D. Bulushev - Academia.edu (original) (raw)

Papers by D. Bulushev

Research paper thumbnail of Formation of the surface NO during N2O interaction at low temperature with iron-containing ZSM-5

The journal of physical chemistry. B, Jan 12, 2006

Interaction of N2O at low temperatures (473-603 K) with Fe-ZSM-5 zeolites (Fe, 0.01-2.1 wt %) act... more Interaction of N2O at low temperatures (473-603 K) with Fe-ZSM-5 zeolites (Fe, 0.01-2.1 wt %) activated by steaming and/or thermal treatment in He at 1323 K was studied by the transient response method and temperature-programmed desorption (TPD). Diffuse reflectance infrared fourier transform spectroscopy (DRIFTS) of NO adsorbed at room temperature as a probe molecule indicated heterogeneity of surface Fe(II) sites. The most intensive bands were found at 1878 and 1891 cm(-1), characteristic of two types mononitrosyl species assigned to Fe2+(NO) involved in bi- and oligonuclear species. Fast loading of atomic oxygen from N2O on the surface and slower formation of adsorbed NO species were observed. The initial rate of adsorbed NO formation was linearly dependent on the concentration of active Fe sites assigned to bi- and oligonuclear species, evolving oxygen in the TPD at around 630-670 K. The maximal coverage of a zeolite surface by NO was estimated from the TPD of NO at approximatel...

Research paper thumbnail of Implication of the acid–base properties of V/Ti-oxide catalyst in toluene partial oxidation

Journal of Molecular Catalysis A: Chemical, 2002

The work presents the effect of K-doping on V/Ti-oxides taking into account: the surface acid-bas... more The work presents the effect of K-doping on V/Ti-oxides taking into account: the surface acid-base properties and the structure of surface vanadia species in respect to the catalyst performance and deactivation. The structure of active surface species determines redox properties, which are related to the catalytic performance by the Mars-van Krevelen mechanism. The reducibility of surface vanadia is studied by temperature-programmed reduction (TPR) in H 2 . The molecular structure of surface vanadia is determined by FT-Raman spectroscopy in a controlled atmosphere. Surface acid-base properties are characterised via temperature-programmed desorption (TPD) of pyridine with mass spectrometric analysis of the products.

Research paper thumbnail of Dynamics of N2O decomposition over HZSM-5 with low Fe content

Journal of Catalysis, 2004

The dynamics of N2O decomposition to gaseous nitrogen and oxygen over HZSM-5 catalysts with a low... more The dynamics of N2O decomposition to gaseous nitrogen and oxygen over HZSM-5 catalysts with a low iron content (200 and 1000 ppm) was studied by the transient response method in the temperature range 523–653 K. The active catalysts were prepared from HZSM-5 with Fe in the framework on its steaming at 823 K followed by thermal activation in He at 1323 K. Two main

Research paper thumbnail of Formation of Active Sites for Selective Toluene Oxidation during Catalyst Synthesis via Solid-State Reaction of V2O5 with TiO2

Journal of Catalysis, 2000

Interaction of V 2 O 5 with TiO 2 during the preparation of V/Tioxide catalysts via solid-state r... more Interaction of V 2 O 5 with TiO 2 during the preparation of V/Tioxide catalysts via solid-state reaction has been studied by means of in situ FT-Raman spectroscopy, HRTEM and XPS. This interaction results in the formation of monomeric vanadia species with vanadium in tetrahedral coordination. The bridging oxygen in the V-O-Ti bond is suggested to be responsible for the catalytic activity during the partial oxidation of toluene. The formation of the monomeric vanadia species correlates with the improved catalyst performance, characterized by reaction rate and selectivity to benzaldehyde and benzoic acid. Mechanical activation by intensive grinding of V 2 O 5 /TiO 2 mixture via ball milling was necessary for the interaction of the oxides during the calcination. The monomeric species formation was observed at a temperature as low as 523 K. The dynamics of V 2 O 5 /TiO 2 interaction strongly depends on the presence of moisture during the calcination. In dry oxidative atmosphere, a part of the monomeric species is rapidly formed. Then, the process slows down and becomes diffusion-controlled. During the calcination in humid oxidative atmosphere, quick amorphization of bulk crystalline V 2 O 5 was observed followed by slow formation of the monomeric vanadia species.

Research paper thumbnail of Benzene hydroxylation over FeZSM-5 catalysts: which Fe sites are active?

