Highly Dispersed Copper over Î²-Mo2C as an Efficient and Stable

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Highly dispersed Copper over #-Mo2C as Efficient and Stable Catalysts for RWGS Reaction Xiao Zhang, Xiaobing Zhu, Lili Lin, Siyu Yao, Mengtao Zhang, Xi Liu, Xiaoping Wang, Yongwang Li, Chuan Shi, and Ding Ma ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b02991 • Publication Date (Web): 20 Dec 2016 Downloaded from http://pubs.acs.org on December 20, 2016

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Highly dispersed Copper over β-Mo2C as Efficient and Stable Catalysts for RWGS Reaction Xiao Zhang1, Xiaobing Zhu1, Lili Lin2, Siyu Yao2, Mengtao Zhang2, Xi Liu3, Xiaoping Wang3, Yong-Wang Li3, Chuan Shi*,1, Ding Ma*,2

1

State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024,

China 2

College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China.

3

Syncat@Beijing, Synfuels China Co. Ltd, Beijing Huairou, 101407, China.

*

To whom correspondence should be addressed: Chuan Shi ([email protected]); Ding Ma ([email protected])

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ABSTRACT

Cu-oxide catalysts tend to deactivate dramatically in reverse water gas shift (RWGS) reaction because of the aggregation of supported copper particles at high temperatures. Herein, β-Mo2C, typical kind of transition metal carbides, has been demonstrated to be capable of dispersing and stabilizing copper particles. Cu/β-Mo2C catalysts exhibit good catalytic activity and stability for RWGS reaction. Under relatively high weight hourly space velocity (WHSV=300,000 ml/g/h), the optimized 1wt% Cu/β-Mo2C exhibits superior activity over traditional oxides-supported Pt and Cu based catalysts. The activity was well maintained in a 40 h stability test and the catalyst shows stable reactivity in a six-cycle start-up cool-down experiment. Detailed structure characterizations demonstrate that the strong interaction between Cu and β-Mo2C effectively promotes the dispersion of supported copper and prevents the aggregation of Cu particles, which accounts for the extraordinary activity and stability for RWGS reaction.

KEYWORDS Molybdenum carbide, Copper, Reverse water-gas shift, CO2 dissociation, CO selectivity.


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1. Introduction Ocean acidification and climate changes caused by excessive CO2 emission are expected to be two of the most vital global environmental concerns in the 21st century.1 Therefore, developing techniques for CO2 capture and catalytic conversion has become an urgent and significant issue to mitigate this threat. Currently, only a little fraction of emitted CO2 has been reused via processes such as urea, salicylic acid and polycarbonates synthesis.2 How to effectively, economically and sustainably utilize CO2 waste as an important carbon resource is still a major challenge in scientific community. CO2 hydrogenation to CO, commonly known as reverse water gas shift reaction, is among the most promising CO2 conversion processes for CO2 utilization. The product CO could convert into various liquid fuels (gasoline, diesel and methanol) and high value oxygenates via Fischer-Tropsch synthesis and other syngas processes.3-6 Cu-based oxide catalysts have been widely used in the field of CO2 hydrogenation because of their excellent activity and selectivity.7-13 Previous studies reported that the catalytic activity of Cu-based catalyst could be considerably enhanced by increasing the copper dispersion,14 because copper dispersion influences the interface of Cu-oxides which is widely accepted as the active sites for RWGS reaction. However, traditional Cu-oxide catalysts tend to deactivate dramatically under working condition because of the fierce aggregation of supported copper particles at high temperature,15 while the regeneration of copper catalysts requires complicated activation process. As a result, the 3 ACS Paragon Plus Environment

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poor stability of Cu-oxide catalysts is the main obstacles that keeps them from industrial application. Discovering support materials that is able to effectively enhance the Cu dispersion and anchor the particles over substrate to prevent high temperature sintering is a major concern for the construction of effective and long life-time RWGS catalysts. Transition metal carbides (TMCs), a serial of inexpensive noble metal like materials, are among the promising candidates of supports.16 Previous studies has demonstrated that TMCs are excellent substrates to disperse metal.17-21 Meanwhile, the special catalytic properties of TMCs in hydrogen dissociation as well as C=O bond scissoring may further promote the RWGS activity.18, 22 In particular, recent studies have demonstrated that the hexagonal β-Mo2C as well as Co/β-Mo2C synthesized via impregnation method exhibits better performance than traditional metal-oxides RWGS catalysts,18 which invokes us to investigate the whether β-Mo2C is a suitable substitution for traditional metal oxides that is capable of achieving high activity and long-term stability in RWGS reaction. Herein, a serial of novel Cu/β-Mo2C catalysts are synthesized via temperature program carburization method by using Cu-MoO3 composites as precursor. Catalytic performance test demonstrates these catalysts exhibit extraordinary RWGS reaction activity and stability. Under optimized condition (600

