Titania-supported Vanadia Catalysts Research Articles

Redox Dynamics of Active VOx Sites Promoted by TiOx during Oxidative Dehydrogenation of Ethanol Detected by Operando Quick XAS

Titania-supported vanadia (VOx/TiO2) catalysts exhibit outstanding catalytic in a number of selective oxidation and reduction processes. In spite of numerous investigations, the nature of redox transformations of vanadium and titanium involved in various catalytic processes remains difficult to detect and correlate to the rate of products formation. In this work, we studied the redox dynamics of active sites in a bilayered 5% V2O5/15% TiO2/SiO2 catalyst (consisting of submonolayer VOx species anchored onto a TiOx monolayer, which in turn is supported on SiO2) during the oxidative dehydrogenation of ethanol. The VOx species in 5% V2O5/15% TiO2/SiO2 show high selectivity to acetaldehyde and an ca. 40 times higher acetaldehyde formation rate in comparison to VOx species supported on SiO2 with a similar density. Operando time-resolved V and Ti K-edge X-ray absorption near-edge spectroscopy, coupled with a transient experimental strategy, quantitatively showed that the formation of acetaldehyde over 5% V2O5/15% TiO2/SiO2 is kinetically coupled to the formation of a V4+ intermediate, while the formation of V3+ is delayed and 10–70 times slower. The low-coordinated nature of various redox states of VOx species (V5+, V4+, and V3+) in the 5% V2O5/15% TiO2/SiO2 catalyst is confirmed using the extensive database of V K-edge XANES spectra of standards and specially synthesized molecular crystals. Much weaker redox activity of the Ti4+/Ti3+ couple was also detected; however, it was found to not be kinetically coupled to the rate-determining step of ethanol oxidation. Thus, the promoter effect of TiOx is rather complex. TiOx species might be involved in a fast electron transport between VOx species and might affect the electronic structure of VOx, thereby promoting their reducibility. This study demonstrates the high potential of element-specific operando X-ray absorption spectroscopy for uncovering complex catalytic mechanisms involving the redox kinetics of various metal oxides.

High Surface Area VOx/TiO2/SBA-15 Model Catalysts for Ammonia SCR Prepared by Atomic Layer Deposition

The mode of operation of titania-supported vanadia (VOₓ) catalysts for NOx abatement using ammonia selective catalytic reduction (NH₃-SCR) is still vigorously debated. We introduce a new high surface area VOₓ/TiO₂/SBA-15 model catalyst system based on mesoporous silica SBA-15 making use of atomic layer deposition (ALD) for controlled synthesis of titania and vanadia multilayers. The bulk and surface structure is characterized by X-ray diffraction (XRD), UV-vis and Raman spectroscopy, as well as X-ray photoelectron spectroscopy (XPS), revealing the presence of dispersed surface VOx species on amorphous TiO₂ domains on SBA-15, forming hybrid Si–O–V and Ti–O–V linkages. Temperature-dependent analysis of the ammonia SCR catalytic activity reveals NOx conversion levels of up to ~60%. In situ and operando diffuse reflection IR Fourier transform (DRIFT) spectroscopy shows N–Hstretching modes, representing adsorbed ammonia and -NH₂ and -NH intermediate structures on Bronsted and Lewis acid sites. Partial Lewis acid sites with adjacent redox sites are proposed as the active sites and desorption of product molecules as the rate-determining step at low temperature. The high NOₓ conversion is attributed to the presence of highly dispersed VOₓ species and the moderate acidity of VOₓ supported on TiO₂/SBA-15.

High Surface Area VOx/TiO2/SBA-15 Model Catalysts for Ammonia SCR Prepared by Atomic Layer Deposition

The mode of operation of titania-supported vanadia (VOx) catalysts for NOx abatement using ammonia selective catalytic reduction (NH3-SCR) is still vigorously debated. We introduce a new high surface area VOx/TiO2/SBA-15 model catalyst system based on mesoporous silica SBA-15 making use of atomic layer deposition (ALD) for controlled synthesis of titania and vanadia multilayers. The bulk and surface structure is characterized by X-ray diffraction (XRD), UV-vis and Raman spectroscopy, as well as X-ray photoelectron spectroscopy (XPS), revealing the presence of dispersed surface VOx species on amorphous TiO2 domains on SBA-15, forming hybrid Si–O–V and Ti–O–V linkages. Temperature-dependent analysis of the ammonia SCR catalytic activity reveals NOx conversion levels of up to ~60%. In situ and operando diffuse reflection IR Fourier transform (DRIFT) spectroscopy shows N–Hstretching modes, representing adsorbed ammonia and -NH2 and -NH intermediate structures on Bronsted and Lewis acid sites. Partial Lewis acid sites with adjacent redox sites are proposed as the active sites and desorption of product molecules as the rate-determining step at low temperature. The high NOx conversion is attributed to the presence of highly dispersed VOx species and the moderate acidity of VOx supported on TiO2/SBA-15.

