Examining the surface of a synergistic Pt-Rh/γ-Al2O3 catalyst using NO as a probe molecule

The interaction of NO and NO + H2 with reduced γ-Al2O3, Pt/γ-Al2O3, Rh/γ-Al2O3, Pt(90%)-Rh/(10%)/γ-Al2O3 (90/10), and Pt(95%)-Rh(5%)/γ-Al2O3 (95/5) was monitored with FTIR spectroscopy to better understand the surface of a synergistic Pt-Rh catalyst (95/5). The surface of the alloy nanoparticles on the synergistic 95/5 catalyst contained both Pt and Rh due to the presence of FTIR peaks representing linear NO on Pt and NO on Rh. The presence of Pt and Rh on the surface of 90/10 was confirmed through the observation of the linear NO-on-Pt peak and the Rh nitrosyl peak, respectively. The presence of the Rh nitrosyl peak at 150 °C and 200 °C on 90/10 indicates the presence of partially oxidized Rh on the surface of the supported Pt-Rh alloy particles at these temperatures. The following relative order of NO reduction activity was observed for the prepared catalysts: 95/5 > Pt/γ-Al2O3 ≈ 90/10 ≫ Rh/γ-Al2O3. The maximum synergistic performance of 95/5 was a five-fold increase over the activity of Pt/γ-Al2O3, and this was only observed after conditioning the catalyst by equilibrating to reaction conditions for 10 h at 250 °C. A discussion of the reaction mechanism and the observed surface species reveals that the differing performance of 95/5 and 90/10 is related to the amount of Rh on the surface of the supported alloy nanoparticles.