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Article

Catalytic Oxidation of Chlorobenzene over Ruthenium-Ceria Bimetallic Catalysts

by
Meng Ye
1,*,
Li Chen
2,3,
Xiaolong Liu
2,
Wenqing Xu
2,
Tingyu Zhu
2 and
Guanyi Chen
1
1
School of Environmental Science and Engineering, Tianjin University, Tianjin 300072, China
2
Beijing Engineering Research Center of Process Pollution Control, National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China
3
Chemistry and Chemical Engineering, Guizhou University, Guiyang 550025, China
*
Author to whom correspondence should be addressed.
Catalysts 2018, 8(3), 116; https://doi.org/10.3390/catal8030116
Submission received: 28 February 2018 / Revised: 13 March 2018 / Accepted: 13 March 2018 / Published: 16 March 2018
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Abstract

:
A series of Ru-based mono and bimetallic materials were prepared and evaluated in the catalytic oxidation of chlorobenzene. Among the different Ru-based catalysts, 1Ru/TiO2(P25) was the most active catalyst and contributed the lowest complete oxidation temperature, suggesting that commercial P25 TiO2 was the best support for Ru catalysts. After ceria oxides were introduced into the Ru catalytic system, the catalytic activity of 1Ru-5Ce/TiO2(Rutile) dramatically improved and that of P25 supported catalysts was decreased. Comparing the chlorobenzene consumption rates for 1Ru/TiO2 and 1Ru-5Ce/TiO2 at 280 °C, it could be concluded that monometallic Ru catalytic system was appropriate for P25 support, and the Ru-Ce bimetallic catalytic system was suitable for the rutile TiO2 support. At 280 °C, for 1Ru-5Ce/TiO2(Rutile) and 1Ru-5Ce/TiO2(P25), the chlorobenzene conversion was stabilized at approximately 91% and 86%, respectively. According to the physicochemical properties of the catalysts as characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), and Hydrogen temperature programmed reduction (H2-TPR), it can be concluded that (a) electrophilic Oads species play an important role in VOCs oxidation; (b) abundant RuO2 nanoparticles on the surface of 1Ru-5Ce/TiO2(Rutile) result in higher catalytic activity and stability; and (c) dispersion is not the major factor for the catalytic activity, rather the unique structure greatly facilitated the catalytic activity and stability.

Graphical Abstract

1. Introduction

Reducing volatile organic compounds (VOCs) emissions has been a major challenge for manufacturers and researchers [1,2]. In comparison to other VOCs, chlorinated volatile organic compounds (CVOCs) are more toxic and difficult to remove from the flue gas [3,4,5,6]. Many strategies have been developed for CVOC abatement, including incineration, catalytic oxidation, and adsorption-based techniques [7,8,9]. Among those methods, catalytic oxidation has been regarded as the most promising due to its advantages of high efficiency and absence of secondary pollution [7,10,11,12].