Journal of Catalysis, 2004

FeZSM-5 with a wide range of Fe content (0.015–2.1 wt%) were studied in the benzene hydroxylation... more FeZSM-5 with a wide range of Fe content (0.015–2.1 wt%) were studied in the benzene hydroxylation to phenol with nitrous oxide (C6H6:N2O = 1:5) at low temperatures (550 K). Catalysts were activated before the reaction by steaming and/or calcinations in He (1323 K). High selectivity of benzene-to-phenol transformation (>98%) was obtained within 3 h without any deactivation of the catalyst. Three

Research paper thumbnail of Characterization of Surface Vanadia Forms on V/Ti–Oxide Catalyst via Temperature-Programmed Reduction in Hydrogen and Spectroscopic Methods

Journal of Catalysis, 2002

Surface vanadia species formed on vanadia/titania catalysts consisting of 0.2-2.6 monolayers (ML)... more Surface vanadia species formed on vanadia/titania catalysts consisting of 0.2-2.6 monolayers (ML) of VO x have been characterized by FT-Raman spectroscopy under controlled atmosphere, temperature-programmed reduction in hydrogen (TPR), and solubility in diluted HNO 3 . Three types of species were observed with the maximum peak temperatures as follows: isolated monomeric species (≤770-780 K), polymeric species (810 K), and bulk amorphous V 2 O 5 (852 K). During the reduction, the V= =O bond of the monomeric species with tetracoordinated vanadium disappears as shown by diffuse reflectance infrared Fourier transform spectroscopy. A formation of new hydroxyl groups with a basic character was observed. The monomeric species was found to be chemically stable with respect to diluted HNO 3 . Bulk amorphous V 2 O 5 and polymeric vanadia were soluble in HNO 3 and removed from the surface. The state of vanadium in the oxidized catalysts was mainly pentavalent, as shown by XPS, and did not change after acid treatment. Reduction by hydrogen of monolayer vanadia in a 0.2-ML V/TiO 2 catalyst was studied by temperature-programmed reduction (TPR) at different heating rates. A one-site kinetic model is able to account for the TPR data, in spite of the presence of the monomeric and polymeric species. This indicates that these species could be considered equal with respect to the interaction with hydrogen. The activation energy was determined for the catalyst reduction (98 ± 5 kJ/mol).

Research paper thumbnail of Water Vapor Effects in N 2 O Decomposition over Fe−ZSM-5 Catalysts with Low Iron Content

Industrial & Engineering Chemistry Research, 2007

ABSTRACT Transient response method, temperature-programmed desorption (TPD), and a temporary anal... more ABSTRACT Transient response method, temperature-programmed desorption (TPD), and a temporary analysis of products (TAP) reactor were used to study an inhibiting effect of water vapor in N2O decomposition to N2 and O2 over Fe−ZSM-5 catalysts with low iron content. The zeolites were activated in He at 1323 K for dehydroxylation and formation of active Fe(II) sites. At temperatures lower than 673 K the adsorbed water affects three main reaction steps involved in N2O decomposition, namely, N2O reversible adsorption, atomic oxygen (O)Fe loading from N2O, and oxygen desorption. The amounts of reversibly adsorbed N2O and (O)Fe loaded from N2O considerably decreased when water was preadsorbed on the zeolite. The oxygen recombination/desorption is the rate-determining step of N2O decomposition. During the TPD runs, oxygen desorption from the water-saturated catalyst as compared to the dry one took place at 300 K higher temperature. For the first time it was shown that water adsorption on the catalyst loaded with (O)Fe at 523 K induces the concomitant desorption of molecular oxygen. Moreover, the (O)Fe loaded on wet zeolite was shown to be inactive in CO oxidation, indicating transformation of active Fe(II) sites in the presence of N2O and H2O into an inactive Fe(III) hydroxylated form.

Research paper thumbnail of Transient kinetics of toluene interaction with V/Ti-oxides in anaerobic conditions

Chemical Engineering Journal, 2005

Toluene interaction with the catalysts consisting of 0.35, 0.62, 0.75 and 3.7 monolayers (ML) of ... more Toluene interaction with the catalysts consisting of 0.35, 0.62, 0.75 and 3.7 monolayers (ML) of VO x supported on anatase-titania, containing potassium, was studied by transient response techniques at 523-673 K. FT-Raman spectroscopy under dehydrated conditions was used to determine the state of vanadia. K-perturbed (1020 cm −1 ) and K-doped (990 cm −1 ) monomeric vanadia species as well as "amorphous" KVO 3 (960-940 cm −1 ) were found at vanadia coverage less than a monolayer. Bulk V 2 O 5 (994 cm −1 ) was present only in the 3.7 ML V/TiO 2 catalyst as a dominant species. Benzaldehyde (BA), total oxidation products and surface carbon-containing species were the main products of the toluene interaction. The proposed reaction network involves five steps and two types of oxygen sites. Both the BA and CO 2 formation increased with the concentration of vanadia. The former is determined mainly by nucleophilic-lattice oxygen that is involved in the monolayer vanadia species. The latter as well as the formation of the main part of surface carbon-containing species increased much more steeply being dependent, probably, from electrophilic oxygen abundant in polymerised vanadia species and V 2 O 5 . The performed kinetic modelling satisfactorily describes the response curves of BA, CO 2 and toluene obtained during the toluene interaction with the pre-oxidised 0.35-0.75 ML V/TiO 2 catalysts. The presence of bulk V 2 O 5 in the 3.7 ML V/TiO 2 catalyst seems to provide some change in the reaction mechanism demanding a modification of the reaction scheme.

