Monomeric Vanadyl Species Research Articles

Uncovering the Dinuclear Mechanism of NO2-Involved NH3-SCR over Supported V2O5/TiO2 Catalysts.

Commercial vanadium oxide catalysts exhibit high efficiency for the selective catalytic reduction (SCR) of NO with NH3, especially in the presence of NO2 (i.e., occurrence of fast NH3-SCR). The high-activity sites and their working principle for the fast NH3-SCR reaction, however, remain elusive. Here, by combining in situ spectroscopy, isotopic labeling experiments, and density functional theory (DFT) calculations, we demonstrate that polymeric vanadyl species act as the main active sites in the fast SCR reaction because the coupling effect of the polymeric structure alters the elementary reaction step and effectively avoids the high energy barrier of the rate-determining step over monomeric vanadyl species. This study unveils the high-activity dinuclear mechanism of the NO2-involved SCR reaction over vanadia-based catalysts and provides a fundamental basis for developing high-efficiency and low V2O5-loading SCR catalysts.

Sn-doped rutile TiO2 for vanadyl catalysts: Improvements on activity and stability in SCR reaction

Sn-doped rutile TiO2 supports (SnxTi1−xO2, X = 0.1, 0.2, 0.5) were designed and V2O5/SnxTi1−xO2 catalysts were synthesized by incipient wetness impregnation. The V2O5/Sn0.2Ti0.8O2 exhibited higher low-temperature SCR activity and better H2O and SO2 resistance than the traditional V2O5/TiO2 (anatase) catalyst. In situ DRIFTs, XPS, H2-TPR and NH3/NO+O2-TPD results showed that Sn doping improved the redox property, NH3/NO adsorption and the amount of surface chemisorbed oxygen. The kinetic study indicated that Sn doping had little effect on SCR reaction pathways, but promoted SCR reaction, mainly through Eley-Rideal mechanism. More importantly, the V2O5/Sn0.2Ti0.8O2 showed better thermal stability than V2O5/TiO2. For V2O5/TiO2, the aggregation and sintering of the catalyst occurred, and the surface area decreased significantly after thermal aging. The monomeric vanadyl species partially transferred to crystalline V2O5. But for V2O5/Sn0.2Ti0.8O2, the surface area did not decrease as much as that for V2O5/TiO2, and the monomeric vanadyl species partially transferred to the polymeric vanadyl species.

Textural and morphology changes of mesoporous SBA-15 silica due to introduction of guest phase

Abstract The research focuses on study of guest phase effect on the surface area and pore volume of SBA-15 with the emphasis on elucidation of reasons for these changes. The changes of surface area and pore volume are evident from evaluated N2 adsorption isotherms of VOx-SBA-15 even for samples with relative low content of supported guest phase, which is “atomically” spread on the surface in the form of anchored monomeric vanadyl species. These species cannot block the pore with diameter of 10 nm, nevertheless the presence of such phase causes decrease in adsorbed nitrogen during physisorption. Comparison of guest phase amount with differences in adsorbed amount of nitrogen led to conclusion that each vanadyl complex prevents adsorption of about one or two N2 molecules in the layer and influences two adsorption layers. Significant pore blocking occurs in the VOx-SBA-15 materials only in the case of presence bulk oxide-like nanospecies. Re-structuralization of silica mimicking phase separation phenomena relying on spinodal decomposition of a system was observed by SEM/TEM analysis and adsorption isotherms inspection for materials with high vanadium content.

Catalytically Active Vanadia Species on Silica: Effect of Oxygen and Water

Energies and free energies are calculated by density functional theory for the reaction of one and two water molecules with monomeric vanadyl species anchored on silica surface models. Whereas monohydrated species may be present depending on the local surface structure, dihydrated species are disfavored by entropy. Vibrational frequencies are calculated and compared with observed Raman spectra in the range between 900 and 1100 cm–1. Moreover, the oxidation of vanadyl species to vanadium peroxo species and the formation of vanadium oxo peroxo species anchored by one V–O–Si bond to the support (“umbrella model”) is studied. The calculated energies show that the formation of peroxo species is unlikely for surface models.

