FORMATE SYNTHESIS ON CU(100)

Formate has been synthesized from a high pressure CO2:H2 gas-mixture on a well defined Cu(100) single crystal, and the important kinetic parameters have been determined. The formate was synthesized in a high pressure cell incorporated into the UHV system and subsequently studied by XPS, HREELS, and TPD. The formate synthesis was studied over a pressure range of 0.5-5.0 bar and a temperature range of 323-363 K. The rate of synthesis was found to be dependent on gas composition (CO2:H2) and weakly dependent on total pressure. At 363 K, the initial rate of formate synthesis on the Cu(100) surface in a 70:30 (CO2:H2) gas-mixture at 2.3 bar was found to be (4.8 + 0.6) x 10(4) formate molecules per copper atom per second. The overall activation energy for the formate synthesis on the clean surface was found to be 55.6 J 8.0 kJ mol-1. A kinetic model for formate synthesis is presented which reproduces the experimental results for varying synthesis conditions. The model suggests that the rate limiting step in the synthesis is the reaction of adsorbed atomic hydrogen with adsorbed CO2 forming the surface formate.