Research paper thumbnail of N 2 O Decomposition over Fe-ZSM-5 Studied by Transient Techniques

Chemical Engineering & Technology, 2009

N 2 O decomposition to gaseous N 2 and O 2 catalyzed by a commercial Fe-ZSM-5 has been studied by... more N 2 O decomposition to gaseous N 2 and O 2 catalyzed by a commercial Fe-ZSM-5 has been studied by different transient techniques: (i) via the transient response methods at ambient pressure, (ii) via the temporal analysis of products (TAP) reactor under vacuum, and (iii) by temperature-programmed desorption (TPD) under vacuum. The catalyst was activated in He at 1323 K. Two main steps can be distinguished within the transient period of N 2 O decomposition under constant N 2 O feed at 603 K: the first step consists of molecular N 2 formation and surface atomic oxygen (O) Fe . It follows a period of stoichiometric N 2 O decomposition to gaseous N 2 and O 2 with increasing conversion until steady state is reached. The observed rate increase is assigned to a slow accumulation on the surface of NO x,ads species formed from N 2 O and participating as co-catalyst in the N 2 O decomposition. The NO x,ads species accelerates the atomic oxygen recombination/ desorption, which is the rate-determining step of N 2 O decomposition. The formation and accumulation of NO x,ads species during N 2 O interaction with the catalyst was confirmed by TAP studies. The amount of NO x,ads was found to depend on the number of N 2 O pulses injected into the TAP reactor. In the presence of adsorbed NO x on the catalyst surface (NO x,ads ) the desorption of dioxygen is facilitated. This results in a shift of the oxygen desorption temperature from 744 K to considerably lower temperatures of 580 K in TPD experiments. Pulses of gaseous NO had a similar effect leading to the formation NO x,ads , thus facilitating the oxygen recombination/desorption.

Research paper thumbnail of Transient kinetics of toluene partial oxidation over V/Ti oxide catalysts

Catalysis Today, 2000

Transient kinetics in the toluene oxidation over V/Ti oxide catalysts prepared by grafting and im... more Transient kinetics in the toluene oxidation over V/Ti oxide catalysts prepared by grafting and impregnation have been compared. V 4+ cations are supposed to be the sites for the formation of electrophilic oxygen species participating in deep oxidation. Another oxygen species (probably nucleophilic) present on the oxidised catalyst surface are responsible for benzaldehyde formation. Selectivity of 80-100% can be obtained during the initial period of the reaction on the grafted catalysts in the presence of gaseous oxygen and during the interaction of toluene (without O 2 in the mixture) with partially reduced catalysts.

Research paper thumbnail of Effect of potassium doping on the structural and catalytic properties of V/Ti-oxide in selective toluene oxidation

Applied Catalysis A: General, 2000

Small addition of potassium to V/Ti-oxide catalyst (K:V=0.19), consisting of 3.7 monolayer VO x ,... more Small addition of potassium to V/Ti-oxide catalyst (K:V=0.19), consisting of 3.7 monolayer VO x , increased activity and selectivity in partial oxidation of toluene. In order to elucidate the nature of vanadia species formed on the surface of V/Ti-oxide upon potassium doping, the catalysts were studied by transient kinetics method. The transient product responses during toluene oxidation by the oxygen present in the catalyst were compared for K-doped and non-doped samples. The formation of CO 2 decreased and formation of benzaldehyde increased with addition of potassium. This suggests a lower surface concentration of electrophilic oxygen (O − , O 2 − ), which is usually responsible for the deep oxidation, and a higher concentration of nucleophilic oxygen (O 2− ), responsible for the partial oxidation.

