Mechanistic implications of lanthanum-modification on gold-catalyzed formic acid decomposition under SCR-relevant conditions

The use of formate-based ammonia precursors as alternatives to urea in the selective catalytic reduction (SCR) process requires that formic acid released upon their thermolysis in the hot exhaust is rapidly decomposed to carbon dioxide. This work aims at the rational development of a dedicated catalyst that is highly active and selective towards formic acid decomposition to carbon dioxide under SCR-relevant conditions, i.e. under lean conditions and in presence of a large amount of water. The incremental addition of a basic oxide (lanthana) to Au/TiO2 revealed an optimum in the base-induced promotional effect. The base-modification of Au/TiO2 induced a C-H bond weakening of the bidentate formates, which are the dominant surface species and the kinetically relevant intermediates for carbon dioxide formation. At 15 wt% lanthana loading, monodentate formates were substantially suppressed leading to similar to 85% reduction in carbon monoxide production. Very high lanthanum surface concentrations lowered the relative coverage of oxygen-derived surface species that are crucial for the decomposition of the abundantly present formates. The linearity of the Constable-Cremer relationship between the apparent activation energy and the natural log of the pre-exponential factor indicates the mechanistic similarity in formic acid decomposition on gold supported on unmodified and lanthanum-modified titania catalysts. Such mechanistic insights helped derive an optimal catalyst. The optimal catalyst exhibited close to three-fold higher activity for ammonium formate decomposition while still maintaining 100% selectivity to ammonia.