Energy Dispersive EXAFS Research Articles

Combined spatially resolved operando spectroscopy: New insights into kinetic oscillations of CO oxidation on Pd/γ-Al2O3

Spatially resolved, combined energy dispersive EXAFS (EDE) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements have been performed over a fixed catalyst bed of Pd/γ-Al2O3 during kinetic oscillations of CO oxidation. The kinetic oscillations of CO oxidation over Pd (or for that matter Pt or Rh) catalysts are a complicated phenomenon that require characterisation techniques with high time resolution and spatial resolution in order to make links between catalyst structure and surface reactivity. By measuring the extent of Pd oxidation at the nanoparticle surface, from Pd K-edge EDE, and matching this with the CO coverage, from DRIFTS spectra, at multiple positions of the fixed bed reactor it is found that the majority of the catalyst undergoes a sharp transition from the CO poisoned catalyst to the highly active, oxidised Pd surface. This transition occurs initially at the end of the catalyst bed, nearest the outlet, and propagates upstream with increasing temperature of the reactor. The oscillations in Pd surface oxide formation and CO coverage are observed only in the first ∼1 mm of the bed, which gives rise to oscillations in CO2 and O2 concentrations observed by end-pipe mass spectrometry after the light-off temperature. The catalyst initially exists as less active, CO poisoned metallic Pd nanoparticles before light-off which transition to a highly active state after light-off when the Pd nanoparticle surface becomes dominated by chemisorbed oxygen. Kinetic oscillations only occur at the front of the catalyst bed where there is sufficient concentration of CO in the gas phase to compete with O2 for adsorption sites at the catalyst surface. We demonstrate the complex nature of the evolving catalyst structure and surface reactivity during catalytic operation and the need for spatially resolved operando methods for understanding and optimising catalyst technologies.

Time resolved operando spectroscopic study of the origin of phosphorus induced chemical aging of model three-way catalysts Pd/Al2O3

The origin of phosphorus (P) induced chemical aging of model three-way catalyst Pd/Al2O3 is studied by transient CO and NO pulses. The fresh catalyst Pd/Al/F (calcined at 500°C for 4h) is subjected to thermal treatment in air at 700°C for 5h with (7.5P/Pd/Al) and without (Pd/Al/700) P poison. The catalyst surface and reaction dynamics during CO/NO pulses at 300°C are captured in a single shot by time resolved operando energy dispersive-extended X-ray absorption fine structure (ED-EXAFS), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and mass spectrometry (MS). The results reveal the occurrence of Pd redox cycle during the temporal analysis of the catalysts by CO/NO pulses. However a secondary process, presumably PdCx formation, is hampered under these experimental conditions on the P aged catalyst 7.5P/Pd/Al. Additionally on 7.5P/Pd/Al, reactions involving Pd redox cycle are not as efficient as on Pd/Al/700 as evident from ED-EXAFS, MS and DRIFTS data. Furthermore, DRIFTS data show a larger population of isocyanate species (reflected by bands at 2180, 2228 and 2250cm−1) on Pd/Al/700 than on 7.5P/Pd/Al that exhibits only a single band at 2260cm−1. The missing low frequency bands at 2180 and 2228cm−1 are attributed to the clogging of the surface Pd sites by P and formation of AlPO4 species at the expense of a fraction of surface Al3+ sites of the support, respectively. Consequently, the production of CO2 (and also N2) is higher on Pd/Al/700 than on 7.5P/Pd/Al as evident by MS data confirming that the chemical aging caused by P is more detrimental than thermal aging for three-way catalytic efficiency of the model Pd/Al2O3 as reported in [S.K. Matam, et al., Catal. Today 184 (2012) 237].

Energy dispersive x-ray absorption spectroscopy for studies of catalysts in solution

The millisecond time resolution achieved by Energy Dispersive EXAFS (EDE) and the high stability provided by the unmoved optical components make this technique very attractive to study chemical reactions and elucidate reactive intermediate species. Great progress has been made on the EDE study of the problematic in situ experiments of reactions in solution. Herein, limitations and solutions for the study of these systems are given. Two examples of catalytic reactions followed in situ and carried out in a stopped-flow/UV-Vis spectroscopy/EDE set-up are shown.

