocating a higher content and others preferring fewer. Our recent study underscored the preference of a regular Pd surface for non-polar bond hydrogenation, while oxidized alterations, generating interfaces like Pd/oxide, favor polar bond reactions [42]. Hence, the polarity of the unsaturated bonds emerges as a crucial factor, yet much research focuses on the activity of individual kinds of substrates. Additionally, prior studies have often failed to distinguish between the impact of changes in Pd particle size (known as the size effect) and variations in the number of oxygen-containing groups in the carrier on catalytic activity. Herein, we utilize temperature-programmed reduction (TPR) to reduce a Pd/C catalyst in a hydrogen atmosphere, thereby altering the content of the surface oxygen species. Temperature-programmed desorption (TPD) was employed to examine the impact of heat on the surface oxygen species of the Pd/C catalyst in the absence of hydrogen. In situ mass spectrometry (MS) facilitated the real-time detection of gases released during the TPR and TPD processes, enabling an analysis of changes in surface oxygen species. The changes in oxygen-containing groups (-O-, -OH, -COOH, -COOC, and -CHO) in the Pd/C catalyst were assessed using X-ray photoelectron spectroscopy (XPS) to corroborate the MS data. The structural changes in Pd in the Pd/C catalyst following TPR were analyzed using CO titration, diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS), transmission electron microscopy (TEM), and scanning electron microscopy (SEM). X-ray diffraction (XRD) was employed to characterize changes in Pd crystallinity. Furthermore, we assessed the hydrogenation activity of the phenylacetylene and acetophenone of reduced Pd/C catalysts to understand the influence of oxygen-containing groups. It was observed that the reduction of Pd/C decreased the hydrogenation activity for acetophenone but increased it for phenylacetylene. By incorporating mathematical descriptions [43], we differentiated that, apart from Pd particle size, the quantity of oxygen-containing groups of Pd/C indeed influences the catalytic activity.

