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

Abstract

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.

Numerical Modeling of Hydroperoxyl-Mediated Oxidative Dehydrogenation of Formic Acid under SCR-Relevant Conditions

“…The coverage of hydroxyls, which are involved in multiple reaction steps, undergoes very little change with varying temperature. The low site occupancy of these species is in agreement with their high degree of depletion observed using in situ DRIFTS [43]. 1.…”

“…Complementary to the fall in the formate coverage, the proportion of free active sites experienced a slight increase with increasing temperature. Such a trend is in line with the in situ DRIFTS results[11,43], which…”

“…Such an explanation is in line with the close agreement observed at 200 °C in Figure 5. The formate coverage at the carbon monoxide producing sites on the support was negligibly low at this temperature owing to competitive adsorption and site blockage by water as shown in our previous works [11,43].…”

“…One possible explanation for the deviation at 300 °C is that the proposed scheme considers only one type of active site that can adsorb formicderived species (formates) and produce carbon dioxide. However, as shown in our previous works [11,43], other sites exist on the support away from the gold, which deprotonate formic acid and decompose the formate to carbon monoxide. These carbon monoxide-producing sites exhibited formic acid orders ranging between 0.0 and 0.5 suggesting that they harbor formate coverages equal to or lower than the saturation coverage.…”

“…Figure 7 plots the influence of oxygen on the experimentally and predicted formic acid conversion at 200 °C and 300 °C. Previously, we demonstrated that oxygen is indispensable for carbon dioxide production, without which gold does not catalyze formic acid decomposition [43]. Additionally, the order in oxygen was positive.…”