Alexander M. Volodin

Photoinduced phenomena on the surface of wide-band-gap oxide catalysts

Abstract This article reviews papers devoted to the investigation of photoinduced processes on non-semiconductor oxide catalysts. The main results of our ESR in situ studies of such systems are presented. Wide-band-gap oxides are shown to constitute a new and very promising class of photocatalysts capable of functioning under illumination with visual light. It is shown that in many cases the light is initially absorbed by charge transfer complexes with the following generation of ion-radical intermediates with high reactivity. A very important feature of photoinduced reactions in the absorption band of charge transfer complexes is a significant shift of their red edge to longer wavelengths in comparison with similar processes in homogeneous systems. In many cases, the formation of such complexes is caused by the existence of strong acceptor or donor sites typical for many oxide catalysts with pronounced acidic or basic properties.

Nanocrystalline ultra high surface area magnesium oxide as a selective base catalyst

Nanocyrstals of MgO having 4 nm average crystallite sizes with 450–550 m2/g surface area were prepared by the Aero-gel preparation method (AP-MgO). Dehydrohalogenation of 1-chlorobutane was performed using AP-MgO as a catalyst; And 1-butene is the selective product formed even after several cycles of 1-chlorobutane injections. AP-MgO was allowed to react with Cl2, which resulted in the formation of a highly reactive adduct which has a capability to selectively chlorinate propane.

ESR study of nanocrystalline aerogel-prepared magnesium oxide

Two signals have been observed in the ESR spectra of nanocrystalline MgO synthesized by an aerogel technique. These signals have been assigned to F+ centers in the nanocrystals and –O–˙CH2 radicals formed from residual methoxide groups present at the points of contact between the nanocrystals. The signal of F+ centers was found to be thermally stable up to 500 °C in oxidizing atmosphere. The second signal was somewhat less stable, and could be removed upon heating to 300 °C in the presence of oxygen. Similar organic radicals could be generated by UV irradiation of aerogel prepared (AP) samples, while their concentration appeared to follow the concentration of residual organics in the materials. Irradiation of the samples in the presence of oxygen resulted in the formation of 4-coordinated surface [O−˙O2] radical anions, while no signal typical for 3-coordinated radical anions was observed. Strong Lewis acid and basic sites on the surface of AP-MgO were characterized using the adsorption of 2,2,6,6-tetramethyl-4-oxo-1-oxyl-pyperidine (TEMPONE) and dinitrobenzene, respectively.

Superoxide radical anions on the surface of zirconia and sulfated zirconia: formation mechanisms, properties and structure

In situ ESR spectroscopy has been used for direct comparison of different thermal and light-induced processes leading to generation of superoxide radical anions on the surface of various zirconia and sulfated zirconia materials. For materials of both types the magnetic resonance parameters of the radical anions were found to be practically independent of the generation method, except for oxygen coadsorption with NO that yields radicals with somewhat smaller gz values. The parameters appear to depend mostly on the state of the surface zirconia cations stabilizing the radical anions, so that the g tensor anisotropy is significantly smaller over sulfated zirconia. It is shown that light-induced formation of superoxide radical anions in the presence of coadsorbed hydrocarbons can be initiated with visible light. Original SIET reaction mechanisms are suggested for the formation of superoxide radical anions by coadsorption with hydrocarbons and illumination after such coadsorption to extend the previously known ones to account for the observed phenomena. Cluster model DFT calculations of magnetic resonance parameters of O2- radical anions stabilized on the surface of zirconium dioxide showed that the adsorption complexes have a -shape rather than linear structure. The magnetic resonance parameters obtained by calculations practically match experimental data and adequately describe their changes after the surface modification with sulfates.

Radical cations of aromatic molecules with high ionization potentials on the surfaces of oxide catalysts: Formation, properties, and reactivity

The results obtained by the authors on the formation and properties of the radical cations of benzene and other molecules with high ionization potentials on the surfaces of ZSM-5 zeolites and sulfated zirconium dioxide are analyzed in this paper. It was found that radical cations of aromatic compounds can be obtained both by a thermal process and under illumination with visible light. Radical cations were found to be key intermediates in the low-temperature polycondensation of aromatic compounds on these catalysts. A possible mechanism of the formation of radical cations and the nature of the sites responsible for these processes are discussed.

