Valerii I. Bukhtiyarov

Metal-support interactions in cobalt-aluminum co-precipitated catalysts: XPS and CO adsorption studies

Cobalt-aluminum catalysts were prepared using either the precipitation of Co 2+ in the presence of freshly prepared Zn-Al hydrotalcite (the promoted sample) or the co-precipitation of Co 2+ and Al 3+ (the unpromoted samples). The evolution of the initial hydrotalcite-like structure was monitored during its calcination and the reductive treatment by means of XPS. It was shown that at 480 ◦ C the reduction of the calcined samples results in the formation of Co 0 species, the further reduction at 650 ◦ C results in an increase of the amount of the Co 0 species. The samples reduced at 650 ◦ C chemisorb readily carbon monoxide at 77 K, while the sample reduced at 480 ◦ C does not chemisorb CO at 77 K. At elevated temperatures, all reduced samples are found to be able to chemisorb CO. Terminal CO moieties as well as monodentate carbonates, formates and carboxyl species were detected at the surface of the reduced samples at their exposure to the CO medium at the elevated temperature. The intensity of the IR absorption bands of chemisorbed CO are found proportional to the surface fraction of the Co 0 species, measured by XPS. The apparent red shift of the IR absorption bands is observed for CO adsorbed on the samples reduced at 480 ◦ C. The obtained data correlate with the catalytic properties of the Co-Al samples in hydrogenation reactions. The conclusion on the existence of a strong metal–support interaction in the samples under the study is made.

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.

X-ray absorption and photoemission studies of the active oxygen for ethylene epoxidation over silver

Activation of clean polycrystalline silver by a C2H4 + O2 reaction mixture has been studied by XANES, XPS, and UPS. In situ monitoring of the O K-edge XAS spectrum of the pre-treated silver surface revealed a broad signal at 10–20 eV above the threshold. The comparison of the X-ray absorption spectra with O 1s and valence band photoemission data allowed us to attribute this XAS signal to “electrophilic” oxygen (Eb(O 1s) = 530.4 eV) which is known to be active in ethylene epoxidation. The complete absence of XAS features in the photon energy range typical for π* and σ* transitions of molecular oxygen (530–535 eV) indicates both the atomic origin of the electrophilic oxygen and the absence of molecular species on the catalyst surface under the present reaction conditions.

In situ study of selective oxidation of methanol to formaldehyde over copper

Selective methanol oxidation to formaldehyde over polycrystalline copper has been studied with the use of in situ XPS combined with mass spectrometry. It has been shown that the copper surface completely covered by methoxy groups exhibits low activity in methanol oxidation, whereas the metallic copper with sub-oxide oxygen is active in the selective oxidation of methanol to formaldehyde. The concentration of the sub-oxide oxygen species seems to correlate with the rate of formaldehyde production.

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.

Ethylene Epoxidation over Silver Catalysts

The ethylene epoxidation over Ag catalysts is one of the most important industrial processes converting several billion US dollars annually due to the importance of ethylene oxide (EO) as a versatile chemical intermediate. Despite the intensive application of this process, the reaction mechanism is still under debate in the literature. One reason for the lack of detailed understanding of the heterogeneous catalytic reaction is the existence of many different oxygen species on the silver catalyst surface. The application of new in-situ methods like ambient-pressure X-ray photoelectron spectroscopy allows investigation of the role of the different oxygen species in the ethylene epoxidation reaction. The formation and transformation of the oxygen species and particle-size effects will be discussed in terms of the catalytic reaction.