2. Results and Discussion

2.1. Catalyst Characterization

Seven types of Pd/C (Table S1) were prepared under different atmospheres and temperatures to analyze the changes in oxygen-containing groups in Pd/C. The oxygen-containing species were first monitored by TPR-MS (Figure 1). The results from the in situ MS analysis indicated that the TPR process of Pd/C catalysts can be divided into two stages. In the first stage (40–330 °C), PdO is rapidly reduced by hydrogen gas to produce H2O. Notably, the partial decomposition of the carbon support releases H2O, CO, and CO2. The second stage (500–800 °C) involves further decomposition of the carbon support, leading to the release of CO. This observation suggests that the carbon support harbors not only carbon, but also a substantial percentage of oxygen-containing groups (e.g., -O-, -OH, -COOH, COOC, and -CHO). These functional groups undergo decomposition during the TPR process.
To discern the role of H2 during TPR, the alterations in the catalyst were monitored during the temperature-programmed desorption (TPD) process in a N2 environment (Figure S1). The outcomes of the in situ MS reveal a close resemblance between the TPD and the TPR processes. The initial stage (40–400 °C) demonstrates the partial decomposition of the carbon support that releases H2O, CO, and CO2. Subsequently, in the second stage (400–600 °C), further decomposition of the carbon support leads to the liberation of CO. This indicates that a significant portion of the oxygen-containing groups (-O-, -OH, -COOH, COOC, and -CHO) present in the carbon support primarily decompose under the influence of the heating process. Therefore, the catalysts that underwent heating at 400 °C and 600 °C in a N2 atmosphere were also prepared to evaluate the effect of oxygen-containing groups in Pd/C.
The XPS analysis further supported the identification of carbon–oxygen species liberation during the TPR process in Pd/C catalysts. As shown in Figures S2–S6 and Table S2, a notable decrease from 12.1% to 6.5% in the surface oxygen content of Pd/C catalysts was evident post-high-temperature reduction. The analysis of the O 1s XPS spectra (Figure S2c,d) highlighted a reduction from 65% to 56% in the abundance of C-O species (-O- and -OH) after reduction at 500 °C, indicating the facile decomposition of specific C-O species at this temperature. This observation correlates with findings documented in the literature [38]. Moreover, after the high-temperature treatment at 800 °C, the content of C=O species (-COOH, COOC, and -CHO) notably decreased from 44% to 22% [44], suggesting the decomposition of C=O species within the 500 °C to 800 °C range during the reduction process.
The reduction in oxygen-containing groups induced the agglomeration of large Pd particles, resulting in a decreased ratio of surface Pd. Indeed, the XPS analysis also revealed a decline in the surface Pd content from 2.0% to 0.6% (Figure S2a and Table S2), highlighting the influence of oxygen-containing groups in stabilizing Pd particles [45]. The analysis of metal dispersion (Figure S7 and Table 1) indicated a decrease from 20.2% to 3.6% following reduction. Moreover, the Pd particle size calculated from the metal dispersion aligned with the results obtained from the TEM and SEM (Figure 2 and Figures S8–S11). These findings suggest a gradual increase from 5.5 nm to 31 nm in Pd particle size during the reduction process.
The XRD patterns in Figure 3a illustrate that the initial state of Pd in the Pd/C catalyst was PdO, which transformed into metallic Pd upon hydrogen reduction at 60 °C. With increased reduction temperature, the XRD signal of the metallic Pd(111) diffraction peaks displayed a narrower full-width half maximum, indicating a transition of Pd particles from a low-crystallinity state to a highly crystalline state. CO diffuse reflectance infrared Fourier-transform spectroscopy (CO-DRIFTS) analysis was conducted and is presented in Figure 3b. Post-reduction at 500 °C, the atop CO adsorption signal at 1997 cm−1 on the surface of the Pd/C catalyst disappeared, replaced by a relatively lower frequency signal at 1850 cm−1 and a minimal peak at 2123 cm−1. This observation suggests that, after high-temperature reduction, a larger portion of low-crystallinity Pd transforms into a metallic-Pd regular surface resembling Pd NCs (1845 cm−1) [42]. Due to the reduced metal dispersion after reduction at 800 °C in the Pd/C catalyst, measuring the CO adsorption infrared signal becomes challenging. Nonetheless, the TEM results (Figure 2) clearly depict Pd agglomerating into larger spherical particles.
Based on the presented evidence, it can be concluded that, during the TPR process, the decomposition of oxygen-containing groups in the carbon support consistently results in the enlargement of Pd particles. Consequently, active sites on the catalyst undergo a transition from the surface, edges, and interfaces of low-crystallinity small Pd particles to the larger Pd particles exhibiting higher crystallinity.

2.2. Mathematical Analysis of the Size Effect

Researchers in previous studies have noted a simultaneous correlation between the quantity of oxygen-containing groups and the size of Pd particles [39,40,41]. However, there has been a challenge in differentiating the impacts of Pd particle size (known as the size effect) from variations in the quantity of oxygen-containing interfacial sites on catalytic activity. The size effect encompasses geometric, electronic, and surface/interface structural changes in metal nanoparticles, significantly influencing the catalytic performance. Eliminating the interference caused by the size effect is complex. Theoretical fittings of the relationship between catalyst activity and particle diameter offer insights beyond the size effect’s influence.
Christopher et al. synthesized diverse catalysts featuring different-sized Ni, Pd, and Pt particles loaded on CeO2 and investigated the correlation between the activity of various active sites and particle size [43]. Based on this study, assuming a single atom as an active site and equating the catalytic activity of a single particle to the number of active sites, a catalytic metal particle with a regular polyhedron adsorbed on the support forms. As shown in Figure 4 and Table 2 and Tables S3–S6, the relationship between the activity per amount of substance (k = X/N) and particle diameter (d) reveals the distribution of active catalytic sites on the metal particles: surface (k ∝ d−1), perimeter (k ∝ d−2), and corner (k ∝ d−3). Christopher et al. derived relationships from a stepped-particle model: surface (k ∝ d−0.9), perimeter (k ∝ d−1.9), and corner (k ∝ d−2.6). The experimental value of −2.3, falling within the range of −1.9 to −2.6, implies that the metal atoms at the nexus of the metal, support, and atmosphere were active sites [43].
Aibing Yu et al. conducted a study involving the synthesis of Pd NCs of varying sizes loaded on Al2O3-SiO2 for the hydrogenation of benzaldehyde, acetophenone, and butyrophenone [17]. Employing the previously mentioned method, the analysis of the relationship between the activity per amount of substance (k) and particle diameter (d) was performed using the results obtained from this research. The fitted results for the nine sets of activity data in the paper indicated exponents ranging between 1.59 and 2.25. This indicates that, for the catalytic hydrogenation reaction of aldehyde/ketone, active sites are generally accepted to be at the metal/oxide interface.