Numerous catalysts have been reported for CVOC purification, with the research mainly focused on the noble metals [13,14,15,16,17] and transition metal oxides [18,19,20,21,22,23,24,25,26,27]. Abundant polychlorinated by-products were generated in the catalytic oxidation of CVOCs over the noble metals Pt and Pd [28,29]. V2O5/TiO2 has been widely studied in the complete oxidation of chlorinated organic compounds, such as the chlorobenzenes, chlorophenols, and polychlorinated biphenyl (PCBs), as well as polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) [30,31,32,33]. Recently, much attention has been devoted to the transition metal oxides (Cr, Mn, Co, Fe), where catalyst deactivation was usually observed. Hence, it is still very meaningful to explore novel catalysts with the advantages of higher ability for anti-chlorine poisoning and less polychlorinated by-products.
Ruthenium-based catalysts have been well demonstrated in ammonia synthesis [34,35], CO oxidation [36,37,38], and the deacon reaction [39,40,41,42,43]. Under oxygen-rich conditions, a highly active RuO2 layer was generated on the catalyst surface, and the oxidation reaction was highly promoted [44]. Sumitomo Chemical Corporation has built a demonstration plant for Cl2 production through the deacon process over Ru-based catalysts, and the demo-plant has been running smoothly for two years, revealing that Ru catalysts exhibit excellent stability in chlorine-containing gas flow.
Recently, Ru-based catalysts have gained increasing importance in VOC oxidation, including ethyl acetate [45], propane [46,47,48], benzene [49], chlorobenzene [50,51], trichloroethylene [33,52], methyl bromide [53], and others. It is noteworthy that Ru catalysts contributed apparently higher catalytic activity than many reported catalysts with comparable loadings in CVOC oxidation [50,51,54].
Among the Ru-based catalysts, Ru/TiO2 was recognized as the most promising catalyst in chlorine-containing oxidation reactions, such as the deacon reaction and CVOC oxidation. The supports commonly include the anatase, rutile, and P25 (mixed phase) TiO2. P25 and rutile have been recognized as the most promising supports due to the strong interactions between rutile TiO2 and RuO2 due to their similar lattice spacing. Our group studied the oxidation of trichloroethylene over Ru/TiO2 (anatase, rutile, and P25), and concluded that Ru/TiO2-P25 contributed the highest catalytic activity and stability [33].
Bimetallic catalysts commonly show higher catalytic activity, selectivity, and anti-poisoning ability than monometallic materials due to the synergistic effect [49,55,56,57,58,59,60,61,62]. Yashnik et al. prepared Pd-Mn catalysts employed in methane oxidation, and a Pd-Mn synergistic effect was observed. Recently, CeO2 has been widely studied due to its unique properties of high oxygen mobility and oxidative ability. Hence, it is of great interest to study the bimetallic catalysts Ru-Ce/TiO2 (anatase, rutile, and P25) for CVOC oxidation.
Herein, a series of Ru-based mono- and bimetallic materials were prepared and evaluated in the catalytic oxidation of chlorobenzene. XRD, XPS, TEM, HAADF-STEM and H2-TPR characterizations were conducted. Catalytic activities were well correlated with observed physicochemical properties.

