Applying Dynamic and Synchronous DRIFTS/EXAFS to the Structural Reactive Behaviour of Dilute (≤1 wt%) Supported Rh/Al2O3 Catalysts using Quick and Energy Dispersive EXAFS

Abstract

Examples of the application of a synchronous DRIFTS/EXAFS/mass spectrometry (MS) methodology to the study of dilute (B 1wt%) Rh/Al 2 O 3 catalysts are discussed. These are used to explore the potential of this approach for understanding of the behaviour of supported metal catalysts ''in a single shot'', and in the often preferred regime of low (\1 wt%) loadings of active precious metals. Firstly, the sequential interaction of NO (323 K) and then CO (373 K) with reduced, 0.5 wt% Rh/Al 2 O 3 catalysts is studied. Infrared spectroscopy indicates that two surface species (a bent Rh(NO -) and Rh(CO) 2 species) can be created using this sequential gas absorption/reaction method with minimal interference from other carbonyl or nitrosyl species. As such the potential for a reliable structural characterisation of the local structure of these species by EXAFS becomes possible. However, in contrast to the infrared spectroscopy, analysis of the EXAFS data also indicates that, even for such low loaded Rh systems, oxidative disruption of the Rh by the NO and CO is not complete and that bonding typical of small Rh clusters persists in both cases. The possible sources of this apparent spectroscopic difference of opinion are discussed. Secondly, 1wt% Rh/Al 2 O 3 catalysts are studied using dispersive EXAFS at 573 K with 100 ms time resolution, during a redox switching event involving a reducing feedstock comprising just 3000 ppm of CO and 3000 ppm of NO. It is shown that highly useful and insightful time resolved and synchronously obtained XANES/EXAFS/IR data can be obtained even in this dilute Rh and more ''realistic'' case. Additional data, regarding the overall performance of the experiment, as currently implemented at the ESRF, along with a discussion of where enhanced performance might be yet still be gained, are also given.

Modulation Excitation X-Ray Absorption Spectroscopy to Probe Surface Species on Heterogeneous Catalysts

“…The sample bed provided a X-ray transmission path length through the sample bed of 5 mm mounted within a reactor system designed for combined transmission EXAFS/DRIFTS measurements [21]. The X-ray beam was vertically defocused (to ca.…”

“…Experiments were performed using a cell designed for combined EXAFS-IR experiments [21] and detailed results are published elsewhere [16]. The average time-resolved spectra of the reduced catalyst during CO vs. NO pulsing (Fig.…”

Catalysing Sustainable Fuel and Chemical Synthesis

“…To conclusively establish whether oxide or metal is responsible for alcohol selox catalysed by dispersed palladium nanoparticles, a multi-dimensional spectroscopic investigation of vapour phase crotyl alcohol selox was undertaken (since XAS is an averaging technique a complete understanding of catalyst operation requires multiple analytical techniques [162][163][164]). Synchronous, timeresolved DRIFTS/MS/XAS measurements of supported and colloidal palladium were performed in a bespoke environmental cell [165] to simultaneously interrogate adsorbates on the catalyst surface, Pd oxidation state and reactivity under transient conditions in the absence of competitive solvent effects [166,167].…”

Mechanistic Studies of Alcohol Selective Oxidation

“…Using an innovative reactor design [158,159], catalyst oxidation state (XAS), reactively formed surface intermediates (DRIFTS), and evolved gas-phase products (MS) were simultaneously followed on a subsecond timescale (Figure 2.10). Adopting this transient approach uncovered a flexible (temperature-sensitive) surface restructuring from Pd oxide to metal on contacting with crotyl alcohol, which could be reversed under dioxygen.…”