Dmitri A. Bulushev

Vanadia/titania catalysts for gas phase partial toluene oxidation : Spectroscopic characterisation and transient kinetics study

Formation of vanadia species during the calcination of ball milled mixt. of V2O5 with TiO2 was studied by Raman spectroscopy in situ and at ambient conditions. Calcination in air leads to fast (1-3 h) spreading of vanadia over TiO2 followed by a slower process giving a monolayer vanadia. The calcinated catalyst showed higher activity during toluene oxidn. than the uncalcinated one, but the selectivity towards C7-oxygenated products (benzaldehyde and HOBz) remains unchanged. The activity of the catalysts is ascribed to the formation of vanadia species in the monolayer. The details of the parallel-consecutive reaction scheme of toluene oxidn. are presented from steady-state and transient kinetics studies. Different O species seem to participate in the deep and partial oxidn. of toluene. Coke formation was obsd. during the reaction presenting an av. compn. C2nH1.1n. The amt. of coke on the catalyst was not dependent on the calcination step and the V content in the catalyst. Coke formation is responsible for the deactivation of the catalyst. [on SciFinder (R)]

Effect of Potassium Doping on the Structural and Catalytic Properties of V/Ti Oxide in Selective Toluene Oxidation

Small addition of potassium to V/Ti-oxide catalyst (K:VD0.19), consisting of 3.7 monolayer VOx , 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 CO2 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. The catalysts were characterised by means of HRTEM, FT-Raman spectroscopy and 51 V NMR. Potassium addition introduces a disorder in the crystalline structure of bulk V 2O5 particles resulting in better spreading of V 2O5 over TiO2 surface. The interaction of V2O5 with TiO2 was facilitated upon K-doping, leading to the increased formation of monomeric vanadia species, which are the active sites in toluene partial oxidation to benzaldehyde.

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

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 H2. 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. Transient response techniques with continuous monitoring of the composition of gaseous phase are applied to follow the catalyst surface transformations. Evolution of benzaldehyde (BA) formed during interaction of toluene with the pre-oxidised catalyst (without gaseous oxygen) gives information about the nucleophilicity of surface oxygen. Addition of potassium to surface vanadia leads to an increased oxygen nucleophilicity, resulting in a higher selectivity towards BA formation. In general, increase in surface basicity decreases catalytic activity, but at the same time the catalyst deactivation due to coking is suppressed. This allows catalyst optimisation in view of a better control of the partial oxidation process.

Ruthenium Clusters on Carbon Nanofibers for Formic Acid Decomposition: Effect of Doping the Support with Nitrogen

The catalytic properties of 1 wt % Ru catalysts with the same mean Ru cluster size of 1.4–1.5 nm supported on herringbone‐type carbon nanofibers with different N contents were compared for H2 production from formic acid decomposition. The Ru catalyst on the support with 6.8 wt % N gave a 1.5–2 times higher activity for the dehydrogenation reaction (CO2, H2) than the catalyst on the undoped support. The activity in the dehydration reaction (CO, H2O) was the same. As a result, the selectivity to H2 increased significantly from 83 to 92 % with N‐doping, and the activation energies for both reactions were close (55–58 kJ mol−1). The improvement could be explained by the presence of Ru clusters stabilized by pyridinic N located on the open edges of the external surface of the carbon nanofibers. This N may activate formic acid by the formation of an adduct (>NH+HCOO−) followed by its dehydrogenation on the adjacent Ru clusters.

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

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 VOx 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, CO2 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.

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

Transient kinetics in the toluene oxidn. over V/Ti oxide catalysts prepd. by grafting and impregnation have been compared. V4+ cations are supposed to be the sites for the formation of electrophilic oxygen species participating in deep oxidn. Another oxygen species (probably nucleophilic) present on the oxidized 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 O2 in the mixt.) with partially reduced catalysts. [on SciFinder (R)]

Structural and catalytic properties of vanadia-based fibrous catalysts in toluene partial oxidation

Structured vanadia-based catalysts in the form of woven fabrics were successfully synthesised by incipient-wetness impregnation and sol–gel methods. The catalysts were investigated by Raman, X-ray photo-electron spectroscopies as well as high-resolution transmission electron microscopy (HRTEM). The spectroscopic results revealed that the elemental fibres consisted of a silica core covered by vanadia/titania. The modification of fibre silica surface by alumina resulted in a support with an increased dispersion of active vanadia/titania layer. Crystalline anatase was not found to be necessary for the formation of active surface vanadia species. Drying of titania layer without calcination led to a higher vanadia dispersion on titania. Structured fibrous vanadia-based catalysts demonstrated comparable activity and identical selectivity towards the products of toluene partial oxidation (benzaldehyde and benzoic acid) as granulated vanadia/titania with the same vanadium coverage. The transient response technique was applied to elucidate the role of the support on the active species. Thus, the V 2O5/TiO2/SiO2 woven fibre fabrics show promise for hydrocarbon partial oxidation, providing advantages due to its open macro-structure suitable for design of structured catalytic beds.

DRIFTS and transient-response study of vanadia/titania catalysts during toluene partial oxidation

The work was aimed on the determination of the role of bulk V2O5 present in vanadia/titania catalysts for toluene partial oxidation. Two catalysts with 1.8 and 11.1 wt% of V were studied by in situ DRIFTS and transient-response methods. Bulk V2O5 was found by FT-Raman spectroscopy only in the 11.1 wt% V/TiO2 catalyst while monolayer vanadia species (monomeric and polymeric) in both samples. Toluene interaction in the presence of gaseous oxygen showed that the selectivity to benzaldehyde and benzoic acid was similar for the two catalysts. The nature of adsorbed species was also similar. Toluene interaction with the oxidised samples in the absence of gaseous oxygen showed rapid formation of coordinatively adsorbed benzaldehyde (∼1625 cm−1) and benzoate/carboxylate species (1600–1400 cm−1), accompanied by the disappearance of the monomeric vanadia species (∼2037 cm−1). The catalyst containing V2O5 could convert toluene more deeply than the catalyst without V2O5, namely to adsorbed benzoic acid (∼1670 cm−1). Additionally, large quantities of gaseous products (COx, benzaldehyde, H2O) were obtained with the former catalyst, while almost none with the latter. Thus, bulk V2O5 could supply oxygen for the monolayer vanadia species easily reducible by toluene. This oxygen is required for the product formation and desorption.