2. Results and Discussion

2.1. Catalytic Oxidation of Chlorobenzene

The catalytic oxidation of chlorobenzene was conducted over different Ru-based samples, such as 1Ru/TiO2(P25), 1Ru/ZrO2, 1Ru/γ-Al2O3, and 1Ru/SiO2. The results are summarized in Figure 1a. Among those catalysts, 1Ru/TiO2(P25) contributed the lowest complete oxidation temperature at 280 °C, revealing that commercial P25 TiO2 was the best support for Ru catalysts in chlorobenzene oxidation, and it was believed that the rutile phase played an important role in the P25 support due to the similar interplanar lattice spacings for RuO2 and rutile (110) of TiO2. Besides, the intrinsic physicochemical properties of the supports are also of great concern on their catalytic performance. Considering that P25 exists as a mixed phase containing the anatase and rutile phases, 1Ru/TiO2(P25), 1Ru/TiO2(Anatase), and 1Ru/TiO2(Rutile) were prepared and compared in chlorobenzene oxidation. As shown in Figure 1b, 1Ru/TiO2(P25) was still the most active catalyst. 1Ru/TiO2(Rutile) gave far lower catalytic activity than the other two samples, and its conversion was below 50% at 350 °C. Similar phenomenon has been reported in our previous work on the catalytic oxidation of trichloroethylene over Ru/TiO2 catalysts [33]. RuO2 was commonly well-dispersed on rutile TiO2, whereas RuO2 sintering occurred on Ru/TiO2(P25) and Ru/TiO2(Anatase). However, particle size effect was often observed for noble metal catalysts. In this catalytic system, Ru/TiO2(P25) showed the highest activity than Ru/TiO2(Rutile) and Ru/TiO2(Anatase) according to the combined effects of dispersity and particle size effect.
Bimetallic catalysts commonly showed higher activity than monometallic catalysts due to the synergistic effect. To further increase in the catalytic activity of 1Ru/TiO2 materials, ceria oxides were introduced into the Ru catalytic system (Figure 2). The T90 of 1Ru-5Ce/TiO2(Rutile), 1Ru-5Ce/TiO2(P25), and 1Ru-5Ce/TiO2(Anatase) were 279, 283, and 290 °C, respectively. Surprisingly, the catalytic activity of 1Ru-5Ce/TiO2(Rutile) was dramatically improved. However, the catalytic activity of P25 supported catalysts decreased with Ce addition. 1Ru-5Ce/TiO2(Anatase) showed slight catalytic improvement in comparison to 1Ru/TiO2(Anatase). It could be concluded that the support crystal phase plays an important role in Ru-catalyzed chlorobenzene oxidation, which is consistent with previous reports.
The chlorobenzene consumption rates for 1Ru/TiO2 and 1Ru-5Ce/TiO2 catalysts were compared at 280 °C (Figure 3). It is noteworthy that 1Ru/TiO2(P25) contributed the highest chlorobenzene consumption rate (0.37 μmol/(g s)) as compared to the other samples, demonstrating that the monometallic Ru catalytic system was appropriate for P25 support. Obviously, 1Ru-5Ce/TiO2(Rutile) gave a far higher chlorobenzene consumption rate (0.34 μmol/(g s)) than that of 1Ru/TiO2(Rutile) (0.01 μmol/(g s)), revealing that the Ru-Ce bimetallic catalytic system was very suitable for rutile TiO2 support. Considering the lower cost of rutile TiO2 as compared to P25, it is very important to conduct systematic studies for rutile TiO2 supported Ru-Ce bimetallic catalysts. It is noteworthy that Ru-5Ce/TiO2(Rutile) showed comparable catalytic performance to Ru/TiO2(P25), and the RuO2 active species could be highly trapped and stabilized by TiO2(Rutile) and CeO2, which was proved in the HAADF-STEM images.
To evaluate catalytic stability, on-stream chlorobenzene oxidation experiments were carried out for 1Ru-5Ce/TiO2(P25), 1Ru-5Ce/TiO2(Rutile), and 1Ru-5Ce/TiO2(Anatase). As shown in Figure 4a, 1Ru-5Ce/TiO2(Rutile) gave best catalytic performance at 280 °C. For 1Ru-5Ce/TiO2(Rutile) and 1Ru-5Ce/TiO2(P25), the chlorobenzene conversion stabilized at approximately 91% and 86%, respectively. Although 1Ru-5Ce/TiO2(Anatase) activity showed an increasing trend over time, its catalytic activity was far lower than other two materials. Besides, light-off curves with cycle experiments were conducted. As shown in Figure 4b, the catalytic activity slightly decreased within the fourth runs. However, the catalytic activity was then stabilized, and the conversion in 16th run was basically comparable to the 4th run, indicating excellent stability for 1Ru-5Ce/TiO2(Rutile).
For VOCs purification applications, CO2 selectivity was also an important factor for the catalyst. In the catalytic oxidation of chlorobenzene over 1Ru/TiO2(Rutile) and 1Ru-5Ce/TiO2(Rutile), organic byproducts such as multi-chlorinated benzenes were basically not observed. Hence, the CO2 selectivity was calculated based on CO2 and CO. As shown in Figure 5, 1Ru-5Ce/TiO2(Rutile) gave excellent CO2 selectivity (98–100%), which was apparently better than 1Ru/TiO2(Rutile), revealing that Ru-Ce bimetallic catalytic system was beneficial to enhencing CO2 selectivity.

