XPS investigation of surface changes during thermal aging of Natural Gas Vehicle catalysts: Influence of Rh addition to Pd

Abstract

This study investigates thermal aging induced effects on the surface properties of model and commercial automotive catalysts for Natural Gas Vehicles, by means of X-ray photoelectron spectroscopy and infrared spectroscopy of chemisorbed CO as a probe molecule for noble metals. Post-combustion catalysts for this application are essentially composed of palladium containing low amounts of rhodium and additives to improve the oxygen storage properties. The aging procedure involved exposing the catalysts to an oxidizing atmosphere containing air and 10 vol.% H 2 O at 980• C for 4 h in order to simulate surface changes which may occur under usual three-way operating conditions. It was found that sintering processes led to a significant loss of Pd concentration on model and commercial catalysts. Regarding Rh, diffusion processes into alumina also take place, which are partially inhibited on model catalysts when Rh interacts with Pd. Greater stabilization of Rh species is suggested on commercial catalysts in the presence of oxygen storage materials.

Thermal CO Oxidation and Photocatalytic CO2 Reduction over Bare and M-Al2O3 (M = Co, Ni, Cu, Rh, Pd, Ag, Ir, Pt, and Au) Cotton-Like Nanosheets

“…For Pt-Al 2 O 3 nanosheets, Pt 4d 3/2 and Pt 4d 5/2 XPS peaks were observed at BEs of 332.9 eV and 314.9 eV, respectively, with a spin-orbit splitting of 18.0 eV. The XPS peaks were attributed to metallic Pt [34,35], which was well-fitting with the XRD result of metallic Pt. For Au-Al 2 O 3 nanosheets, the Au 4f 7/2 and Au 4f 5/2 XPS peaks were observed at BEs of 87.3 eV and 83.6 eV, respectively, with a spin-orbit splitting of 3.7 eV.…”

“…For Rh-Al 2 O 3 nanosheets, Rh 3d 3/2 and Rh 3d 5/2 XPS peaks were observed at BEs of 314.3 eV and 309.7 eV, respectively, with a spin-orbit splitting of 4.6 eV. The XPS BEs were attributed to an oxidation state of Rh 3+ [ 32 , 34 ]. As discussed above, the lattice distances in the HRTEM image confirmed orthorhombic Rh 2 O 3 .…”

“…An additional weak shoulder peak was seen around 308 eV for Rh 3d 5/2 peak. This could be due to metallic Rh [ 32 , 34 ]. On the basis of the XPS and HRTEM data, Rh-species appeared to be consistent with Rh@Rh 2 O 3 core-shell type structure.…”

“…For Pd-Al 2 O 3 nanosheets, Pd 3d 3/2 and Pd 3d 5/2 XPS peaks were observed at BEs of 341.7 eV and 336.2 eV, respectively, with a spin-orbit splitting of 5.5 eV. The XPS peaks were attributed to an oxidation state of Pd 2+ [ 32 , 34 , 35 ]. There was a good coincidence between the oxidation state of the XPS and the XRD profiles of tetragonal PdO.…”

Effect of preparation method of Co-promoted Pd/alumina for methane combustion

“…4 shows XPS spectra of Pd(3d) for catalysts with Pd and Co loading of 1 wt%. The binding energy of Pd(3d 5/2 ) was 336.8 eV, which can be assigned to Pd 2+ stabilized in PdO (336.1-336.9 eV) [62,63]. Although the top of the peak was not varied, slight shift (∼0.3 eV) of the peak edge to lower binding energy was observed after Co-addition.…”