Steven G. Fiddy

Rapid phase fluxionality as the determining factor in activity and selectivity of highly dispersed, Rh/Al2O3 in deNOx catalysis.

The nature of oxide-supported metal catalysts may change in oxidising conditions: Studies on the correlation between the Rh phase in the structure of the Rh/AL2O3 catalyst and the catalytic performance for the reduction of NO by H2 to N2 (on reduced, metallic sites) and N2O (on oxidised sites) reveal that the phases of the supported metal species can be interconverted on time scales that can be deterministic in temrs of the activity and selectivity of the catalysts.

Structure–performance relationships of Rh and RhPd alloy supported catalysts using combined EDE/DRIFTS/MS

Energy dispersive extended X-ray absorption fine structure spectroscopy (ED-XAFS), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and mass spectrometry (MS), have been combined for the structure-function study of Rh and RhPd supported catalysts for the reduction of NO by CO. The combined results show that although alloying of Rh with Pd prevents the dissociative oxidation of the Rh by NO, it does not prevent the extensive disruptive oxidation of Rh by CO. The influence of oxidative disruption by molecular CO in such systems may therefore be far more pervasive and catalytically important than has been previously observed. The overall metal particle size observed in the RhPd alloy system during the CO/NO reaction is significantly larger than for the Rh-only system for the entire temperature range employed. The catalytically active sites, however, are likely to be similar, with the overall activity of the alloy system to be reduced due to inactive RhPd alloy nanoparticles.

Identification of the surface species responsible for N2O formation from the chemisorption of NO on Rh/alumina.

Energy dispersive EXAFS (EDE) and diffuse reflectance infrared spectroscopy (DRIFTS) are combined synchronously at high time resolution (17 Hz) to probe how NO(g) reacts with gamma-Al(2)O(3) supported, metallic Rh nanoparticles of an average 11 A diameter; a bent nitrosyl species is considered to be the key to the formation of N(2)O.

High-quality energy-dispersive XAFS on the 1 s timescale applied to electrochemical and catalyst systems.

A long program involving the development of a fast linear and stable detector, and the recent upgrade of the bent monochromator to a 4-point one at station 9.3 Daresbury Laboratory is showing exciting results such that it has now become possible to obtain analysable XAFS data on 50mM solutions in time scales of a second or less. Three examples of such data are shown: a homogeneous Ni catalyst reaction using a stopped flow system, a heterogeneous catalyst reduction showing time dependent data collected in a microreactor, and an electrochemical system showing the oxidation of small Pt particles as the potential is changed. The improvements have come from two major developments the monochromator and the detector.

Particle development and characterisation in Pt(acac)2 and Pt(acac)2/GeBu4 derived catalysts supported upon porous and mesoporous SiO2: effect of reductive environment, and support structure

In situ, time resolved energy dispersive EXAFS (EDE) has been used in conjunction with temperature programmed reduction/decomposition (TPR/TPD), transmission electron microscopy (TEM), powder X-ray diffraction (PXRD), and diffuse reflectance infrared spectroscopy (DRIFTS) to probe the evolution and character of Pt and PtGe alloy particles, derived from Pt(acac)2 and GeBu4 precursors, supported upon amorphous and mesoporous silicas. The reduction kinetics and final particle size distributions obtained are found to be functions of the reductive environment, the support architecture, and the presence of GeBu4. Reduction of the Pt-only systems in H2 is found to have an autocatalytic character resulting in the rapid formation of large (N1Pt–Pt>8) particles at ca. 390 K. Reduction in the absence of hydrogen and/or in the presence of co-adsorbed GeBu4, results in a considerable retardation of particle growth and shows a dependence upon the support architecture. Both EDE and DRIFTS show that, in the case of PtGe alloy system elemental Pt particles form first (in the region of 400–500 K) and it is only at temperatures in excess of 500 K that significant alloy (Pt3Ge) formation is observed along with a concomitant reduction in average particle size. This same pattern of behaviour is also observed when Pt particles are pre-formed prior to the introduction of GeBu4 and subsequent reduction. These results are discussed in terms of the reductive processes at work in these systems, the support architecture, and the effects of retained carbonaceous materials on developing particles.

In situ energy dispersive EXAFS (EDE) of low loaded Pt(acac)2/HI SiO2 catalyst precursors on a timescale of seconds and below

We demonstrate second and sub-second time resolved energy dispersive EXAFS (EDE) for in situ interrogation of phase changes in a supported metal catalyst precursor [(⩽5 wt%) Pt(acac)2/HI SiO2] during thermal decomposition and reduction; the formation of Pt particles is radically altered by the presence H2, in both kinetic and structural senses.

Susceptibility of a heterogeneous catalyst, Rh/γ-alumina, to rapid structural change by exposure to NOElectronic supplementary information (ESI) available: data showing monitoring positions in Fig. 2 and data showing reduction of the NO oxidised Rh adduct on alumina. See http://www.rsc.org/suppdata/cc/b1/b106846f/

Metal particles in a Rh/γ-Al2O3 catalyst of differing particle size are oxidised by NO/He within 5 seconds at 313 K; rapid, highly exothermic dissociative chemisorption of NO is the initial step.

Probing the Structure of Operating Fuel Cell Cathode Catalysts using XAS

A proton exchange membrane fuel cell has been designed to enable correlation of cathode catalytic activity with structural parameters determined by in situ XAS investigations in transmission mode. The cell design is described and validated for oxygen reduction activity measurement using both Pt and PtCo alloy catalysts. Findings from in situ XAS investigations at the Pt LIII edge are presented for a 60% Pt/C cathode catalyst operating at OCV, under load with oxygen feed, and during time resolved potential step experiments with oxygen and nitrogen feeds. In nitrogen XANES and EXAFS analysis show increasing oxide formation and particle disruption with increasing potential up to 1.0 V. In oxygen the catalyst structure remained unchanged irrespective of the applied potential and therefore load drawn from the cell. Differences in coordination number at the same applied potential in oxygen compared to nitrogen indicate that a particle morphology change occurs during the ORR.

Redox behaviour of Pd-based TWCs under dynamic conditions: analysis using dispersive XAS and mass spectrometry

The redox behaviour of Pd-based TWCs is studied under dynamic, cycling conditions (e.g. lambda oscillations) on a 50 millisecond scale. Pd temporal response to gas inlet mixture changes is governed by metal-promoter interface properties.

In situ, time resolved, and simultaneous multi-edge determination of local order change during reduction of supported bimetallic (Pt–Ge) catalyst precursors using energy dispersive EXAFS

In situ, time resolved, simultaneous multi-edge energy dispersive EXAFS (EDE) reveals Ge precursor induced perturbations of the reduction of supported Pt(acac)2 to form Pt particles, and details the subsequent thermally induced formation of PtGe species.