Nonhydrolytic vanadia-titania xerogels: Synthesis, characterization, and behavior in the selective catalytic reduction of NO by NH3

V2O5-TiO2 catalysts with V2O5 contents ranging from 6 to 18wt% were obtained by calcination at 773K of xerogels prepared by a nonhydrolytic sol–gel route from VOCl3, TiCl4, and iPr2O. These materials were characterized by X-ray diffraction, micro-FT-Raman spectroscopy, static 51V NMR, SEM, N2 physisorption, temperature-programmed desorption of ammonia (NH3-TPD), and temperature-programmed reduction by H2 (H2-TPR). In all cases mesoporous solids were obtained with specific surface areas up to 87m2g−1. Raman spectroscopy indicated the presence of the same vanadia species as in conventional vanadia-titania catalysts. Up to a critical V2O5 loading (between 10 and 12wt%), all the vanadium was highly dispersed in the form of monomeric vanadyl and polymeric vanadate species, whereas for higher loadings V2O5 crystallites also formed. Most of the vanadium species (80–90%) were accessible to reduction by H2. The catalytic properties of these materials were evaluated in the selective catalytic reduction of NO by NH3. The estimated turnover frequency ‘TOF’ at 473K (mole of NO converted per mole of V per hour) went through a maximum value (16.6h−1) at a V loading of 8wt%, corresponding to an estimated surface vanadia loading of about 10μmolm−2. The N2O selectivity increased upon V loading as expected. A compromise between activity and high N2 selectivity (98% at 623K) could be found with the material containing 6wt% V2O5.

Effect of promoters including tungsten and barium on the thermal stability of V 2O 5/sulfated TiO 2 catalyst for NO reduction by NH 3

The effect of tungsten and barium on the thermal stability of V 2O 5/TiO 2 catalyst for NO reduction by NH 3 was examined over a fixed bed flow reactor system. The activity of V 2O 5/sulfated TiO 2 catalyst gradually decreased with respect to the thermal aging time at 600 °C. The addition of tungsten to the catalyst surface significantly enhanced the thermal stability of V 2O 5 catalyst supported on sulfated TiO 2. On the basis of Raman and XRD measurements, the tungsten on the catalyst surface was identified as suppressing the progressive transformation of monomeric vanadyl species into crystalline V 2O 5 and of anatase into rutile phase of TiO 2. However, the NO removal activity of V 2O 5/sulfated TiO 2 catalyst including barium markedly decreased after a short aging time, 6 h at 600 °C. This may be due to the transformation of vanadium species to inactive V–O–Ba compound by the interaction with BaO which was formed by the decomposition of BaSO 4 on the catalyst surface at high reaction temperature of 600 °C. The addition of SO 2 to the feed gas stream could partly restore the NO removal activity of thermally aged V 2O 5/sulfated TiO 2 catalyst containing barium.

On the role of monomeric vanadyl species in toluene oxidation over V2O5/TiO2 catalysts: a kinetic study using the TAP reactor

V2O5/TiO2 catalysts with V2O5 loadings of 1wt.% (Eurocat EL10V1) and 8wt.% (Eurocat EL10V8) as well as the corresponding NH3-washed samples (EL10V1w, EL10V8w) exhibiting exclusively monomeric vanadyl species were used as model catalysts in order to elucidate the role of different vanadia species in toluene oxidation. The catalysts were characterized by elemental analysis and X-ray photoelectron spectroscopy. The catalytic performance in toluene oxidation over EL10V1 and EL10V1w was investigated using a temporal analysis of products (TAP) reactor system under high-vacuum conditions. When oxidizing toluene over the untreated sample EL10V1 the products benzaldehyde (BA), benzoic acid (BAc), maleic acid anhydride (MA), and COx were detected. BA was observed at low temperatures followed by BAc at temperatures higher than about 500K, and finally COx above 600K indicating a consecutive oxidation mechanism. However, only the products BA and COx were observed for the EL10V1w catalyst. Previous DRIFTS and TPR/TPO studies had shown that the monomeric vanadyl species are able to adsorb toluene dissociatively, and that in the absence of gas-phase oxygen the desorption of oxygenated products from these species cannot occur, since the capability of fast oxygen insertion into toluene and desorption of oxygenates is associated with polymeric and crystalline vanadia species. Thus, a concerted mechanism for the selective oxidation of toluene in the presence of gas-phase oxygen over the ammonia-washed catalysts is suggested, i.e., the desorption of BA and the reoxidation of the monomeric vanadyl species are assumed to occur simultaneously.

