V. N. Korchak

Oxidation of co with catalyst oxygen and with oxygen from the gas phase over CuO/CeO2 according to TPD and IR spectroscopic data

The forms of CO adsorption on the 5 and 10% CuO/CeO2 samples were studied by temperature-programmed desorption and IR spectroscopy. It was established that CO forms bridging and mono- and bidentate carbonate complexes on CuO clusters at 20°C; these complexes are decomposed with the desorption of CO2 at Tmax = 160 and 240–350°C, respectively. The Cu+ cations, on which Cu+-CO carbonyls are formed, are produced simultaneously with carbonates. They are decomposed at 110°C in a vacuum with the desorption of CO and oxidized at 20°C in an atmosphere of oxygen to form bridging carbonates. These latter are decomposed in a range of 20–150°C with the release of CO2. Adsorbed oxygen decreases the decomposition temperature of the carbonates formed upon the adsorption of CO by 50°C and keeps the sample in an oxidized state, which is active with respect to the subsequent adsorption and oxidation of CO. The oxidation of CO by the catalyst oxygen decreases the activity of the sample in these processes and increases the thermal stability of carbonate complexes. Based on the properties of the adsorption complexes of CO, a conclusion was made that the bridging carbonates participate in the reaction of carbon monoxide oxidation by the catalyst oxygen and oxygen from a gas phase in a range of 20–150°C. The decomposition of the carbonates with an activation energy of 64 kJ/mol is the rate-limiting step.

A temperature-programmed desorption and IR spectroscopic study of the mechanism of carbon monoxide oxidation on copper-containing catalysts

With the use of the temperature-programmed desorption of CO and IR spectroscopy, it was found that, after the adsorption of carbon monoxide on the oxidized 5% CuO/CeO2, 5% CuO/ZrO2, 5% CuO/Al2O3, and 5% CuO/SiO2 samples at 20°C, the greatest amount of adsorbed molecules (2.5 × 1020 g−1) was present on 5% CuO/ZrO2. Of these molecules, 1.0 × 1020 and 1.4 × 1020 g−1 formed carbonyl and carbonate adsorption complexes with the participation of copper-containing sites, respectively. The oxidized sample of 5% CuO/ZrO2 contained only the oxidation sites Cu2+O2−, which participated in the formation of carbonates upon the adsorption of CO. A portion of Cu2+ cations was reduced to Cu+ in the course of reaction, and two types of the carbonyl complexes Cu+CO were formed on them. They were characterized by the presence of absorption bands at 2110 and 2107 cm−1 in the IR spectrum and decomposed with the desorption of CO at 100 and 170°C. Carbonyls were oxidized by adsorbed oxygen at 20°C to carbonates. The temperature of their decomposition accompanied by the desorption of CO2 (Tmax = 170°C) was lower than that of carbonates (240 and 350°C) formed upon the adsorption of CO on the oxidized surface in the absence of oxygen from a gas phase. The properties of adsorption complexes and their participation in the reaction CO + O2 → CO2 at low temperatures, in particular, with the use of a hydrogen-containing mixture, were considered. The oxidation of CO on CuO clusters in 5% CuO/CeO2 and 5% CuO/ZrO2 were discussed.

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−.

Mechanochemical synthesis of CuO-CeO2 catalysts for the preferential oxidation of CO in the presence of H2

Mechanochemical activation was used in the synthesis of CuO-CeO2 catalysts for the preferential oxidation of CO in the presence of excess H2. Catalysts similar in properties to supported CuO/CeO2 systems were prepared from mixtures of copper oxide (5 or 10 wt % CuO) and cerium dioxide with the use of mechanochemical activation. It was found that the time of mechanochemical activation influences the catalytic properties: a maximum conversion of CO into CO2 (97%) at 140°C was achieved with a sample of 10 wt % CuO-90 wt % CeO2 after mechanochemical activation in a ball mill for 90 min. Changes in the phase compositions of the catalysts depending on mechanochemical activation time and reaction mixture composition were studied by X-ray diffraction. The interaction of the oxides of copper and cerium in the process of mechanochemical activation with the formation of new Cu-O-Ce surface structures, which, supposedly constitute active sites for CO oxidation, was found using differential scanning calorimetry and differential thermogravimetric analysis.

The Study of Self-Oscillations During CH 4 Oxidation Over Ni by the Pulse Method: Is it Possible?

