CO2 activation on MnOx /Pd(111) model catalyst

Abstract

CO2 is an atmospheric pollutant (i.e., a greenhouse gas) and it can be converted into valuable chemicals such as methanol, methane, and formic acid. However, CO2 reduction is a challenging process due to the thermodynamic stability of CO2. In this work, we focus on the activation of CO2 by using an atomically well-defined MnOx/Pd(111) planar model catalyst. Pd(111) surface can dissociatively adsorb hydrogen molecules, but CO2 does not strongly bind to the Pd(111) surface. On the other hand, MnOx nano-structures can facilitate the activation of CO2 due to the presence of acid and base sites on the metal oxide surface. Therefore, MnOx/Pd(111) was chosen as a model catalyst to investigate catalytic CO2 activation. A multifunctional ultra-high vacuum system with quadrupole mass spectrometer (QMS), X-ray photoelectron spectrometer (XPS), and low energy electron diffraction (LEED) was used to perform the experiments. Manganese thin film growth mechanism on Pd(111) surface was determined by using XPS. Manganese was evaporated on Pd(111) substrate at two different temperatures (i.e., 85 K and 300 K). Formation of products after the dosing of the reactants on the MnOx/Pd(111) surface was examined via temperature programmed desorption (TPD). For both cases, formed manganese oxide thin film was investigated by using XPS to estimate the relative, Mn0, Mn2+ and Mn3+ surface concentrations. Prepared manganese film on Pd(111) at 300 K could activate CO2 to CO, which is a valuable chemical for the chemical industry. To prepare smaller clusters, manganese was evaporated on the Pd(111) single crystal surface at 85 K. At moderate manganese coverage, carbonate CO32- formation was detected on the MnOx/Pd(111) interfacial sites.