o

C, WHSV=300,000 ml/g/h), 1wt%

Cu/β-Mo2C catalyst shows mass specific rate of 47.7*10-5 molCO2/gcat/s, and maintains 85% of initial activity after 40 h on stream. No obvious deactivation is observed in a six-cycle start-up cool-down test. The excellent performance of Cu/β-Mo2C shows great 4 ACS Paragon Plus Environment

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advantages over traditional Cu based catalysts. After detailed characterizations, we discover that the strong interaction between copper and hexagonal β-Mo2C substrate greatly enhances the metal dispersion and prevents the agglomeration of supported Cu particles under the working reaction condition, which accounts for the extraordinary catalytic performances. 2. Experimental 2.1 Catalyst preparation The β-Mo2C catalyst was synthesized using a TPC procedure. Ammonium paramolybdate ((NH4)6Mo7O24·4H2O; 81~83% as MoO3; Alfa Aesar) was heated to 500 o

C at 10 oC/min and held at this temperature for 4 h to prepare precursor MoO3 in a

muffle furnace. The precursor was crushed and sieved to retain particles with sizes between 300 and 450 µm (40-60 mesh). The powder was then supported in a tubular quartz reactor with quartz wool and placed in a vertical furnace. The powder exposed to 20% CH4/H2 flowing at 150 mL/min was heated from room temperature (RT) to 300 oC at 5 oC /min and then increased from 300 oC to 700 oC at a rate of 1 oC /min. The sample was maintained at 700 °C for 2 h. The resulting material was cooled to room temperature in Ar and then passivated using a 1% O2/Ar mixture for 12 h. The precursor of the Cu/β-Mo2C catalyst was prepared using a co-precipitation method. An aqueous solution containing ammonium paramolybdate ((NH4)6Mo7O24·4H2O; 81~83% as MoO3; Alfa Aesar) was mixed with the Cu(NO3)2 solution at room 5 ACS Paragon Plus Environment

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temperature. The mixed solution was stirred for 4 h and dried using a water bath at 80 oC, followed by drying at 110 oC in a vacuum oven overnight. The resulting material was calcined at 500 oC for 4 h to obtain the precursor. Then the Cu/β-Mo2C catalyst was prepared via a temperature-programmed reaction similar to TPC method addressed above apart from the 5 oC/min rate when the temperature raised from 300 oC to 700 oC. Cu/ZnO/Al2O3 catalyst (HiFUELTM W220) were purchased from Alfa Aesar. 2.2 Catalyst characterization ICP: The actual copper contents in the samples were determined by inductively coupled plasma optical emission spectroscopy (ICP-OES) (Optima 2000, PerkinElmer). XRD: X-ray powder diffraction (XRD) analysis was conducted using a D/MAX-2400 equipment (Rigaku, Japan) with Cu Kα radiation. BET: The surface area was measured on a JW-BK122W surface area analyzer by nitrogen absorption at −196 oC using the Brunauer-Emmett-Teller (BET) method. HRTEM: Scanning Transmission Electron Microscopy (S/TEM) images of the catalysts were obtained on a Talos F200X equipped with Super-X EDX, operating at 200 kV. All samples were suspended in ethanol by ultra-sonication and the obtained suspensions were dropped onto a nickle-carbon TEM grids. The grids were left to dry at room temperature prior to TEM measurements. XPS: XPS analysis was done over a Leybold Max 200 spectrometer (Leybold, Germany) using Al Kα radiation as excitation source. 6 ACS Paragon Plus Environment

