The performance of two bimetallic Pt-Rh catalysts Pt(95%)-Rh(5%)/γ-Al2O3 (95/5) and Pt(90%)-Rh(10%)/γ-Al2O3 (90/10) was compared to Pt/γ-Al2O3 and Rh/γ-Al2O3 for the reduction of NO with H2 resulting in the following order of relative activity: 95/5 > Pt/γ-Al2O3 ≈ 90/10 > Rh/γ-Al2O3. A conditioning step, equilibrating the catalyst to reaction conditions at 250C for 10 h, was necessary to observe the maximum synergistic performance of 95/5, a five-fold increase over the activity of Pt/γ-Al2O3. Prior to conditioning, 95/5 had similar activity to Pt/γ-Al2O3. The prepared catalysts were characterized with in-situ FTIR spectroscopy and electron microscopy to gain insight into the difference in performance between 95/5 and 90/10. In-situ FTIR studies were conducted using NO and NO + H2 as probes to qualitatively examine the surfaces of the supported Pt-Rh alloy nanoparticles. Results indicate that metallic Pt and Rh are both present on the surface of 95/5, while the surface of 90/10 contains oxidized Rh in addition to metallic Pt. The non-synergistic performance of 90/10 is attributed to the presence of oxidized Rh and/or an increased surface fraction of Rh.
Tags: bimetallic catalysts, synergy
Effective bimetallic catalysts often exhibit synergy. Synergy is defined as the bimetallic catalyst being more active and/or selective than either of its constituent metals. The activity and selectivity of bimetallic catalysts is dependent upon their particle surface compositions which in turn is a function of individual particle composition, pretreatment conditions, and other factors. This study compares the activity and surface composition of a Pt(95%)-Rh(5%)/gamma-Al2O3 catalyst (95/5) to Pt/gamma-Al2O3, Rh/gamma-Al2O3, and gamma-Al2O3. Reacting NO and H2 over each of the catalysts resulted in the following order of relative NO reduction activity 95/5 > Pt/gamma-Al2O3 > Rh/gamma-Al2O3 >> Al2O3. The maximum synergistic performance of 95/5, a five-fold increase over the activity of Pt/gamma-Al2O3, was obtained after conditioning the catalyst by reacting it at 250oC for 10 h, and the synergistic performance of 95/5 was unaffected by a 24 h reduction at 300oC. The surface composition of the prepared catalysts was investigated with in-situ FTIR spectroscopy at 100oC, 150oC and 200oC using NO as a probe molecule. The obtained spectra indicate that both Pt and Rh are present on the surface of the synergistic 95/5 catalyst. The amount of Rh on the catalyst surface increased as a function of NO adsorption temperature. Rh present on the surface of the alloy nanoparticles was in a reduced state at 150oC and 200oC, while on a monometallic Rh/gamma-Al2O3 catalyst some of Rh present was partially oxidized. Analytical electron microscopy has been used to show that the metals on the surface of synergistic Pt-Rh bimetallics exist as a single phase of Pt-rich Pt-Rh alloy nanoparticles, while both Pt-rich and Rh-rich alloy phases are present on non-synergistic Pt-Rh bimetallics.
Tags: bimetallic catalysts, synergy