Igor P. Prosvirin

Chapter 4 X‐Ray Photoelectron Spectroscopy for Investigation of Heterogeneous Catalytic Processes

Abstract X‐ray photoelectron spectroscopy (XPS) is commonly applied for the characterization of surfaces in ultrahigh vacuum apparatus, but the application of XPS at elevated pressures has been known for more than 35 years. This chapter is a description of the development of XPS as a novel method to characterize surfaces of catalysts under reaction conditions. This technique offers opportunities for determination of correlations between the electronic surface structures of active catalysts and the catalytic activity, which can be characterized simultaneously by analysis of gas‐phase products. Apparatus used for XPS investigations of samples in reactive atmospheres is described here; the application of synchrotron radiation allows the determination of depth profiles in the catalyst, made possible by changes in the photon energy. The methods are illustrated with examples including methanol oxidation on copper and ethene epoxidation on silver. Correlations between the abundance of surface oxygen species and yields of selective oxidation products are presented in detail. Further examples include CO adsorption and methanol decomposition on palladium and CO oxidation on ruthenium.

On the nature of the interaction of H2PdCl4 with the surface of graphite-like carbon materials

Abstract Adsorption of H2PdCl4 from aqueous solutions on the surface of graphite-like carbon materials proceeds through two competitive pathways: reduction, which gives rise to Pd ° particles of 6–100nm in size, and formation of π-complexes of PdCl2 with >CC PdCl 2 C +2 HCl ← (2) H 2 PdCl 4 + C → (1) Pd C + 2 Cl C +2 HCl Process (1) is localized near the exterior surface of the porous carbon particles. Process (2) takes place on the whole surface of these particles. The quantitative ratio of the adsorbed palladium species (Pd2+ and Pd °) and also the dispersion and morphology of the metallic particles depend on the mode of contact of the H2PdCl4 solution with the carbon, the composition of the gas phase, the size of the carbon particles, the chemical state of the carbon surface and the conditions of drying. The results obtained are explained in terms of the electrochemical theory of carbon reactivity.

High‐Resolution 3D Proton MRI of Hyperpolarized Gas Enabled by Parahydrogen and Rh/TiO2 Heterogeneous Catalyst

Several supported metal catalysts were synthesized, characterized, and tested in heterogeneous hydrogenation of propene with parahydrogen to maximize nuclear spin hyperpolarization of propane gas using parahydrogen induced polarization (PHIP). The Rh/TiO2 catalyst with a metal particle size of 1.6 nm was found to be the most active and effective in the pairwise hydrogen addition and robust, demonstrating reproducible results with multiple hydrogenation experiments and stability for ≥1.5 years. 3D (1) H magnetic resonance imaging (MRI) of 1 % hyperpolarized flowing gas with microscale spatial resolution (625×625×625 μm(3) ) and large imaging matrix (128×128×32) was demonstrated by using a preclinical 4.7 T scanner and 17.4 s imaging scan time.

XPS, TPD and TPR studies of Cs–O complexes on silver: their role in ethylene epoxidation

Abstract Various Cs oxides formed on the Ag(111) and Ag(110) single crystals depending on cesium coverage, oxygen pressure have been identified by XPS and TPD. The comparison of the determined characteristics with the data for the Ag/Al2O3 catalysts allows us to select those of the Cs–O structures which are common for both types of samples: cesium peroxide and cesium suboxide. Mechanisms of their influence onto routes of ethylene oxidation are discussed.

Study of reactivity of oxygen states adsorbed at a silver surface towards C2H4 by XPS, TPD and TPR

Reactivity of oxygen states absorbed at the surface of a silver foil towards ethylene has been studied by XPS, TPD and TPR. Isotope experiments with “ionic” oxygen (BE9(C 1s)=528.4 eV) labelled by 18O2 and the “covalent” oxygen (BE(O 1s)=530.5 eV) labelled by 16O2 have been carried out. It has been revealed that both oxygen states are desorbed separately. Following the ethylene experiment indicated that the “ionic” oxygen is active in total oxidation only, while the “covalent” oxygen takes part in the ethylene epoxidation.

