The La2O3/Si thin films have been deposited by reactive DC magnetron sputtering. Amorphous state of La2O3 layer has been shown by RHEED observation. Top surface chemistry of the a-La2O3 has been ...evaluated with layer-by-layer depth profiling by ion bombardment and XPS measurements. It was found by core level spectroscopy that the top surface of the a-La2O3 film consists of hydrocarbon admixture, lanthanum carbonate, and hydroxides that formed as a result of contact with air atmosphere. Thickness of this top surface modified layer is below 1 nm for a contact time of ∼1.5 h with air at normal conditions.
•The oxidation of methanol over a monolayer V2O5/TiO2 catalyst was examined.•At low temperatures, dimethoxymethane and methyl formate are the main products.•Under mild conditions, the production of ...formaldehyde is greatly inhibited.•According to in situ X-ray photoelectron spectroscopy, the reaction involves reversible reduction of V5+ cations.•The oxidation of methanol proceeds through the classical Mars–van Krevelen mechanism.
The oxidation of methanol over highly dispersed vanadia supported on TiO2 (anatase) has been investigated using in situ Fourier transform infrared spectroscopy (FTIR), near ambient pressure X-ray photoelectron spectroscopy (NAP XPS), X-ray absorption near-edge structure (XANES), and a temperature-programmed reaction technique. The data were complemented by kinetic measurements collected in a flow reactor. It was found that dimethoxymethane competes with methyl formate at low temperatures, while the production of formaldehyde is greatly inhibited. Under the reaction conditions, the FTIR spectra show the presence of non-dissociatively adsorbed molecules of methanol, in addition to adsorbed methoxy, dioxymethylene, and formate species. According to the NAP XPS and XANES data, the reaction involves a reversible reduction of V5+ cations, indicating that the vanadia lattice oxygen participates in the oxidation of methanol via the classical Mars–van Krevelen mechanism. A detailed mechanism for the oxidation of methanol on vanadia catalysts is discussed.
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•Cu-Fe-Al oxide nanocomposites are promising catalysts for catalytic combustion of solid fuels in a fluidized bed.•The mechanism of promotion of iron oxide by Cu and Al was ...elucidated.•Aluminum is a textural promoter stabilizing the high specific surface area of iron oxide based catalysts.•Copper is a chemical promoter which increases the catalytic activity in the CO oxidation.
The Fe-Al and Cu-Fe-Al oxide nanocomposites active in the oxidation of CO were studied by XRD, TEM, EDX, Raman spectroscopy, TPR-H2 and differential dissolution techniques. The nanocomposites were prepared by fusion of aluminum, iron, and copper salts that leads to their inhomogeneity. The Al3+ cations partially dissolve in the Fe2O3 lattice, which leads to a significant decrease in size of iron oxide. Copper locates in the Al-rich agglomerates to form CuO nanoparticles and partially dissolves in alumina. The dissolution of copper in iron oxide and formation of a (Cu,Al,Fe)3O4 spinel was observed only at a high Cu loading. Hence, aluminum plays the role of a textural promoter, preventing sintering and stabilization of the high specific surface area of oxide. Copper does not act as a textural promoter, as it does not affect the crystal size of iron oxide. The addition of Cu increases the catalytic activity of iron oxide-based catalysts.
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•The deactivation of catalysts by acidity of pyrolysis liquid is a specific drawback to be addressed.•The “accelerated” deactivation of novel sol-gel Ni-based catalyst was studied by ...treating in acetic acid solution (pH = 2–3).•Oxidation of metalliс species Cu and Ni was shown to obtain Cu2O, NiO and Ni(OH)2- like phases.•Decreasing activity of pre-treated catalysts in gas-phase hydrodeoxygenation of propanoic acid was caused by metal leaching.
The main reasons of catalysts deactivation in hydro-processing pyrolysis liquids are by coke deposition, poisoning by bio-oil impurities (S, N, K, Cl, etc.), leaching of catalyst components, structural degradation in the presence of H2O, and sintering. The deactivation of catalysts by the acidity of the pyrolysis liquid is a specific concern, and this deactivation mechanism was studied by treating newly developed NiCuMo-SiO2 catalysts in 1 M acetic acid water solution (pH = 2–3). The activity of the acid-treated catalysts was subsequently investigated in the hydrodeoxygenation of gaseous propionic acid, in a tubular reactor at 225 °C with n-hexane and n-octane serving as diluent and internal standard, respectively.