Research paper thumbnail of Deactivation kinetics of V/Ti-oxide in toluene partial oxidation

Applied Catalysis A: General, 2001

Deactivation kinetics of a V/Ti-oxide catalyst was studied in partial oxidation of toluene to ben... more Deactivation kinetics of a V/Ti-oxide catalyst was studied in partial oxidation of toluene to benzaldehyde (BA) and benzoic acid (BAc) at 523-573 K. The catalyst consisted of 0.37 monolayer of VO x species and after oxidative pre-treatment contained isolated monomeric and polymeric metavanadate-like vanadia species under dehydrated conditions as was shown by FT Raman spectroscopy. Under the reaction conditions via in situ DRIFTS fast formation of adsorbed carboxylate and benzoate species was observed accompanied by disappearance of the band of the monomeric species (2038 cm −1 ) (polymeric species were not controlled). Slow accumulation of maleic anhydride, coupling products and/or BAc on the surface caused deactivation of the catalyst during the reaction. Temperature-programmed oxidation (TPO) after the reaction showed formation of high amounts of CO, CO 2 and water. Rate constants for the steps of the toluene oxidation were derived via mathematical modelling of reaction kinetics at low conversion and constant oxygen/toluene ratio of 20:1. The model allows predicting deactivation dynamics, steady-state rates and selectivity. The highest rate constant was found for the transformation of BA into BAc explaining a low BA yield in the reaction.

Research paper thumbnail of Benzene to phenol hydroxylation with N2O over Fe-Beta and Fe-ZSM-5: Comparison of activity per Fe-site

Applied Catalysis A: General, 2007

Fe-Beta catalysts with iron content of 0.045-2.0 wt.% were studied in the benzene to phenol trans... more Fe-Beta catalysts with iron content of 0.045-2.0 wt.% were studied in the benzene to phenol transformation with N 2 O and compared to similar Fe-ZSM-5 catalysts to understand the influence of zeolite structure on the Fe-sites activity. The Fe-containing zeolites were prepared either by a direct hydrothermal synthesis or by a post-synthesis Fe deposition followed by activation (steaming or high temperature treatment in He). Total amount of Fe(II) active sites able to form atomic oxygen (O) Fe , from N 2 O, was quantified by the transient response method at 523 K measuring the released N 2 . The fraction of the (O) Fe active in CO oxidation was determined via the amount of CO 2 released. The catalyst activity in the benzene to phenol transformation over the activated isomorphously substituted Fe-Beta and Fe-ZSM-5 zeolites was directly proportional to the amount of the (O) Fe active in CO oxidation. The turnover frequency (TOF) over Fe-Beta and Fe-ZSM-5 catalysts was found to be close indicating a similarity in the structures of Fe active sites. The observed $2-fold difference can be attributed to the influence of the zeolite host lattices. The difference between Fe-Beta and Fe-ZSM-5 was also observed in DRIFT spectra of NO adsorbed on iron sites at room temperature. The bands of NO adsorbed on Fe-Beta and Fe-ZSM-5 were at 1873 and 1878/1891 cm À1 , respectively. The areas of these bands correlate with the amount of (O) Fe active in CO oxidation. Comparison of the DRIFT spectra of adsorbed NO on the zeolites with the spectra on some iron containing compounds allowed to attribute the adsorption sites to Fe(II) sites and not to Fe(III) sites. # 2006 Published by Elsevier B.V.

Research paper thumbnail of Influence of Potassium Doping on the Formation of Vanadia Species in V/Ti Oxide Catalysts

Langmuir, 2001

The influence of potassium on the formation of surface vanadia species on V/Ti oxide catalysts co... more The influence of potassium on the formation of surface vanadia species on V/Ti oxide catalysts containing from 0.2 to 5 monolayers of vanadia (K/V atomic surface ratio e1) has been investigated by temperature programmed reduction in hydrogen and by FT-Raman spectroscopy under dehydrated conditions. In the pure catalysts, monomeric and polymeric (metavanadate-like) species, "amorphous" and bulk crystalline V2O5 were detected depending on the surface vanadia loading. In the K-doped catalysts, vanadia species formed on the surface depend also on the K/V atomic ratio. Even at small K/V ratios, K inhibits the formation of the polymeric species in favor of the "K-doped" and/or "K-perturbed" monomeric species. These species possess lengthened VdO bonds with respect to the monomeric species in the undoped V/Ti oxides. At K/V ) 1, the "K-doped" monomeric species and "amorphous" KVO3 are mainly present on the surface. Reduction of vanadia forms in the K-doped catalysts takes place at higher temperatures than in the catalysts where potassium was absent. The monomeric and polymeric species, which are the active sites in partial catalytic oxidation, have the lowest reduction temperature. Vanadia species formed on the commercial titania, containing K, were also elucidated. The catalysts were characterized via X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, and Brunauer-Emmett-Teller surface area measurements.