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N2O Chemisorption: The Cu dispersion of Cu/β-Mo2C and the Cu/ZnO/Al2O3 catalysts were detected by N2O surface titration method, which was carried out on the AutoChem II 2920 with a thermal conductivity detector (TCD) at atmospheric pressure.23 2.3 Catalyst evaluation for RWGS The catalytic performance evaluation is done in a continuous flow fixed-bed quartz reactor at atmospheric pressure. The reactor was a 4 mm I.D. quartz U-tube in which 20 mg catalyst (40-60 mesh) diluted with inert SiO2 (80 mg) was sandwiched between quartz wool layers in the tube reactor. Before performing the experiment, the as-prepared catalysts were pretreated in 15% CH4/H2 at 100 mL/min at 590 oC for 2 h (heating rate: 10 oC/min), while the Cu/ZnO/Al2O3 catalyst was pretreated in a flow of 20% H2/N2 mixture at 250 oC for 2 h (heating rate: 5 oC/min). Following the pretreatment, the catalysts were exposed to the feed gas (a stream of H2/CO2 at 100 ml/min with a 2:1 ratio). At each temperature, the products are analyzed after 60 min steady-state reaction. The product was analyzed by using a gas chromatograph (GC7890Ⅱ, Tianmei, China) with a TCD detector. The outlet gas flow rate was determined by the inner standard method, in which the CH4, CO, and CO2 were calculated based on the flow rate of inner gas N2. Performances of the catalysts were characterized in terms of CO2 conversion. The CO2 conversion was defined as the molar ratio of CO2 removed in the reaction to the feed CO2. CO2 conversion and CO selectivity were defined as follows: 7 ACS Paragon Plus Environment

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(%) = (%) =

∗ 100%

∗ 100%

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Equation 1 Equation 2

where , , and are the flow rates of CO, CO2, CH4 in the outlet reformate, !" and was the flow rate of CO2 in the feed.

2.4 CO2 dissociation and two-step temperature programmed surface reaction CO2 dissociations at different temperatures over 36wt% Cu/ZnO/Al2O3, β-Mo2C and 1wt% Cu/β-Mo2C catalysts were performed. Activated samples were exposed to 1% CO2/Ar (100ml/min) at a proper temperature for 10 min. The effluent gases were measured online using an infrared absorption spectrometer (SICK-MAIHAK-S710, Germany). Two-step temperature programmed surface reaction was performed over 1wt% Cu/β-Mo2C catalyst. After 1% CO2/Ar (100 ml/min) exposure at 300 oC for 10 min, following Ar purging (100 ml/min), the sample was reduced in 2% H2/Ar (100ml/min) at 300 oC for 10 min. The effluent gases were detected online using a mass spectrometer (Pfeiffer Vacuum GSD 301). 3. Results and Discussion The XRD profiles of the as-prepared β-Mo2C and Cu/β-Mo2C catalysts are shown in Figure 1. The peaks at 2θ of 34.8 o, 38.4 o, and 39.8 o are attributed to the diffraction of β-Mo2C with hexagonal closest packing (HCP) crystal structure (PDF#65-8766). In addition to the patterns of molybdenum carbide, diffractions of metallic copper 8 ACS Paragon Plus Environment

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crystallites appear at 43.3 o and 50.4 o in the 5 and 10 wt% Cu/β-Mo2C samples. The absence of these peaks in 1 and 3wt% Cu/β-Mo2C catalysts indicates that copper is highly dispersed over the molybdenum carbide support with the particle sizes much smaller than 4 nm (the detection limitation of lab diffractometer). Further increasing copper loading will render the aggregation of copper domains and thus the diffractions associated with metallic Cu are visible in the corresponding XRD profiles. The BET surface area of Cu modified β-Mo2C catalysts and the unsupported β-Mo2C are shown in Table 1. The specific surface area of pure β-Mo2C support is determined to be 5.8 m2/g. When doping with copper, the surface area of Cu/β-Mo2C catalysts increases dramatically. Especially for the 1wt% Cu/β-Mo2C catalyst, the BET surface area reaches 46.0 m2/g. This phenomenon could be attributed to the facilitation of catalytic dissociating of the carbon source and hydrogen during the high temperature carburization process by the doped Cu species.24 With the increasing Cu loading, the surface area drops from 46.0 to 13.6 m2/g, the reason of which is not clear, although some reports attribute it to the coke formation during the carburization process.25 The Cu dispersion is determined using N2O titration method. The result suggests that 1wt% Cu/β-Mo2C has the highest Cu dispersion (61.8%), which means Cu species are highly dispersed over β-Mo2C substrate. This result is agreeing well with the XRD observation, where the diffractions of Cu were not observed on 1wt% Cu/β-Mo2C catalyst. While on the contrary, the higher the Cu loading, the lower the Cu dispersion, which indicates that with more Cu added into the 9 ACS Paragon Plus Environment