Electron Transfer at the Gas−Solid Interface: Reaction of Gas-Phase MoCl5 with Vanadium Oxide Supported on Silica

Electron transfer between two transition metals, MoCl5 and VOx supported on silica, is studied in situ at the gas−solid interface by spectroscopy (electron paramagnetic resonance, diffuse reflectance UV−vis−near-IR, XPS, Raman), electron microscopy, and chemical analysis for a long period of time (∼9000 h). Initially, the VOx/SiO2 (VV, 3d0) solid phase is composed of V2O5 crystallites and monomeric tetrahedral vanadyl species grafted onto silica ((⋮SiO)3VO). Upon reaction at 30 °C with gas-phase MoCl5 (MoV, 4d1), the solid undergoes drastic changes in color, which define four stages. Each stage is accompanied by variations in the chemical state of the supported phase. The principal results are as follows. (i) No electron transfer and no reaction takes place between MoCl5 and grafted (⋮SiO)3VO species. (ii) In contrast, V2O5 crystallites undergo both electron transfer from MoV to VV and chemical changes leading to a change in the speciation of V and to the formation of a new and amorphous VIV−O−MoVI−Cl pha...

Temperature-programmed reduction and oxidation experiments with V2O5/TiO2 catalysts

Temperature-programmed reduction (TPR) and oxidation (TPO) experiments were performed with V2O5/TiO2 catalysts with vanadia loadings of 1 wt.% (Eurocat EL10V1) and 8 wt.% (Eurocat EL10V8) and with unsupported V2O5. It was possible to correlate the redox properties with the presence of different vanadia species: crystalline and polymeric vanadia species known to be present on EL10V8 were found to be the most difficult to reduce but the easiest to reoxidize. Monomeric vanadyl species known to be present on EL10V1 were the easiest to reduce but the most difficult to reoxidize. In a further set of experiments toluene was used as reducing agent under isothermal conditions at 623 K to probe the catalytic properties of the various vanadia species. In the subsequent TPO experiments the oxygen mass balance including O2, CO, CO2 and H2O was solved thus allowing us to distinguish between the oxidative degradation of adsorbates and the reoxidation of the catalyst. The O2 consumption at lower temperatures was shown to originate from the total oxidation of adsorbates, observed for all three samples, whereas the O2 consumption at higher temperatures, observed only for EL10V8 and V2O5, was caused by the reoxidation of these catalysts. Thus, monomeric vanadyl species were able to adsorb toluene dissociatively, but no oxygenated products were released into the gas phase. In contrast, fast oxygen insertion into toluene and desorption of oxygenates occurred when crystalline and polymeric vanadia species were present.

On the role of monomeric vanadyl species in toluene adsorption and oxidation on V 2O 5/TiO 2 catalysts: a Raman and in situ DRIFTS study

Supported model catalysts exclusively exposing monomeric vanadyl species were prepared by washing V 2O 5/TiO 2 samples (Eurocat EL10V1 and EL10V8) with aqueous solutions of ammonia. FT-NIR Raman spectroscopy confirmed that V 2O 5 crystallites and polymeric species which had been present on both samples were removed by the washing treatment, leaving behind monomeric vanadyl species. The adsorption of toluene was studied using in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) in order to elucidate the role of monomeric vanadyl species in hydrocarbon activation and transformation. Oxidative adsorption occurred in all samples, suggesting monomeric vanadyl species to be crucial for this step. Toluene oxidation had proceeded to a lower extent on the washed samples than on the unmodified Eurocat samples which is consistent with the capability of fast oxygen insertion associated with polymeric vanadia species and V 2O 5 crystallites. These species also proved to be essential to make gas-phase oxygen available to adsorbed hydrocarbons.

