O. N. Sil’chenkova

Properties of surface compounds in methanol conversion on γ-Al2O3: Data of in situ IR spectroscopy

In situ IR spectroscopic studies show that a formate, an aldehyde-like complex, and bridging and linear methoxy groups exist on the alumina surface involved in methanol conversion. In the absence of methanol in the gas phase, the interaction between two bridging methoxy groups yields dimethyl ether in the gas phase. When methanol is present in the gas phase, it interacts with methoxy groups on the surface. This reaction makes the main contribution to the formation of dimethyl ether. The linear methoxy group undergoes conversion via several routes. The main route is desorption with methanol formation in the gas phase, and no more than 10% of the linear methoxy groups are converted into formate and aldehyde, which are CO2 sources in the gas phase. In the absence of methanol in the gas phase, the conversion rate of the methoxy groups is independent of the presence of water and oxygen. A scheme of the surface reactions is suggested to explain the conversion of the methoxy groups.

Oxidation of carbon monoxide in the presence of hydrogen on the CuO, CoO, and Fe2O3 oxides supported on ZrO2

The catalytic activity of the CuO/ZrO2, CoO/ZrO2, Fe2O3/ZrO2, and CuO/(CoO, Fe2O3)/ZrO2 systems in the reaction of selective CO oxidation in the presence of hydrogen was studied at 20–450°C over the oxide concentration range of 2.5–10 wt % on the surface of ZrO2. The conversion of CO on the CoO/ZrO2 systems was almost independent of the concentration of CoO: 88 or 90% for 2.5 or 10% CoO, respectively. TPR data allowed us to relate the catalytic activity of CoO/ZrO2 to Co-O-Zr clusters, the amount of which was almost constant over the test range of CoO concentrations. The conversion of CO on 2.5% CuO/ZrO2 was 32% (190°C) or 62–66% on 5–10% CuO/ZrO2 (170°C). According to TPR data, clusters like Cu-O-Zr occurred on the surface of ZrO2, and the amount of these clusters reached a maximum upon supporting 5% CuO. The catalytic properties of 5% CuO/5% CoO/ZrO2 and 5% CoO/5% CuO/ZrO2 samples were identical to those of 5% CuO/ZrO2 samples. It is likely that the formation of active reaction sites upon consecutively supporting the oxides occurred on the same surface sites of ZrO2. In this case, Co and Cu oxides competed for cluster formation, and the copper cation can displace the cobalt cation from the formed clusters. The Fe2O3 samples were inactive; a maximum conversion of 34% (290°C) was observed on 10% Fe2O3/ZrO2. The catalytic properties of CuO/Fe2O3/ZrO2 were also identical to those of CuO/ZrO2, and they depended on the presence of Cu-O-Zr clusters on the surface.

Spectroscopic study of the properties of surface compounds in methanol conversions on Cu/γ-Al2O3

The reactions of methanol on the (10% Cu)/γ-Al2O3 surface were studied by the spectrokinetic method (simultaneous measurements of the conversion rates of surface compounds and the product formation rates). Bridging and linear methoxy groups result from the interaction of methanol with surface hydroxyl groups. Formate and aldehyde-like complexes form by the oxidative conversion of the linear methoxy groups. Hydrogen forms via the recombination of hydrogen atoms on copper clusters, and the hydrogen atoms result from interconversions of surface compounds. The source of CO2 in the gas phase is the formate complex, and the source of CO is the aldehyde complex. In the absence of methanol in the gas phase, dimethyl ether forms by the interaction between two bridging methoxy groups. When present in the gas phase, methanol reacts with methoxy groups on the surface. The roles of oxygen and water vapor in the conversions of surface compounds are discussed.

Properties of surface compounds in methanol conversion on copper-containing catalysts based on CeO2 according to in situ IR-spectroscopic data

Formate and carbonate complexes and bridging and linear methoxy groups were detected on the surfaces of CeO2 and 5.0% Cu/CeO2 under the reaction conditions of methanol conversion using IR spectroscopy. The reaction products were H2, methyl formate, CO, CO2, and H2O. The bridging and linear methoxy groups were the sources of formation of bi- and monodentate formate complexes, respectively. Methyl formate was formed as a result of the interaction of the linear methoxy group and the formate complex. The study demonstrated that the recombination of hydrogen atoms on copper clusters and the decomposition of methyl formate were the main reactions of hydrogen formation. Formate and carbonate complexes were the source of CO2 formation in the gas phase, and the decomposition of methyl formate was the source of CO. It was found that the addition of water vapor to the reaction flow considerably decreased the rate of CO formation at a constant yield of hydrogen. The effects of water vapor and oxygen on the course of surface reactions and the formation of products are discussed. To explain the mechanism of methanol conversion, a scheme of surface reactions is proposed.