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substrate, it tends to aggregate into larger particles. Indeed, for 5wt% Cu/β-Mo2C, the average particles size increased to around 8.1 nm (Table 1). Further increasing the concentration of Cu to 10wt%, the dispersion of copper dropped to only 5.3% and the estimated Cu particle size is around 15.3 nm. As the catalysts have undergone 590 °C treatment, it is interesting that the loaded Cu is not seriously aggregated under this harsh condition, especially for the 1wt% Cu/β-Mo2C. The well dispersion of Cu species especially at low Cu loading maximized Cu-β-Mo2C interface, which may benefit its reaction performance. The catalytic performance in RWGS of as prepared catalysts was evaluated at various temperature under a relatively high space velocity of 300,000 ml/g/h. As shown in Table 2, Cu/β-Mo2C exhibits superior activity over literature reported metal-oxides catalysts in RWGS reaction. At 300 °C, while β-Mo2C itself can catalyze RWGS reaction at this low temperature, the addition of Cu has greatly increased the activity. The mass specific rate of 1wt% Cu/β-Mo2C reaches 7.3*10-5 molCO2/gcat/s, nearly 5 times higher than that of the commercial Cu/ZnO/Al2O3, and at least one order of magnitude more active than that of β-Mo2C (0.1*10-5 molCO2/gcat/s). When the temperature arises to 600 °C, the activity further increases to 47.7*10-5 molCO2/gcat/s. Considering the relatively low Cu loading of the 1wt% Cu/β-Mo2C, the intrinsic activity of Cu/β-Mo2C based on the active Cu species is far more active than that of traditional Cu-oxides catalysts.


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The influence of copper loading on activity and selectivity is also investigated (Figure 2a and 2b). Again, one can see that the bare β-Mo2C support is active to RWGS reaction. However, only 15% CO2 has been transformed into CO over β-Mo2C at 600 oC. Doping copper on β-Mo2C changes the reaction profile, with all the Cu/β-Mo2C catalysts more reactive than the bare β-Mo2C. Significantly, the results show that the optimized Cu loading is 1wt%, while excessive Cu leads to the gradual drop of activity. On the selectivity side, although the CO selectivities of Cu/β-Mo2C catalysts are slightly lower than that of the Cu/ZnO/Al2O3 catalyst, they are still higher than 96% at the whole temperature range over all Cu/β-Mo2C catalysts. The main side product is methane which comes from the methanation reaction of CO2. The stability of 1wt% Cu/β-Mo2C catalyst is also excellent. Under WHSV of 300,000 ml/g/h at 600 oC (Figure 2c), it maintains 85% of its initial activity after 40 h. In comparison, the Cu/ZnO/Al2O3 catalyst loses more than 60 % of its original activity (from 42.5% to 15%) within 15 h reaction under same reaction condition. In cyclic stability test from 300 oC to 600 oC (6 cycles in total), 1wt %Cu/β-Mo2C catalyst can reproduce its activity in each cycle (Figure 3), indicating that the Cu modified β-Mo2C catalyst at low Cu loading is stable not only under the steady state but also the start-up cool-down condition. Also, we performed the cyclic stability test (4 cycles in total) of 10wt% Cu/β-Mo2C and long-time stability tests of 3wt%, 5wt%, 10wt% Cu/β-Mo2C


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catalysts (Figure S1 and S2). The good results prove β-Mo2C is a better support material than metal oxides that could effectively stabilize Cu species. We suppose that the Cu-β-Mo2C interface tunes the electronic structure of loaded Cu species and thus make the catalyst more active in RWGS reaction. To check this, X-ray photoelectron spectroscopy (XPS) method is used to investigate the surface oxidation state of 1wt% Cu/β-Mo2C catalyst. As both Cu and β-Mo2C are keen to air, in order to prevent the undesirable oxidation, the test sample was treated in a flow of 15% CH4/H2 at 590 oC for 2 h in the pretreatment chamber of in-situ XPS spectrometer before measurement. The Cu 2p XPS spectrum (Figure 4a) shows that the peak at 932.6 eV dominates the spectrum. The absence of the strong satellite peaks in the region from 940 to 950 eV suggests that there is no Cu (II) species in 1wt% Cu/β-Mo2C catalyst after the in-situ activation. As it is difficult to distinguish Cu (I) and Cu (0) in XPS experiment, the Cu LMM Auger electron spectroscopy (AES) is collected simultaneously (Figure 4b). Interestingly, AES spectra exhibits peaks at both 568.2 and 569.8 eV, which indicates that the co-existence of Cu+ and Cu0 species in the 1wt% Cu/β-Mo2C sample. And this is in agreement with the computed results of Cu dispersion over iron carbides surface where Cu is slightly positively charged.26-28 The presence of Cu+ demonstrates that Cu and Mo2C has strong interaction and part of electron has transferred from Cu to Mo2C during the preparation and activation processes. The modulated electronic structure makes the


