Saifutdinov B.R.,Samara State Technical University
Russian Chemical Bulletin | Year: 2014
The thermodynamics of the adsorption of heteroaromatic compounds of the 1,3,4-oxadiazole and 1,2,4,5-tetrazine series from aqueous acetonitrile solutions was studied in Henry's region under the conditions close to equilibrium using the dynamic method. The applicability of the adsorption retention models to the description of medium-property relationships at different temperatures was evaluated. The effect of the composition of the solution on the thermodynamic characteristics of adsorption was studied. The effect of the acetonitrile concen-tration on the enthalpy and entropy of adsorption cannot be described by simple plots, since it is determined by the nature of the adsorbate. At the same time, the Gibbs free energy change of adsorption is characterized by a monotonic dependence on the acetonitrile concentration. Linear relationships between changes in the enthalpy and entropy of adsorption of the same substance from water-acetonitrile media of different compositions were established. The character of these relationships is determined by the adsorbate nature and explains the dependence of its thermodynamic characteristics of adsorption on the solution composition. The compensation of the values of enthalpy and entropy changes in a series of the studied adsorbates measured for the constant composition of the solution was discovered. The concentration of acetonitrile significantly affects the character of this compensation. © 2014 Springer Science+Business Media New York.
Enikeev R.,Samara State Technical University
Food Chemistry | Year: 2012
An easy-to-use method for quantitative analysis of bacterial exopolysaccharides (EPS) was developed. The acid treatment was resultant in removing proteins and lactose and allowed to avoid long-term operation as dialysis. It was shown that, as opposed to anthron and phenol-sulphuric acid methods, reducing sugars method after EPS hydrolysis with glucose as standard provided reliable results, no matter to the EPS monosaccharide composition. EPS quantity in kefir of different brands was varied greatly, approximately at 50-200 mg/l, showing potential to enhance kefiran production by alteration of technological process. The temperature optimum for EPS production by kefir grains and kefir starter was about 30 °C, while the rise of temperature increased soluble EPS in culturing media by means of extraction from kefir grain body. © 2012 Elsevier Ltd. All rights reserved.
Kogan V.M.,RAS N. D. Zelinsky Institute of Organic Chemistry |
Nikulshin P.A.,Samara State Technical University |
Rozhdestvenskaya N.N.,RAS N. D. Zelinsky Institute of Organic Chemistry
Fuel | Year: 2012
A new mechanistic model for molybdenum sulfide catalysts with cobalt and nickel promoters under hydrodesulfurization reaction conditions is proposed, which includes dynamic migration of sulfur and promoter atoms between adjacent sulfide layers. These migrations are caused by heterolytic dissociation of gas-phase hydrogen and formation of hydride hydrogen on a promoter atom. Hydride hydrogen adsorbed on a promoter induces electron density transfer from the bonding promoter to Mo, changing its catalytic activity. The proposed concept of dynamic migrations provides new interpretation for the "rim-edge" model, which is based on the formation of two types of active sites: highly active ("rapid") and less active ("slow") sites. The "slow" sites are proposed to be associated with a non-promoted cluster on the edge of a MoS 2 slab layer ("rim"), which catalyzes hydrogenation. The "rapid" sites are proposed to be associated with a combination of promoted and non-promoted clusters on edges of the same (type I) or adjacent (type II) MoS 2 layers. The "rapid" sites catalyze C-S bond hydrogenolysis. Adsorbed hydrogen induces migration of sulfur and promoter atoms between adjacent clusters. This dynamic migration causes a transformation of "rapid" into "slow" sites and vice versa and, therefore, influences catalytic activity. Migration of Co or Ni promoter atoms to Mo sites on the rim is more likely than to Mo sites on the edge of Co(Ni)MoS layers because the former can accept promoter atoms from both upper and lower layers. The proposed model provides critical information for rational design of improved hydrotreating catalysts and selection of preferred catalytic reaction conditions for various types of hydrocarbon feedstocks through optimization of the density and ratio of "rapid" and "slow" catalyst active sites. © 2012 Elsevier Ltd. All rights reserved.
Kogan V.M.,RAS N. D. Zelinsky Institute of Organic Chemistry |
Nikulshin P.A.,Samara State Technical University
Catalysis Today | Year: 2010
On the basis of the fundamental concepts of catalysis by transition metal sulfides (TMSs) a new dynamic model of the promoted Mo-sulfide catalysts functioning under HDS conditions has been developed. According to the model under hydrogen atmosphere sulfidation degree of the edges of different layers of the slab can vary and surface sulfur can migrate from one layer to a neighboring layer so that average sulfidation state of the slab remains constant under given reaction conditions. The model describes oscillations by sulfur and promoter atoms between the adjacent layers of the slab on the atomic scale level. The driving force of such processes is the heterolytic dissociation of the gas phase hydrogen and formation of hydride hydrogen linked to a promoter atom. This hydride hydrogen triggers an electron transfer from a promoter to Mo with the formation of the vacancy. This model correlates with radioisotopic data and an earlier suggested "forcing out" mechanism and gives some new interpretations of the important statements of the electron theory of promotion and the "rim-edge" model. © 2009 Elsevier B.V. All rights reserved.
Volokitin A.I.,Julich Research Center |
Volokitin A.I.,Samara State Technical University |
Persson B.N.J.,Julich Research Center
Physical Review Letters | Year: 2011
We investigate the van der Waals friction between graphene and an amorphous SiO2 substrate. We find that due to this friction the electric current is saturated at a high electric field, in agreement with experiment. The saturation current depends weakly on the temperature, which we attribute to the quantum friction between the graphene carriers and the substrate optical phonons. We calculate also the frictional drag between two graphene sheets caused by van der Waals friction, and find that this drag can induce a voltage high enough to be easily measured experimentally. © 2011 American Physical Society.