Atomic XAFS Research Articles

The atomic AXAFS and Δμ XANES techniques as applied to heterogeneous catalysis and electrocatalysis

X-Ray absorption spectroscopy (XAFS) is an attractive in situ and in operando technique. In recent years, the more conventional extended X-ray absorption fine structure (EXAFS) data analysis technique has been complemented by two newer analysis methods: the 'atomic' XAFS (AXAFS) technique, which analyzes the scattering from the absorber atom itself, and the Delta(mu) XANES technique, which uses a difference method to isolate the changes in the X-ray absorption near edge structure (XANES) due to adsorbates on a metal surface. With AXAFS it is possible to follow the electronic effect a support has on a metal particle; with Delta(mu) XANES it is possible to determine the adsorbate, the specific adsorption sites and adsorbate coverage on a metal catalyst. This unprecedented new information helps a great deal to unravel the complex kinetic mechanisms operating in working reactors or fuel cell systems. The fundamental principles and methodology for applying the AXAFS and Delta(mu) XANES techniques are given here, and then specific applications are summarized, including H adsorption on supported Pt in the gas phase, water activation at a Pt cathode and methanol oxidation at a Pt anode in an electrochemical cell, sulfur oxidation on Pt, and oxygen reduction on a Au/SnO(x) cathode. Finally, the future outlook for time and/or space resolved applications of these techniques is contemplated.

Application of AXAFS Spectroscopy to Transition‐Metal Oxides: Influence of the Nearest and Next Nearest Neighbour Shells in Vanadium Oxides

The influence of changes in coordination number, interatomic distances, and oxidation state on the intensity and centroid position of the Fourier transform (FT) of the atomic X-ray absorption fine structure (AXAFS) peak of vanadium oxide bulk model compounds and alumina-supported vanadium oxide clusters has been investigated. Using Na3VO4 and V2O5 as model compounds, it has been shown that the nearest neighbour shells have a pronounced influence on the AXAFS intensity; specifically, a 40 % decrease in intensity was observed between these two compounds. Secondly, the influence of partial reduction of the vanadium oxide species has been determined; this led to a 50 % decrease in the AXAFS intensity and to an increase in the centroid position. Furthermore, the influence of the vanadium oxide loading has been assessed. A non-linear relationship between the vanadium oxide loading and the AXAFS intensity has been found, indicating that the AXAFS intensity is sensitive to the formation of V-O-V bridging bonds between the vanadium VO4 clusters. The results show that AXAFS can be used to probe the relative energy level of the vanadium valence orbitals.

Atomic XAFS as a Tool To Probe the Reactivity of Metal Oxide Catalysts: Quantifying Metal Oxide Support Effects

The potential of atomic XAFS (AXAFS) to directly probe the catalytic performances of a set of supported metal oxide catalysts has been explored for the first time. For this purpose, a series of 1 wt % supported vanadium oxide catalysts have been prepared differing in their oxidic support material (SiO2, Al2O3, Nb2O5, and ZrO2). Previous characterization results have shown that these catalysts contain the same molecular structure on all supports, i.e., a monomeric VO4 species. It was found that the catalytic activity for the selective oxidation of methanol to formaldehyde and the oxidative dehydrogenation of propane to propene increases in the order SiO2 < Al2O3 < Nb2O5 < ZrO2. The opposite trend was observed for the dehydrogenation of propane to propene in the absence of oxygen. Interestingly, the intensity of the Fourier transform AXAFS peak decreases in the same order. This can be interpreted by an increase in the binding energy of the vanadium valence orbitals when the ionicity of the support (increasing electron charge on the support oxygen atoms) increases. Moreover, detailed EXAFS analysis shows a systematic decrease of the V-Ob(-M(support)) and an increase of a the V-O(H) bond length, when going from SiO2 to ZrO2. This implies a more reactive OH group for ZrO2, in line with the catalytic data. These results show that the electronic structure and consequently the catalytic behavior of the VO4 cluster depend on the ionicity of the support oxide. These results demonstrate that AXAFS spectroscopy can be used to understand and predict the catalytic performances of supported metal oxide catalysts. Furthermore, it enables the user to gather quantitative insight in metal oxide support interactions.