Research paper thumbnail of Formation of the surface NO during N2O interaction at low temperature with iron-containing ZSM-5

The journal of physical chemistry. B, Jan 12, 2006

Interaction of N2O at low temperatures (473-603 K) with Fe-ZSM-5 zeolites (Fe, 0.01-2.1 wt %) act... more Interaction of N2O at low temperatures (473-603 K) with Fe-ZSM-5 zeolites (Fe, 0.01-2.1 wt %) activated by steaming and/or thermal treatment in He at 1323 K was studied by the transient response method and temperature-programmed desorption (TPD). Diffuse reflectance infrared fourier transform spectroscopy (DRIFTS) of NO adsorbed at room temperature as a probe molecule indicated heterogeneity of surface Fe(II) sites. The most intensive bands were found at 1878 and 1891 cm(-1), characteristic of two types mononitrosyl species assigned to Fe2+(NO) involved in bi- and oligonuclear species. Fast loading of atomic oxygen from N2O on the surface and slower formation of adsorbed NO species were observed. The initial rate of adsorbed NO formation was linearly dependent on the concentration of active Fe sites assigned to bi- and oligonuclear species, evolving oxygen in the TPD at around 630-670 K. The maximal coverage of a zeolite surface by NO was estimated from the TPD of NO at approximatel...

Research paper thumbnail of Implication of the acid–base properties of V/Ti-oxide catalyst in toluene partial oxidation

Journal of Molecular Catalysis A: Chemical, 2002

The work presents the effect of K-doping on V/Ti-oxides taking into account: the surface acid-bas... more The work presents the effect of K-doping on V/Ti-oxides taking into account: the surface acid-base properties and the structure of surface vanadia species in respect to the catalyst performance and deactivation. The structure of active surface species determines redox properties, which are related to the catalytic performance by the Mars-van Krevelen mechanism. The reducibility of surface vanadia is studied by temperature-programmed reduction (TPR) in H 2 . The molecular structure of surface vanadia is determined by FT-Raman spectroscopy in a controlled atmosphere. Surface acid-base properties are characterised via temperature-programmed desorption (TPD) of pyridine with mass spectrometric analysis of the products.

Research paper thumbnail of Dynamics of N2O decomposition over HZSM-5 with low Fe content

Journal of Catalysis, 2004

The dynamics of N2O decomposition to gaseous nitrogen and oxygen over HZSM-5 catalysts with a low... more The dynamics of N2O decomposition to gaseous nitrogen and oxygen over HZSM-5 catalysts with a low iron content (200 and 1000 ppm) was studied by the transient response method in the temperature range 523–653 K. The active catalysts were prepared from HZSM-5 with Fe in the framework on its steaming at 823 K followed by thermal activation in He at 1323 K. Two main

Research paper thumbnail of Formation of Active Sites for Selective Toluene Oxidation during Catalyst Synthesis via Solid-State Reaction of V2O5 with TiO2

Journal of Catalysis, 2000

Interaction of V 2 O 5 with TiO 2 during the preparation of V/Tioxide catalysts via solid-state r... more Interaction of V 2 O 5 with TiO 2 during the preparation of V/Tioxide catalysts via solid-state reaction has been studied by means of in situ FT-Raman spectroscopy, HRTEM and XPS. This interaction results in the formation of monomeric vanadia species with vanadium in tetrahedral coordination. The bridging oxygen in the V-O-Ti bond is suggested to be responsible for the catalytic activity during the partial oxidation of toluene. The formation of the monomeric vanadia species correlates with the improved catalyst performance, characterized by reaction rate and selectivity to benzaldehyde and benzoic acid. Mechanical activation by intensive grinding of V 2 O 5 /TiO 2 mixture via ball milling was necessary for the interaction of the oxides during the calcination. The monomeric species formation was observed at a temperature as low as 523 K. The dynamics of V 2 O 5 /TiO 2 interaction strongly depends on the presence of moisture during the calcination. In dry oxidative atmosphere, a part of the monomeric species is rapidly formed. Then, the process slows down and becomes diffusion-controlled. During the calcination in humid oxidative atmosphere, quick amorphization of bulk crystalline V 2 O 5 was observed followed by slow formation of the monomeric vanadia species.

Research paper thumbnail of Benzene hydroxylation over FeZSM-5 catalysts: which Fe sites are active?

