The reactions of carboxylic acids on UO2(1 1 1) single crystal surfaces. Effect of gas-phase acidity and surface defects

The interaction of formic acid with the stoichiometric and oxygen-defected (1 1 1) surfaces of UO2 single crystal (the [1 1 1] plane is the natural cleavage of uranium dioxide) has been investigated. HCOOH adsorbs with a high sticking coefficient (s(0.33)=0.6 at 305 K). The sticking coefficients for acetic and propionic acids were also measured for comparison. Both showed lower values, as expected from their weaker gas-phase acidity. The surface coverage and the degree of surface stoichiometry affected the reaction products of formic acid. On the stoichiometric surface, formate species decomposed via both dehydrogenation (CO2 and H2) and dehydration (CO and H2O) pathways, with the yield of the latter far higher than that of the former. The desorption profile of these products was found to be very complex. Each route desorbed in two distinct temperature domains (ca. 630 K––the α-state––and ca. 700 K––the β-state) and with two distinct CO to CO2 ratios. The CO2 yield of the α-state (small) was relatively insensitive to surface coverage while that of the β-state (large) dramatically increased with increasing adsorbate surface coverage. Conversely, the CO yield of the α-state (large) was sensitive to surface coverage while that of the β-state (small) was relatively independent. On the oxygen-deficient surface the main route was dehydrogenation to CO2, although reduction to formaldehyde (together with considerable amounts of ethylene) occurred. Formaldehyde desorbed in two different temperature domains, at ca. 550 and 630 K. The first is attributed to U cations with multiple coordinative unsaturations, while the second is most likely due to reduction of formates by randomly distributed surface defects.