Study On Monolithic NiC_x/Ni-foam Catalyst For Water Gas Shift Reaction

Up to now,the water-gas shift reaction（WGS）undoubtedly plays a vital role in syngas（H₂+CO）and H₂ production industry.In most scenarios,the feed gas enters a high-temperature WGS reactor from the kinetic perspective of accelerating the reaction rate.The Fe-Cr catalysts are extensively used at high-temperature stage,but the existence of Cr is harmful to environment.In recent years,Ni-based catalysts have gained great consideration to replace the Fe-Cr catalysts due to their high catalytic activity and nontoxicity,but suffers from the undesirable CH₄ formation.Transition metal carbides are proven to be effective doe the WGS reaction.Following the track of above findings,carburization of nickel foam to form NiC_x/Ni-foam might be possible to tailor a qualified Ni-based WGSR catalyst,by making the catalyst development and reaction engineering go hand in hand toward unique coupling of the carbide-improved H₂O adsorption/dissociation and high heat/mass transfer of Ni-foam,but was scarcely explored.In this work,a promising NiC_x/Ni-foam to be used for the HTS reaction has been developed,which achieves CO conversion close to thermodynamic equilibrium with high selectivity and shows good stability.The effects of catalyst preparation and reaction conditions on their catalytic performance for the WGSR process were investigated.Such foam-structured catalysts are obtainable by hydrothermal growth of NiC₂O₄·2H₂O on a Ni-foam and subsequent reduction-assisted carburization.The catalyst performance is dependent on the catalyst preparation conditions（both hydrothermal treatment and reduction-assisted carburization）,in nature,being tightly linked with the amount of nickel carbide formed and mounted on the Ni-foam.The preferred catalyst with high content of NiC_x is capable of converting 63.6%or 94.3%CO at 350℃or 400℃with trace or（27）0.8%CH₄ formation for a feed gas with H₂O/CO/Ar molar ratio of 4/1/9,and is stable for at least 110 h.The existence of non-stoichiometric nickel carbides（NiC_x）is clearly identified with the aid of XRD,SEM,TEM and XPS technioques.In-situ FT-IR experiments indicate that the WGSR proceeds over the NiC_x sites through the carboxyl pathway.The NiC_x sites facilitate the adsorption and dissociation of water thereby leading to the enhancement of catalytic activity.Of considerable interest is that the NiC_x sites show another advantage over the Ni sites in significantly suppressing the formation of Ni（CO）_n（n=2 or 3;the active precursors for CH₄ formation）,thus leading to significant improvement of the reaction selectivity.The above findings are further supported by DFT theoretical calculations.