Catalytic Purification of Exhaust Gases Over Pd–Rh Alloy Catalysts

Three-way catalysts with low content of Pd–Rh alloy particles used as active components were synthesized and studied. Monometallic and bimetallic mixed catalysts with corresponding precious metals loading were chosen as reference samples. The catalytic activity was tested in oxidation of carbon monoxide, hydrocarbons, and in nitrogen oxides reduction. The stability of the samples was estimated by prompt thermal aging in situ technique. Ethane hydrogenolysis testing reaction was used to determine the surface concentration of Pd and Rh, and to confirm the alloy formation. Photoluminescence and electron paramagnetic resonance spectroscopy were applied to clarify the possible reasons of deactivation and to elucidate the mechanism of stabilization. It was shown that Pd–Rh alloyed catalyst is characterized by comparable activity and enhanced stability. While the Pd and Rh particles of monometallic samples were found to interact with support at high temperatures resulting in sintering and bulk diffusion, the particles of alloy type kept their initial state of dispersion and catalytic activity.

Catalytic properties of massive iron-subgroup metals in dichloroethane decomposition into carbon products

The formation of nanocarbon materials on massive nickel, nichrome, and some other alloys via the carbide cycle mechanism is reported using 1,2-dichloroethane decomposition as an example. The role of the physical stage of the carbide cycle is elucidated, and massive metal surface activation methods ensuring the realization of this stage are considered. The surface layer of massive nickel or some nickel alloys is most effectively activated by the action of chlorine resulting from the catalytic decomposition of 1,2-dichloroethane. It has been demonstrated by ferromagnetic resonance (FMR) spectroscopy that the activation of the massive metal surface in 1,2-dichloroethane decomposition to nanocarbon is due to the surface undergoing crystal chemical restructuring. The microstructuring of the surface yields fine Ni particles similar in size (0.2–0.3 μm) and shape, whose FMR spectra are anisotropic and have similar magnetic resonance parameters. Both chlorine-free and chlorinated hydrocarbons decompose over these particles via the carbide cycle mechanism. It is demonstrated that it is possible to design catalytic reactors packed with massive nickel or its alloy. The nanocarbon material obtained in such a reactor will not be contaminated by components of conventional catalyst supports (Al, Mg, etc.). The stable performance temperature of the catalyst will be increased, and this will allow the equilibrium outlet methane concentration to be reduced.

Photostimulated Formation of Radicals on Oxide Surfaces

AbstractWe present results ofin situ EPR investigations of the mechanism of photostimulated processes resulting in radical and ion-radical particle formation on the surfaces of oxide dielectrics (magnesium, calcium, aluminum oxides, zeolites). Three types of reactions are discussed:1.Formation of oxygen anion-radicals on MgO and CaO surfaces.2.Formation of benzene cation-radicals on ZSM-5 zeolites.3.Formation of radical particles from aromatic nitrocompounds adsorbed on alumina. On the basis of investigation of the spectral relationships and the properties of surface active centre, it is concluded that light is absorbed by coordinatively unsaturated surface sites in the first system, whereas in the other processes, electron donor-acceptor (EDA) complexes between adsorbed molecules and surface active sites are supposed to be key intermediates. These EDA complexes are shown to incorporate donor solvent molecules as well. In this case the energetic characteristics of the photoprocesses are substantially determined by the ionization potential of solvent molecules.Mechanisms of photo- and thermostimulated processes are compared and possible similarities are discussed for all the reactions studied.

CATION RADICAL INTERMEDIATES IN BENZENE AND TOLUENE POLYCONDENSATION ON SULFATED ZIRCONIA

Benzene dimer cation radicals were detected on sulfated zirconia at low temperature and benzene coverage. Two temperature regions for their transformations were observed. Below 473 K the radical concentration remained stable reflecting the amount of strong surface acceptor sites. Above 623 K it increased drastically.

O - Radical Anions on Oxide Catalysts: Formation, Properties, and Reactions

A systematic in situ EPR study of processes yielding O− radical anions on the surface of oxide dielectrics (MgO, CaO), semiconductors (ZnO, TiO2), supported systems (V/SiO2), and zeolite FeZSM-5 is reported. Methodological approaches to the study of O-radical anions are considered for the cases in which these species are directly undetectable by EPR. Particular attention is focused on the development of methods of investigation of so-called α-oxygen on the FeZSM-5 surface, which is an O− radical anion stabilized on the paramagnetic ion Fe3+. The reactions involving α-oxygen and the analogous reactions known for O-radical anions stabilized on the oxide surface are demonstrated to occur in similar ways. The photostimulated formation of spatially separated electron and hole centers on the surface of oxide systems is most likely due not to charge separation, but to the spatial separation of the radicals resulting from the homolytic photodissociation of chemisorbed water. A scheme is suggested for this process on the partially hydroxylated MgO surface.