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Cu species more active in CO2 activation and the following transformation reactions, which is critical for the good RWGS performance.29 The next question is whether the Cu-Mo2C surface structure do make the 1wt% Cu/β-Mo2C highly active and stable? Indeed, as shown above, the introduction of Cu and thus the creation of the Cu-Mo2C interface is responsible for the improved activity. Moreover, the larger specific surface area and the better copper dispersion of 1wt% Cu/β-Mo2C catalyst are the most important factors, which renders a large Cu-Mo2C interface. To make this much clearer, the distribution of different elements in 1wt% Cu/β-Mo2C catalyst is characterized by using STEM-EDX, which clearly shows that Cu species are almost homogeneously distributed over the β-Mo2C as no aggregation or sintering of Cu is identified (Figure 5). This observation is in good agreement with the XRD and chemisorption results, which again confirmed that Cu forms a kind of highly dispersed species over the surface of the molybdenum carbide substrate. The highly dispersed nature of Cu species gives the catalyst a large Cu-Mo2C interface, and makes it very reactive in RWGS reaction. Besides the reaction stability evaluation, the stability of the Cu species is also checked by chemisorption method. N2O titration experiments demonstrated that the dispersion of Cu species over molybdenum carbide only slightly decreased (from 61.8% to 51.3%) after a 40 h RWGS reaction at high temperature (600 o

C) and high WHSV (300,000 mL/g/h). Obviously, no sintering is observed in the spent

1wt% Cu/β-Mo2C catalyst as revealed by STEM-EDX method (Figure S3) and XRD 13 ACS Paragon Plus Environment

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patterns (Figure S4). On the other hand, for Cu/ZnO/Al2O3 catalyst, although it has been reacted under the same reaction condition but with less reaction time (15 h), the Cu NPs of the Cu/ZnO/Al2O3 catalyst already aggregated (Figure S5), with the average size grows from 8.7 nm to 16.4 nm (determined by Scherrer's equation). Therefore, β-Mo2C has much stronger interaction with copper than ZnO-Al2O3 mixed oxide. Besides rendering the excellent activity, it greatly increased the dispersion of Cu species and its stability during the harsh reaction conditions. As 1wt% Cu/β-Mo2C is very active at low temperature, to further understand the relationship between the excellent low temperature catalytic performance and the Cu-Mo2C interaction in the 1wt% Cu/β-Mo2C catalyst, CO2 dissociation experiments under low reaction temperature were performed to distinguish the role of bare β-Mo2C support and that loaded with low concentration of Cu. As shown in Figure 6a, CO generates instantly when introducing CO2 (1% CO2/Ar, 100 ml/min) into the pretreated 1wt% Cu/β-Mo2C catalyst at 250 oC. This indicates that the catalyst can dissociate CO2 even at low temperature, which is the first step for RWGS reaction.18, 30 Instead, we did not observe the formation of large amount of CO when β-Mo2C was exposed to CO2 stream under the same condition (Figure 6a). This fact demonstrates that molybdenum carbide itself has very weak ability to active CO2 at this low temperature. Moreover, the commercial Cu/ZnO/Al2O3 catalyst, although with a much high Cu concentration (36 wt%), cannot dissociate CO2 efficiently at this temperature as well. Therefore, the 14 ACS Paragon Plus Environment

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integration of Cu with β-Mo2C is pivotal for the generation of active center for the low temperature CO2 activation, which is believe to be the key step in RWGS reaction. We determine the amount of CO generated from CO2 dissociation at different temperatures quantitatively. As shown in the Figure 6b, with the increase of the temperature, the amount of CO production increases for all three catalysts. It is noteworthy that the Cu-β-Mo2C combination greatly enhanced its capacity for CO2 dissociation much better than β-Mo2C and Cu/ZnO/Al2O3 catalyst at all temperature range, again, confirming the efficiency of the Cu-Mo2C interface in the CO2 activation process. Surface oxygen atoms left from CO2 dissociation could be reduced in the following step by H2 to produce H2O (Figure S6), without CO generated in this phase, which suggests that the RWGS reaction might follow a redox mechanism rather than intermediate mechanism over Cu/β-Mo2C catalyst. 4. Conclusion In summary, we have successfully synthesized a highly efficient and stable Cu/β-Mo2C catalyst for the RWGS reaction. The Cu-Mo2C interaction renders ultra-high dispersion of copper species on the β-Mo2C substrate, which contributes to the high catalytic activity. The interaction also effectively anchors the Cu species, giving the catalyst extraordinary stability.