2.2. Catalyst Characterization

The XRD patterns of Ru-based monometallic and bimetallic catalysts were collected. As shown in Figure 6a, 1Ru/TiO2(P25) and 1Ru/ZrO2 showed no peaks ascribed to ruthenium species, possibly due to being well dispersed. For 1Ru/SiO2 and 1Ru/γ-Al2O3, peaks attributable to RuO2 were observed at 28.0°, 35.1°, and 54.3°, revealing that RuO2 was easily aggregated on the SiO2 and γ-Al2O3 surface, suggesting that this could be the main reason for their poor catalytic activities. In Figure 6b, 1Ru/TiO2 and 1Ru-5Ce/TiO2 showed similar XRD patterns, and no ceria species were observed.
XPS spectra for the catalysts 1Ru/TiO2 and 1Ru-5Ce/TiO2 were obtained; the Ru 3d and O 1s spectra are illustrated in Figure 7. As shown in Figure 7a, the supports of P25 and anatase gave the main peaks at similar a binding energy, whereas the rutile TiO2 support showed a major peak at a higher binding energy (BE) value. The main peaks showed apparent shifts to the high BE value after the addition of the CeO2. Considering that the assignments and definitions of Ru peaks were inconsistent in previous studies, and apparent shifts were observed, Ru 3d spectra were not de-convoluted in this research. The XPS data were summarized in Table 1. It could be seen that 1Ru/TiO2(Rutile) contributed the highest Ru content on the catalyst surface, whereas giving the lowest catalytic activity, revealing that the dispersion was not the only factor influencing the catalytic performance. As illustrated in Figure 7b, O 1s were de-convoluted into two peaks at 529.8 and 531.8 eV, ascribed to Olatt (lattice oxygen) and Oads (adsorbed oxygen, e.g., O2−, O22−, or O−), respectively. It was generally believed that the electrophilic Oads species play an important role in VOCs oxidation. For the three 1Ru-5Ce/TiO2 catalysts, 1Ru-5Ce/TiO2(Rutile) contributed the highest Ru/Ce ratio (0.3) and Oads /Olatt ratio (0.2) possibly due to the synergistic effect between Ru and Ce.
TEM and high resolution transmission electron microscopy (HRTEM) characterizations were conducted to identify the morphology of 1Ru/TiO2(Rutile) and 1Ru-5Ce/TiO2(Rutile). As shown in Figure 8a,b, RuO2 species were not observed on the surface of 1Ru/TiO2(Rutile), possibly due to its thin layer structure which usually showed a low contrast in TEM images. However, for 1Ru-5Ce/TiO2(Rutile), abundant RuO2 nanoparticles were observed (Figure 8c), revealing that RuO2 nanoparticles were easily formed in the presence of CeO2. Interplanar lattice spacings for RuO2 and rutile (110) of TiO2 were observed, and it could be seen that they exhibit similar lattice spacing values. Hence, RuO2 nanoparticles could be highly stabilized by TiO2(Rutile), leading to higher catalytic activity and stability than other bimetallic materials.
To further explore the distributions of monometallic Ru and bimetallic Ru-Ce species, HAADF-STEM and STEM-energy dispersive X-ray spectroscopy (STEM-EDS) mapping images were collected for 1Ru/TiO2(Rutile) and 1Ru-5Ce/TiO2(Rutile). As shown in Figure 9, the distributions for Ru (green), Ti (red), and Ce (purple) are presented. For 1Ru/TiO2(Rutile), Ru and Ti showed a similar distribution, indicating that RuO2 was highly dispersed on the rutile TiO2 support due to their similar crystal lattice spacing values. However, the highly dispersed catalyst gave poor catalytic performance, revealing that dispersion was not the major factor for the catalytic activity, which was different from the other catalytic systems. For 1Ru-5Ce/TiO2(Rutile), it was interestingly observed that Ru species were surrounded by CeO2 and therefore forming the trapped RuO2. In consideration of its high activity, it was proposed that this unique structure greatly facilitated catalytic activity and stability.
In order to research the reducibility of the catalysts, H2-TPR profiles of 1Ru-5Ce/TiO2(Rutile), 1Ru/TiO2(Rutile), and 5Ce/TiO2(Rutile) were collected and summarized in Figure 10. A single reduction peak at 150 °C for 1Ru/TiO2(Rutile), which was ascribed to the reduction of RuO2 to metallic Ru, and the actual H2 consumption was 0.187 mmol/g, which was close to the theoretical value (0.198 mmol/g) calculated by assuming that all Ru atoms in the catalysts existed as Ru4+. For 1Ru-5Ce/TiO2(Rutile), a broad peak at 128 °C was observed assigned to the overlapping of the reduction of RuO2 and the active oxygen species in CeO2. Apparently, the reducibility was enhanced in the Ru-Ce bimetallic catalyst, suggesting a synergistic effect between Ru and Ce in 1Ru-5Ce/TiO2(Rutile). The hydrogen spillover effect was commonly observed in the low temperature reduction of a catalyst composed of transition metal oxide and noble metal [49].

