Water-assisted oxygen activation during gold-catalyzed formic acid decomposition under SCR-relevant conditions

Owing to the problems associated with the use of urea as an ammonia precursor compound in the selective catalytic reduction process in automobiles, alternative compounds such as ammonium formate, methanamide, and guanidinium formate have been suggested as more efficient and cleaner candidates. For the application of such formate-based ammonia precursors, a fundamental understanding of catalytic formic acid decomposition in the presence of the diesel exhaust components water and oxygen is required. Oxygen activation is a reaction step common to many catalytic processes. We observed that oxygen activation over titania and base-modified titania-supported gold catalysts is greatly enhanced in the presence of water, resulting in a significant increase in carbon dioxide production from the decomposition of formic acid. Unlike the supports, gold catalyzed the selective production of carbon dioxide. Monodentate and bidentate formates are the kinetically relevant surface species for carbon monoxide and carbon dioxide production, respectively. The support acts as a reservoir, storing bidentate formates that do not react in the steady state when formic acid and oxygen (and water) are co-fed. However, during transient experiments, when the feed is switched from formic acid to oxygen (and water), they are reactivated upon reverse spillover to the active site associated with gold, where they decompose to carbon dioxide. In the presence of oxygen and water, carbon monoxide oxidation and the water gas shift reaction do not produce carbon dioxide. Instead, a direct oxidative-dehydrogenation-type pathway proceeds, which strongly differs from stoichiometric formic acid decomposition. A kinetically consistent mechanism is proposed in which the hydroperoxy species facilitate the C-H bond cleavage of formates to release carbon dioxide and water in the rate-determining step. (C) 2017 Elsevier Inc. All rights reserved.