Journal of Catalysis, 2004

FeZSM-5 with a wide range of Fe content (0.015–2.1 wt%) were studied in the benzene hydroxylation... more FeZSM-5 with a wide range of Fe content (0.015–2.1 wt%) were studied in the benzene hydroxylation to phenol with nitrous oxide (C6H6:N2O = 1:5) at low temperatures (550 K). Catalysts were activated before the reaction by steaming and/or calcinations in He (1323 K). High selectivity of benzene-to-phenol transformation (>98%) was obtained within 3 h without any deactivation of the catalyst. Three

Research paper thumbnail of Characterization of Surface Vanadia Forms on V/Ti–Oxide Catalyst via Temperature-Programmed Reduction in Hydrogen and Spectroscopic Methods

Journal of Catalysis, 2002

Surface vanadia species formed on vanadia/titania catalysts consisting of 0.2-2.6 monolayers (ML)... more Surface vanadia species formed on vanadia/titania catalysts consisting of 0.2-2.6 monolayers (ML) of VO x have been characterized by FT-Raman spectroscopy under controlled atmosphere, temperature-programmed reduction in hydrogen (TPR), and solubility in diluted HNO 3 . Three types of species were observed with the maximum peak temperatures as follows: isolated monomeric species (≤770-780 K), polymeric species (810 K), and bulk amorphous V 2 O 5 (852 K). During the reduction, the V= =O bond of the monomeric species with tetracoordinated vanadium disappears as shown by diffuse reflectance infrared Fourier transform spectroscopy. A formation of new hydroxyl groups with a basic character was observed. The monomeric species was found to be chemically stable with respect to diluted HNO 3 . Bulk amorphous V 2 O 5 and polymeric vanadia were soluble in HNO 3 and removed from the surface. The state of vanadium in the oxidized catalysts was mainly pentavalent, as shown by XPS, and did not change after acid treatment. Reduction by hydrogen of monolayer vanadia in a 0.2-ML V/TiO 2 catalyst was studied by temperature-programmed reduction (TPR) at different heating rates. A one-site kinetic model is able to account for the TPR data, in spite of the presence of the monomeric and polymeric species. This indicates that these species could be considered equal with respect to the interaction with hydrogen. The activation energy was determined for the catalyst reduction (98 ± 5 kJ/mol).

Research paper thumbnail of Water Vapor Effects in N 2 O Decomposition over Fe−ZSM-5 Catalysts with Low Iron Content

Industrial & Engineering Chemistry Research, 2007

ABSTRACT Transient response method, temperature-programmed desorption (TPD), and a temporary anal... more ABSTRACT Transient response method, temperature-programmed desorption (TPD), and a temporary analysis of products (TAP) reactor were used to study an inhibiting effect of water vapor in N2O decomposition to N2 and O2 over Fe−ZSM-5 catalysts with low iron content. The zeolites were activated in He at 1323 K for dehydroxylation and formation of active Fe(II) sites. At temperatures lower than 673 K the adsorbed water affects three main reaction steps involved in N2O decomposition, namely, N2O reversible adsorption, atomic oxygen (O)Fe loading from N2O, and oxygen desorption. The amounts of reversibly adsorbed N2O and (O)Fe loaded from N2O considerably decreased when water was preadsorbed on the zeolite. The oxygen recombination/desorption is the rate-determining step of N2O decomposition. During the TPD runs, oxygen desorption from the water-saturated catalyst as compared to the dry one took place at 300 K higher temperature. For the first time it was shown that water adsorption on the catalyst loaded with (O)Fe at 523 K induces the concomitant desorption of molecular oxygen. Moreover, the (O)Fe loaded on wet zeolite was shown to be inactive in CO oxidation, indicating transformation of active Fe(II) sites in the presence of N2O and H2O into an inactive Fe(III) hydroxylated form.

Research paper thumbnail of Transient kinetics of toluene interaction with V/Ti-oxides in anaerobic conditions

Chemical Engineering Journal, 2005

Toluene interaction with the catalysts consisting of 0.35, 0.62, 0.75 and 3.7 monolayers (ML) of ... more Toluene interaction with the catalysts consisting of 0.35, 0.62, 0.75 and 3.7 monolayers (ML) of VO x supported on anatase-titania, containing potassium, was studied by transient response techniques at 523-673 K. FT-Raman spectroscopy under dehydrated conditions was used to determine the state of vanadia. K-perturbed (1020 cm −1 ) and K-doped (990 cm −1 ) monomeric vanadia species as well as "amorphous" KVO 3 (960-940 cm −1 ) were found at vanadia coverage less than a monolayer. Bulk V 2 O 5 (994 cm −1 ) was present only in the 3.7 ML V/TiO 2 catalyst as a dominant species. Benzaldehyde (BA), total oxidation products and surface carbon-containing species were the main products of the toluene interaction. The proposed reaction network involves five steps and two types of oxygen sites. Both the BA and CO 2 formation increased with the concentration of vanadia. The former is determined mainly by nucleophilic-lattice oxygen that is involved in the monolayer vanadia species. The latter as well as the formation of the main part of surface carbon-containing species increased much more steeply being dependent, probably, from electrophilic oxygen abundant in polymerised vanadia species and V 2 O 5 . The performed kinetic modelling satisfactorily describes the response curves of BA, CO 2 and toluene obtained during the toluene interaction with the pre-oxidised 0.35-0.75 ML V/TiO 2 catalysts. The presence of bulk V 2 O 5 in the 3.7 ML V/TiO 2 catalyst seems to provide some change in the reaction mechanism demanding a modification of the reaction scheme.

