Insight Into The Effect Of Co, Fe, Cu-doped Surface Structure Of Ni-based Catalysts On Perfomance Of Carbon Elimination In CH₄/CO₂Reforming

Catalytic reforming of CH4with CO2is a key reaction in the process ofclean coal transformation technique. However, the rapid coke accumulation onthe Ni-based catalyst and coke-induced deactivation are the main drawbackrestricting its industrial application. The production of deposited carbon is acomplex process in CH4/CO2reforming, and can be generalized as carbonformation and elimination. If the produced carbon can’t be eliminated, theadsorbed carbon tends to diffuse, accumulate on the catalyst surface and inducedeactivation of the catalyst. For carbon formation, present studies widelybelieved that CH4complete dissociation is dominantly responsible for carbonformation. For carbon elimination, previous expremental and theritical studieshave suggested that the oxygen intermediates, such as O and OH species (Ospecies from CO2dissociation, and OH species is formed via RWGS reactionand CO2dissociation with H-assisted), largely act as surface cleaner to scavengethe deposited carbon. Therefore, C+O and C+OH reactions are believed to bethe key step to understand carbon removal.It has been suggested that the metal-doped bimetallic catalysts display the excellent catalytic performance and the distinctive carbon-resistance ability.Among them, NiM(M=Co, Cu, Fe) bimetallic catalysts have been one of themost widely investigated non-noble metal catalysts at present. On the other hand,the reaction atmosphere and temperature obviously affect the arrangement ofactive component, and modify the surface morphology of Ni-based alloys. Themodification of surface morphology will in turn have an effect on the adsorptionproperties of key intermediates, further on the reactivity of the deposited carbonwith oxygen intermediates and the performance of carbon elimination.In this work, a systematic density functional theory (DFT) calculation hasbeen carried out to investigate the mechanism of C+O(OH) reaction on the alloyNiM(111) surfaces, including the homogeneous and segregated surfaces,respectively, and the obtained results are expected to probe into the effects ofsecond metal composition and NiM(111) surface structure on the adsorption ofC, O, OH, CO and COH involved in carbon elimination, as well as themechanism of C+O(OH) reaction. The main conclusions are as follows:（1）For the effect of NiM(M=Co, Cu, Fe) surface structure on theadsorption of key intermediates, our results show that compared to the pureNi(111), the incorporation of second metal Co into Ni increase the adsorptionstability of C, O and OH species, but weakens the adsorption stability of COspecies, and has few effect on COH adsorption. The introduction of Fe into Ni isbeneficial for the adsorption of O, OH and C species; the addition of Fe weakensthe adsorption ability of CO, and has few effect on COH adsorption when Fe surface coverage less than3/4ML, while when Fe surface coverage is equal to1ML, the most favorable adsorption site for CO and COH transfers from thethreefold hollow site to the Fe top site and Fe-Fe bridge site, and thecorresponding adsorption energies increase, indicating that the excessivesegregation of Fe enhances the adsorption of CO and COH species. Theincorporation of Cu into Ni decreases the adsorption ability of C, O, OH, COand COH species; the decreased adsorption stability of C, O and OH speciesmay increase the diffusion ability of C, O and OH species to interact with eachother, further improve the ability of carbon elimination.（2）On the NiCu sufurce, O and OH binding at the threefold site consistedof Ni is more favorable, while on the NiCo and NiFe surfaces, O and OH preferto adsorb at the threefold site composed of three Co or Fe atoms, and thecorresponding adsorption energy is significantly increased, indicating that Coand Fe has stronger affinity for oxygen species.（3）On the pure Ni(111), Co(111), Fe(110), Cu(111) and the alloyNiM(M=Co, Fe, Cu) surfaces, OH species have a stronger ability to eliminatethe deposited carbon. However, on the homogeneous NiFe surface, O species ismore effective for carbon elimination than OH species.（4）The incorporation of second metal Co and Fe isn’t in favor of carbonelimination, namely, carbon deposits can’t be effectively improved on the alloyNiCo(111) and NiFe(111) surfaces. The introduction of Cu is beneficial forcarbon elimination, indicating that the NiCu(111) surface promotes carbon elimination.（5）On the NiCu(111) surface, the segregation of Cu is obviouslybeneficial for carbon elimination. On NiCu(111) with the segregated surface,especially for the moderate segregated NiCu(111) surface with Cu surfaceenrichment, carbon formation is inhibited, and carbon elimination is accelerated;as a result, once carbon deposition occurs on the moderate segregated NiCu(111)with Cu surface enrichment, carbon deposits can be easily eliminated by O andOH species. An efficient periodic cycle of carbon formation and elimination onthe segregated NiCu alloy with Cu surface enrichment may be one of thecontributions leading to the stable catalytic performance. This may serve as thekey factor that NiCu bimetallic catalyst with Cu surface enrichment displaysexcellent catalytic performance and carbon-resistance ability in CH4/CO2reforming.