2.3. In Situ FTIR Studies and Reaction Mechanism

To further investigate the reaction mechanism of chlorobenzene oxidation over 1Ru-5Ce/TiO2(Rutile), in situ FTIR spectra were collected. As shown in Figure 11, the band at 1892 cm−1 was tentatively ascribed to the trace maleic anhydride coordinated to Ruδ+ at the corner sites of the RuOx clusters according to the previous research [49]. The band at 1598 cm−1 was ascribed to the phenolate species. The band at 1731 cm−1 was assigned to the C=O from quinone or other ketone species [23]. The bands between 1568–1526 cm−1 were due to the COO-antisymmetric stretching vibration of (chlorinated)-maleate and acetates, and the band at 1404 cm−1 was attributable to the COO-symmetric stretching vibrations of (chlorinated)-maleate and acetates. The band at 1367 cm−1 was assigned to the –CH2–stretching vibration of (chlorinated)-acetates.
Accordingly, the reaction mechanism was proposed. As shown in Figure 12, in comparison to 1Ru/TiO2(Rutile), 1Ru-5Ce/TiO2(Rutile) contributed a much higher catalytic efficiency due to its trapped RuO2 structure caused by CeO2. During the oxidation, CeO2 also play an important role for affording the active oxygen species, which facilitated the oxidation of chlorobenzene over RuO2 centers. For 1Ru-5Ce/TiO2(Rutile) (A), chlorobenzene (B) was firstly oxidized into phenolate species (C). Then, the phenolate species are further oxidized into o-benzoquinone (D) and p-benzoquinone (E). Subsequently, small organic intermediates are generated through the ring-opening process, and the intermediates are easily chlorinated by the chlorine released from catalyst surface (C), leading to (chlorinated)-maleate (F) and acetates (G). Finally, the intermediates are decomposed into CO2, H2O, HCl, and Cl2.

3. Materials and Methods

3.1. Catalyst Preparation

1Ru/Support samples were prepared using an impregnation method with 1 wt % Ru. In a standard preparation of 1Ru/TiO2-P25, commercial P25 TiO2 (Degussa, Essen, Germany) was mixed with an aqueous solution of Ru(NO)(NO3)3 (1.5 mg/mL, Aladdin, Shanghai, China). The mixture was stirred for 5 h at room temperature. Then, the solvent was removed under vacuum, and the solid was dried at 110 °C for 5 h. Subsequently, the resulting solid was calcined at 350 °C for 4 h, giving the final product of 1Ru/TiO2(P25). The samples of 1Ru/SiO2, 1Ru/γ-Al2O3, 1Ru/TiO2(Anatase), and 1Ru/TiO2(Rutile) were prepared using the same method by changing the supports (SiO2, γ-Al2O3, TiO2(Anatase), and TiO2(Rutile) were purchased from Aladdin, Shanghai, China).
The 1Ru-5Ce/TiO2 catalysts were prepared through a step-impregnation method. The percentage of elemental Ru was 1 wt % and that of CeO2 was 5 wt %. In a standard preparation of 1Ru-5Ce/TiO2-P25, commercial P25 TiO2 (Degussa, Essen, Germany) was mixed with the solution of Ce(NO3)3 6H2O (Aladdin, Shanghai, China), and the solid was dried at 110 °C for 5 h. Subsequently, the resulting solid was calcined at 350 °C for 4 h, giving 5Ce/TiO2-P25. Subsequently, an aqueous solution of Ru(NO)(NO3)3 (1.5 mg/mL, Aladdin, Shanghai, China) was mixed with the suspension of 5Ce/TiO2, and the mixture was stirred for 5 h at room temperature. Then, the solvent was removed under vacuum, and the resulting solid was dried at 110 °C for 5 h. Finally, the solid was calcined at 350 °C for 4 h, giving the final product of 1Ru-5Ce/TiO2-P25.

3.2. Catalyst Characterization

The catalysts were characterized using various techniques. X-ray diffraction (XRD) patterns of the catalysts were collected with a powder diffractometer (Rigaku D/Max-RA, Rigaku, Tokyo, Japan) using Cu Kα radiation (40 kV and 120 mA). The surface area and pore diameter were characterized by N2 adsorption at 77 K in an automatic surface area and porosity analyzer (Autosorb iQ, Quantachrome, Boynton Beach, FL, USA). Transmission electron microscopy (TEM), high resolution transmission electron microscopy (HR-TEM), and high-angle, annular, dark-field scanning TEM (HAADF-STEM) images were recorded on an FEI Tecnai G2 F20 field emission electron microscope (FEI, Hillsboro, OR, USA) operating at 200 kV. X-ray photoelectron spectroscopy (XPS) analyses were performed with a Thermo Scientific ESCALAB 250Xi (Thermo Fisher Scientific, Waltham, MA, USA) X-ray using an Al Kα source. Hydrogen temperature programmed reduction (H2-TPR) was carried out on an AutoChemII 2920 apparatus (Micromeritics, Atlanta, GA, USA) with a flow-type reactor. FTIR spectra were collected using an FTIR spectrometer (Nicolet 6700, Thermo Fisher Scientific, Waltham, MA, USA) equipped with an MCT detector (cooled by liquid nitrogen) and a stainless steel IR cell (CaF2 windows).

