RESUMO
The dehydrogenation of methylcyclohexane to toluene was investigated over high-loading monometallic Ni-SiO2 and bimetallic Zn/Ni-SiO2 catalysts. The catalysts were prepared by the impregnation coupled with the advantageous heterophase sol-gel technique. Their performance was tested in a fixed-bed flow reactor at 250-350 °C, 0.1 MPa pressure, equimolar ratio H2/Ar (24 nL h-1 in total), and a methylcyclohexane feed rate of 12 mL h-1. Information regarding the structure of Ni-Zn catalysts was obtained by N2 and CO adsorption, temperature-programmed reduction, high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, in situ X-ray diffraction, and in situ X-ray absorption spectroscopy. The results have shown that the addition of zinc leads to the hindrance of Ni reducibility along with weakening the Ni interaction with the silica matrix. This behavior particularly indicated the formation of solid oxide nickel-zinc solutions. The catalytic properties of Zn-modified catalysts in the dehydrogenation of methylcyclohexane appeared significantly superior to their Ni-Cu counterparts. For example, the selectivity of Zn/Ni-SiO2 catalysts toward toluene formation increased markedly with a decrease in the Ni : Zn mass ratio, achieving 97% at 350 °C over the sample with Ni : Zn = 80 : 20. This is attributed to the promoting geometric and electronic effects arising from the formation of bimetallic Ni-Zn solid solutions. Moreover, a deeper reduction of zinc and a more efficient formation of solid bimetallic solutions are observed after the catalytic tests.
RESUMO
A series of Mn-Co mixed oxides with a gradual variation of the Mn/Co molar ratio were prepared by coprecipitation of cobalt and manganese nitrates. The structure, chemistry, and reducibility of the oxides were studied by X-ray diffraction (XRD), X-ray absorption spectroscopy, X-ray photoelectron spectroscopy (XPS), and temperature-programmed reduction (TPR). It was found that at concentrations of Mn below 37 atom %, a solid solution with a cubic spinel structure is formed. At concentrations above 63 atom %, a solid solution is formed on the basis of a tetragonal spinel, while at concentrations in a range of 37-63 atom %, a two-phase system, which contains tetragonal and cubic oxides, is formed. To elucidate the reduction route of mixed oxides, two approaches were used. The first was based on a gradual change in the chemical composition of Mn-Co oxides, illustrating slow changes in the TPR profiles. The second approach consisted in a combination of in situ XRD and pseudo-in situ XPS techniques, which made it possible to directly determine the structure and chemistry of the oxides under reductive conditions. It was shown that the reduction of Mn-Co mixed oxides proceeds via two stages. During the first stage, (Mn, Co)3O4 is reduced to (Mn, Co)O. During the second stage, the solid solution (Mn, Co)O is transformed into metallic cobalt and MnO. The introduction of manganese cations into the structure of cobalt oxide leads to a decrease in the rate of both reduction stages. However, the influence of additional cations on the second reduction stage is more noticeable. This is due to crystallographic peculiarities of the compounds: the conversion from the initial oxide (Mn, Co)3O4 into the intermediate oxide (Mn, Co)O requires only a small displacement of cations, whereas the formation of metallic cobalt from (Mn, Co)O requires a rearrangement of the entire structure.
RESUMO
The heightened interest in liquid organic hydrogen carriers encourages the development of catalysts suitable for multicycle use. To ensure high catalytic activity and selectivity, the structure-reactivity relationship must be extensively investigated. In this study, high-loaded Ni-Cu catalysts were considered for the dehydrogenation of methylcyclohexane. The highest conversion of 85% and toluene selectivity of 70% were achieved at 325 °C in a fixed-bed reactor using a catalyst with a Cu/Ni atomic ratio of 0.23. To shed light on the relationship between the structural features and catalytic performance, the catalysts were thoroughly studied using a wide range of advanced physicochemical tools. The activity and selectivity of the proposed catalysts are related to the uniformity of Cu distribution and its interaction with Ni via the formation of metallic solid solutions. The method of introduction of copper in the catalyst plays a crucial role in the effectiveness of the interaction between the two metals.
RESUMO
A series of CuO x -TiO2 photocatalysts were prepared using fresh and thermally activated Evonik Aeroxide P25 titanium dioxide. The photocatalysts were characterized by X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, XANES, diffuse reflectance spectroscopy, and N2 adsorption technique. Photocatalytic activity of the samples was tested in hydrogen production from aqueous-alcoholic solutions of methylene blue under UV radiation (λ = 386 nm). It was found for the first time the synergistic effect of hydrogen production from two substrates-dye and ethanol. The maximum hydrogen production rate in the system water-ethanol-methylene blue was 1 µmol min-1, which is 25 times higher than a value measured in a 10% solution of ethanol in water. The thermal activation of titania also leads to a change in the rate of hydrogen production. The highest catalytic activity was observed for a CuO x -TiO2 photocatalyst based on titania thermally-activated at 600 °C in air. A mechanism of the photocatalytic reaction is discussed.