2.3. Relationship between Oxygen-Containing Groups in Pd/C and the Hydrogenation Activity of Acetophenone and Phenylacetylene

Catalytic activity was evaluated via the hydrogenation of acetophenone (C=O, polar) and phenylacetylene (C≡C, non-polar). Notably, the 60 °C-H2-Pd/C displayed outstanding catalytic activity for acetophenone (Figure 5a,b and Figure S12). However, following high-temperature reduction, a substantial decline in catalytic activity was observed. The examination of the relationship between activity (k) and particle diameter (d) (Figure 5c) indicated the following: k ∝ d−4.4 (−4.4 < −3). This finding suggests that the reduction in activity is not exclusively due to increased Pd size (a reduction in active sites at the edges and corners). The additional decline in catalytic activity is attributed to the reduction in Pd/oxide sites. The carbon support contains diverse oxygen-containing groups (-O-, -OH, -COOH, COOC, -CHO, etc.) that contribute to the formation of Pd/oxide interfaces. However, the decomposition of these oxygen-containing groups at high temperatures diminishes the Pd/oxide active sites within the catalyst, further diminishing its catalytic activity.
The observations from Figure 5d,e and Figure S12 indicate a notable distinction in the hydrogenation behavior between phenylacetylene and acetophenone. With an increase in reduction temperature, the rate of phenylacetylene hydrogenation also increases. The examination of the relationship between activity (k) and particle diameter (d) (Figure 5f) indicated the following: k ∝ d0.76 (0.76 > −1). Contrary to the decrease in catalytic activity due to the diminished surface Pd sites following the size increase, the activity enhancement for phenylacetylene hydrogenation is attributed to the reduction of Pd/oxide sites alongside the tuned electronic property conducive to promoting the hydrogenation of non-polar bonds [46].
The findings depicted in Figure S13 reveal that the changes observed in catalytic activity and the XRD of the Pd/C catalyst after heating in N2 are nearly indistinguishable from those obtained under the H2 atmosphere. This suggests that analogous transformations occur within the Pd/C catalyst when subjected to heating in a N2 environment, akin to the alterations witnessed during hydrogen reduction. For the homemade catalysts of Pd/C2, 60 °C-H2-Pd/C2 showed outstanding catalytic activity for acetophenone. Post-reduction at 500 °C, the catalytic activity decreased (Figure S14a) and the atop CO adsorption signal at 2130 cm−1 on the surface of the 60 °C-H2-Pd/C2 catalyst disappeared (Figure S14b). As shown in Figures S15 and S16, the TPD and TPR analyses indicate the decomposition and reduction of C2 and Pd/C2 in temperature ranges of 40–200 °C and 400–800 °C. These properties are similar to commercial Pd/C catalysts. However, due to the large size of Pd particles in 60 °C-H2-Pd/C2 (Figure S17), no significant XRD changes were observed (Figure S18).