Physical startup of energy dispersive EXAFS spectrometer at Kurchatov Center of synchrotron radiation and nanotechnologies

The energy dispersive EXAFS spectrometer was created by a joint effort of the Joint Institute for Nuclear Research, Belarus State University, the Kurchatov Institute Russian Research Center, and the Institute of Crystallography of the Russian Academy of Sciences. A brief description of the spectrometer is given. The first results of a complex adjustment of the spectrometer units on the synchrotron radiation beam are presented.

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

Examples of the application of a synchronous DRIFTS/EXAFS/mass spectrometry (MS) methodology to the study of dilute (≤ 1wt%) Rh/Al2O3 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/Al2O3 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/Al2O3 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.

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.

Rapid Monitoring of the Nature and Interconversion of Supported Catalyst Phases and of Their Influence upon Performance: CO Oxidation to CO2 by γ‐Al2O3 Supported Rh Catalysts

Spatially and temporally resolved energy-dispersive EXAFS (EDE) has been utilised in situ to study supported Rh nanoparticles during CO oxidation by O2 under plug-flow conditions. Three distinct phases of Rh supported upon Al2O3 were identified by using EDE at the Rh K-edge during CO oxidation. Their presence and interconversion are related to the efficiency of the catalysts in oxidising CO to CO2. A metallic phase is only found at higher temperatures (>450 K) and CO fractions (CO/O2 > 1); an oxidic phase resembling Rh2O3 dominates the active catalyst under oxygen-rich conditions. Below about 573 K, and in CO-rich environments, high proportions of isolated Rh(I)(CO)2 species are found to co-exist with metallic Rh nanoparticles. Alongside these discrete situations a large proportion of the active phase space comprises small Rh cores surrounded by layers of active oxide. Confinement of Rh to nanoscale domains induces structural lability that influences catalytic behaviour. For CO oxidation over Rh/Al2O3 there are two redox phase equilibria alongside the chemistry of CO and O adsorbed upon extended Rh surfaces.

Detectors for energy-dispersive EXAFS (EDE) experiments

EXAFS, X-ray absorption spectroscopy fine structure, has been an important tool within synchrotron radiation science for some years. It has been used extensively in step scanning mode to provide information on the local environment within both static and slowly changing samples. Energy-dispersive EXAFS (EDE) allows similar information to be collected in a significantly shorter time frame, enabling the study of truly dynamically changing samples. The technique uses a bent crystal monochromator to focus an energy-dispersed X-ray beam onto a sample which then impinges upon an array detector positioned behind the sample. This allows the absorption over a range of energies to be collected in a very short period. Until recently only few systems have existed for this technique, typically employing commercially available detectors originally constructed for other applications. These presented limitations in speed, energy resolution, linearity and dynamic range and could only follow changes in the millisecond range. Using UK ESPRC grant funds, a new detector has been developed (XSTRIP) which shows great improvements over the above limitations and has demonstrated the potential to open new areas of science to researchers. This paper will present the technique, some of the historical limitations, the performance increase afforded by the XSTRIP system and an overview of the future direction of detectors for this most exciting and important technique.

In situ monitoring of oxide-supported platinum-group metal catalysts by energy dispersive EXAFS

