Influence of adsorbate interactions on heterogeneous reaction kinetics. Formic acid decomposition on nickel

The catalytic decomposition of formic acid on Ni catalysts has been studied as a model system to show the influences of adsorbate interactions on heterogeneous reaction kinetics. Formic acid decomposition on a Ni(111) surface was studied under ultrahigh-vacuum (UHV) conditions. It was found that when formic acid adsorbs as a dimer it dehydrates to leave CO and formate adsorbed on the surface. Formic acid monomer dehydrogenates on Ni(111) to an adsorbed formate. Adsorbate interactions strongly influenced the decomposition kinetics of adsorbed formate. Formate-formate interactions were found to be attractive, reducing the reaction rate with increasing formate coverage. Formate-CO interactions were strongly repulsive, causing a significant acceleration of the reaction rate with increasing CO coverage. Steady-state reaction kinetics of formic acid decomposition on a Ni wire were well represented by the UHV kinetics. The attractive formate-formate interactions caused a maximum in the isothermal decomposition rate as a function of formic acid pressure. A significant enhancement of the steady-state formic acid decomposition rate was observed upon addition of small amounts of CO. It is shown that CO acts as a cocatalyst for heterogeneous formic acid decomposition by causing phase separation between adsorbed CO and adsorbed formate on the nickel surface.