3.3. Catalytic Evaluation

The catalysts were evaluated in a fixed-bed, quartz micro-reactor (i.d. = 4 mm) from 100 to 350 °C with 100 mg of catalyst (60–80 mesh). In the middle of the quartz microreactor, a quartz sieve was fixed, and the catalyst was placed on the quartz sieve. Chlorobenzene was introduced from a gas cylinder, and its concentration, as part of the total flow (500 ppm Chlorobenzene + 20% O2 + Ar (balance)), was calibrated by a gas chromatography (GC 2010 Plus, Shimadzu, Kyoto, Japan) using a bypass. The total flow ratio of the reactant mixture was 100 mL/min, and the weight hourly space velocity (WHSV) was 60,000 mL/(g h). The reactants and products were analyzed on-line with a gas chromatography (GC 2010 Plus, Shimadzu, Kyoto, Japan) equipped with a flame ionization detector (FID). The conversion of chlorobenzene was calculated using Equation (1).
X = [ C ( in ) − C ( out ) ] C ( in ) × 100 % ,
where X is the conversion, and C(in) and C(out) are the inlet and outlet concentrations of chlorobenzene, respectively. The reactants and products (CO2 and CO) were analyzed on-line with a gas chromatography (GC 2010 Plus, Shimadzu, Kyoto, Japan) equipped with a methanizer (MTH, Shimadzu, Kyoto, Japan) furnace and two flame ionization detectors (FID). In the catalytic oxidation of chlorobenzene, organic byproducts were not observed by GC. Hence, the CO2 selectivity was calculated using the equation: CO2 selectivity = [C(CO2)/(C(CO2) + C(CO))].

4. Conclusions

Among the different Ru-based catalysts, 1Ru/TiO2(P25) showed the greatest activity in chlorobenzene oxidation, revealing that commercial P25 TiO2 was the best support for Ru catalysts. After ceria oxides were introduced into the Ru catalytic system, the T90 of 1Ru-5Ce/TiO2(Rutile), 1Ru-5Ce/TiO2(P25), and 1Ru-5Ce/TiO2(Anatase) were 279, 283, and 290 °C, respectively, revealing that the support crystal phase plays an important role in Ru-catalyzed chlorobenzene oxidation. Comparing the consumption rates for the 1Ru/TiO2 and 1Ru-5Ce/TiO2 catalysts, it can be concluded that the optimum support for each catalytic system was different. For 1Ru-5Ce/TiO2(P25), 1Ru-5Ce/TiO2(Rutile), and 1Ru-5Ce/TiO2(Anatase), and 1Ru-5Ce/TiO2(Rutile) gave the best catalytic performance at 280 °C; the chlorobenzene conversion of 1Ru-5Ce/TiO2(Rutile) and 1Ru-5Ce/TiO2(P25) were found to be approximately 91% and 86%, respectively. According to XRD patterns, it can be concluded that the poor catalytic activities of Ru/SiO2 and 1Ru/γ-Al2O3 were due to RuO2 easily aggregating on the supports. According to the XPS spectra, it is believed that the electrophilic Oads species play an important role in VOCs oxidation. Compared to the 1Ru/TiO2(Rutile), abundant RuO2 nanoparticles were observed on the surface of 1Ru-5Ce/TiO2(Rutile), thereby leading to higher catalytic activity and stability. According to the HAADF-STEM and STEM-EDS mapping images, dispersion was not the major factor for the catalytic activity; the unique structure greatly facilitated the catalytic activity and stability. Compared with 1Ru/TiO2(Rutile), 1Ru-5Ce/TiO2(Rutile) contributed a much higher catalytic efficiency because of its trapped RuO2 structure, caused by CeO2. Additionally, the reaction mechanism was proposed according to the intermediates observed in the in situ FTIR studies.