Energy dispersive EXAFS (EDE), using a Laue monochromator on ID24 of the ESRF, and multiple ion monitoring mass spectrometry have been used to monitor the structure and function of alumina-supported rhodium and palladium catalysts within a microreactor. Metallic 5%-Rh/Al2O3 was preformed in situ by hydrogen reduction, and shown to react rapidly with NO at room temperature yielding an oxidised phase. In order for this material to catalyse the reduction of NO by H2 (to N2), it was shown that the metallic phase must be reformed. The “light-off” temperature at which the metallic phase was formed and catalysis commenced varied with NO : H2 stoichiometry, this temperature lowering in more reducing conditions. N2O formation was concomitant with the conversion from the oxidised to the metallic phase.The EDE configuration allowed the simultaneous monitoring of structure at both the rhodium and palladium K-edges. Incorporation of a 20% palladium mole fraction into 5%-Rh-Pd/Al2O3 protected rhodium from oxidation and also reduced the light-off temperature, and this is reflected in low coordination number values for the composite Pd to Rh/Pd shell. This favours palladium being preferentially on the surface of the nanoparticles. At higher temperatures there are changes in coordination numbers than imply a redistribution of metal sites under catalytic conditions. Future plans to include infrared spectroscopy as a third simultaneous technique will be outlined.

Oxidation/reduction kinetics of supported Rh/Rh2O3 nanoparticles in plug flow conditions using dispersive EXAFS

The kinetics of oxidation and reduction of Al(2)O(3) supported Rh nanoparticles have been determined on a 50 millisecond timescale using energy dispersive EXAFS (EDE).

Synchronous, time resolved, diffuse reflectance FT-IR, energy dispersive EXAFS (EDE) and mass spectrometric investigation of the behaviour of Rh catalysts during NO reduction by CO

Synchronous, time resolved, infra-red, XAFS, and mass spectroscopies are simultaneously applied in situ to the investigation of the dynamic behaviour of Rh/Al2O3 catalysts during NO reduction by CO; NO conversion, and its kinetic character are closely correlated to the conversion of Rh(I)(predominantly RhI(CO)2) to Rh(0).

Limits and Advantages of X-ray Absorption Near Edge Structure for Nanometer Scale Metallic Clusters

We consider the analysis of the K edge of 3d or 4d transition metals when nanometer-scale metallic clusters are considered. From a practical point of view, numerical simulation of the XANES part of the K absorption spectrum of most elements of the periodic table can be performed through full multiple scattering calculations. Then, on the basis of a linear combination of the XANES spectra of reference compounds, the presence of the different phases present inside the materials can be quantified. Here, we show that for nanometer scale metallic clusters, it is not sufficient to consider only the electronic state of the metal of interest to perform a linear combination analysis. In the case of these peculiar materials, special attention has to be paid to different structural parameters, for example, the size and morphology of the cluster, the interatomic distance (taking into account contraction/dilatation processes), and the presence of heterometallic bonds (in the case of bimetallic clusters). Moreover, this approach is not specific to the metallic state. As a conclusion, the quantitative measurement of the structural parameters coming from EXAFS analysis constitutes an invaluable starting point for the FEFF-PCA simulation. The fact that major results coming from the emergence of dynamical studies, namely, Quick-EXAFS or energy dispersive EXAFS, are now obtained will lead to significant breakthroughs in the understanding of the genesis/reactivity of nanometer-scale entities.

Structural characterisation of solution species implicated in the palladium-catalysed Heck reaction by Pd K-edge X-ray absorption spectroscopy: palladium acetate as a catalyst precursor

Energy dispersive EXAFS (EDE), Quick EXAFS (QEXAFS), 13C NMR and X-ray crystallography have been used to probe the co-ordination sphere of palladium in the course of the phosphine free Heck reaction. [Pd2I6][NBu3H]2 has been isolated from the precatalytic solution and its crystal structure determined. EDE and QEXAFS spectra of the complexes Pd(OAc)2, Pd(PPh3)4 and [Pd2I6][NEt3H]2 illustrated the value of the technique in structure elucidation. EXAFS of the precatalytic solution detects [Pd2I6]2− and no co-ordinated carbon. EXAFS of the catalytic solution shows a first co-ordination sphere of 2 carbon atoms and a second of 2–2.5 iodines. A scheme involving an equilibrium between the oxidative addition product and the olefin co-ordination species, has been proposed to explain these results.

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.

