Correlating the Reverse Water-Gas Shift Reaction with Surface Chemistry: The Influence of Reactant Gas Exposure to Ni(100)

Using ambient pressure X-ray photoelectron spectroscopy(APXPS)and mass spectrometry (MS) while varying the temperature, we wereable to directly understand how the sequence of exposing reactantgases, CO2 and H-2, influenced the surface speciesand reverse water-gas shift reaction product formation on Ni(100).When first dosing CO2 at room temperature (RT), the Ni(100)surface shows a significant amount of surface oxide due to CO2 spontaneously dissociating into CO and O and this is nearlyunchanged upon introducing H-2. However, when H-2 is dosed first to the nickel surface and CO2 is subsequentlyintroduced, the spontaneous dissociation of CO2 still continuesyet it forms less surface oxide due to the pre-adsorbed hydrogen onthe surface. Interestingly, the major product of CO2 dissociationfrom both reactions is a surface oxide, while only a small amountof OH species is observed in either exposure case. This observationconfirms that the dominant pathway of CO2 dissociationon Ni(100) follows the redox mechanism. As the temperature increases,the adsorbed CO further dissociates into atomic carbon and oxygen.As the temperature continues to increase, the influence of the reactantgas exposure sequence no longer exists: the desorption of CO as CO(g)takes place in both cases. It is to note that the surface oxide ispresent on a nickel surface even at high temperatures over 400 & DEG;C,indicating kinetic rate differences between the adsorbed CO desorptionand surface oxide hydrogenation to H2O. Our results revealhow the sequence of dosing gases affects the surface reactivity ofthe catalyst, providing important insight into the behavior of thecatalyst surfaces and their product formation.