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ASSOCIATED CONTENT

Supporting Information RWGS activity of 10wt% Cu/β-Mo2C catalyst for 4 cycles (Figure S1); Stability performance of 1wt% Cu/β-Mo2C, 3wt% Cu/β-Mo2C and 10wt% Cu/β-Mo2C catalysts in RWGS reaction (Figure S2); STEM-EDX images of the used 1wt% Cu/β-Mo2C sample (Figure S3); XRD patterns of the fresh and the used 1wt% Cu/β-Mo2C catalyst (Figure S4); XRD patterns of the fresh and the used Cu/ZnO/Al2O3 catalyst (Figure S5); CO2 dissociation followed by H2 reduction over 1wt% Cu/β-Mo2C catalyst at 300 oC (Figure S6).

AUTHOR INFORMATION

Corresponding Author * To whom correspondence should be addressed: Chuan Shi ([email protected]); Ding Ma ([email protected]).

ACKNOWLEDGMENT

The work was supported by the National Natural Foundation of China (Nos. 21373037, 21577013 and 21673273), the Fundamental Research Funds for the Central Universities (No. DUT15TD49 and DUT16ZD224).


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(11) Grabow, L. C.; Mavrikakis, M. ACS Catal. 2011, 1 (4), 365-384. (12) Graciani, J.; Mudiyanselage, K.; Xu, F.; Baber, A. E.; Evans, J.; Senanayake, S. D.; Stacchiola, D. J.; Liu, P.; Hrbek, J.; Fernandez Sanz, J.; Rodriguez, J. A. Science 2014, 345 (6196), 546-50. (13) Porosoff, M. D.; Yan, B.; Chen, J. G. Energy Environ. Sci. 2016, 9 (1), 62-73. (14) Reske, R.; Mistry, H.; Behafarid, F.; Roldan Cuenya, B.; Strasser, P. J. Am. Chem. Soc. 2014, 136 (19), 6978-6986. (15) Yue, H.; Zhao, Y.; Zhao, S.; Wang, B.; Ma, X.; Gong, J. Nat. Commun. 2013, 4, 2339. (16) Levy, R. B.; Boudart, M. Science 1973, 181 (4099), 547-549. (17) Li, J.; Liu, L.; Liu, Y.; Li, M.; Zhu, Y.; Liu, H.; Kou, Y.; Zhang, J.; Han, Y.; Ma, D. Energy Environ. Sci. 2014, 7 (1), 393-398. (18) Porosoff, M. D.; Yang, X.; Boscoboinik, J. A.; Chen, J. G. Angew. Chem. Int. Ed. 2014, 53 (26), 6705-6709. (19) Schweitzer, N. M.; Schaidle, J. A.; Ezekoye, O. K.; Pan, X.; Linic, S.; Thompson, L. T. J. Am. Chem. Soc. 2011, 133 (8), 2378-2381. (20) Vidal, A. B.; Feria, L.; Evans, J.; Takahashi, Y.; Liu, P.; Nakamura, K.; Illas, F.; Rodriguez, J. A. J. Phys. Chem. Lett. 2012, 3 (16), 2275-2280. (21) Sabnis, K. D.; Cui, Y.; Akatay, M. C.; Shekhar, M.; Lee, W.-S.; Miller, J. T.; Delgass, W. N.; Ribeiro, F. H. J. Catal. 2015, 331, 162-171. 18 ACS Paragon Plus Environment