Towards a structure–activity relationship for oxide supported metals

The chemistry of [RhCl(CO) 2] 2 on rutile single crystal surfaces and high area oxides is described. Dissociative chemisorption leads to RhCl(CO) 2/oxide in most cases, but when the favoured site is removed, as in rutile(1 1 0)(1×2), reductive decomposition to the metal is rapid. In that favoured site, the surface provides a Lewis base site (O) for coordination to Rh and a Lewis acid site (Ti) for Cl. Metal particles formed by the thermolysis can attain SMSI effects. Using energy dispersive EXAFS (EDE) in conjunction with mass spectrometry, the reactions of Rh(CO) 2Cl/alumina and Rh/alumina with NO have been studied in detail using the Rh K-edge. Both react rapidly with NO under mild conditions. For the dicarbonyl, the kinetics of the changes in the Rh structure and the gas phase compositions reveal two different stages in the reaction towards a bent nitrosyl complex, formulated as RhCl(NO)/alumina. For Rh/alumina, there is rapid disruption of the metal particles; O 2 similarly rapidly disrupts supported rhodium. This fluidity of the metal structures strengthens the importance of synchronous structure/reactivity observations. It has long been the aim to derive a molecular level understanding of the heterogeneous catalysts. Such descriptions are often thwarted by the structural complexity and also by the absence of techniques that can be suitably applied to such materials under reaction conditions. We have sought to tackle this problem by a multi-facetted approach stretching from detailed studies of the chemisorption of organo-rhodium compounds onto the surfaces of single-crystal surfaces through the interaction of organometallics onto the surfaces of high area supports to the improved characterisation of oxide-supported metal catalysts. In this report, we review progress towards the elusive structure–reactivity relationship.

Simultaneous Determination of Structural and Kinetic Parameters Characterizing the Interconversion of Highly Dispersed Species: the Interaction of NO with RhI(CO)2/γ-Al2O3

Energy-dispersive EXAFS (EDE), combined with mass spectrometry and a flow microreactor system, has been used to investigate the reaction of an Al2O3-supported RhI(CO)2 species with NO. This combined in situ approach uniquely permits a priori analysis of the structures of the species involved (on a time scale of ca. 2 s) and simultaneous determination of reaction mechanism and kinetic parameters. In the current case, it is found that the Al(O)RhI(CO)2Cl species reacts to form an intermediate Al(O)Rh(NO)2Cl Cl species (ν ≈ 0.357 ± 0.125 s-1, Eact ≈ 11 ± 1.25 kJ mol-1), which subsequently forms an (AlO)2Rh(NO)-Cl species and N2O(g) (ν ≈ 2 ± 0.5 × 104 s-1, Eact ≈ 40 ± 3.5 kJ mol-1) showing a bent (134°) RhNO bond. This combination of rapid and complementary techniques should be applicable to a wide range of disciplines where quantitative structural and kinetic determinations are of importance.

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.

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.

Characterisation of Reticon and Hamamatsu Photodiode array and the subsequent development of high performance VME-based detector systems optimised for Energy Dispersive EXAFS

Energy Dispersive EXAFS is an established and successful technique employed at Daresbury Laboratory for the study of dynamic experiments. At the heart of this technique is an in house developed high-performance VME-based detector system using a photodiode array. This system originally used a Reticon RL1024S array but extensive investigation of three other photodiode arrays namely the Reticon 512T, 512SB and the Hamamatsu S3904 has enabled their characterisation and the subsequent development of optimised drive and signal processing electronics. This has provided two further systems which exhibit improved signal to noise, excellent linearity and increased operational speed.

Experience of 3 Types of Commercially Available Photodiode Array in the Daresbury Laboratory Energy Dispersive EXAFS Detector System

The technique of Energy Dispersive EXAFS is well established as an excellent tool for dynamic measurements. Daresbury Laboratory has developed a photodiode array based detector system for synchrotron radiation Energy Dispersive EXAFS experiments which allows high quality data to be collected in time scales of less than a second [1]. In order to provide the highest quality system the performance of 3 different commercially available photodiode arrays has been evaluated. This paper presents the results of these evaluations and indicates the future direction of this project.