For the first time, the pulse method has been applied to the study of the self-oscillatory behaviour. For methane oxidation over Ni foil the response to a sequence of equal pulses has been strictly periodic. The pulse method allowed to obtain some new information about the origin of oscillations in this reaction.Graphical Abstract

Synchronization of Local Oscillators in Oxidation Reactions of C 1 -C 4 Hydrocarbons over Metal Catalysts

The synchronization of reaction rate oscillations in the oxidation of C1–C4 hydrocarbons over polycrystalline nickel, cobalt, and palladium foils has been investigated. The synchronization of foil temperature oscillations during the reaction takes place via the diffusion of the reactants in the gas phase. For the nickel catalysts, the synchronization of the oscillators occurs in the same phase, while for the palladium catalysts, both in-phase and antiphase oscillations are observed. This distinction between the dynamic behaviors of the systems of two coupled oscillators is due to the fact that the mechanism of reaction rate oscillations varies from one metal to another.

Broadband infrared photoluminescence of TlCdI3 iodide doped with bismuth

The Bridgman–Stockbarger method is used to prepare single-crystal TlCdI3 samples doped with bismuth. The material exhibits a broadband photoluminescence in the near-IR range with a maximum intensity at a wavelength of 1175 nm. The properties of bismuth-doped TlCdI3 were compared with previously studied chlorides and bromides containing Bi+ impurity centers. It is demonstrated that the luminescence center in TlCdI3 is not the monovalent bismuth cation.

CO oxidation by oxygen of the catalyst and by gas-phase oxygen over (0.5–15)%CoO/ZrO 2

CO adsorption on (0.5–15)%CoO/ZrО2 catalysts has been investigated by temperature-programmed desorption and IR spectroscopy. At 20°С, carbon monoxide forms carbonyl and monodentate carbonate complexes on Com2+On2- clusters located on the surface of crystallites of tetragonal ZrO2. With an increasing CoO content of the clusters, the amount of these complexes increases and the temperature of carbonate decomposition, accompanied by CO2 desorption, decreases from 400 to 304°С. On the 5%CoO/ZrО2 sample, the carbonyls formed on the Со2+ and Со+ cations and Со0 atoms decompose at 20, 90, and 200–220°С, respectively, releasing CO. At 20°С, they are oxidized by oxygen to monodentate carbonates, which decompose at 180°С. Adsorbed oxygen decreases the temperature of their decomposition on oxidation sites by ~40°C, and the sample remains in an oxidized state ensuring the possibility of subsequent CO adsorption and oxidation. The rate of the oxidation of 5%CoO/ZrО2 containing adsorbed CO by oxygen is higher than the rate of the oxidation of the same sample reduced by carbon monoxide, because the latter reaction is an activated one. In view of the properties of the complexes, it can be concluded that the carbonates decomposing at 180°С are involved in CO oxidation by oxygen from the gas phase in the presence of hydrogen, a process occurring at 50–200°С. The rate-limiting step of this process the decomposition of the carbonates, which is characterized by an activation energy of 77–94 kJ/mol.

Self-Sustained Oscillations as a Method to Increase an Active Surface and Catalytic Activity of Ni and Pd

The effect of Ni and Pd surface development during catalytic self-oscillatory oxidation of C1–C4 alkanes on the activity of these two metals in other catalytic reactions was studied. Scanning electron microscopy investigations revealed that the surface of bulk Ni and Pd (foil or powder) developed significantly faster during alkane oxidation in a self-oscillatory regime than under stationary conditions. Thanks to increase in available metal surface achieved during such self-oscillatory pretreatment, catalytic activity of Ni in methane dry reforming and in ethylene hydrogenation and that of Pd in total methane oxidation increased by an order of magnitude compared to the untreated metals. With time on stream, the activity dropped to some stationary level that was still significantly higher than the activity of the fresh metals. Morphological changes of Ni during the pretreatment were caused by periodic oxidation–reduction of the surface atomic layers whereas in case of Pd redox cycles were accompanied by carbon dissolution-removal. The amount of carbon dissolved in Pd during self-oscillatory oxidation of C1–C4 alkanes decreased with increasing chain length, likewise the metal surface development. Supported Pd/Al2O3 catalyst did not exhibit significant activity changes after the self-oscillatory pretreatment suggesting that the morphology of Pd particles remained unaltered.Graphical Abstract

Thermal stability of surface nitrogen–oxygen complexes and phase transitions in ZrO2

The IR spectra of surface compounds observed in the course of the temperature-programmed desorption (TPD) of NOx and the TPD spectra are compared. The high-temperature peaks of desorption are related to the decomposition of surface nitrites and nitrates. The low-temperature peaks of NOx desorption with maximums below 140°C are caused by the decomposition of surface nitrosyls. On the heating of surface nitrosyls, the following two reaction paths are possible: desorption at low temperatures and conversion into nitrates. The shape of the TPD spectra of NO depends on the phase composition of test samples. The transition of a tetragonal phase into a monoclinic one occurred upon the surface dehydroxylation of polycrystalline particles with the formation of particles with a tetragonal nucleus and a monoclinic crust. This transition is reversible. The cooling of a sample in a moist atmosphere leads to the transition of the monoclinic crust to the tetragonal phase.