Effect of grafting sequence on the behavior of titania-supported v V 2O 5-WO 3 catalysts in the selective reduction of NO by NH 3

V 2 O 5 -WO 3 /TiO 2 catalysts have been prepared by subsequent, alternating, and simultaneous grafting of vanadia and tungsta onto titania. The performance of these catalysts in the selective catalytic reduction (SCR) of NO by NH 3 was compared with corresponding titania-supported single oxides prepared by the same method. For a given composition, the activity of the catalysts depended only marginally on the sequence of grafting for catalysts with low loadings (up to an experimental monolayer, ca. 10 μ mol V/m 2 and 6 μ mol W/m 2 , respectively). Increase of the calcination temperature from 573 to 773 K decreased the activity of catalysts with low loading. This behavior is attributed to spreading of the vanadia species over the titania surface, resulting in an increase of less active monomeric vanadyl species. For catalysts with higher loading (> experimental monolayer), the interaction between vanadia and tungsta species was intensified with increasing calcination temperature, affording higher activity and new species with hydroxyl groups characterized by anIR-band at a frequency ≤3600 cm −1 . The formation of weaker acid sites from which ammonia desorbed at temperatures 2 O 5 -WO 3 /TiO 2 catalysts, indicating strong interaction between vanadia and tungsta species, which results in a higher activity compared to the corresponding titania-supported single oxides. The studies demonstrate that high activity can be achieved with ternary V 2 O 5 -WO 3 /TiO 2 catalysts if the total loading exceeds an experimental monolayer and the catalyst is calcined at 773 K, or with catalysts derived from WO 3 TiO 2 calcined at 1023 K before vanadia deposition.

Characterization of modified V 2O 5-ZrO 2 catalysts for the oxidative dehydrogenation of propane

V 2O 5-ZrO 2 catalysts of different composition were investigated by means of X-ray diffraction, Laser-RAMAN spectroscopy, temperature programmed reduction and catalytic measurements. At low vanadia contents monomeric vanadyl species and polyvanadate species were found. Alkalimetal additives lead to an increase of the VO bond length in the sequence of nonpromoted < Li < Na < K. The catalytic activity in the oxidative dehydrogenation of propane is connected to the kind of VO x surface species and correlates with the reducibility.

Raman spectroscopic studies of the thermal decomposition mechanism of ammonium metavanadate

Thermal decomposition of NH4VO3 under N2 and NH3+ H2O has been studied on a molecular level by in situ Raman spectroscopy in the temperature range 150–400 °C. Under N2 , the decomposition mechanism leading to formation of V2O5 has been observed to proceed via two intermediates: viz. an amorphous, transitional (NH4)2V4O11 and NH4V3O8 . Under NH3+H2O, a slightly different decomposition pathway, rationalized in terms of stability of intermediates, leading to the formation of V2O3 has been observed. The identification of Raman-inactive V2O3 is further supported by the formation of V2O4and V2O5 , along with monomeric vanadyl species, through oxidation of V2O3 at 400 °C.

Vanadia supported on titania aerogels morphological properties and catalytic behaviour in the selective reduction of nitric oxide by ammonia

Vanadia on titania-aerogel catalysts with a wide range of surface areas (s bet 70–190 m 2 g −1) and vanadia loadings (2.5–17μmol m −2) have been prepared and tested for the selective catalytic reduction of nitric oxide with ammonia (SCR). The high surface area of the titania aerogels allows the immobilisation of larger amounts of active vanadia species per gram of support, which results in higher overall SCR activities, compared to catalysts based on conventionally prepared titania. Transmission electron microscopy. X-ray diffraction measurements and the SCR activities indicate total dispersion of the vanadia in the VO x/TiO 2-aerogel samples up to a theoretical monolayer of V 2O 5. Raman spectroscopy showed that monomeric vanadyl species and two-dimensional polyvanadate layers were present on the titania surface. Increase of the vanadia loading favoured the formation of polyvanadate species. The intrinsic SCR activity depends on the vanadia (surface) concentration, whereas the morphology and acidity of the titania (anatase) support seem to have little influence. Temperature-programmed desorption after exposure to SCR feed gas (SCR-TPD) of both the titania supports and the VO x/TiO 2 catalysts, indicated that the acidity of the titania aerogel supports depends on the crystallite size, as smaller crystallites ( d c < 10nm) result in increased acidity. Upon deposition of less than 6 μmol m −2 (V) on the TiO 2-aerogels, the adsorption capacity of the samples decreases and centres are formed that bind ammonia more strongly. These catalysts show low SCR activity. Further vanadia immobilisation restores the total amount of adsorbed species and leads to the formation of adsorption centres with moderate acidity, which results in up to ten times higher SCR activity. The occurrence of oxidation products of adsorbed ammonia with high vanadia loadings reflects the enhanced oxidation capability of these catalysts at elevated temperatures.

