Understanding the Surface Structure and Catalytic Activity of SnOx/Au(111) Inverse Catalysts for CO2 and H2 Activation

Abstract

Carbon dioxide hydrogenation is a promising approach for the reduction of greenhouse gas pollution via the production of fuels and high-value chemicals utilizing C1 chemistry. In this process, the activation of nonpolar molecules, CO2 and H2, at mild conditions is challenging. Herein, we report a well-defined inverse SnOx/Au(111) catalyst that shows the ability to activate both CO2 and H2 at room temperature. Scanning tunneling microscopy (STM) and ambient pressure X-ray photoemission spectroscopy (AP-XPS) are combined to understand the surface structure, growth mode, chemical state, and activity of SnOx/Au(111) surfaces. Nanostructures of SnOx at the sub-monolayer level were prepared by depositing Sn on Au(111) followed by O2 oxidation. For the as-prepared SnOx/Au(111), two-dimensionally formed SnOx thin films on a Au(111) substrate were observed with STM of two different moieties, discernible based on their height: clusters (∼0.4 Å) and nanoparticles (NPs, 1–2.5 Å), which are assigned to Sn–Au alloys and SnOx, respectively, in corroboration with XPS analysis. Furthermore, SnOx/Au(111) was annealed under UHV to test its thermal stability. Upon annealing at 400–600 K, a disappearance of SnOx NPs and reappearance of highly dispersed Sn clusters were clearly noticeable from the STM and XPS results, identifying the thermal decomposition of SnOx and subsequent formation of Sn–Au alloys on the surface due to the recombination of Sn clusters with Au. We investigated the reactivity of the SnOx/Au(111) surfaces toward CH4, CO2, and H2. The SnOx/Au(111) surfaces have excellent CO2 and H2 activation abilities even at room temperature with negligible reactivity for methane activation. Our AP-XPS results show that H2 can be activated on the SnOx NPs by the reduction to Sn. For CO2, the activation and further dissociation are identified by a reoxidation of Sn with newly formed Sn–O bonds and the formation of surface carbon. Therefore, we propose that SnOx is a potential catalyst or additive to achieve CO2 hydrogenation under mild conditions.