PNNL

Abstract

The dimer saddle point searching method has been used with density functional theory calculations to study the reactivity of formate (HCOO) on the Cu(111) surface. Possible reaction paths for the HCOO decomposition (or synthesis) and hydrogenation in the presence of co-adsorbed H atom have been identified without assuming their final states. Starting from the most stable bidentate HCOO adsorption configuration, we have calculated the reaction rates and pre-exponential factors of the identified HCOO reaction and diffusion paths using harmonic transition state theory. In agreement with previous experimental and theoretical studies, HCOO is found to form by gaseous CO2 and adsorbed H through the Eley-Rideal (ER) mechanism. Barriers for direct HCOO synthesis from CO via both the ER and Langmuir-Hinshelwood (LH) mechanisms are high, 1.44 and 2.45 eV respectively. This indicates that the reaction pathway is unfavorable on the Cu(111) surface. The decomposition rate of HCOO to HCO+O is low while the reverse rate is high, indicating that the reaction path of HCOO decomposition to HCO is also unlikely. On the other hand, the reaction route for HCOO hydrogenation to H2COO in the presence of a co-adsorbed H atom has been identified and found to be favorable with an activation energy of 1.24 eV. This is comparable to the barrier of 1.30 eV for the ER reaction route of HCOO decomposition to CO2(g). Except for two fast HCOO diffusion mechanisms, our results show that and HCOO hydrogenation to H2COO are the dominant reaction paths on the Cu(111) surface.