Partial oxidation of toluene to benzaldehyde and benzoic acid over model vanadia/titania catalysts: Role of vanadia species

Pure and K-doped vanadia/titania prepared by different methods have been studied in order to elucidate the role of vanadia species (monomeric, polymeric, bulk) in catalytic toluene partial oxidation. The ratio of different vanadia species was controlled by treating the catalysts in diluted HNO3, which removes bulk vanadia and polymeric vanadia species, but not the monomeric ones, as was shown by FT-Raman spectroscopy and TPR in H2. Monolayer vanadia species (monomeric and polymeric) are responsible for the catalytic activity and selectivity to benzaldehyde and benzoic acid independently on the catalyst preparation method. Bulk V2O5 and TiO2 are considerably less active. Therefore, an increase of the vanadium concentration in the samples above the monolayer coverage results in a decrease of the specific rate in toluene oxidation due to the partial blockage of active monolayer species by bulk crystalline V2O5. Potassium diminishes the catalyst acidity resulting in a decrease of the total rate of toluene oxidation and suppression of deactivation. Deactivation due to coking is probably related to the Brønsted acid sites associated with the bridging oxygen in the polymeric species and bulk V2O5. Doping by K diminishes the amount of active monolayer vanadia leading to the formation of non-active K-doped monomeric vanadia species and KVO3

Transient response method for characterization of active sites in HZSM-5 with low content of iron during N2O decomposition

The surface active Fe-sites in HZSM-5 with low content of iron (<1000 ppm) activated by steaming and high temp. (up to 1323 K) calcination in inert lead to the formation of surface oxygen (O)ad species from N2O and were characterized quant. by transient response method. Only a part of (O)ad deposited on zeolite by decompg. N2O was active in CO oxidn. at 523 K. A binuclear Fe-center is suggested as an active center, featuring a \"diamond core\" structure, similar to that of the monooxygenase enzyme. The active O-atoms were assigned to the paired terminal oxygen atoms each bound to one Fe-site in the binuclear [Fe2O2H]+-cluster. Zeolite pre-satd. by water vapor at 473-523 K generates (O)ad species from N2O completely inactive in the CO oxidn. The total amt. of the oxygen, (O)ad, deposited on the pre-satd. by water zeolite corresponds to a half of stoichiometric amt. of the surface Fe-atoms and suggests that water blocks a half of the binuclear [Fe2O2H]+-center, the remaining acting as a single Fe-site. [on SciFinder (R)

Low Temperature Decomposition of Nitrous Oxide over Fe/ZSM-5: Modelling of the Dynamic Behaviour

The kinetics of N2O decomposition to gaseous nitrogen and oxygen over HZSM-5 catalysts with low content of iron (360 °C. The amount of surface NO during the transient increases with the reaction temperature, the reaction time, and the N2O concentration in the gas phase up to a maximum value. The maximum amount of surface NO was found to be independent on the temperature and N2O concentration in the gas phase. This leads to a first-order N2O decomposition during the transient period, and to a zero-order under steady state. A kinetic model is proposed for the autocatalytic reaction. The simulated concentration–time profiles were consistent with the experimental data under transient as well as under steady-state conditions giving a proof for the kinetic model suggested in this study

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

The dynamics of N2O decompn. 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 temp. range 523-653 K. The active catalysts were prepd. 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 steps were distinguished in the dynamics of N2O decompn. The first step represents N2O decompn. forming gaseous nitrogen and surface at. oxygen. The second step is assocd. with surface oxygen recombination and desorption. At 523-553 K only the first step is obsd. Above 573 K the decompn. of N2O to O2 and N2 in stoichiometric amts. starts at a rate increasing with time until a steady-state value is reached. This increase was assigned to the catalysis by adsorbed NO formed slowly on the catalyst surface from N2O, as indicated by temp.-programmed desorption. The catalytic effect of the adsorbed NO was also confirmed by transient expts. with forced addn. of NO in the stream of N2O during its decompn. A simplified kinetic model is proposed to explain the autocatalytic reaction. Catalyst pretreatment in O2 did not affect N2O decompn., but irreversible water vapor adsorption at 603 K resulted in a twofold decrease in surface oxygen loading from N2O and complete inhibition of the oxygen desorption. [on SciFinder (R)

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

The influence of potassium on the formation of surface vanadia species on V/Ti oxide catalysts contg. from 0.2 to 5 monolayers of vanadia (K/V at. surface ratio ?1) has been investigated by temp. programmed redn. in hydrogen and by FT-Raman spectroscopy under dehydrated conditions. In the pure catalysts, monomeric and polymeric (metavanadate-like) species, \"amorphous\" and bulk cryst. 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 at. 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 V:O 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. Redn. of vanadia forms in the K-doped catalysts takes place at higher temps. than in the catalysts where potassium was absent. The monomeric and polymeric species, which are the active sites in partial catalytic oxidn., have the lowest redn. temp. Vanadia species formed on the com. titania, contg. K, were also elucidated. The catalysts were characterized via XPS, high-resoln. transmission electron microscopy, and Brunauer-Emmett-Teller surface area measurements. [on SciFinder (R)

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

Highly dispersed gold on activated carbon fibers for low temperature CO oxidation

Gold nanoparticles of 2–5 nm supported on woven fabrics of activated carbon fibers (ACF) were effective during CO oxidation at room temperature. To obtain a high metal dispersion, Au was deposited on ACF from aqueous solution of ethylenediamine complex [Au(en)2]Cl3 via ion exchange with protons of surface functional groups. The temperature-programmed decomposition method showed the presence of two main types of functional groups on the ACF surface: the first type was associated with carboxylic groups easily decomposing to CO2 and the second one corresponded to more stable phenolic groups decomposing to CO. The concentration and the nature of surface functional groups was controlled using HNO3 pretreatment followed by either calcination in He (300–1273 K) or by iron oxide deposition. The phenolic groups are able to attach Au3+ ions, leading to the formation of small Au nanoparticles (9 nm) Au agglomerates after reduction by H2. These catalysts demonstrated lower activity as compared to the ones containing mostly small Au nanoparticles. Complete removal of surface functional groups rendered an inert support that would not interact with the Au precursor. The oxidation state of gold in the Au/ACF catalysts was controlled by X-ray photoelectron spectroscopy before and after the reduction in H2. The high-temperature reduction in H2 (673–773 K) was necessary to activate the catalyst, indicating that metallic gold nanoparticles are active during catalytic CO oxidation

N2O Decomposition over Fe-ZSM-5 Studied by Transient Techniques

N2O 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 N2O decomposition under constant N2O 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 N2O 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 NOx,ads species formed from N2O and participating as co-catalyst in the N2O decomposition. The NOx,ads species accelerates the atomic oxygen recombination/desorption, which is the rate-determining step of N2O decomposition. The formation and accumulation of NOx,ads species during N2O interaction with the catalyst was confirmed by TAP studies. The amount of NOx,ads was found to depend on the number of N2O pulses injected into the TAP reactor. In the presence of adsorbed NOx on the catalyst surface (NOx,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 NOx,ads thus facilitating the oxygen recombination/desorption