Quantum mechanical analysis of CO2-ZrO2 interaction and operando FTIR-DRIFTS analysis of WGS on Au- and Pt-based catalysts

The aims of the current study are to attempt to understand the nature of CO2 activation on a commercial ZrO2, stoichiometric m-ZrO2(-111), by using DFT calculations on periodic models for the adsorption of CO2, CO and O; to investigate the reasons of superior WGS activity and selectivity over Au and Pt-based catalysts supported on CeO2, and to scrutinize WGS reaction mechanism over those catalysts through utilizing a series of in-situ and transient operando FTIR-DRIFTS-MS analyses. Though DFT calculations on periodic models revealed that bidentate and tridentate carbonates were formed at the possible active sites upon CO2 adsorption on stoichiometric m-ZrO2(-111), dissociative CO2 adsorption did not take place. Results of the DFT simulations performed on O–ZrO2 system have shown that m-ZrO2 surface enables oxygen mobility. Under ideal feed flow (1.36% CO-2.72% H2O-He) at 300°C, negligible WGS activity was observed on CeO2, 1%V/CeO2 and 0.5%Re-1%V/CeO2, while the Au based (1%Au/CeO2 and 1%Au-0.5%Re/CeO2) and Pt based (1%Pt/CeO2 and 1%Pt-0.5%Re-1%V/CeO2) catalysts exhibited consistent WGS activity with the latter demonstrating higher activity. Over Au-based catalysts at 300oC under ideal feed flow, CO adsorption was identified as the rate-determining step of WGS reaction, while for Pt-based catalysts at 350oC, it was determined to be H2O dissociation. Though the mechanism over Au-based catalysts were found to be consistent with redox mechanism under ideal feed flow at 300oC, the main reaction path determined over Pt-based catalysts was the associative carboxyl mechanism under ideal feed flow at 350oC. However, associative carboxyl mechanism and redox mechanism for Au- and Pt-based catalysts, respectively, cannot be ruled out with certainty. Additionally, associative formate mechanism was observed to be a minor reaction path of WGS for both Au- and Pt- based catalysts tested.