Determination of O[H] and CO Coverage and Adsorption Sites on PtRu Electrodes in an Operating PEM Fuel Cell

A special in situ PEM fuel cell has been developed to allow X-ray absorption measurements during real fuel cell operation. Variations in both the coverage of O[H] (O[H] indicates O and/or OH) and CO (applying a novel Deltamu(L3) = mu(L3)(V) - mu(L3)(ref) difference technique), as well as in the geometric (EXAFS) and electronic (atomic XAFS) structure of the anode catalyst, are monitored as a function of the current. In hydrogen, the N(Pt)(-)(Ru) coordination number increases much slower than the N(Pt)(-)(Pt) with increasing current, indicating a more reluctant reduction of the surface Pt atoms near the hydrous Ru oxide islands. In methanol, both O[H] and CO adsorption are separately visible with the Deltamu technique and reveal a drop in CO and an increase in OH coverage in the range of 65-90 mA/cm(2). With increasing OH coverage, the Pt-O coordination number and the AXAFS intensity increase. The data allow the direct observation of the preignition and ignition regions for OH formation and CO oxidation, during the methanol fuel cell operation. It can be concluded that both a bifunctional mechanism and an electronic ligand effect are active in CO oxidation from a PtRu surface in a PEM fuel cell.

Atomic XAFS as a Tool to Probe the Electronic Properties of Supported Noble Metal Nanoclusters

Atomic XAFS is a very attractive technique for probing electronic properties of supported metal nanoclusters. For platinum nanoparticles on different supports, the technique is found to be in good agreement with infrared CO adsorption measurements. The advantages of AXAFS, however, are that no probe molecule is required and that real-time measurements under reaction conditions are possible.

AXAFS as a probe of charge redistribution within organometallic complexes.

Atomic XAFS on [PtCl(NCN)-Z] pincer complexes shows it to be a sensitive probe for the determination of the electron density on the metal atom, similar yet complementary to NMR.

Separation of double-electron and atomic XAFS contributions in x-ray absorptionspectra of Pt foil and Na2Pt(OH)6

Both double-electron excitation (DEE) and atomic XAFS (AXAFS) contributionsare shown to be present in L23 x-ray absorption spectra of Pt foil and Na2Pt(OH)6.In the Fourier transform of the spectra, the DEE feature appears at lowerRvalues than the AXAFS feature. A background subtraction procedure is developedthat leaves the DEE contribution in the background and the AXAFS in theoscillatory part of the spectrum. The separation of the DEE and theAXAFS contributions is possible only when using a cubic spline functionroutine in which the smoothing parameter can be continuously adjusted.

Nature of the Metal–Support Interaction in Supported Pt Catalysts: Shift in Pt Valence Orbital Energy and Charge Rearrangement

Conversion of neopentane (hydrogenolysis and isomerization on Pt/LTL, Pt/SiO2–Al2O3, Pt/MgO–Al2O3) and tetralin (hydrogenation on Pt/Y) catalytic data, combined with spectroscopic Pt atomic XAFS (AXAFS) data, and theoretical calculations are utilized to elucidate the nature of the metal–support interaction for Pt-supported catalysts. The turnover frequency (TOF) of both the neopentane and tetralin conversion strongly depends on the composition of the support. The TOF increases with increasing acidity, polarization power of the charge-compensating cations (Na+, H+, K+, La3+), Si/Al ratio, and the presence of extra-framework Al. The intensity of the experimental Pt atomic XAFS correlates with the TOF. Ab initio scattered wave cluster calculations on a Pt4O3 cluster were performed using the FEFF7 code. The electron charge on the three “support” oxygens was changed from +0.05 to −0.01 electron to mimic changes in the support Madelung potential, which for the cluster is dominated by the nearest neighbor oxygen charge. The trends found in these theoretical AXAFS results are in excellent agreement with the experimental Pt AXAFS data and suggest that a metal cluster–support potential model is adequate for describing the changes seen in the experimental AXAFS. The experimental AXAFS results can also be understood using a molecular orbital scheme. This molecular orbital scheme further indicates that the metal–support interaction not only changes the ionization potential of the Pt valence orbitals but also induces a charge rearrangement from the Pt 6s orbitals within the particle to the oxygens of the Pt-support interface and vice versa. This charge rearrangement is also indicated by the AXAFS through the shift, ΔR, in AXAFS peak position. Both effects influence the electronic s tructure of the Pt particles. The changes in the electronic structure alter the catalytic properties of the Pt surface atoms by varying the bond strength and bond order (single or bridged) to the catalytic intermediates. The consequences of the metal–support interaction for tailor-made supported metal catalysts will be discussed.

