Effect of adsorbate proximity on surface reactions: Synthesis and decomposition of the formate intermediate in UHV from coadsorbed CO, H2O, and O on Rh(100)

Abstract

The coadsorption of CO, H2O, and O on the Rh(100) surface has been studied using temperature programmed electron energy loss spectroscopy (TP-EELS), temperature programmed reaction spectroscopy (TPRS), and low energy electron diffraction (LEED). Following exposure at 90 K both H2O and CO are adsorbed without decomposition on the O-covered surface. As the temperature is increased to between 150 and 200 K, TP-EELS reveals that hydroxyl (OH) is formed (with bending mode at 114 meV and stretching mode at 394 meV) and disappears as gas phase water is evolved at 260 K. Beginning at 220 K and continuing to 260 K, two new modes develop at 94 and 164 meV which we identify as the scissor [δ(OCO)] and symmetric stretch [νs(OCO)] modes of bidentate formate (HCOO). TP-EELS and TPRS establish that the formate decomposes near 290 K with gas phase CO2 and H2O as products. Based on EELS intensity vs temperature, the kinetics of formate production (activation energy E=8±2 kcal⋅mol−1, and preexponential ν=103±2 cm2⋅s−1) and decomposition (E=26±3 kcal⋅mol−1, ν=1017±2 s−1) were determined. The effect of varying the initial reactant concentrations on the rate of formate production indicates that adsorbate concentrations high enough to force CO and OH into adjacent sites are required for the reaction to proceed. The role of preadsorbed O is both to facilitate production of OH through reaction with coadsorbed H2O and to help provide the crucial crowding of the surface necessary for HCOO formation.