The samples treated by acid at different times (15–360 min) were characterized by X-ray diffraction (XRD), high resolution transition electron microscopy (HRTEM), X-ray fluorescence (XRF), CO chemisorption, N2 physical adsorption, and X-ray photoelectron spectroscopy (XPS). XRF and HRTEM studies together with the residual mass of catalyst pointed out at gradual leaching of catalyst components. Among the catalyst components, dissolution of nickel was the most pronounced, while molybdenum content decreased to a lesser extent. This is due to the formation of more acid stable molybdenum blues. The amount of copper decreased only slightly, due its higher electrochemical potential. Oxidation of metalliс species Cu and Ni is shown to obtain Cu2O, NiO and Ni(OH)2-like phases. Interestingly, the acidic treatment resulted in increasing active surface of the catalyst, nevertheless, the catalyst activity in propionic acid conversion irreversibly decreased in time by the acetic acid treatment due to loss of the active components (substantially nickel).
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•The oxidation of ethanol over vanadia supported catalyst was examined.•The monolayer V2O5/TiO2 catalyst demonstrated the best activity in the selective oxidation of ethanol.•The ...reaction involves reversible reduction of V5+ cations whereas titanium cations remain in the Ti4+ state.•At 100–150°C acetaldehyde is the main product; at higher temperatures the reaction shifts toward acetic acid.
The catalytic performance of vanadia supported on silica, alumina, zirconia, and titania was investigated in the selective oxidation of ethanol. It was shown that the activity and product distribution strongly depend on the support material, which determines the structure of supported vanadia species. On silica and alumina, low-active V2O5 crystallites were mainly formed regardless of the vanadium content. These catalysts demonstrated high selectivity toward only acetaldehyde. In contrast, monomeric surface vanadia species and polymeric surface vanadia species were mainly formed over TiO2 when the vanadium content did not exceed what is necessary for the ideal monolayer. Over zirconia, both the surface vanadia species and the V2O5 crystallites existed regardless of the vanadium content. It was found that the surface vanadia species are more active in the selective oxidation of ethanol than the V2O5 crystallites. The highest activity was observed for the polymeric vanadia species and, correspondingly, the best catalytic performance was achieved on the monolayer V2O5/TiO2 catalyst. At low temperatures between 110 and 150°C, this catalyst demonstrated high activity in the oxidation of ethanol to acetaldehyde with the selectivity ranging between 80% and 100%. At temperature near 200°C, the same catalyst was active in the oxidation of ethanol to acetic acid with the selectivity of approximately 65%. The surface intermediates and the catalyst state were also studied in situ by X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. It was shown that under reaction conditions near 100°C, non-dissociatively adsorbed molecules of ethanol, ethoxide species, and adsorbed acetaldehyde exist on the catalyst surface, while at higher temperatures, V2O5/TiO2 is mainly covered with acetate species. Titanium cations remained in the Ti4+ state, whereas V5+ cations underwent a reversible reduction under reaction conditions. On the basis of the in situ data complemented by the results of kinetic measurements, a reaction mechanism for the selective oxidation of ethanol to acetaldehyde and acetic acid over the monolayer catalysts was proposed.
Self-sustained reaction rate oscillations in the oxidation of propane and in the propane steam reforming with oxygen over nickel foil have been studied in situ by near-ambient pressure X-ray ...photoelectron spectroscopy and mass-spectrometry. It was found that regular relaxation-type oscillations in both reactions proceed under similar conditions. In the former case, the peaks of CO, CO
2
, H
2
, and H
2
O were detected by mass-spectrometry as gas-phase products. In contrast, in the latter case, after addition of water to the reaction feed, the mass-spectrometric signal of water decreased simultaneously with the signals of O
2
and C
3
H
8
, whereas the signals of CO, CO
2
, and H
2
increased. It means that in the presence of water in the reaction feed, the propane steam reforming proceeds with a significant rate. In both cases, the oscillations arise due to spontaneous oxidation and reduction of the catalyst. According to the Ni2
p
and O1
s
core-level spectra measured in situ, the high-active catalyst surface is represented by nickel in the metallic state, and the transition to the low-active state is accompanied by the growth of a NiO film on the catalyst surface. The oscillations in the gas phase are accompanied by oscillations in the catalyst temperature, which reflects proceeding endothermic and exothermic processes. An oscillatory mechanism, which can be common for oxidative catalytic reactions over transitional metals, is discussed.
