Promotion of Ammonium Formate and Formic Acid Decomposition over Au/TiO₂ by Support Basicity under SCR-Relevant Conditions

Abstract

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.

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

“…The formic acid orders for carbon dioxide production (Table 1) were negative on both catalysts under all conditions. Consistent with the higher surface basicity [11], Au/La-TiO 2 exhibited more negative formic acid orders than Au/TiO 2 . The orders became less negative in the regime of lower gas-phase formic acid concentration and higher temperature.…”

“…The preparation and testing of the titania-supported gold catalysts are described elsewhere [8,11,12]. Briefly, anatase titania (DT51, Crystal Global) and anatase titania modified with 10 wt % La 2 O 3 (DT57, Crystal Global) were impregnated with a 0.032 M solution of HAuCl 4…”

“…For experiments to determine the kinetic orders, the concentrations of formic acid, carbon monoxide, oxygen, carbon dioxide, and water were varied in the range 0-2,200 ppm, 0-750 ppm, 0.25-4.00 vol %, 0-750 ppm, and 0.01-1.50 vol%, respectively. External and internal mass transfer limitations were ruled out under the investigated conditions [11]. The formulas used for the calculation of conversion, product selectivity, and yield are described in our previous work [11].…”

“…External and internal mass transfer limitations were ruled out under the investigated conditions [11]. The formulas used for the calculation of conversion, product selectivity, and yield are described in our previous work [11]. The C-balance was closed within an accuracy of ±3% by summing carbon monoxide, carbon dioxide and formic acid.…”

“…By thermal aging of the Au/La-TiO 2 catalyst to grow the gold particles to a size range similar to that of Au/TiO 2 , we disentangled the two effects and showed that Au/La-TiO 2 retained higher activity compared to Au/TiO 2 even after thermal treatment. Furthermore, carbon dioxide formation via oxidation of carbon monoxide was excluded based on the low activity of Au/TiO 2 and Au/La-TiO 2 for carbon monoxide oxidation ( Figure S2) [11]. Hence, a direct pathway is responsible for carbon dioxide production and is associated with the presence of gold.…”