3. Materials and Methods

3.1. Materials

Considering the diverse nature of Pd/C catalysts, the initial Pd/C utilized in our study was a commercially available catalyst (Alfa Aesar, Haverhill, MA, USA; palladium, 5% on carbon powder, Type 490; H33217; LOT: M11256; CAS:7440-05-3; EINECS: 231-115-6). In the chosen commercial Pd/C catalyst, Pd exists in the form of PdO, ensuring safety and stability during catalyst utilization. However, upon hydrogen gas reduction, it becomes highly flammable, akin to regular highly active Pd/C, necessitating careful handling and precautionary measures. Activated carbon (C2, coconut shell charcoal), ethanol (AR), phenylacetylene (AR), acetophenone (AR), and Na2PdCl4 (>98%) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). H2 (99.99%), CO (99.99%), Ar (99.99%), 5% H2/Ar, and 5% CO/Ar were purchased from Linde Gas (Dublin, Ireland). All the reagents were used without further purification.

3.2. Catalyst Preparation

Pd/C (0.1 g) was reduced under 5% H2/Ar (30 mL·min−1) at 60 °C for 2 h. After the catalyst was naturally cooled to 40 °C, the gas flow was changed into Ar (30 mL·min−1) and purged for 1 h. The catalyst obtained was marked as 60 °C-H2-Pd/C. A known amount of Pd/C (0.1 g) was dried at 40 °C for 1 h with an Ar gas flow rate of 30 mL·min−1, then reduced under 5% H2/Ar (30 mL·min−1). After the catalyst was heated to 500 °C with a heating rate of 5 °C·min−1, it was naturally cooled in the instrument to 50 °C; then, the gas flow was replaced with Ar (30 mL·min−1) and blown for 1 h. The catalyst obtained was marked as 500 °C-TPR-Pd/C; 800 °C-TPR-Pd/C was synthesized following a similar process to that described above.
Pd/C (0.1 g) was dried at 40 °C for 1 h with N2 (30 mL·min−1), then heated at 400 °C for 2 h with a heating rate of 5 °C·min−1. After the catalyst was naturally cooled to 40 °C, the gas flow was changed to Ar (30 mL·min−1) and purged for 1 h. The catalyst obtained was marked as 400 °C-N2-Pd/C; 600 °C-N2-Pd/C was synthesized following a similar process. These various treatments are summarized in Table 1.
C2 (0.1 g) was dispersed in 5 mL water. A Na2PdCl4 solution (0.5 mL, 0.1 mol/L, 0.05 mmol Pd) was then added into the C2 dispersion under stirring. After stirring for 2 h, the black powder was collected through filtration, washed with water thoroughly, and dried in the air at 90 °C for 12 h. The black powder of Pd/C2 was obtained after natural cooling. Pd/C2 (0.1 g) was reduced under 5% H2/Ar (30 mL·min−1) at 60 °C for 2 h. After the catalyst was naturally cooled to 40 °C, the gas flow was switched into Ar (30 mL·min−1) and purged for 1 h. The catalyst obtained was marked as 60 °C-H2-Pd/C2. The Pd/C2 (0.1 g) catalyst was dried at 40 °C for 1 h with Ar (30 mL·min−1), then reduced under 5% H2/Ar (30 mL·min−1) with a heating rate of 5 °C·min−1. After the catalyst was heated to 500 °C, the catalyst was naturally cooled to 40 °C, and the gas flow was changed to Ar (30 mL·min−1) and purged for 1h. The catalyst obtained was marked as 500 °C-TPR-Pd/C.

