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

Sridhar, Manasa, Mantzaras, John, Anton van Bokhoven, Jeroen, Kröcher, Oliver
Industrial & engineering chemistry process design and development 2018 v.57 no.31 pp. 10206-10215
active sites, ammonia, carbon dioxide, catalysts, catalytic activity, chemical bonding, cleavage (chemistry), dehydrogenation, formates, formic acid, gold, guanidinium, hydrolysis, mechanistic models, oxygen, process design, spectral analysis
Formic acid decomposition to carbon dioxide is an important step during the decomposition of alternative formate-based ammonia precursor compounds, such as ammonium formate, guanidinium formate, and methanamide, in the selective catalytic reduction (SCR) process. At oxygen and water concentrations prevalent in a typical SCR feed, an oxidative dehydrogenation (ODH) type pathway of formic acid decomposition proceeds on titania-supported gold catalysts. Using the surface perfectly stirred reactor (SPSR) model, the ODH mechanism is demonstrated to satisfactorily predict the experimentally observed conversions. The single-site mechanistic model captured the negative trend in conversion with increasing formic acid concentrations, stemming from an extensive coverage of the active sites by formates. This in turn rendered the active sites unavailable for the formation of the active hydroperoxyls required for the rate-determining step of C–H bond scission of formates. The positive order dependency on oxygen concentration and the promotional effect of water are qualitatively and semi-quantitatively described by the model. Predicted trends in the relative surface coverages of different reaction intermediates are in agreement with previously reported kinetic and spectroscopic measurements. These results are important fundamentally for the understanding of formic acid decomposition chemistry and in general catalysis by gold, as well as for the design of dedicated hydrolysis catalysts for applications in the SCR process.