Characterization of Surface Structure and Oxidation/Reduction Behavior of Pd–Pt/Al₂O₃Model Catalysts

Abstract

Structural and morphological characterisation of bimetallic Pd-Pt/Al 2 O 3 model catalysts are performed using X-ray diffraction, X-ray absorption spectroscopy, transmission electron microscopy and CO chemisorption. Further, the catalysts were studied under oxidising and reducing conditions using both X-ray absorption spectroscopy and low-energy ion scattering spectroscopy. For the as-prepared catalysts, the existence of alloyed bimetallic Pd-Pt particles and of (tetragonal) PdO were found for the samples calcined at 800 C. PdO is present in form of crystals at the surface of the Pd-Pt particles or as isolated PdO crystals on the support oxide. Bimetallic Pd-Pt nanoparticles were only formed on the Pd-Pt catalysts after calcination at 800 C. The results show that the Pd-Pt nanoparticles undergo reversible changes in surface structure composition and chemical state in response to oxidising or reducing conditions. Under oxidising conditions Pd segregates to the shell and oxidises forming PdO, while under reducing conditions regions with metallic Pd and Pd-Pt alloys were observed at the surface.No bimetallic Pd-Pt nanoparticles were observed for the sample initially calcined at 500 C, but instead isolated monometallic particles, where small Pt particles are easily oxidised under O 2 treatment. In the monometallic catalysts, the Pd is found to be completely oxidised already after calcination and to consist of metallic Pd after reductive treatment.

Study of methane oxidation over alumina supported Pd–Pt catalysts usingoperandoDRIFTS/MS andin situXAS techniques

“…There is an increase in the white line intensity during the lean periods for all samples and the spectra resemble the PdO spectrum, suggesting the formation of PdO, while there is a fast decrease of the white line intensity at the switch to rich periods suggesting a reduction to a metallic Pd state, in agreement to our previous findings on the oxidation/reduction behavior of Pd-Pt catalysts [22]. Therefore, we expect a restructuring of the surface of the bimetallic nanoparticles to occur during the dynamic methane oxidation experiments.…”

“…For simplicity, the samples will hereafter be referred to by their sample ID. The as prepared catalysts have been characterised ex situ by X-ray diffraction and transmission electron microscopy (TEM) as described in more detail in our recent publication [22]. Operando FTIR and MS, and in situ XAS were employed to study the methane oxidation over the bimetallic Pd-Pt catalysts as described below.…”

“…Particle sizes were estimated from TEM analysis of the as-prepared samples. [22,27]. At ESRF, methane oxidation measurements were performed at 360 • C with 3 min length pulses of 1.5% O 2 in an otherwise constant flow of 0.1% CH 4 .…”

“…This can have dramatic effects on the overall performance of the catalyst as the catalytic properties change correspondingly. In our recent publication [22] we have studied the influence of catalyst calcination conditions on the oxidation and reduction behavior of bimetallic Pd-Pt catalyst. It is shown that bimetallic Pd-Pt nanoparticles are formed after calcination at high temperature, 800 • C, both in dry and wet air.…”

Deactivation of a Pd/Pt Bimetallic Oxidation Catalyst Used in a Biogas-Powered Euro VI Heavy-Duty Engine Installation

“…The bi-metallic catalysts can be deactivated by poisoning and sintering of precious metal particles similar to mono-metallic catalysts. However, the bi-metallic catalysts may in addition suffer from segregation of the metals as shown by Gremminger et al The Pd/Pt bi-metallic catalyst is dynamic and presents surface species that are dependent on the reaction condition [20], and when exposed to water, these Pd/Pt species could segregate [21]. This segregation of platinum and palladium into mono-metallic particles impacts the stability and activity of the catalyst.…”