Different Reaction Mechanisms of Ammonia Oxidation Reaction on Pt/Al₂O₃ and Pt/CeZrO₂ with Various Pt States

Abstract

NH3 emissions were limited strictly because of the threat for human health and sustainable development. Pt/Al2O3 and Pt/CeZrO2 were prepared by the impregnation method. Differences in surface chemical states, reduction ability, acid properties, morphological properties, reaction mechanisms, and ammonia oxidation activity were studied. It indicated that Pt species states were affected by different metal–support interactions. The homogeneously dispersed Pt species over Pt/Al2O3 exposed Pt(111) because of weak metal–support interactions; there even existed an obvious interface between Pt and Al2O3. While obscure even an overlapped interface was observed over Pt/CeZrO2, resulting in the formation of PtO because of the oxygen migration from CeZrO2 to Pt species (confirmed by CO-FTIR, the cycled H2-TPR and transmission electron microscopy results). It was noteworthy that different reaction mechanisms were induced by different states of Pt species; NH was the key intermediate species for ammonia oxidation reaction over Pt/Al2O3, but two kinds of intermediates, N2H4 and HNO, were observed for Pt/CeZrO2. It consequently resulted in the obvious distinction of the NH3-SCO catalytic performance; the light-off temperatures of NH3 over Pt/Al2O3 and Pt/CeZrO2 were 231 and 275 °C, respectively, while the maximum N2 selectivity (65%) was obtained over Pt/CeZrO2, it was obviously better than that over Pt/Al2O3.

The Effects of Platinum Dispersion and Pt State on Catalytic Properties of Pt/Al2O3 in NH3 Oxidation

“…[11b,25a] We also note that the mechanism of NH 3 oxidation over oxidized clusters and PtO x nanoparticles may differ from that for metallic platinum. [26] This assumption is confirmed by a significant difference (> 50°C, Figure 5) in the onset temperature of ammonia oxidation reaction on preoxidized and prereduced catalysts.…”

“…This proposal is based on results obtained by Sun et al. who compared Pt/Al 2 O 3 and Pt/CeZrO 2 catalysts [26] . These catalysts displayed different trends in dissociation of O 2 and interaction with NH 3 .…”

“…Drawing an analogy with our catalysts, it can be assumed that NH 3 oxidation over N‐Ox‐600, Cl‐Red‐550, and Cl‐Red‐600 catalysts with predominantly metallic platinum particles occurs via the NH mechanism. On the contrary, over the N‐Ox‐400 catalyst with oxidized platinum species the N 2 H 4 and HNO mechanisms are dominating, similar to that on Pt/CeZrO 2 with PtO species [26] . Since O 2 is not activated on ionic platinum, it can be assumed that activation requires reduction of PtO to Pt 0 with abstraction of a hydrogen atom from NH 3 .…”

“…The ammonia oxidation reaction over Pt/Al 2 O 3 followed the NH mechanism, whereas the N 2 H 4 mechanism and HNO mechanism coexisted over Pt/CeZrO 2 . The NH mechanism would easily lead to the formation of byproduct NO, while N 2 was promoted by N 2 H 4 and HNO mechanism [26] . The stepwise ammonia activation by adsorbed atomic oxygen is lumped into one stoichiometric reaction assuming the first step (NH 3 +O) to be the rate determining and a first‐order reaction with respect to NH 3 and O over metallic platinum [27]…”

“…Indeed, the platinum surface was found to be deactivated at low temperatures due to the formation of strongly bonded N, NH x , NO and OH species [5,38] . Atomic oxygen reacts with NH x species to form OH groups, [25a,b,26,39] which are formed after abstraction of all H atoms from the adsorbed NH 3 molecule [38] . Molecular nitrogen is formed as a result of recombination of N ad (ad=adsorbed) species, while the interaction of N ad and O ad leads to the appearance of NO ad [38] .…”