Search Results

You are looking at 1 - 10 of 12 items for

  • Author or Editor: Viorel Chihaia x
  • Refine by Access: All Content x
Clear All Modify Search
Reaction Kinetics, Mechanisms and Catalysis
Authors: Viorel Chihaia, Karl Sohlberg, Karl Sohlberg, and Niculae I. Ionescu
Restricted access

Abstract

Various tungstated zirconia catalysts with a WO3 loading of about 16 wt% were characterized both in their acid and oxidation properties. The samples have been characterized in their micro-structural and surface properties by BET, X-ray diffraction, Raman spectroscopy, temperature programmed reduction, elemental chemical analysis. The surface acidity was determined by the techniques of NH3 adsorption microcalorimetry and pyridine infrared spectroscopy (FT-IR). Improved acidity has been detected upon addition of WO3 to zirconia by both techniques. The global acid strength and the total number of acid sites increased greatly with the formation of WOx clusters on the zirconia support. This acidity increase can be attributed to the creation of Br⊘nsted acid sites generated by the well dispersed WOx domains, as observed by FT-IR pyridine desorption.

Restricted access

Abstract

The properties of defects on the (100) MoS2 surface have been investigated by the perturbed cluster method. The perturbed cluster method provides an accurate description of the local defect properties while taking into account the interaction between the defect and the surrounding crystal. The surface energies, including correlation correction, of different defect structures of various sizes on the (100) MoS2 surface are reported and compared with the energy of a reference surface cluster. The results, in conjunction with calculations of the electronic properties and electrostatic potential of the different defect sites, show that the chemistry of the defects differs from that of the perfect (100) “as-cleaved” surface. The enhanced reactivity of the defects is ascribed to the anisotropy in the electrostatic potential. The presence of “nodes” in the surface electrostatic potential suggests that the adsorption of small polarizable molecules will preferentially take place in the vicinity of these defects.

Restricted access

Abstract

The reduction kinetics with CO of the 12-molybdophosphoric—HPMo, 1-vanado-11-molybdophosphoric—HPVMo acids and their salts with NH4 +, K+ and Cs+ cations were studied for reduction/reoxidation cycles with mixtures of CO:Ar and O2: Ar, by means of “in situ” UV–Vis–DRS measurements. The reflectivity versus time curves registered during the reduction/reoxidation processes for the HPMo and HPVMo and its salts with NH4 +, K+ and Cs+ cations, at the constant wavelength of 620 nm and different reaction temperatures between 523 and 623 K, were processed as the Kubelka–Munk function versus time. The linear shape of Kubelka–Munk function versus time curves for the reduction process suggests apparent zeroth order kinetics and it was used for the calculation of apparent activation energy. The kinetic compensation effect between the apparent activation energy and the pre-exponential factor was observed. The Kubelka–Munk function versus time curves for the reoxidation process consist of two steps, the first with a very fast reaction rate and the second with a slow reaction rate. An explanation for their shape is proposed. The heteropoly oxomolybdates reach a degree of reoxidation higher than heteropoly compounds containing vanadium together with molybdenum.

Restricted access

Abstract

Horseradish peroxidase immobilization inside molecular sieves with pores hexagonally structured as such or functionalized with copper ions is presented in comparison with the bidimensional immobilization onto aerosil A380 particles. Immobilization was performed under mild conditions that do not lead to enzyme denaturation. Guest–host interactions were pursued spectrophotometrically and by thermal analysis measurements. FTIR spectroscopy showed no major secondary structural change for the entrapped enzyme despite some influence of the support functionalization that can be supposed. The activity of the immobilized enzyme was tested in the oxidation of Alizarin Red S (ARS) with hydrogen peroxide. It was found that the dye oxidation is catalyzed by the enzyme in homogeneous reaction but the rate becomes much slower in heterogeneous catalytic processes with prepared catalysts. It seems that horseradish peroxidase immobilization in simple porous silica or alumino-silica post synthesis blocks the enzyme activity. In addition, copper ions make a stable complex with ARS hindering its oxidation as well.

Restricted access
Reaction Kinetics, Mechanisms and Catalysis
Authors: Viorel Chihaia, Karl Sohlberg, M. Scurtu, S. Mihaiu, M. Caldararu, and M. Zaharescu

Abstract

Tin and cerium based oxide ceramics, due to their peculiar properties, are good candidates for using as sensors, solid electrolytes in fuel cells, and catalysts. In the present work, Sn–Ce–O powders with the composition of interest for catalysis applications were obtained by solid state reactions and by thermal decomposition of the different tin and cerium precursors. The structural characterization of the resulted samples was performed by X-ray diffraction. Morphological characteristics were evaluated from X-ray microstructural parameters and BET surface areas measurements. Surface evolution of the studied powders was investigated by electrical measurements in various atmospheres in the 25–400 °C temperature range. The samples studied behave as a n-type semiconductor. The catalytic activity for CO oxidation was measured in gas flow between 25 and 400 °C. The conversion degree depends on the preparation method of the samples. Magnetic susceptibility measurements at room temperature of the samples before and after catalytic test indicated a paramagnetic behavior. Higher values of magnetic susceptibility of the samples after catalytic test compared to initial ones could indicate the influence of CeO2 on the catalytic activity.