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(22) Porosoff, M. D.; Kattel, S.; Li, W.; Liu, P.; Chen, J. G. Chem. Commun. 2015, 51 (32), 6988-6991. (23) Yuan, Z.; Wang, L.; Wang, J.; Xia, S.; Chen, P.; Hou, Z.; Zheng, X. Appl. Catal., B 2011, 101 (3–4), 431-440. (24) Jung, K. T.; Kim, W. B.; Rhee, C. H.; Lee, J. S. Chem. Mater. 2003, 16 (2), 307-314. (25) Ma, Y.; Guan, G.; Hao, X.; Zuo, Z.; Huang, W.; Phanthong, P.; Kusakabe, K.; Abudula, A. RSC Adv. 2014, 4 (83), 44175-44184. (26) Tian, X.; Wang, T.; Yang, Y.; Li, Y.-W.; Wang, J.; Jiao, H. PCCP 2014, 16 (48), 26997-27011. (27) Tian, X.; Wang, T.; Yang, Y.; Li, Y.-W.; Wang, J.; Jiao, H. J. Phys. Chem. C 2014, 118 (38), 21963-21974. (28) Tian, X.; Wang, T.; Yang, Y.; Li, Y.-W.; Wang, J.; Jiao, H. J. Phys. Chem. C 2015, 119 (13), 7371-7385. (29) Wang, Z.-Q.; Xu, Z.-N.; Peng, S.-Y.; Zhang, M.-J.; Lu, G.; Chen, Q.-S.; Chen, Y.; Guo, G.-C. ACS Catal. 2015, 5 (7), 4255-4259. (30) Dietz, L.; Piccinin, S.; Maestri, M. J. Phys. Chem. C 2015, 119 (9), 4959-4966. (31) Kim, S. S.; Lee, H. H.; Hong, S. C. Appl. Catal., A 2012, 423–424, 100-107. (32) Furimsky, E. Appl. Catal., A 2003, 240 (1–2), 1-28. (33) Chen, C.-S.; Cheng, W.-H.; Lin, S.-S. Catal. Lett. 2000, 68 (1), 45-48. 19 ACS Paragon Plus Environment

ACS Catalysis

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(34) Wang, L.; Zhang, S.; Liu, Y. J. Rare. Earth. 2008, 26 (1), 66-70.


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ACS Catalysis

Table 1. Characteristic of the as-prepared Cu/β-Mo2C catalysts with different Cu loading

Sample

Cu loading a

Surface area 2

b

Cu dispersion c

Cu crystallite

(wt%)

(m /g)

(%)

size (nm)d

β-Mo2C

N/A

5.8

N/A

N/A

1% Cu/β-Mo2C

1.3

46.0

61.8

N/A

3% Cu/β-Mo2C

4.1

30.3

20.7

N/A

5% Cu/β-Mo2C

6.9

24.9

6.8

8.1

10% Cu/β-Mo2C

13.5

13.6

5.1

15.3

a

Determined by ICP-OES; b Determined by N2 adsorption; chemisorption method; d Estimated by XRD profile.


c

Determined by N2O

ACS Catalysis

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Page 22 of 30

Table 2. Comparison of CO2 conversion rate and CO selectivity for the as-prepared and literature reported catalysts

Catalyst

Loading

H2:CO2

Temp.

Pressure

Rate

CO sel.

(wt %)

Ratio

(oC)

(MPa)

(10-5 molCO2/gcat/s)

(%)

0.1

100

1.4

100

This

7.5

100

work

600

20.0

100

300

7.3

96.5

24.3

97.6

This

500

37.9

99.0

work

600

47.7

99.2

300 400 β-Mo2C

Cu/β-Mo2C

N/A

1.3

2:1

2:1

0.1

500

400

0.1

Ref.

Co/β-Mo2C

7.5

2:1

300

0.1

1.41

98.1

18

Pt-Al2O3

0.97

1.4:1

400

N/A

0.16

N/A

31

Pt-TiO2

0.98

1.4:1

400

N/A

0.47

N/A

31

Cu-SiO2

9

1:1

500

0.1

3.34

N/A

32

Cu-K-SiO2

9/1.9

1:1

500

0.1

6.32

N/A

32

Cu-Al2O3

10

1:9

500

N/A

0.9

100

33

1.5

100

36

2:1

This

Cu-Zn-Al

26.1

100

work

Ni-CeO2

2

1:1

6.69

77

34

300 0.1

500 400

0.1


111

100

002

10% Cu/β-Mo2C 5% Cu/β -Mo2C

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

101

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3% Cu/β-Mo2C 1% Cu/β -Mo2C β-Mo2C

PDF#65-8766 (β -Mo2C) PDF#89-2838 (Cu)

30

35

40

45

50

55

60

65

70

75

80

85

o

2 Theta( )

Figure 1. XRD patterns of β-Mo2C and the as-prepared Cu modified β-Mo2C with different Cu loadings. Patterns of crystal β-Mo2C and metallic Cu are plotted as references.


