This work demonstrates the rational design of a dedicated hydrolysis catalyst for application in the selective catalytic reduction (SCR) of NOx. 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 CO2 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,E-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 lanthanum-modified catalysts. Gold is essential to achieve selectivity to CO2; its absence yields CO. The introduction of lanthanum to the catalytic system preferentially promoted the CO2 formation mechanism, enabling complete decomposition of formic acid selectively to CO2 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.