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Reactivity of Surface Species in Heterogeneous Catalysts Probed by In Situ X-ray Absorption Techniques

Nanocatalysis: size- and shape-dependent chemisorption and catalytic reactivity

Hydrogen is dissociatively adsorbed on the gold particles in Au/Al2O3 catalysts, as demonstrated by a combination of in-situ X-ray absorption spectroscopy, chemisorption, and H/D exchange experimen...

Hydrogen chemisorption on Al2O3-supported gold catalysts.

Activation of oxygen on gold/alumina catalysts: in situ high-energy-resolution fluorescence and time-resolved X-ray spectroscopy.

Knowledge of the structure of catalysts is essential to understand their behavior, which further facilitates development of an active, selective, and stable catalyst. Determining the structure of a functioning catalyst is essential in this regard. The structure of a catalyst is prone to change during the catalytic process and needs to be determined in its working conditions. In this tutorial review, we have summarized studies done at synchrotron radiation facilities that illustrate the capability to determine catalyst structure using X-ray absorption spectroscopy (XAS) and X-ray emission spectroscopy (XES). These studies aim at facilitating the determination of the dynamic structure-performance relationships during a catalytic process.

/pdf/advanced-x-ray-absorption-and-emission-spectroscopy-in-situ-4i9uzf7b35.pdf

Advanced X-ray absorption and emission spectroscopy: in situ catalytic studies

Pt L3 X-ray absorption edge data on small supported Pt particles (N < 6.5) reveals that at very low H2 pressure or high temperature the strongest bonded H is chemisorbed in an atop position. With decreasing temperature or at higher H2 pressure only n-fold (n = 2 or 3) sites are occupied. At high H2 pressure or low temperature, the weakest bonded H is positioned in an “ontop” site, with the chemisorbing Pt already having a stronger bond to a H atom in an n-fold site. DFT calculations show that the adsorption energy of hydrogen increases for Pt particles on ionic (basic) supports. The combination of the DFT calculations with hydrogen chemisorption data and the analysis of the Pt L3 X-ray absorption spectra implies that both the H coverage and/or the type of active Pt surface sites, which are present at high temperature catalytic reaction conditions, strongly depend on the ionicity of the support. The consequences for Pt catalyzed hydrogenolysis and hydrogenation reactions will be discussed.

/pdf/three-site-model-for-hydrogen-adsorption-on-supported-293w4vxuf6.pdf

Three-site model for hydrogen adsorption on supported platinum particles: influence of support ionicity and particle size on the hydrogen coverage.

The influence of the support ionicity on the adsorption of H, CHx (x = 2, 3), and O on supported Pt particles is investigated using density functional theory (DFT). The DFT calculations reveal that the importance of the 6s,p states for the Pt adsorbate bonding increases in the order Pt−O < Pt−CH2 < Pt−CH3 < Pt−H. Both the Pt−H and Pt−O bond strengths are strongly affected by changes in the support ionicity whereas the difference for the Pt−CHx bond strength is much smaller. The Pt−H bond strength is higher for ionic supports, whereas the Pt−O bond strength is higher for acidic or more covalent supports. These bond strength changes occur because the support ionicity has a large impact on both the Pt 6s,p and 5d states. On ionic supports with electron rich oxygen atoms, the 6s,p states are pushed toward the surface of a metal cluster where it can participate in H bonding, whereas on supports with electron poor oxygen atoms (existing in acidic supports or for supports with more covalent cations) the 6s,p sta...

/pdf/theoretical-study-on-pt-particle-adsorbate-bonding-influence-1ls0b1h0c8.pdf

Theoretical Study on Pt Particle Adsorbate Bonding: Influence of Support Ionicity and Implications for Catalysis

The development of a new in situ probe of metallic character in supported metal clusters utilizing X-ray absorption spectroscopy is described. The technique is based on the extent of screening of the core-hole left in the neutral final state after the X-ray absorption. The technique allows for the clear differentiation between local interatomic charge transfer and more delocalized metallic screening. The particle size at the metal insulator transition is found to depend strongly on the electron richness of the support oxygen atoms (i.e., ionic vs covalent oxides). Pt particles on supports with electron poor oxygen atoms (covalent) show metallic screening for sizes as small as 12 A in diameter. In contrast, on supports with electron rich oxygen atoms (ionic) the Pt particles do not show metallic behavior until around 20 A. The wide variation of previously reported estimates of the particle size at which the insulator to metal transition occurs is explained, giving a consistent picture for the onset of metallic character, and the reasons for the strong support effect.

/pdf/strong-support-effects-on-the-insulator-to-metal-transition-4hys72rahu.pdf

Strong Support Effects on the Insulator to Metal Transition in Supported Pt Clusters as Observed by X-ray Absorption Spectroscopy

https://dspace.library.uu.nl/bitstream/handle/1874/26048/J.Catal-Oudenhuijzen-2003.pdf?sequence=1

Michiel K. Oudenhuijzen

Papers

Three-site model for hydrogen adsorption on supported platinum particles: influence of support ionicity and particle size on the hydrogen coverage.

Theoretical Study on Pt Particle Adsorbate Bonding: Influence of Support Ionicity and Implications for Catalysis

Strong Support Effects on the Insulator to Metal Transition in Supported Pt Clusters as Observed by X-ray Absorption Spectroscopy

Support-induced compensation effects in H/D exchange of cyclopentane