3.3. Catalyst Characterization

Temperature-programmed reduction (TPR) and temperature-programmed desorption (TPD) were carried out via a Micromeritics AutoChem II 2920 chemical adsorption instrument (Micromeritics, Norcross, GA, USA) equipped with a mass spectrometry (MS) detector. CO titration measurements were performed with a Micromeritics AutoChem II 2920 chemical adsorption instrument with a TCD detector. For each CO titration measurement, 100 mg catalyst was reduced under 5% H2/Ar (30 mL·min−1) at 200 °C for 0.5 h, then purged with Ar (30 mL·min−1) at 200 °C for 0.5 h and cooled down to 50 °C. The exact 0.51 mL pulses of a mixture of 5% CO in Ar (30 mL·min−1) were delivered to the reactor, and the time between pulses was 5 min until the consumption peaks became stable. Transmission electron microscopy (TEM) images were obtained with JEOL JEM-2100plus (JEOL, Tokyo, Japan). Scanning electron microscopy (SEM) images were obtained with Zeiss GeminiSEM 500 (Zeiss, Jena, Germany). X-ray photoelectron spectroscopy (XPS) was achieved using a PHI Quantum 2000 Scanning ESCA Microprobe instrument (Physical Electronics, Chanhassen, MN, USA) equipped with an Al Kα X-ray source (hν = 1486.6 eV) and binding energies referenced to C1s (284.8 eV). X-ray diffraction (XRD) patterns were recorded using Rigaku D-2550VL/PC (Rigaku, Tokyo, Japan) with Cu Kα radiation from 5° to 80° (scanning rate, 10·min−1). Diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS) spectra were conducted on a ThermoFisher IS50 Spectrometer with a 4 cm−1 resolution (ThermoFisher, Waltham, MA, USA).

3.4. Catalytic Tests

The catalytic reactions were performed in a glass pressure vessel with a volume of 20 mL. Prior to its utilization, the reaction vessel was cleansed with ultrapure water and subsequently dried in an oven at 90 °C for an hour. Typically, 4.0 mg of the catalysts, 1 mmol of acetophenone (4 mmol for phenylacetylene), and 5 mL of ethanol were introduced into the vessel. Additionally, a polytetrafluoroethylene magnetic stir bar, which was olive-shaped and 15 mm in length, was incorporated. The vessel was subjected to heating and stirring within a water bath. The heater employed for heating the reaction vessel in the water bath was a heating magnetic stirrer (IKA C-MAG HS7) furnished with a contact electronic thermometer (ETS-D5-IKA). The water bath temperature was fixed at 60 °C and the rotational speed was regulated at 1000 rpm. The catalysts contained the same amount (0.2 mg) of Pd in each measurement. At specific time intervals during the catalytic reaction, 100 μL of the reaction mixture was withdrawn and centrifuged, and the supernatant was analyzed using a gas chromatograph (Fuli 9790 II) equipped with an HP-5 column and an FID detector (Fuli Instruments, Wenling, China). As illustrated in the chromatograms in Figures S19 and S20, the final mixture contained multiple reduction products. For the hydrogenation of acetophenone, the product is phenylethanol, and the byproduct is ethylbenzene. For the hydrogenation reaction of phenylacetylene, the reaction product is styrene, and the byproduct is ethylbenzene.