Acknowledgments

This work was supported by Natural Science Foundation of China (21607154).

Author Contributions

Meng Ye and Tingyu Zhu conceived and designed the experiments; Meng Ye, Li Chen, Xiaolong Liu, Wenqing Xu, and Guanyi Chen performed the experiments; Meng Ye and Tingyu Zhu analyzed the data; and Meng Ye wrote the paper.

Conflicts of Interest

The authors declare no conflicts of interest. The founding sponsors had no role in the design of the study; the collection, analyses, or interpretation of data; the writing of the manuscript; or the decision to publish the results.

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Figure 1. Chlorobenzene conversion as a function of reaction temperature over (a) 1Ru/TiO2(P25), 1Ru/ZrO2, 1Ru/γ-Al2O3, and 1Ru/SiO2, and (b) 1Ru/TiO2(P25), 1Ru/TiO2(Anatase), and 1Ru/TiO2(Rutile) under the conditions of chlorobenzene concentration = 500 ppm, chlorobenzene/O2 molar ratio = 1/400, and weight hourly space velocity (WHSV) = 60,000 mL/(g h).
Figure 1. Chlorobenzene conversion as a function of reaction temperature over (a) 1Ru/TiO2(P25), 1Ru/ZrO2, 1Ru/γ-Al2O3, and 1Ru/SiO2, and (b) 1Ru/TiO2(P25), 1Ru/TiO2(Anatase), and 1Ru/TiO2(Rutile) under the conditions of chlorobenzene concentration = 500 ppm, chlorobenzene/O2 molar ratio = 1/400, and weight hourly space velocity (WHSV) = 60,000 mL/(g h).
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Figure 2. Chlorobenzene conversion as a function of reaction temperature over 1Ru-5Ce/TiO2 catalysts under the conditions of chlorobenzene concentration = 500 ppm, chlorobenzene/O2 molar ratio = 1/400, and WHSV = 60,000 mL/(g h).
Figure 2. Chlorobenzene conversion as a function of reaction temperature over 1Ru-5Ce/TiO2 catalysts under the conditions of chlorobenzene concentration = 500 ppm, chlorobenzene/O2 molar ratio = 1/400, and WHSV = 60,000 mL/(g h).
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Figure 3. Chlorobenzene consumption rates for 1Ru/TiO2 and 1Ru-5Ce/TiO2 catalysts at 280 °C.
Figure 3. Chlorobenzene consumption rates for 1Ru/TiO2 and 1Ru-5Ce/TiO2 catalysts at 280 °C.
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Figure 4. (a) Chlorobenzene conversion as a function of on-stream reaction time at 280 °C over the 1Ru-5Ce/TiO2 catalysts; (b) cycle experiments over 1Ru-5Ce/TiO2(Rutile) under the conditions of chlorobenzene concentration = 500 ppm, chlorobenzene/O2 molar ratio = 1/400, and WHSV = 60,000 mL/(g h).
Figure 4. (a) Chlorobenzene conversion as a function of on-stream reaction time at 280 °C over the 1Ru-5Ce/TiO2 catalysts; (b) cycle experiments over 1Ru-5Ce/TiO2(Rutile) under the conditions of chlorobenzene concentration = 500 ppm, chlorobenzene/O2 molar ratio = 1/400, and WHSV = 60,000 mL/(g h).
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Figure 5. CO2 selectivity of chlorobenzene oxidation over 1Ru/TiO2(Rutile) and 1Ru-5Ce/TiO2(Rutile) under the conditions of chlorobenzene concentration = 500 ppm, chlorobenzene/O2 molar ratio = 1/400, and WHSV = 60,000 mL/(g h).
Figure 5. CO2 selectivity of chlorobenzene oxidation over 1Ru/TiO2(Rutile) and 1Ru-5Ce/TiO2(Rutile) under the conditions of chlorobenzene concentration = 500 ppm, chlorobenzene/O2 molar ratio = 1/400, and WHSV = 60,000 mL/(g h).