Measurements of photon interference X-ray absorption fine structure (piXAFS).

Experimental data are presented which demonstrate the existence of a fine structure in extended X-ray absorption spectra due to interference effects in the initial photon state (piXAFS). Interference occurs between the incident electromagnetic wave and its coherently scattered waves from neighboring atoms. Using fine platinum and tungsten powders as well as polycrystalline platinum foil, piXAFS was measured in high-precision absorption experiments at beamline X1 at HASYLAB/DESY over a wide energy range. piXAFS is observed below and above absorption-edge positions in both transmission and total-electron-yield detection. Based on experimental data it is shown that piXAFS is sensitive to geometric atomic structure. Fourier-transformed piXAFS data carry information, comparable with that of EXAFS, about the short-range-order structure of the sample. Sharp structures occur in piXAFS when a Bragg backscattering condition of the incident X-rays is fulfilled. They allow precise measurement of long-range-order structural information. Measured data are compared with simulations based on piXAFS theory. Although piXAFS structures are similarly observed in two detection techniques, the importance of scattering off the sample for the measurements needs to be investigated further. Disentangling piXAFS, multielectron photoexcitations and atomic XAFS in high-precision measurements close to absorption edges poses a challenge for future studies.

An atomic X-ray absorption fine structure study of the influence of hydrogen chemisorption and support on the electronic structure of supported Pt particles

The physical principles of atomic XAFS (AXAFS) are presented along with important details on how to isolate the AXAFS contribution from the experimental XAFS data. Intuitive illustrations are given showing how various interactions of the absorber atom with its neighbours influence the AXAFS contribution. Hydrogen chemisorbed on the surface of the supported metal particles is shown to have a strong influence on the amplitude of the Fourier transform AXAFS peak. The effect of the support (with different amounts and types of charge compensating ions (H + ,K + ), different Si/Al ratio and extra-framework Al) on the experimental AXAFS spectra of Pt dispersed in zeolites (LTL,Y) and on flat supports (Al 2O3‐SiO2, MgO‐Al2O3) are summarised. It is shown that the essence of the metal‐support interaction, as revealed in the AXAFS, is a shift in the ionisation potential of the valence d-orbital electrons brought about by the polarisation induced primarily by the changing charge on the support oxygen atoms.

The metal–support interaction in Pt/Y zeolite: evidence for a shift in energy of metal d-valence orbitals by Pt–H shape resonance and atomic XAFS spectroscopy