4. Conclusions

Based on the above results, Pd/C catalysts present diverse active sites comprising various-sized Pd particle surfaces, perimeter and corner sites, and Pd/oxide interfaces. When the carbon carrier within Pd/C bears a high oxygen content, and Pd exists in the form of low-crystallinity small particles, the primary active sites are predominantly the edge and corner sites of Pd particles, along with Pd/oxide interfaces. Such a structure promoted the catalytic hydrogenation of acetophenone (C=O, polar) and reduced activity for phenylacetylene (C≡C, non-polar). On the other hand, upon heating in a reducing or inert atmosphere to eliminate oxygen, Pd transforms into highly crystalline large particles. In this scenario, the predominant active site was the Pd(0) surface, manifesting decreased hydrogenation activity for acetophenone and enhanced activity for phenylacetylene. Hence, it is deduced that the polarity of the catalytic active sites correlates with the polarity of the substrate. The correlation between the polarity of catalytic active sites and the substrate can guide the design of tailored catalysts for specific reactions. In industrial applications, selecting the appropriate Pd/C catalyst with either a high Pd(0) ratio or significant Pd/oxide interfaces can optimize catalytic performance, reduce costs, and improve product yields. Future research should investigate the structure of various carbon supports in relation to the distribution and nature of Pd active sites. By using different carbon materials with varying oxygen content and structural properties, it will be possible to further understand how carbon support influences catalytic activity and selectivity. This could lead to the development of more efficient Pd/C catalysts.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal14080545/s1. The Supplementary Information includes detailed experimental data and analysis of the Pd/C catalyst under various conditions. Table S1: Summary of treatment of the samples; Table S2: The XPS analysis of Pd/C catalyst after reduction at different temperatures; Figure S1: The MS signals of the TPD process for the Pd/C in N2; Figure S2: (a) The elemental content ratios analyzed by XPS of the Pd/C catalyst reduced at 60 °C in 5% H2/Ar and then temperature-programmed to 500 °C and 800 °C for reduction. Peak analysis results of the O 1s XPS spectra for (b) the Pd/C catalyst reduced at 60 °C in 5% H2/Ar and temperature-programmed to (c) 500 °C and (d) 800 °C for reduction; Figure S3: XPS spectrum of Pd/C catalyst reduced at 60 °C in 5% H2/Ar; Figure S4: XPS spectrum of Pd/C catalyst reduced at 500 °C in 5% H2/Ar; Figure S5: XPS spectrum of Pd/C catalyst reduced at 800 °C in 5% H2/Ar; Figure S6: (a) C 1s XPS spectra, (b) O 1s XPS spectra, and (c) Pd 3d XPS spectra of the Pd/C catalyst reduced at 60 °C in 5% H2/Ar and then temperature-programmed to 500 °C and 800 °C for reduction; Figure S7: CO titration analysis s of 60 °C-H2-Pd/C, 500 °C-TPR-Pd/C and 800 °C-TPR-Pd/C; Figure S8: TEM images of Pd/C catalyst reduced at (a–c) 60 °C in 5% H2/Ar and temperature-programmed to (d–f) 500 °C and (g–i) 800 °C for reduction; Figure S9: SEM images of Pd/C catalyst reduced at 60 °C in 5% H2/Ar acquired form In-lens detector (a, c, e, g) and (b, d, f, h) ESB; Figure S10: SEM images of Pd/C catalyst temperature-programmed to 800 °C in 5% H2/Ar for reduction. The images are acquired from (a, c, e, g) In-lens detector and (b, d, f, h) ESB, respectively; Figure S11: SEM images of Pd/C catalyst temperature-programmed to 800 °C in 5% H2/Ar for reduction. The detectors are (a, c) Inlens and (b, d) ESB; Table S3: The values of the data points in Figure 4a (FCC, regular tetrahedron) and their corresponding calculation formulas; Table S4: The values of the data points in Figure 4b (FCC, regular tetrahedron) and their corresponding calculation formulas; Table S5: The values of the data points in Figure 4c (FCC, cube) and their corresponding calculation formulas; Table S6: The values of the data points in Figure 4d (BCC, cube) and their corresponding calculation formulas; Figure S12: Catalytic activity for (a–c) acetophenone and (d–f) phenylacetylene hydrogenation of Pd/C catalysts reduced at 60 °C in 5% H2/Ar and via temperature programming to 500 °C and 800 °C; Figure S13: (a) Catalytic activity for acetophenone hydrogenation of Pd/C catalyst reduced at 60 °C in 5% H2/Ar and at 400 °C and 600 °C in N2 for 2 h. (b) XRD of initial Pd/C, Pd/C reduced at 60 °C in 5% H2/Ar, and Pd/C reduced at 400 °C and 600 °C in N2 for 2 h; Figure S14: (a) Catalytic activity for acetophenone hydrogenation and (b) CO-DRIFTS of 60 °C-H2-Pd/C2 and 500 °C-TPR-Pd/C2; Figure S15: (a)The in situ MS signals of the TPD process for the Pd/C2 in N2. (b) The in situ MS signals of the TPR process for the 500 °C-TPD-Pd/C2 in H2; Figure S16: The in situ MS signals of the TPR process for the C2 in H2; Figure S17: TEM images of 60 °C-H2-Pd/C2 catalyst; Figure S18: (a) Original XRD and (b) partially enlarged XRD of 60 °C-H2-Pd/C2 and 500 °C-TPR-Pd/C2. Figure S19: Chromatographic spectrum for acetophenone hydrogenation of Pd/C catalysts reduced at 60 °C in 5% H2/Ar. Figure S20: Chromatographic spectrum for phenylacetylene hydrogenation of Pd/C catalysts reduced at 60 °C in 5% H2/Ar.

