Structure sensitivity of alumina- and zeolite-supported platinum ammonia slip catalysts

Abstract

The influence of the size and structure of Pt particles on the catalytic performance in selective ammonia oxidation for emission control applications is hardly understood. There is a lack of in situ structural characterization to explain the catalyst performance under atmospheric pressure and relevant reactant ratios. In this study we complemented conventional laboratory tests with operando X-ray absorption spectroscopy (XAS) to determine the activity- and selectivity-governing factors in ammonia slip catalysts with different Pt particle sizes supported on γ-alumina and a ZSM-5 zeolite. A previously reported narrow activity range of platinum catalysts was extended by using catalysts with atomically and sub-nanometre dispersed Pt. The increase in activity with particle size was mainly caused by the availability of favourable Pt ensembles on the surface, probably B5 sites. Below the required size for the existence of these ensembles (∼2 nm) Pt possessed poor activity for ammonia oxidation, while upon reaching it, the activity drastically rose and was further moderately dependent on particle size. Spectroscopic data revealed the same reaction mechanism for particles ≥∼2 nm. It included initial reduction during heating in the reaction mixture and subsequent re-oxidation at high temperatures. For Pt single sites/small clusters, the mechanism was different and involved only gradual reduction without further re-oxidation. The individual spectral components for the two types of mechanisms were resolved. Their evolution correlated with catalyst activity and selectivity change. While ammonia oxidation proceeded through the Ostwald mechanism on active catalysts with sufficiently large particles, single or low-coordinated sites were rather likely to catalyse selective catalytic reduction.