The turnover frequency (TOF) for conversion of neo-pentane was determined for Pt in Y zeolite with different numbers of protons and La +3 ions, different Si/Al ratios and with non-framework Al being present. Comparing Pt/NaY to Pt/H-NaY and Pt/K-USY with Pt/H-USY, respectively, shows an increase in the ln (TOF) which is proportional to the number of protons. Compared to NaY, the TOF of Pt in non-acidic NaLaY zeolite is about 25 times higher, which indicates also a strong influence on the charge of the cations on the TOF of Pt. The 20 times increase in the Pt TOF for K-USY compared to NaY is attributed to the effect of a higher Si/Al ratio and non-framework Al in the K-USY. EXAFS data collected on Pt/NaY and Pt/H-USY showed platinum particles consisting of 14–20 atoms on an average. These results were confirmed by HRTEM, which also showed that the Pt particles were dispersed inside the zeolite. The EXAFS data indicate that the metal particles are in contact with the oxygen ions of the support. The peak in the Fourier transform of the atomic XAFS (AXAFS) spectrum of the Pt/H-USY is larger in intensity than the corresponding peak of the Pt/Na-Y data. A detailed analysis of the L 2 and the L 3 X-ray absorption near edge structure revealed a shape resonance due to the Pt–H anti-bonding state (AS) induced by chemisorption of hydrogen on the surface of the platinum metal particles. The difference in energy ( E res) between the AS and the Fermi-level ( E F) is 4.7 eV larger for Pt/H-USY than for Pt/NaY. Both the AXAFS spectra and the shape resonances of the Pt-NaY and the Pt/H-USY catalysts provide direct experimental evidence of how the support properties determine the electronic structure of the platinum metal particles. Previous AXAFS and shape resonance work lead to a model in which the position in energy of the Pt valence orbitals is directly influenced by changes in the potential (i.e. electron charge) of the oxygen ions of the support and how the proton density affects this oxygen charge. This work shows that the potential of the oxygen ions is also a function of the Si/Al ratio of the support and the polarisation power of the charge compensating cations (H +, Na +, La 3+ and extra-framework Al); the metal particles experience an interaction which is determined by several properties of the support. The data further reveal how the change in the Pt electronic structure directly influences the catalytic properties of the catalyst. While the TOF is dependent on the metal–support interaction, the hydrogenolysis selectivity is determined by the Pt particle size, and increases linearly with increasing dispersion, or decreasing particle size.

A New Model Describing the Metal–Support Interaction in Noble Metal Catalysts

The catalytic activity and spectroscopic properties of supported noble metal catalysts are strongly influenced by the acidity/alkalinity of the support but are relatively independent of the metal (Pd or Pt) or the type of support (zeolite LTL or SiO2). As the alkalinity of the support increases, the TOF of the metal particles for neopentane hydrogenolysis decreases. At the same time, there is a decrease in the XPS binding energy and a shift from linear to bridge bonded CO in the IR spectra. Analysis of the shape resonance in XANES spectra indicates that in the presence of chemisorbed hydrogen the difference in energy between the Pt–H antibonding orbital and the Fermi level decreases as the alkalinity of the support increases. Based on the results from the IR, XPS, and shape resonance data a new model is proposed in which the interaction between the metal and support leads to a shift in the energy of the metal valence orbitals. The EXAFS structural analysis indicates that the small metal particles are in contact only with the oxide ions of the support. Finally, a new spectroscopic characterisation, Atomic XAFS, is presented which provides new insights into the origin of the electronic changes in the metal. As the alkalinity of the support increases, there is decrease in the metal ionisation potential. The primary interaction is a Coulomb attraction between metal particle and support oxygen ions, which affects the metal interatomic potential. This model for the metal–support interaction explicitly excludes the need for electron transfer, and it can account for all observed changes in the catalytic, electronic, and structural properties of the supported metal particles induced by support acidity ranging from acidic to neutral to alkaline.

XAFS spectroscopy in catalysis research: AXAFS and shape resonances.

Two new techniques in X-ray absorption spectroscopy (XAS) have recently been developed which provide previously unobtainable information on supported noble metal catalysts. Atomic X-ray absorption fine structure (AXAFS) provides direct information on the average changes in interatomic potential of the metal particles induced by metal-support effects. Changes in the interatomic potential can now be directly related to changes in catalytic reactivity. Analysis of shape resonances near the X-ray absorption edge provides information on the changes in the nature of the bonding of adsorbates (like hydrogen) to the metal cluster as induced by the properties of the support. The strong decrease in activity of supported platinum clusters for neo-pentane hydrogenolysis with increasing alkalinity of the support can be ascribed to a decrease in ionization potential of the metal particles directly influenced by the alkalinity of the support.

'Atomic' XAFS as a probe of electronic structure.

