This work demonstrates the rational design of a dedicated hydrolysis catalyst for application in the selective catalytic reduction (SCR) of NO x . Modification of titania by lanthanum prior to gold deposition entailed highly improved catalytic activities for ammonium formate (AmFo) and formic acid decomposition under SCR-relevant conditions stemming from dual phenomena: particle size effect and base effect. Smaller gold particles were stabilized, and there was higher uptake of CO 2 and formic acid, as demonstrated by HAADF-STEM and in situ DRIFT analyses, respectively. The difference in the activities between the lanthanum-modified, unmodified, and tungsten-modified catalysts was implicitly dictated by the formic acid coverage, which was in turn greatly increased in the presence of base. In situ DRIFT studies under reaction conditions identified formate as a relevant reaction intermediate, under reaction conditions. Higher E a,app alongside a higher pre-exponential factor (A), describe an underlying compensation effect originating from the contribution of enthalpy associated with the desorption of the strongly adsorbed formate, which is consistent with the highly negative formic acid orders observed in the case of the lanthanummodified catalysts. Gold is essential to achieve selectivity to CO 2 ; its absence yields CO. The introduction of lanthanum to the catalytic system preferentially promoted the CO 2 formation mechanism, enabling complete decomposition of formic acid selectively to CO 2 at significantly lower gold loading and lower contact times, making it a promising candidate for decomposition of formate-based ammonia precursors in the SCR process.

Owing to the problems associated with the usage of urea as the ammonia precursor compound in the selective catalytic reduction (SCR) 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-dehydrogenationtype (ODH) 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.