Brønsted acid enhanced hexagonal cerium phosphate for the selective catalytic reduction of NO with NH3: In situ DRIFTS and DFT investigation

Abstract

The possible effect of optimized acid sites on NH3-SCR performance and the fundamental mechanism are barely illustrated. In this work, we report two model catalysts of hexagonal (h-CPO) and monoclinic (m-CPO) cerium phosphate with disparate acidity that show different NH3-SCR activities under the same reaction conditions. Brønsted acid sites were found to be crucial for NH3-SCR performance at both low and high temperature. The electron localization discrepancy of h-CPO was more pronounced as compared with m-CPO, leading to the enrichment of P-OH (Brønsted acid site) which could strongly absorb NH3 and then generate NH4+ to participate in fast SCR via Langmuir-Hinshelwood mechanism, resulting in good activity at low temperature. The zeolitic water stored in the open channels of h-CPO could be released as supplement for P-OH sites which prevent the depletion and non-selective oxidation of NH3 thus maintaining its high activity at high temperature via the Eley-Rideal mechanism. Meanwhile, as DFT calculation revealed, cerium is the electron deficient center which can easily fix NO and NO2 from the intake, generating active NO2(ad) or nitrites and facilitating fast SCR by reacting with NH4+ species. Hence, the superior protonation ability to form P-OH and low energy barrier to generate active nitrites of h-CPO led its T80 NOx conversion to a broaden temperature of 150–450 oC under high GHSV of 177,000 h-1. Furthermore, experimental and DFT calculation also demonstrated that the enriched Brønsted acid sites over h-CPO have largely suppressed SO2 adsorption, thus significantly reducing the formation of metal sulfates and achieving great SO2 resistance. The ammonium sulfate deposits can be storage of NH3, supplying additional reductant to promote high temperature activity and selectivity.