ACS Catalysis

(a)

55 50 45

CO2 Conversion (%)

40 35

β-Mo2C

1wt% Cu/β-Mo2C 3wt% Cu/β-Mo2C 5wt% Cu/β-Mo2C

eq

10wt% Cu/β-Mo2C

um bri uili

line

36wt% Cu/ZnO/Al2O3

30 25 20 15 10 5 0 300

350

400

450

500

550

600

o

Temperature ( C)

(b) 100

80

CO Selectivity (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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60

β-Mo2C

40

1wt% Cu/β-Mo2C 3wt% Cu/β-Mo2C 5wt% Cu/β-Mo2C

20

10wt% Cu/β-Mo2C 36wt% Cu/ZnO/Al2O3

0 300

350

400

450

500 o

Temperature ( C)


550

600

Page 25 of 30

(c)

50

600 °C

45 40

CO2 Conversion/%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

35 30 25 20 15 10

1wt% Cu/β-Mo2C

5

36wt% Cu/ZnO/Al2O3

0 0

5

10

15

20

25

30

35

40

Time/h

Figure 2. CO2 conversion (a) and CO selectivity (b) of β-Mo2C and Cu modified β-Mo2C catalysts in RWGS reaction (Reaction condition: atmospheric pressure, 300-600 o

C, CO2:H2 = 1:2, WHSV = 300,000 ml/g/h) (c) Stability performance of 1wt%

Cu/β-Mo2C catalyst and the commercial 36wt% Cu/ZnO/Al2O3 catalyst in RWGS reaction. (Reaction condition: atmospheric pressure, 600 oC, CO2:H2 = 1:2, WHSV = 300,000 mL/g/h)


ACS Catalysis

(a) 1wt% Cu/β -Mo2C

50

CO2 Conversion(%)

40

30

1st 2nd 3rd 4th 5th 6th

20

10

0 300

350

400

450

500

550

600

Temperature(°C)

(b) 100

80

CO Selectivity(%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 30

60

40

20

1st 2nd 3rd 4th 5th 6th

1wt% Cu/β -Mo2C

0 300

350

400

450

500

550

600

Temperature(°C)

Figure 3. CO2 conversion (a) and CO selectivity (b) over 1wt% Cu/β-Mo2C for 6 start-up cool-down cycles (Reaction condition: atmospheric pressure, 300-600 oC, CO2:H2 = 1:2, WHSV = 300,000 ml/g/h)


Page 27 of 30

(a) Cu 2p

Intensity

932.5

∆Ε=19.8

955

950

945

940

935

930

Binding Energy/eV

(b)

Cu LMM

0

568.2

Cu

+

Cu +

0

Cu /Cu =0.68

574

572

569.8

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

570

568

566

564

Binding Energy (eV)

Figure 4. Cu 2p XPS spectra (a) and Cu LMM AES spectra (b) over 15% CH4/H2-treated 1wt% Cu/β-Mo2C catalyst.


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 5. The electron microscopic characterizations of the as-prepared 1wt% Cu/β-Mo2C sample. (a) TEM image (scale bar, 10 nm); (b) STEM images and (c, d, e and f) element mapping of C, Mo and Cu.


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(a) 12000

CO2

CO2

CO2

10000

8000

10000

8000

6000

Cu-ZnO-Al2O3

β-Mo2C

1% Cu/β-Mo2C

250°C

250°C

250°C

6000

4000

4000

2000

2000

0

2

4

6

CO

CO

CO

0

8 10

0

Time/min

2

4

6

8 10

0

Time/min

2

4

6

Concentration of CO or CO2 /ppm

Concentration of CO or CO2 /ppm

12000

0

8 10

Time/min

(b) 35000

Cu/ZnO/Al2O3 30000

Concentration of CO/ppm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

32130

β-Mo2C

1% Cu/β-Mo2C 25000 20000 15000 11494

12589

10000 5000 0

3841 1585 122122

250

766 150

3331 550

300

400 Temperature/°C

1152

500

Figure 6. (a) CO2 dissociation at 250 oC over the 36wt% Cu/ZnO/Al2O3, β-Mo2C and 1wt% Cu/β-Mo2C catalysts detected by IR analyser; (b) Concentration of CO generated during CO2 dissociation at 250 oC - 500 oC.


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

TOC Graphic


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Highly Dispersed Copper over Î²-Mo2C as an Efficient and Stable

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