Effect of ammonia on the decomposition of ammonium formate over Au/TiO2 under oxidizing conditions relevant to SCR: Enhancement of formic acid decomposition rate and CO2 production

Abstract

Ammonium formate (AmFo) and formic acid decomposition were carried out in the presence of excess O2 and H2O over 0.5wt% Au/TiO2 anatase monolithic catalysts using various contact times and temperatures between 160°C and 300°C, under spray conditions in a dedicated setup. A systematic investigation of ammonia influence on formic acid decomposition revealed a highly beneficial influence on the reaction rate and the CO2 yield in the temperature range 160–300°C. Ammonia oxidation did not occur at any of the studied temperatures and space velocities. Both AmFo and a stoichiometric ammonia–formic acid mixture exhibited identical homogeneous gas phase as well as heterogeneous catalytic decomposition behavior. With the introduction of ammonia at a concentration of only 0.25 molar equivalents, the pseudo-first-order rate constants for formic acid decomposition experienced close to 110% and 15% increase at 160°C and 260°C, respectively and dosing 12 molar equivalents of ammonia in the gas phase, the rate constants underwent nearly 8-fold increase at 160°C, while at 260°C, only two times increase was achieved. Increasing the ammonia to formic acid molar ratio from 0 to 12 lead to a steep increase in the CO2 yield from 18% to 75% at 160°C, while a relatively smaller rise from 60% to 75% was observed at 260°C. Activity testing of bare titania revealed an inhibitory effect of ammonia on formic acid decomposition to CO. Overall, it can be concluded that the presence of gold is critical for the realization of such an ammonia-induced enhancement of rate and CO2 yield. The obtained results are relevant for the application of formate-based ammonia precursor compounds in the selective catalytic reduction of NOx in Diesel exhaust gases.