Vol 59, No 2 (2018)

Synergetic effect of sodium borohydride (NaBH4) addition to ammonia borane (NH3BH3) hydrolysis reaction had been studied and iron-borate (FeB) was used to catalyze the reaction. Hydrogen generation performance of the hydrolysis reactions was compared for three different operating conditions: (1) in the presence of NaBH₄ with FeB catalyst, (2) with FeB without NaBH₄ addition and (3) in the presence of NaBH₄ without FeB. It was found that addition of NaBH₄ to the NH₃BH₃ hydrolysis reaction catalyzed by FeB resulted in the synergetic effect (synergetic factor (SF) > 0) and improved the hydrogen generation performance. Kinetic analysis showed that NaBH4 addition decreases the activation energy (E_a) from 52.11 ± 0.85 to 27.19 ± 0.44 kJ/mol. Simulation of hydrolysis kinetics curves indicated that addition of NaBH₄ (the mole fraction of NaBH₄ added to NH₃BH₃ is (1)) changed the three-dimensional diffusion mechanism to the one-dimensional one and brought on better hydrolysis properties in terms of higher hydrogen generation rate and lower induction time.

Olefin hydrocarbons are valuable raw materials for petrochemical and polymer manufacturing. Highly effective, but toxic chromium-containing catalytic materials are the most widely used catalysts to obtain olefins in industry. In this regard, the urgent challenge to increase the efficiency of oil processing is to develop the catalysts with low content of harmful active component. In the present study, the catalysts with low chromium content (1 theoretical monolayer = 5 Cr atoms per nm² of support) were synthesized by incipient wetness impregnation of the supports (Al₂O₃, ZrO₂, CeO₂, and Ce_xZr_(1–x)O₂). The samples obtained were characterized by low-temperature nitrogen adsorption, X-ray diffraction and H2-temperature-programmed reduction methods. The catalytic properties of the catalysts were tested in isobutane dehydrogenation reaction. It was shown that the state of chromium on the surface is different over different supports. For the CrO_x/CeO₂ catalyst, the formation of Cr₂O₃ particles with low activity in the dehydrogenation reaction was observed. For other samples, a highly disperse X-ray amorphous state of chromium was characteristic. The catalyst based on Ce_xZr_(1–x)O₂ was the most active in isobutane dehydrogenation reaction due to possible stabilization of chromium as Cr(V) state.

A new ecologically clean method for the solid-phase synthesis of oxide copper–ceria catalysts with the use of the mechanochemical activation of a mixture of Cu powder (8 wt %) with CeO₂ was developed. It was established that metallic copper was oxidized by oxygen from CeO₂ in the course of mechanochemical activation. The intensity of a signal due to metallic Cu in the X-ray diffraction analysis spectra decreased with the duration of mechanochemical activation. The Cu¹⁺, Cu²⁺, and Ce³⁺ ions were detected on the sample surface by X-ray photoelectron spectroscopy. The application of temperature-programmed reduction (TPR) made it possible to detect two active oxygen species in the reaction of CO oxidation in the regions of 190 and 210–220°C by a TPR-H₂ method and in the regions of 150 and 180–190°C by a TPR-CO method. It is likely that the former species occurred in the catalytically active nanocomposite surface structures containing Cu–O–Ce bonds, whereas the latter occurred in the finely dispersed particles of CuO on the surface of CeO₂. The maximum conversion of CO (98%, 165°C) reached by the mechanochemical activation of the sample for 60 min was almost the same as conversion on a supported CuO/CeO₂ catalyst.

The dependence of the activity of СuO/ZrO₂ catalysts in the CO oxidation reaction with oxygen in the presence of an excess of hydrogen and adsorption of СО over them on the CuO content (0.5 to 15%) and the structure of the support ZrO₂, monoclinic (М), tetragonal (Т), or mixed (М + Т) has been studied. It has been found that the activity of CuO/ZrO₂ is associated with the adsorption capacity of the samples for СО at 20°С. Thus, 5%CuO/ZrO₂(Т + М) and 5% CuO/ZrO₂(Т) samples, which exhibit the maximum activity (the СО conversion over them is 80–85% at 160°С), also possess a high chemisorption capacity towards CO (~2.2 × 10²⁰ molecules/g). At the same time, CuO/ZrO₂(М) samples with the CuO contents of 1 and 5% do not chemisorb СО and are inactive in the reaction at 160°С. The СО conversion over them does not exceed 32–36% at 250°С. On the basis of the data obtained by X-ray phase analysis, temperature-programmed reduction with Н₂, temperature-programmed СО desorption, and electron paramagnetic resonance, a conclusion has been made that at low temperatures СО oxidation proceeds over Cu_nO_m clusters that are located on ZrO₂(Т) crystallites. With the increase in the copper oxide content from 0.5 to 5%, the activity of the clusters increases, while the reaction temperature decreases. CuO_m oxo complexes and particles of the СuO phase do not exhibit catalytic activity. The reasons for the low activity of the CuO/ZrO₂(М) samples with the CuO contents of 1 and 5% in the СО oxidation and adsorption processes are discussed. The mechanism of the low-temperature СО oxidation with oxygen in an excess of hydrogen over a 5% CuO/ZrO₂(Т + М) catalyst is considered.

The shapes, sizes, and electronic structures of platinum nanoparticles supported onto highly oriented pyrolytic graphite and oxidized silicon by different methods and their adsorptive properties with respect to hydrogen, oxygen, water, and ammonia were established. The apparent activation energy of the reduction of single oxidized platinum nanoparticles with molecular hydrogen was determined. The possibility of controlling the rate of ammonia decomposition by a nanostructured platinum coating by the application of electric potentials of different values and polarities to it was demonstrated.

Three methods of introduction of modifiers based on Cu and Zn compounds into the CrO_x/Al₂O₃ catalysts for dehydrogenation of light paraffin hydrocarbons are considered: Introduction from sol, introduction using successive impregnation technique and introduction of modifiers by impregnation along with precursor of chromium oxide. The obtained samples are studied by a complex of physical-chemical methods (XRD, UV-Vis spectroscopy, temperature-programmed reduction with hydrogen (TPR-H₂), X-ray fluorescent (XRF) spectrometry, low-temperature N₂ sorption). The catalytic properties of the samples are studied in kinetic mode in isobutane dehydrogenation. Cu- and Zn-modifiers are shown to influence on the peculiarities of reduction of Cr⁶⁺ and, hence, specify the state of active surface of CrO_x/Al₂O₃ catalysts formed in the reductive reaction medium. Not only do the states of modifiers influence on the initial activity of the catalyst, but also on its activity after oxidative regeneration. Introduction of modifiers by successive impregnation method results in formation of copper and zinc aluminates or defective spinels on the Al₂O₃ surface. When the active component is introduced, the modified surface of the support promotes formation and stabilization of Cr⁶⁺ sites that can undergo reversible reduction–oxidation and provide high activity and selectivity towards formation of isobutylene (>98%).

The process of formation of a Fe/C catalyst was considered as a sequence of topochemical reactions. It was demonstrated that potassium promotion accelerated the formation of magnetite at the stage of catalyst calcination in a flow of argon. The particle size of iron oxides decreased with the concentration of potassium. Potassium promotion also led to an increase in the concentration of Hägg carbide in the course of catalyst reduction by synthesis gas.