An XPS study of the composition of iridium films obtained by MO CVD

Abstract Iridium films were deposited on flat quartz substrates by MO CVD from iridium tris-acetilacetonate; these processes were performed at atmospheric pressure in the presence of hydrogen, and substrate temperatures were varied in the range 350–550°C. X-ray photoelectron spectroscopy was used to study film composition: carbon- and oxygen-containing impurities (up to one monolayer) are present on the surface of all the films. During etching by argon ions up to the film-substrate boundary only iridium lines were observed in the spectra. Subsequent etching results in the decrease of the intensity of the iridium lines and the appearance of lines of the substrate material — Si2p, Si2s and O 1s. In the region of the IrSiO 2 boundary there is a transition layer where a compound of the type IrSi x O y is formed. The thickness of this layer increases with increasing deposition temperature. For films deposited at T > 500°C differential charging effects were found, i.e. the samples contain two phases - conductive and nonconductive ones. This might be due to the crystallization of two phases - metallic and “silicate”. The existence of differential charging can also be explained by grain size effects which depend on the deposition temperature.

XPS STUDY OF THE SIZE EFFECT IN ETHENE EPOXIDATION ON SUPPORTED SILVER CATALYSTS

Supported silver catalysts (Ag/α-Al 2 O 3 ) with different particle sizes (100–1000 A) have been prepared and studied by XPS. It has been shown that the increase in the ethene epoxidation rate with silver particle size (size effect) is accompanied by a decrease in the Ag 3d 5/2 binding energy value. The variation in E b (Ag 3d 5/2 ) cannot be explained in terms of a change in the metallic character of the silver (initial state effect), but seems to be determined by the differential charging of the supported silver. The latter effect originates from the easier screening of surface charge induced by photoemission on the silver particles owing to their internal conductivity, rather than that on the dielectric support. The correlation between the internal conductivity of supported silver particles and their catalytic activity is discussed in the context of possible mechanisms for ethene epoxidation.

Model Ag/HOPG catalysts: preparation and STM/XPS study

Model catalysts—Ag on highly oriented pyrolytic graphite (Ag/HOPG)—have been studied using scanning tunneling microscopy (STM), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and X-ray induced Auger electron spectroscopy (XAES). Two types of catalysts were compared: Ag nanoparticles supported on sputtered and non-sputtered HOPG, and the influence of graphite surface defects on the stabilization of Ag nano-sized particles was discussed. A procedure for the preparation of stable (up to 250 °C under a submillibar oxygen/ethylene pressure) silver nanoparticles is suggested. The disappearance of Ag particles in STM images after a sample treatment in the ambient conditions is explained in terms of silver penetration into graphite and the loss of conductivity due to adsorption of contaminants.

XPS, UPS, TPD and TPR studies of oxygen species active in silver-catalysed ethylene epoxidation

The covalent oxygen chemisorbed on silver has been characterised by XPS, UPS, TPD and TPR. It has been shown that this oxygen species active in ethylene epoxidation is characterised by full isotope scrambling in TPD spectra of O2 and the absence of UPS lines located below the silver 4d zone. Both the results testify its atomic origin.

Strong Metal–Support Interactions for Palladium Supported on TiO2 Catalysts in the Heterogeneous Hydrogenation with Parahydrogen

Parahydrogen‐induced polarization (PHIP) was successfully utilized to demonstrate the strong metal–support interaction (SMSI) effect for palladium supported on titania catalysts. Heterogeneous hydrogenation of 1,3‐butadiene over Pd/TiO2 catalysts led to the formation of 1‐ and 2‐butenes and butane, and hyperpolarized products were obtained if parahydrogen was used in the reaction. However, if the catalysts were reduced in H2 flow at 500 °C before the hydrogenation reaction, the observed polarization levels were significantly lower or even zero, which was indicative of the suppression of the pairwise addition of hydrogen route. This observation indicated the possibility to detect the SMSI effect by the PHIP technique. Moreover, by using X‐ray photoelectron spectroscopy it was shown that Pd is partially present as Pdδ+ after reduction under a hydrogen atmosphere at 500 °C. These results were confirmed by transmission electron microscopy, which revealed the formation of Pdδ+ and the dissolution of Pd in the titania lattice.