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Figure 6. XRD patterns of (a) 1Ru/TiO2(P25), 1Ru/ZrO2, 1Ru/γ-Al2O3, and 1Ru/SiO2, and (b) 1Ru/TiO2 and 1Ru-5Ce/TiO2.
Figure 6. XRD patterns of (a) 1Ru/TiO2(P25), 1Ru/ZrO2, 1Ru/γ-Al2O3, and 1Ru/SiO2, and (b) 1Ru/TiO2 and 1Ru-5Ce/TiO2.
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Figure 7. XPS spectra of Ru 3d (a) and O 1s (b) for Ru-based catalysts.
Figure 7. XPS spectra of Ru 3d (a) and O 1s (b) for Ru-based catalysts.
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Figure 8. TEM and HRTEM images of the catalysts: (a,b) 1Ru/TiO2(Rutile), and (c,d) 1Ru-5Ce/TiO2(Rutile).
Figure 8. TEM and HRTEM images of the catalysts: (a,b) 1Ru/TiO2(Rutile), and (c,d) 1Ru-5Ce/TiO2(Rutile).
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Figure 9. Representative HAADF-STEM images (a,d) and STEM-EDS mapping images of (b,e) Ru, (e,g) Ti, and (f) Ce, and (h) Ru-Ce for 1Ru/TiO2(Rutile) (a–c) and 1Ru-5Ce/TiO2(Rutile) (d–h).
Figure 9. Representative HAADF-STEM images (a,d) and STEM-EDS mapping images of (b,e) Ru, (e,g) Ti, and (f) Ce, and (h) Ru-Ce for 1Ru/TiO2(Rutile) (a–c) and 1Ru-5Ce/TiO2(Rutile) (d–h).
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Figure 10. H2-TPR profiles of 1Ru-5Ce/TiO2(Rutile), 1Ru/TiO2(Rutile), and 5Ce/TiO2(Rutile).
Figure 10. H2-TPR profiles of 1Ru-5Ce/TiO2(Rutile), 1Ru/TiO2(Rutile), and 5Ce/TiO2(Rutile).
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Figure 11. In situ FTIR spectra for 1Ru-5Ce/TiO2(Rutile) collected at 290 °C after (a) 1; (b) 3; (c) 10; (d) 20; (e) 30; (f) 60; (g) 90 min on 500 ppm benzene/Ar stream.
Figure 11. In situ FTIR spectra for 1Ru-5Ce/TiO2(Rutile) collected at 290 °C after (a) 1; (b) 3; (c) 10; (d) 20; (e) 30; (f) 60; (g) 90 min on 500 ppm benzene/Ar stream.
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Figure 12. The reaction mechanism of chlorobenzene oxidation over 1Ru-5Ce/TiO2(Rutile).
Figure 12. The reaction mechanism of chlorobenzene oxidation over 1Ru-5Ce/TiO2(Rutile).
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Table
Table 1. XPS data of 1Ru/TiO2 and 1Ru-5Ce/TiO2.
Table 1. XPS data of 1Ru/TiO2 and 1Ru-5Ce/TiO2.
CatalystsRu (at. %)Ce (at. %)Ru/CeOads/Olatt
1Ru/TiO2(P25)0.5--0.2
1Ru/TiO2(Anatase)0.2--0.1
1Ru/TiO2(Rutile)0.7--0.2
1Ru-5Ce/TiO2(P25)0.52.60.20.2
1Ru-5Ce/TiO2(Anatase)0.31.90.10.2
1Ru-5Ce/TiO2(Rutile)0.73.40.30.2

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Ye, M.; Chen, L.; Liu, X.; Xu, W.; Zhu, T.; Chen, G. Catalytic Oxidation of Chlorobenzene over Ruthenium-Ceria Bimetallic Catalysts. Catalysts 2018, 8, 116. https://doi.org/10.3390/catal8030116

AMA Style

Ye M, Chen L, Liu X, Xu W, Zhu T, Chen G. Catalytic Oxidation of Chlorobenzene over Ruthenium-Ceria Bimetallic Catalysts. Catalysts. 2018; 8(3):116. https://doi.org/10.3390/catal8030116

Chicago/Turabian Style

Ye, Meng, Li Chen, Xiaolong Liu, Wenqing Xu, Tingyu Zhu, and Guanyi Chen. 2018. "Catalytic Oxidation of Chlorobenzene over Ruthenium-Ceria Bimetallic Catalysts" Catalysts 8, no. 3: 116. https://doi.org/10.3390/catal8030116

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