Atomic XAFS (AXAFS) offers die potential for a new tool to gain relatively detailed electronic structure and polarization information on systems, in-sit'u, that may not contain long range geometric order. We have shown previously that AXAFS can monitor ch_anges in die charge of less than 0.05e per surface Pt atom on a metallic cluster, and sees interactions from ions 2 and 3 shells removed from die absorber atom. The AXAFS lineshape allows a differentiation between the through space field polarization and the through bond inductive effects. Applications have been made to in situ Pt electrodes, ion.cation interactions in solution, and supported noble metal catalysts.

`Atomic' X-ray absorption fine structure: a new tool for examining electronic and ionic polarization effects

The Zn K-edge X-ray absorption fine structure (XAFS) of Zn(OH) 4 2− ions in basic solution are analyzed. Atomic XAFS (AXAFS) arises from photoelectron scattering off electrons in bonds on the periphery of the absorber. Interactions of the zincate ion with hydrated cations (H 2O) n M + (M=Li +, Na +, K +, and Rb +) are shown to produce significant changes in the AXAFS intensity that result from polarization of the zincate anion by the cation. Both the `through space field' and `through bond inductive' polarization mechanisms are considered; with the inductive mechanism found to dominate in this case.

Conversion Electron/He Ion Yield XAFS of SrTiO3 Thin Films

It is known that the dielectric constant of SrTiO3 thin film deposited on a thick substrate decreases with decreasing the thickness. This may be related with local atomic distortion in SrTiO3 thin film. In order to detect the atomic distortion, Sr and Ti K-edge XAFS spectra were measured with a total-conversion electron/He ion-yield (CEY/CIY) apparatus which is easy to build and to operate. We confirmed the validity of our fabricated CEY/CIY apparatus and applied it to XAFS measurement of SrTiO3 thin films. Fourier transforms of XAFS at different thicknesses suggested the local distortion induced by the internal stress in thin films.

Understanding atomic x-ray absorption fine structure in x-ray absorption spectra

The origin and important parameters determining the intensity of atomic x-ray absorption fine structure (AXAFS) are described both in chemical and physical terms. A full mathematical derivation is presented and new criteria are given for removal of the background to extract the total (EXAFS and AXAFS) from the experimental absorption cross-section. The embedded-atom potential, the interstitial potential and the distribution of the absorber-atom electron density are all found to be important in determining the AXAFS intensity. Application is made to spherical Pt metal clusters, where it is shown that the AXAFS intensity of the central atom is much larger than that of the surface atoms. However, the average AXAFS intensity per platinum atom is found not to depend significantly on cluster size. On the other hand, variation of the metal cluster support does considerably change the intensity as well as the imaginary part of the AXAFS. Hence, AXAFS can be a very useful probe of the effects of metal-support interactions in supported noble-metal catalysts.

Systematic Chemical Effects Observed in “Atomic” X-ray Absorption Fine Structure

We report here, for the first time, large and systematic chemical effects on the “atomic” X-ray absorption fine structure (AXAFS) for a series of Pt−Ru alloys as a function of the composition and for an in situ Pt electrode as the electrode potential is increased. These changes are seen in the Fourier transform of the normal χ(E) function in XAFS, where the AXAFS produces a peak at the characteristic atomic radius. Curved-wave multiple-scattering cluster calculations (FEFF6) confirm the changes with alloy composition and electrode potential, although the theory overestimates the size of the changes. Obvious reasons for this are given. The systematic chemical effects and agreement with theoretical predictions provide strong support for their interpretation as AXAFS features, rather than multielectronic excitations. The AXAFS structure and its interpretation offers the promise of providing a new tool for observing in situ electronic (AXAFS) and geometric (normal EXAFS) structural changes simultaneously in c...

A new method for extracting x-ray absorption fine structure and the atomic background from an x-ray absorption spectrum

A new technique for separating x-ray absorption fine structure (XAFS) and the atomic background in an x-ray absorption spectrum is presented. XAFS oscillations are reliably extracted from the Fourier transform of the nth derivative of the experiment data, without making the usual smooth-background approximation. The atomic background is obtained by the subtraction of the XAFS from the experimental data. Multielectron excitations and atomic XAFS (AXAFS) have been observed in the atomic background. This provides a useful tool for the investigation of multielectron excitations and AXAFS in condensed matter.