Author Contributions

Conceptualization, R.Q.; methodology, P.Y., L.W. and L.Z.; software, R.Q.; validation, P.Y., L.W. and L.Z.; formal analysis, P.Y., L.W. and L.Z.; investigation, P.Y.; data curation, P.Y.; writing—original draft preparation, P.Y.; writing—review and editing, P.Y. and R.Q.; supervision, Y.X. and R.Q.; project administration, Y.X. and R.Q.; funding acquisition, Y.X. and R.Q. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Key R&D Program of China (2022YFA1504500), the Young Scientists Fund of the National Natural Science Foundation of China (22202164), the Natural Science Foundation of Fujian Province (2023J05006) and the Fundamental Research Funds for the Central Universities (20720230002).

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Acknowledgments

We acknowledge the support of ChatGPT, an AI language model from OpenAI, for its role in refining the language and improving the readability of this paper.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. The TPR-MS signals of the Pd/C catalyst recorded in 5% H2/Ar.
Figure 1. The TPR-MS signals of the Pd/C catalyst recorded in 5% H2/Ar.
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Figure 2. (a–c) TEM images and (d–f) particle size distribution statistics for 60 °C-H2-Pd/C, 500 °C-TPR-Pd/C, and 800 °C-TPR-Pd/C, respectively.
Figure 2. (a–c) TEM images and (d–f) particle size distribution statistics for 60 °C-H2-Pd/C, 500 °C-TPR-Pd/C, and 800 °C-TPR-Pd/C, respectively.
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Figure 3. (a) XRD of initial Pd/C, 60 °C-H2-Pd/C, 500 °C-TPR-Pd/C, and 800 °C-TPR-Pd/C. (b) CO-DRIFTS of 60 °C-H2-Pd/C and 500 °C-TPR-Pd/C. Prior to CO-DRIFTS collection, the sample was purged successively with Ar, 5% CO/Ar, and Ar for 10 min each.
Figure 3. (a) XRD of initial Pd/C, 60 °C-H2-Pd/C, 500 °C-TPR-Pd/C, and 800 °C-TPR-Pd/C. (b) CO-DRIFTS of 60 °C-H2-Pd/C and 500 °C-TPR-Pd/C. Prior to CO-DRIFTS collection, the sample was purged successively with Ar, 5% CO/Ar, and Ar for 10 min each.
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Figure 4. Mathematical relationship between particle diameter and activity of a regular polyhedron. (a) FCC, regular tetrahedron; (b) FCC, regular octahedron; (c) FCC, cube; (d) BCC, cube. FCC: Face Center Cubic; BCC: Body Center Cubic.
Figure 4. Mathematical relationship between particle diameter and activity of a regular polyhedron. (a) FCC, regular tetrahedron; (b) FCC, regular octahedron; (c) FCC, cube; (d) BCC, cube. FCC: Face Center Cubic; BCC: Body Center Cubic.
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Figure 5. Catalytic performance of 60 °C-H2-Pd/C, 500 °C-TPR-Pd/C, and 800 °C-TPR-Pd/C. Hydrogenation of acetophenone: (a) catalytic activity, (b) turnover frequency (TOF), and (c) analysis of activity related to particle size. Hydrogenation of phenylacetylene: (d) catalytic activity, (e) TOF, and (f) analysis of activity related to particle size. Reaction conditions: 60 °C, 1 bar H2, 5 mL ethanol, n(acetophenone):n(Pd) = 500:1; n(phenylacetylene):n(Pd) = 2000:1. k = TOF = (the molar of substrates conversion)/(the molar of Pd on the surface)/(conversion time).