The effects of structure on the catalytic activity and selectivity of V 2O 5/TiO 2 for the reduction of NO by NH 3

An investigation of the effect of catalyst structure on the activity and selectivity of TiO 2 (anatase)-supported V 2O 5 for the selective catalytic reduction of NO by NH3 has been carried out. The structure of the catalyst and the adsorbed species present on the surface was characterized by in situ laser Raman spectroscopy (LRS), and the interaction of NH 3 was investigated using temperature-programmed desorption (TPD). At vanadia loadings corresponding to less than a theoretical monolayer, the vanadia is present in the form of monomeric vanadyl and polymeric vanadate species. When the vanadia coverage exceeds a monolayer, crystallites of V 2O 5 form at the expense of the polymeric species. Analysis of the catalytic activity shows that the specific activity of the polymeric vanadates species is about 10 times greater than that of the monomeric vanadyl species. Monomeric species produce N 2 as the principle reaction product, independent of the presence or absence of O 2 in the feed, whereas polymeric vanadates species produce both N 2 and N 2O, with the selectivity to N 2 decreasing with increasing concentrations of O 2 in the feed. LRS experiments reveal that in the absence of O 2 in the feed stream, the catalyst undergoes reduction but that in the presence of O 2, the catalyst remains in a nearly fully oxidized state. TPD experiments indicate that a crucial step in the catalytic reduction of NO is the activation of adsorbed NH 3 to produce NH x ( x = 0 − 2) species. The removal of H atoms from adsorbed NH 3 occurs via reaction with V O groups present in clusters of monomeric vanadyl species and in polymeric vanadate species, the latter being more reactive than the former. The observations reported in this study are interpreted in terms of a reaction mechanism, which accounts for the effects of catalyst structure and oxidation state on the observed properties.

Raman spectroscopy and thermal desorption of ammonia adsorbed on titania (anatase)-supported vanadia

The interaction of NH{sub 3} with TiO{sub 2} (anatase) and a series of TiO{sub 2} (anatase)-supported V{sub 2}O{sub 5} catalysts has been investigated using thermal desorption and in situ Raman spectroscopy. NH{sub 3} adsorbs on TiO{sub 2}(a) by coordinating to Ti{sup 4+} Lewis acid sites. Upon heating, most of the adsorbed NH{sub 3} desorbs intact, with only a small amount of N{sub 2} being produced. At V{sub 2}O{sub 5} loadings well below a monolayer, the dispersed vanadia is present on the surface in the form of monomeric vanadyl and polymeric vanadate species. NH{sub 3} adsorbs on vacant Ti{sup 4+} sites not covered by the vanadia and V{sup {minus}+} sites associated with the monomeric vanadyl species. Temperature-programmed desorption of NH{sub 3} adsorbed on the V{sub 2}O{sub 5} loaded samples indicates that NH{sub 3} is more strongly bound to the surface of these materials and displays a higher tendency to dissociate prior to desorbing. Increasing the coverage of the vanadia species to a monolayer results in a weaker NH{sub 3}-surface bond. Desorption studies show that substantial quantities of NO and N{sub 2}O are evolved by oxidation of the NH{sub 3} and reduction of the sample. Raman spectra recorded in the presence ofmore » NH{sub 3} at high temperatures indicate that the terminal oxygen atoms of the polyvanadate species and clustered monomeric species are removed preferentially, resulting in the reduction of these species. On the other hand, isolated monomeric vanadyls are not reduced by the adsorbing NH{sub 3}. These results suggest that the dissociation of NH{sub 3} is accelerated by the presence of adjacent V=O groups. 32 refs., 13 figs., 1 tab.« less