Restricted access

Abstract

Gold catalysts supported on ceria doped with different metal oxides (Fe, Mn, Co and Sn) were synthesized using two techniques: CP and mechanochemical activation. The catalytic activity in complete benzene oxidation (CBO) was studied. The samples were characterized by means of XRD and high resolution transmission electron microscopy, and non-significant differences in the average size and the distribution of gold particles were observed. This means that the main reason for the different catalytic behavior in CBO has to be searched in the composition and structure of the supports. In spite of the higher hydrogen consumption (e.g., higher oxygen mobility) estimated by means of TPR, the gold catalysts on ceria doped with transition metal oxides generally are less active than gold/ceria catalyst. This observation could be explained taking into consideration that the key factor for the high oxidation activity is not the oxygen supplying but the activation of the very stable benzene molecule. The higher catalytic activity in comparison with that of the gold/ceria catalyst was observed only using a mixed ceria–CoOx support mechanochemically prepared. Very high and stable catalytic activity in CBO was obtained over this sample.

Restricted access
Reaction Kinetics, Mechanisms and Catalysis
Authors: Viorel Chihaia, Karl Sohlberg, B. Grzybowska-Świerkosz, M. Ruszel, R. Grabowski, L. Kępiński, M. A. Małecka, and J. Sobczak

Abstract

Au/MIICr2O4 (MII = Co, Mn, Fe) catalysts have been tested in the total oxidation of ethane, propane, propene at 300 °C and of CO at 35 °C, and characterized by TEM, XPS and hydrogen thermo-programmed reduction (H2TPR). The catalytic activity has been found to depend on the nature of the oxidized compound. For the CO and ethane oxidation, the activity decreases in the sequence Au/CoCr2O4 > Au/MnCr2O4 ≫ Au/FeCr2O4 and follows the sequence of the decreasing reducibility of the supports and the catalysts, estimated from H2TPR measurements. The activity decreases with the increasing Au particle size in the same order, the reactions then seem structure sensitive. No correlations between the activity and reducibility or the particle size are observed for oxidation of the C3 hydrocarbons: the sequence of the activity for propane is: Au/FeCr2O4 > Au/CoCr2O4 > Au/MnCr2O4, whereas activity in propene oxidation does not change markedly within the studied catalysts. The reaction mechanism in the oxidation of propane and propene seems then to be different from that of oxidation of CO and ethane.

Restricted access
Reaction Kinetics, Mechanisms and Catalysis
Authors: Viorel Chihaia, Karl Sohlberg, Rodica Zăvoianu, Anca Cruceanu, Octavian Dumitru Pavel, Emilian Angelescu, Ana Paula Soares Dias, and Ruxandra Bîrjega

Abstract

This contribution presents several aspects concerning the production and characterization of hydrotalcite-like compounds containing Mo-species in the interlayer region (Mo-HT), the corresponding derived mixed oxides (Mo-CHT) and their catalytic activity in the demercaptanization of gasoline contaminated by tert-butanethiol. Mo-HT samples were obtained using two different molybdenum sources, e.g. Na2MoO4 or (NH4)6Mo7O24 and two preparation procedures: (i) ionic exchange and (ii) coprecipitation at pH 10 under high supersaturation. The derived mixed oxides were obtained by calcination at 450 °C during 24 h. The solids have been characterized by chemical analysis, TG/DTA, XRD, SEM–EDX, FTIR, DR-UV–Vis and Raman spectroscopy as well as determination of base sites. The specific area of the solids was determined using the BET method. The best catalysts were found to be those containing higher amounts of Mo species with tetrahedral coordination obtained by calcination of the Mo-HT precursors prepared at pH 10, either by ionic exchange or by co-precipitation using Na2MoO4 as a molybdenum source. Meanwhile, the catalysts containing mainly octahedrally coordinated Mo species (obtained from Mo-HT prepared at pH 10 using (NH4)6Mo7O24·6H2O as molybdenum source) were characterized by a higher concentration of molybdenum species with octahedral coordination, lower number of base sites, smaller surface area, and a poor catalytic activity.

Restricted access
Reaction Kinetics, Mechanisms and Catalysis
Authors: Viorel Chihaia, Karl Sohlberg, Margarita Gabrovska, Rumeana Edreva-Kardjieva, Dorel Crişan, Peter Tzvetkov, Maya Shopska, and Iskra Shtereva

Abstract

The effect of nickel content on the structure and activity of co-precipitated Ni–Al layered double hydroxides (LDHs) as catalyst precursors for CO2 removal by methanation was studied by variation of the Ni2+/Al3+ molar ratio (Ni2+/Al3+ = 3.0, 1.5 and 0.5), and of the reduction and reaction temperatures as well as of the space velocities. Powder X-ray diffraction (PXRD), H2 chemisorption, and temperature programmed reduction (TPR) techniques were applied for physicochemical characterization of the samples. It was specified that the nano-scaled dimensions of the as-synthesized samples also generate nano-metrical metallic nickel particles (PXRD). The existence of readily and hardly reducible Ni2+–O species in the studied samples (TPR), affects catalytic performance. The studied catalysts hydrogenate CO2 effectively to residual concentrations of the latter in the range of 0–10 ppm at reaction temperatures from 400 to 220 °C and space velocities between 22,000 and 3000 h−1. The variation of the CO2 methanation activity with the changes of space velocities depends on the nickel content, and reduction and reaction temperatures. After reduction at 400 and 450 °C, a sample of Ni2+/Al3+ = 3.0 has demonstrated the highest conversion degree at all the reaction temperatures and space velocities, while a catalyst of Ni2+/Al3+ = 0.5 dominated in the methanation activity after reduction within 530–600 °C. The Ni2+/Al3+ = 1.5 catalyst data take intermediate position between Ni2+/Al3+ = 3.0 and Ni2+/Al3+ = 0.5 often closer to Ni2+/Al3+ = 3.0 ones. The studied Ni–Al LDH systems are found to be promising catalyst precursors for fine CO2 removal from hydrogen-rich gas streams through the methanation reaction, depending on the technological regime of catalyst activation.

Restricted access