Research paper thumbnail of N 2 O Decomposition over Fe-ZSM-5 Studied by Transient Techniques

Chemical Engineering & Technology, 2009

N 2 O decomposition to gaseous N 2 and O 2 catalyzed by a commercial Fe-ZSM-5 has been studied by... more N 2 O decomposition to gaseous N 2 and O 2 catalyzed by a commercial Fe-ZSM-5 has been studied by different transient techniques: (i) via the transient response methods at ambient pressure, (ii) via the temporal analysis of products (TAP) reactor under vacuum, and (iii) by temperature-programmed desorption (TPD) under vacuum. The catalyst was activated in He at 1323 K. Two main steps can be distinguished within the transient period of N 2 O decomposition under constant N 2 O feed at 603 K: the first step consists of molecular N 2 formation and surface atomic oxygen (O) Fe . It follows a period of stoichiometric N 2 O decomposition to gaseous N 2 and O 2 with increasing conversion until steady state is reached. The observed rate increase is assigned to a slow accumulation on the surface of NO x,ads species formed from N 2 O and participating as co-catalyst in the N 2 O decomposition. The NO x,ads species accelerates the atomic oxygen recombination/ desorption, which is the rate-determining step of N 2 O decomposition. The formation and accumulation of NO x,ads species during N 2 O interaction with the catalyst was confirmed by TAP studies. The amount of NO x,ads was found to depend on the number of N 2 O pulses injected into the TAP reactor. In the presence of adsorbed NO x on the catalyst surface (NO x,ads ) the desorption of dioxygen is facilitated. This results in a shift of the oxygen desorption temperature from 744 K to considerably lower temperatures of 580 K in TPD experiments. Pulses of gaseous NO had a similar effect leading to the formation NO x,ads , thus facilitating the oxygen recombination/desorption.

Research paper thumbnail of Transient kinetics of toluene partial oxidation over V/Ti oxide catalysts

Catalysis Today, 2000

Transient kinetics in the toluene oxidation over V/Ti oxide catalysts prepared by grafting and im... more Transient kinetics in the toluene oxidation over V/Ti oxide catalysts prepared by grafting and impregnation have been compared. V 4+ cations are supposed to be the sites for the formation of electrophilic oxygen species participating in deep oxidation. Another oxygen species (probably nucleophilic) present on the oxidised catalyst surface are responsible for benzaldehyde formation. Selectivity of 80-100% can be obtained during the initial period of the reaction on the grafted catalysts in the presence of gaseous oxygen and during the interaction of toluene (without O 2 in the mixture) with partially reduced catalysts.

Research paper thumbnail of Effect of potassium doping on the structural and catalytic properties of V/Ti-oxide in selective toluene oxidation

Applied Catalysis A: General, 2000

Small addition of potassium to V/Ti-oxide catalyst (K:V=0.19), consisting of 3.7 monolayer VO x ,... more Small addition of potassium to V/Ti-oxide catalyst (K:V=0.19), consisting of 3.7 monolayer VO x , increased activity and selectivity in partial oxidation of toluene. In order to elucidate the nature of vanadia species formed on the surface of V/Ti-oxide upon potassium doping, the catalysts were studied by transient kinetics method. The transient product responses during toluene oxidation by the oxygen present in the catalyst were compared for K-doped and non-doped samples. The formation of CO 2 decreased and formation of benzaldehyde increased with addition of potassium. This suggests a lower surface concentration of electrophilic oxygen (O − , O 2 − ), which is usually responsible for the deep oxidation, and a higher concentration of nucleophilic oxygen (O 2− ), responsible for the partial oxidation.

