Dimer saddle point searches to determine the reactivity of formate on Cu(111)

We used the dimer saddle point searching method with density functional theory calculations to study the reactivity of formate (HCOO) on the Cu(111) surface. We identified possible reaction paths for the HCOO decomposition (or synthesis) and hydrogenation in the presence of a co-adsorbed H atom without assuming their final states. Starting from the most stable bidentate HCOO adsorption configuration, we calculated the pre-exponential factors and reaction rates of the identified HCOO reaction and diffusion paths using harmonic transition state theory. In agreement with previous experimental and theoretical studies, we found that HCOO was formed by gaseous CO2 and adsorbed H through the Eley–Rideal (ER) mechanism. The activation barriers for direct HCOO synthesis from CO via the ER and Langmuir–Hinshelwood (LH) mechanisms were 1.44 and 2.45 eV, respectively, suggesting that the reaction pathways CO or CO(g) + OH ↔ HCOO were unfavorable on the Cu(111) surface. The decomposition of HCOO to HCO + O was much slower than its reverse recombination. This indicated that the reaction pathway from HCOO to HCO also was unlikely. On the other hand, the reaction route for HCOO hydrogenation to H2COO in the presence of a co-adsorbed H atom had an activation energy of 1.24 eV, suggesting that HCOO hydrogenation was competitive with HCOO decomposition via the ER mechanism with a barrier of 1.30 eV. Except for two fast HCOO diffusion processes, our results showed that HCOO ↔ CO2(g) + H and HCOO + H ↔ H2COO were the dominant reaction pathways on the Cu(111) surface.