Graphical Abstract
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•The oxidation of ethanol over a monolayer V2O5/TiO2 catalyst was examined.•The reaction involves reversible reduction of V5+ cations, whereas titanium cations remain in the Ti4+ ...state.•At 100–150°C: acetaldehyde is the main product; at higher temperatures: the reaction shifts toward acetic acid.•The oxidation of ethanol to acetaldehyde proceeds via the classical Mars–van Krevelen mechanism.•The production of acetic acid from acetate species proceeds only after the partial oxidation of the catalyst.
The selective oxidation of ethanol to acetaldehyde and acetic acid over a monolayer V2O5/TiO2 catalyst has been studied in situ using Fourier transform infrared spectroscopy and near-ambient-pressure X-ray photoelectron spectroscopy (XPS) at temperatures ranging from 100 to 300°C. The data were complemented with temperature-programmed reaction spectroscopy and kinetic measurements. It was found that under atmospheric pressure at low temperatures acetaldehyde is the major product formed with the selectivity of almost 100%. At higher temperatures, the reaction shifts toward acetic acid, and at 200°C, its selectivity reaches 60%. Above 250°C, unselective oxidation to CO and CO2 becomes the dominant reaction. Infrared spectroscopy indicated that during the reaction at 100°C, nondissociatively adsorbed molecules of ethanol, ethoxide species, and adsorbed acetaldehyde are on the catalyst surface, while at higher temperatures the surface is mainly covered with acetate species. According to the XPS data, titanium cations remain in the Ti4+ state, whereas V5+ cations undergo reversible reduction under reaction conditions. The presented data agree with the assumption that the selective oxidation of ethanol over vanadium oxide catalysts occurs at the redox Vn+ sites via a redox mechanism involving the surface lattice oxygen species. A reaction scheme for the oxidation of ethanol over monolayer V2O5/TiO2 catalysts is suggested.
A pair of mesoporous and hierarchical macro/mesoporous alumina-supported catalysts having distinct textural parameters have been chosen to elucidate the effect of texture on activity in ...hydrodesulfurization (HDS) and hydrodemetallization (HDM) of heavy tatar oil possessing extremely high viscosity and sulfur content. For monitoring catalyst properties, the samples have been investigated by XRD, XFS, XPS, EXAFS, SEM, TEM, FTIR, TPD-NH3, mercury porosimetry, and N2 adsorption methods. Among different factors such as support acidity, active component dispersion, and texture, the last one has been found to play the most significant role in this process. The hierarchical macro/mesoporous catalyst shows lower coking rate of the hydrotreated products, as well as higher HDS and HDM conversions despite its lower active component dispersion and decreased support acidity.
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•Ni–Mo alloy favors decarboxylation pathway of ethyl caprate conversion.•The hydrodeoxygenation of ethyl caprate is preferred on Ni–Mo–C sites.•β-Mo2C is responsible for hydrogenation ...reaction of ethyl caprate.•Ni sites present in Ni–Mo alloy favor the hydrodeoxygenation of anisole.•β-Mo2C leads to the formation of a high content of benzene.
Mo- and Ni-containing carbide catalysts were prepared by a Pechini-based method using citric acid as a complexing agent. All the catalysts were characterized by temperature-programmed reduction, X-ray diffraction (XRD), X-ray photoelectron spectroscopy, and N2 and CO sorption. The XRD experiments showed that the bimetallic systems contain phases: β-Mo2C and Mo3Ni2C and an alloy NixMo1−x with a high nickel content. The catalysts were tested in the hydrogenation of ethyl caprate and anisole. For the process with ethyl caprate, it was shown that the catalytic activity rises with an increase of the carbide phase content in the bimetallic catalysts. Further addition of nickel increases the amount of the Ni–Mo alloy, which leads to a drop in the catalytic activity and to an increase in the contribution of the decarboxylation pathway. The study of the carbide catalysts in anisole hydrogenation showed that a rise in the Ni/Mo ratio leads to an increase in activity.
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