Properties of nitrogen-oxygen surface compounds on ZrO2 samples with different phase compositions according to in situ IR spectroscopy data

The zirconium dioxide surface has a wide variety of adsorption sites differing in their nature. The proportions of these sites can be changed by varying the oxide preparation and pretreatment conditions. This fact shows itself as a wide diversity of surface structures resulting from NO and O2 adsorption. Under conditions of the selective catalytic reduction of NOx, the most stable nitrogen oxide species are nitrates that result from the interaction between NOx and the ZrO2 surface. The concentrations of the other nitrogen-oxygen surface compounds are two orders of magnitude lower. The routes of NO3− formation and decomposition on the ZrO2 surface are discussed. In these routes, monodentate nitrates (which show themselves at 1550–1555 cm−1) are considered as intermediates in the formation and decomposition of bidentate NO3−.

Conversion of ethanol on HZSM-5 modified zeolite, according to data from in situ spectrokinetic studies

The conversion of ethanol on zeolite catalysts is studied spectrokinetically in situ. Ethoxy groups and polyene structures (compaction products) are found to be key intermediates on the studied catalysts under reaction conditions. Ethoxy groups are shown to convert into diethyl ether by a bimolecular mechanism at relatively low temperatures in the presence of ethanol in the gas phase. The character of the conversion of ethoxy groups is found to change at temperatures above 200°C: they become a source for the formation of surface polyene structures, which in turn are converted into a complex combination of hydrocarbons.

Interaction between NOx and the surface of supported heteropoly compounds: In situ IR spectroscopic data

The mechanism of activation of nitrogen oxides on unsupported heteropoly compounds and the composition, location, stability, and interconversion mechanisms of adsorption complexes on supported heteropoly compounds have been investigated by in situ IR spectroscopy under thermal desorption conditions. Supporting a small amount of a heteropoly compounds (1% or below) increases NOx adsorption relative to the adsorption observed for the pure support. This effect is most pronounced for CeO2 and least pronounced for ZrO2. The increase in NOx adsorption is due to NO oxidation to NO2 on the supported heteropoly compound. The main adsorption species are nitrite and nitrate complexes, which are located on the support. As the temperature is raised, the nitrite complexes turn into the nitrate complexes. The presence of variable-valence ions in the Keggin anion reduces the nitrate complex-surface binding strength. The ions that are not components of the Keggin anion increase the binding strength. The changes in the nitrate complex-support surface binding strength are due to the support modification taking place via the destruction of part of the supported heteropoly compound.

Mechanism of methanol conversion on ZrO2 and 5% Cu/ZrO2 according to in situ IR spectroscopic data

It is demonstrated by in situ IR spectroscopy that, in methanol conversion on ZrO2 and 5% Cu/ZrO2 catalysts, methoxy groups are present on the catalyst surface, which result from O-H or C-O bond breaking in the methanol molecule. Two types of formate complexes, localized on ZrO2 and CuO, are also observed. The formate complexes form via the oxidative conversion of the methoxy groups. There are two types of linear methoxy groups. First-type linear methoxy groups condense with the formate complex located on CuO to yield methyl formate and then CO and H2. Second-type methoxy groups appear as intermediate products in the formation of dimethyl ether. The main hydrogen formation reactions are the recombination of hydrogen atoms (which result from the interconversion of surface complexes) on copper clusters and the decomposition of methyl formate. The source of CO2 in the gas phase is the formate complex, and the source of CO is methyl formate. The effect of water vapor and oxygen the surface reactions and product formation is discussed.

Methanol conversion over CuO supported on CeO2-ZrO2: An in situ IR spectroscopic study

The main reactions yielding hydrogen are the recombination of hydrogen atoms on copper clusters and methyl formate decomposition. Methyl formate results from the interaction between the linear methoxy group and the formate complex located on CuO. The source of CO2 appearing in the gas phase is the formate complex, and the source of CO is methyl formate. The rates of methoxy group conversion and product formation over supports (ZrO2, CeO2, Ce0.8Zr0.2O2) and copper-containing catalysts (5%Cu/CeO2, 5%Cu/ZrO2, 2%Cu/Ce0.8Zr0.2O2, 2%Cu/Ce0.1Y0.1Zr0.8) are compared. The dominant process in methoxy group conversion over the supports and copper-containing catalysts is methanol decomposition to H2 and CO and to H2 and CO2, respectively. The methoxy group conversion rate is proportional to the H2 and CO2 formation rate and is determined by the concentration of supported copper.

On the properties of surface complexes formed upon the adsorption of NOx, C3H6, and their mixtures with oxygen on ZrO2 according to EPR, TPD, and fourier transform IR spectroscopy data

The adsorption of reactant mixtures is quantitatively and qualitatively different from the adsorption of the individual reactants. Thus, O2 is almost not adsorbed on ZrO2; however, a considerable concentration of molecular oxygen was detected among the products of desorption after the adsorption of a mixture of NO + O2 and the total amount of desorbed molecules was greater by a factor of 10 than their total amount after the individual adsorption of NO and O2. Among the qualitative differences is the formation of the O2- radical anion on the surface only upon the adsorption of the mixture of NO + O2. Similarly, the number of desorbed molecules upon the simultaneous adsorption of C3H6, NO, and O2 was much greater than that upon their individual adsorption; this is related to the formation of paramagnetic and nonparamagnetic NO2–hydrocarbon complexes on the surface, which contained the NO2 group and a hydrocarbon fragment.