Study On Reverse Water Gas Shift Reaction Over Molybdenum Carbide(nitride) Based Catalysts

In the last few years,CO₂ utilization has become a very attractive area of research,not only for environment reasons,but also due to the potential use of CO₂ as an abundant and economical feedstock for making higher value fuels and chemicals.Thus,the global warming problem and the ocean acidification issue will be well alleviated.One of the most attractive route for CO₂ utilization is catalytic conversion of CO₂ into CO via reverse water gas shift（RWGS）reaction,and the produced CO can be further used as feedstock to produce chemicals and synthetic fuels from syngas（CO+H₂）.Therefore,an ideal catalyst for RWGS reaction should have good thermostability,high activity and selectivity towards CO.In this paper,we adopted the temperature program carbonation（TPC）and temperature program nitridation methods to prepare molybdenum carbide（Mo Cx）and molybdenum nitride（Mo₂N）catalysts.And we prepared metal modified Mo₂ N catalysts by incipient-wetness impregnation method.The catalytic performance and stability of these catalysts were evaluated.Meanwhile,we introduced the cold plasma in RWGS reaction to study the synergetic effect of plasma and catalysis.The catalysts have been characterized in terms of XRD,H₂-TPR and CO₂-TPD measurements.We correlated the results of characterization and the performances of the catalyst to understand the reaction mechanism.The results were summarized as follows:（1）Molybdenum carbide catalysts exhibited excellent catalytic performance for RWGS reaction.α-Mo C1-X catalyst with face centered cubic structure（fcc）has much higher activity as compared with β-Mo₂ C with hexagonal close packed structure（hcp）and other reported noble metal catalysts.The CO₂ temperature programmed desorption（CO₂-TPD）and H₂ temperature programmed reduction（H₂-TPR）were conducted to investigate the activation and further reaction of CO₂ and H₂ over both α-Mo C1-x and β-Mo₂ C catalysts.The results suggest that CO₂ was strongly bonded on β-Mo₂ C,and being desorbed and dissociated into CO at much higher temperature as compared to α-Mo C1-x and α-Mo C1-x have a better H₂ reduction ability.Moreover,α-Mo C1-x showed a good stability.The mechanism of RWGS reaction over molybdenum carbide catalysts followed a redox mechanism.（2）We found that Mo₂ N with face centered cubic structure（fcc）was also very active for RWGS reaction with highly activity and CO selectivity（¹00%）.The addition of a small amount of Pt could improve the catalytic performance significantly at high mass space velocity.From the CO₂-TPD and H₂-TPR results,Mo₂ N had a good ability for CO₂ dissociation,but H₂ reduction ability was poor,which indicated the dissociation of H₂ was the rate-determining step for RWGS reaction on these catalysts.The addition of Pt over Mo₂ N not only facilitated the H₂ dissociation obviously,but also built another reaction route（formate decomposition mechanism）for RWGS reaction.（3）In this study,the combination of cold plasma with the Cu/β-Mo₂ C catalyst for CO₂ hydrogenation reaction was investigated at room temperature.Cu/β-Mo₂ C catalyst the most active catalyst for CO₂ hydrogenation to CO because of its high activity,high selectivity to CO and good stability.By changing input power and CO₂/H₂ molar ratio,the CO₂ conversion,CO selectivity and energy efficiency were further improved.The results showed that plasma-catalytic RWGS reaction could obtain high CO₂ conversion at room temperature,which broke through the thermodynamic equilibrium limit.Compared with other reported results,our plasma-catalytic system have the best performance.