Research paper thumbnail of Deactivation kinetics of V/Ti-oxide in toluene partial oxidation

Applied Catalysis A: General, 2001

Deactivation kinetics of a V/Ti-oxide catalyst was studied in partial oxidation of toluene to ben... more Deactivation kinetics of a V/Ti-oxide catalyst was studied in partial oxidation of toluene to benzaldehyde (BA) and benzoic acid (BAc) at 523-573 K. The catalyst consisted of 0.37 monolayer of VO x species and after oxidative pre-treatment contained isolated monomeric and polymeric metavanadate-like vanadia species under dehydrated conditions as was shown by FT Raman spectroscopy. Under the reaction conditions via in situ DRIFTS fast formation of adsorbed carboxylate and benzoate species was observed accompanied by disappearance of the band of the monomeric species (2038 cm −1 ) (polymeric species were not controlled). Slow accumulation of maleic anhydride, coupling products and/or BAc on the surface caused deactivation of the catalyst during the reaction. Temperature-programmed oxidation (TPO) after the reaction showed formation of high amounts of CO, CO 2 and water. Rate constants for the steps of the toluene oxidation were derived via mathematical modelling of reaction kinetics at low conversion and constant oxygen/toluene ratio of 20:1. The model allows predicting deactivation dynamics, steady-state rates and selectivity. The highest rate constant was found for the transformation of BA into BAc explaining a low BA yield in the reaction.

Research paper thumbnail of Benzene to phenol hydroxylation with N2O over Fe-Beta and Fe-ZSM-5: Comparison of activity per Fe-site

Applied Catalysis A: General, 2007

Fe-Beta catalysts with iron content of 0.045-2.0 wt.% were studied in the benzene to phenol trans... more Fe-Beta catalysts with iron content of 0.045-2.0 wt.% were studied in the benzene to phenol transformation with N 2 O and compared to similar Fe-ZSM-5 catalysts to understand the influence of zeolite structure on the Fe-sites activity. The Fe-containing zeolites were prepared either by a direct hydrothermal synthesis or by a post-synthesis Fe deposition followed by activation (steaming or high temperature treatment in He). Total amount of Fe(II) active sites able to form atomic oxygen (O) Fe , from N 2 O, was quantified by the transient response method at 523 K measuring the released N 2 . The fraction of the (O) Fe active in CO oxidation was determined via the amount of CO 2 released. The catalyst activity in the benzene to phenol transformation over the activated isomorphously substituted Fe-Beta and Fe-ZSM-5 zeolites was directly proportional to the amount of the (O) Fe active in CO oxidation. The turnover frequency (TOF) over Fe-Beta and Fe-ZSM-5 catalysts was found to be close indicating a similarity in the structures of Fe active sites. The observed $2-fold difference can be attributed to the influence of the zeolite host lattices. The difference between Fe-Beta and Fe-ZSM-5 was also observed in DRIFT spectra of NO adsorbed on iron sites at room temperature. The bands of NO adsorbed on Fe-Beta and Fe-ZSM-5 were at 1873 and 1878/1891 cm À1 , respectively. The areas of these bands correlate with the amount of (O) Fe active in CO oxidation. Comparison of the DRIFT spectra of adsorbed NO on the zeolites with the spectra on some iron containing compounds allowed to attribute the adsorption sites to Fe(II) sites and not to Fe(III) sites. # 2006 Published by Elsevier B.V.

Research paper thumbnail of Influence of Potassium Doping on the Formation of Vanadia Species in V/Ti Oxide Catalysts

Langmuir, 2001

The influence of potassium on the formation of surface vanadia species on V/Ti oxide catalysts co... more The influence of potassium on the formation of surface vanadia species on V/Ti oxide catalysts containing from 0.2 to 5 monolayers of vanadia (K/V atomic surface ratio e1) has been investigated by temperature programmed reduction in hydrogen and by FT-Raman spectroscopy under dehydrated conditions. In the pure catalysts, monomeric and polymeric (metavanadate-like) species, "amorphous" and bulk crystalline V2O5 were detected depending on the surface vanadia loading. In the K-doped catalysts, vanadia species formed on the surface depend also on the K/V atomic ratio. Even at small K/V ratios, K inhibits the formation of the polymeric species in favor of the "K-doped" and/or "K-perturbed" monomeric species. These species possess lengthened VdO bonds with respect to the monomeric species in the undoped V/Ti oxides. At K/V ) 1, the "K-doped" monomeric species and "amorphous" KVO3 are mainly present on the surface. Reduction of vanadia forms in the K-doped catalysts takes place at higher temperatures than in the catalysts where potassium was absent. The monomeric and polymeric species, which are the active sites in partial catalytic oxidation, have the lowest reduction temperature. Vanadia species formed on the commercial titania, containing K, were also elucidated. The catalysts were characterized via X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, and Brunauer-Emmett-Teller surface area measurements.