Aqueous, Heterogeneous para-Hydrogen-Induced 15N Polarization

The successful transfer of para-hydrogen-induced polarization to 15N spins using heterogeneous catalysts in aqueous solutions was demonstrated. Hydrogenation of a synthesized unsaturated 15N-labeled precursor (neurine) with parahydrogen (p-H2) over Rh/TiO2 heterogeneous catalysts yielded a hyperpolarized structural analogue of choline. As a result, 15N polarization enhancements of over 2 orders of magnitude were achieved for the 15N-labeled ethyltrimethylammonium ion product in deuterated water at elevated temperatures. Enhanced 15N NMR spectra were successfully acquired at 9.4 and 0.05 T. Importantly, long hyperpolarization lifetimes were observed at 9.4 T, with a 15N T1 of ∼6 min for the product molecules, and the T1 of the deuterated form exceeded 8 min. Taken together, these results show that this approach for generating hyperpolarized species with extended lifetimes in aqueous, biologically compatible solutions is promising for various biomedical applications.

Enhanced Sulfur Tolerance of Ceria-Promoted NO x Storage Reduction (NSR) Catalysts: Sulfur Uptake, Thermal Regeneration and Reduction with H2(g)

SOx uptake, thermal regeneration and the reduction of SOx via H2(g) over ceria-promoted NSR catalysts were investigated. Sulfur poisoning and desulfation pathways of the complex BaO/Pt/CeO2/Al2O3 NSR system was investigated using a systematic approach where the functional sub-components such as Al2O3, CeO2/Al2O3, BaO/Al2O3, BaO/CeO2/Al2O3, and BaO/Pt/Al2O3 were studied in a comparative fashion. Incorporation of ceria significantly increases the S-uptake of Al2O3 and BaO/Al2O3 under both moderate and extreme S-poisoning conditions. Under moderate S-poisoning conditions, Pt sites seem to be the critical species for SOx oxidation and SOx storage, where BaO/Pt/Al2O3 and BaO/Pt/CeO2/Al2O3 catalysts reveal a comparable extent of sulfation. After extreme S-poisoning due to the deactivation of most of the Pt sites, ceria domains are the main SOx storage sites on the BaO/Pt/CeO2/Al2O3 surface. Thus, under these conditions, BaO/Pt/CeO2/Al2O3 surface stores more sulfur than that of BaO/Pt/Al2O3. BaO/Pt/CeO2/Al2O3 reveals a significantly improved thermal regeneration behavior in vacuum with respect to the conventional BaO/Pt/Al2O3 catalyst. Ceria promotion remarkably enhances the SOx reduction with H2(g).

Frontispiece: Selective Single-Site Pd−In Hydrogenation Catalyst for Production of Enhanced Magnetic Resonance Signals using Parahydrogen

Pd-In/Al2 O3 single-site catalyst was able to show high selectivity (up to 98 %) in the gas phase semihydrogenation of propyne. Formation of intermetallic Pd-In compound was studied by XPS during reduction of the catalyst. FTIR-CO spectroscopy confirmed single-site nature of the intermetallic Pd-In phase reduced at high temperature. Utilization of Pd-In/Al2 O3 in semihydrogenation of propyne with parahydrogen allowed to produce ≈3400-fold NMR signal enhancement for reaction product propene (polarization=9.3 %), demonstrating the large contribution of pairwise hydrogen addition route. Significant signal enhancement as well as the high catalytic activity of the Pd-In catalyst allowed to acquire 1 H MR images of flowing hyperpolarized propene gas selectively for protons in CH, CH2 and CH3 groups. This observation is unique and can be easily transferred to the development of a useful MRI technique for an in situ investigation of selective semihydrogenation in catalytic reactors.

Combined application of XANES and XPS to study oxygen species adsorbed on Ag foil

Abstract Adsorbed oxygen species realized in the course of ethylene epoxidation over polycrystalline silver have been characterized by X-ray absorption near the edge structure and X-ray photoelectron spectroscopy. Namely, the combined application of XANES and XPS in similar UHV conditions using the same sample allowed us to assign an XAS feature to the nucleophilic and electrophilic oxygen. This is of great significance, since these species are suggested to be included into the active center for ethylene epoxidation. The differences in the oxygen–silver bonding of these oxygen species are discussed.