Figure 5. Catalytic performance of 60 °C-H2-Pd/C, 500 °C-TPR-Pd/C, and 800 °C-TPR-Pd/C. Hydrogenation of acetophenone: (a) catalytic activity, (b) turnover frequency (TOF), and (c) analysis of activity related to particle size. Hydrogenation of phenylacetylene: (d) catalytic activity, (e) TOF, and (f) analysis of activity related to particle size. Reaction conditions: 60 °C, 1 bar H2, 5 mL ethanol, n(acetophenone):n(Pd) = 500:1; n(phenylacetylene):n(Pd) = 2000:1. k = TOF = (the molar of substrates conversion)/(the molar of Pd on the surface)/(conversion time).
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Table 1. The metal dispersion and particle size fitting of Pd/C catalysts after reduction at different temperatures.
Table 1. The metal dispersion and particle size fitting of Pd/C catalysts after reduction at different temperatures.
NameReduction TemperatureMetal DispersionActive Particle Diameter (Hemisphere)TEM Diameter
60 °C-H2-Pd/C60 °C20.2%5.5 nm3.2 nm
500 °C-TPR-Pd/C500 °C9.7%12 nm7.0 nm
800 °C-TPR-Pd/C800 °C3.6%31 nm22 nm
Table 2. Mathematical relationship between activity of a regular polyhedron and particle diameter.
Table 2. Mathematical relationship between activity of a regular polyhedron and particle diameter.
ModelFCC, Regular TetrahedronFCC, Regular OctahedronFCC, CubeBCC, CubeRegular Polyhedron
DDDDDD → ∞ D = αd
ND(D + 1)(D + 2)/6(2D − 1)(2D)(2D + 1)/6 − 4(D − 1)D(D + 1)/6D[D2 + (D − 1)2] + (D − 1)[D(D − 1) + D(D − 1)]D3 + (D − 1)3βD3
X(surface)3(D − 2)(D − 1)/2 + 17(D − 2)(D − 1)/2 + (2D − 1)5 × 2(D − 1)2 − (2D − 3)5(D − 1)2 − (2D − 3)γD2
X(perimeter)3(D − 2)3(D − 2)4(D − 2)4(D − 2)δD1
X(corner)3344εD0
ln kln (X/N)ln (X/N)ln (X/N)ln (X/N)k(surface) = γβ−1α−1d−1 k(perimeter) = δβ−1α−2d−2 k(corner) = εβ−1α−3d−3
ΔDD ( 2 D − 2 ) / 2 + 1 ( 2 D − 2 ) / 3 + 1
d (nm)0.275 Δ0.275 Δ0.275 Δ0.275 Δ
D: the number of atoms on the edge of the model particle; N: the number of atoms in the model particle; X(surface), X(perimeter), and X(corner): the number of atoms on the surface, perimeter, and corner of the model particle; k: activity per amount of substance; Δ: the particle edge length in atomic diameter units; d: the Pd particle edge length in nanometer units; α, β, γ, δ, and ε: different constants for different regular polyhedron.
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You, P.; Wu, L.; Zhou, L.; Xu, Y.; Qin, R. Impact of Oxygen-Containing Groups on Pd/C in the Catalytic Hydrogenation of Acetophenone and Phenylacetylene. Catalysts 2024, 14, 545. https://doi.org/10.3390/catal14080545

AMA Style

You P, Wu L, Zhou L, Xu Y, Qin R. Impact of Oxygen-Containing Groups on Pd/C in the Catalytic Hydrogenation of Acetophenone and Phenylacetylene. Catalysts. 2024; 14(8):545. https://doi.org/10.3390/catal14080545

Chicago/Turabian Style

You, Pengyao, Liming Wu, Lu Zhou, Yong Xu, and Ruixuan Qin. 2024. "Impact of Oxygen-Containing Groups on Pd/C in the Catalytic Hydrogenation of Acetophenone and Phenylacetylene" Catalysts 14, no. 8: 545. https://doi.org/10.3390/catal14080545

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