Behavior of titania-supported vanadia and tungsta SCR catalysts at high temperatures in reactant streams: Tungsten and vanadium oxide and hydroxide vapor pressure reduction by surficial stabilization

In order to meet impending regulations for soot and NOx reduction from mobile diesel engines, advanced emission control systems may require SCR catalysts with substantially improved thermal durability. State of the art vanadia-based SCR catalysts are composed of vanadia, tungsta, and possibly silica, present at relatively low mass fraction and supported on high-surface-area titania. Concern over the possible emission of metals such as vanadia from diesel vehicles fitted with vanadia-based catalysts limits their potential utility. Vanadia and tungsta oxide and hydroxide vapor pressures over the bulk oxides under conditions relevant for the simulated lifetime exposure of catalysts in the mobile SCR application were estimated on the basis of literature data, and the vapor pressures can be consequential. For the bulk tungsta and vanadia, the most volatile component is WO2(OH)2, formed from the reaction of tungsta with water, followed by V4O10 and then VO(OH)3, also a reaction product. An experimental method was developed to measure the vapor-phase transport (a manifestation of vapor pressure) of such inorganic components over real catalysts in representative gas streams by collection on high-surface-area alumina at exposure temperature. In the absence of water at 750°C, only V4O10 was anticipated as the volatile species. However, no V was observed downstream of the catalyst, so that the vapor pressure of titania-supported vanadia was strongly suppressed relative to the bulk oxide. In the presence of water, the results depended on the support. In the case of a support that underwent substantial loss of surface area during exposure, amounts of W were collected consistent with equilibrium vaporization as WO2(OH)2. However, the amounts of V collected were below the amount expected based on equilibrium vaporization as either V4O10 or VO(OH)3. Thus, the reaction of titania-supported vanadia with water at high temperatures was also suppressed. The vapor pressures of the vanadia and tungsta, and the extent of their reaction with water, can be reduced by varying degrees by reducing loss of the surface area of the titania support during exposure and by minimizing the mass fractions of the surficial oxides. New, highly stable titania supports with optimized compositions were found to virtually eliminate the loss of vanadia and tungsta.

ChemInform Abstract: IR and Raman Study of the Surface Acidity of Titania-Supported Vanadia Catalysts.

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

ChemInform Abstract: Magnetic Resonance Studies of Ammonia Adsorption and Decomposition on Titania-Supported Vanadia Catalysts

In-situ proton and deuterion single-resonance NMR techniques were used to study the adsorption and decomposition of ammonia as a function of temperature on a series of V{sub 2}O{sub 5}/TiO{sub 2} catalysts with the weight loading of vanadia ranging from 0 to 6.1 wt % (approximately one monolayer). After analysis of magnetic resonance line shapes, several different molecular species were found to be present on the catalyst surfaces. At low adsorption temperatures on the low-loading samples, the predominant adsorbed species was intact, coordinated ammonia rotating about its C{sub 3} axis. As the weight loading or adsorption temperature was increased, the adsorbed, intact ammonia exhibited an increased H-N-H angle; the total number of protons associated with intact ammonia per gram of sample decreased. At the point where intact ammonia was no longer observed, a new species was observed and assigned to NH{sub 2} moieties. On all samples, ammonium ions were observed at low adsorption temperatures (100{degrees}C) but were found to be easily removed by heating. Their concentration was greater on the higher weight loading samples. The formation of short-lived VOH groups from the protons of dissociated NH{sub 3} was inferred on the higher weight loading samples. Multiple-quantum spin-counting experiments were done tomore » further investigate the nature of the adsorbed species and their groupings on the surfaces. 45 refs., 12 figs., 2 tabs.« less

Comparison of redox isotherms for vanadia supported on zirconia and titania

Zirconia- and titania-supported vanadia catalysts were characterized by coulometric titration, Raman spectroscopy, and rate measurements for methanol oxidation and propane oxidative dehydrogenation (ODH). Redox isotherms obtained at 748 K by coulometric titration of a titania-supported vanadia catalyst with a loading of 2.9 V/nm 2 showed that reduction occurred in a single step at a P O 2 of 10 −22 atm, corresponding to a Δ G of oxidation of −300 kJ/mol; however, reduction of the sample caused changes in the isotherm obtained during re-oxidation consistent with formation of bulk V 2O 5, which requires higher P O 2 for oxidation. With vanadia-zirconia catalysts, increasing the calcination temperature from 748 to 873 K caused changes in both the Raman spectra and in the 748-K redox isotherm consistent with formation of bulk ZrV 2O 7. Rate measurements for methanol oxidation and propane ODH on vanadia-titania, vanadia-zirconia, bulk V 2O 5, and ZrV 2O 7 showed relatively small differences in specific rates.

Single-Step Preparation of Mesoporous, Anatase-Based Titanium−Vanadium Oxide and Its Application

Mesoporous solid solutions of anatase-based titanium-vanadium oxides were synthesized from aqueous solutions. The V/Ti ratio was determined by the composition of the deposition solution, while the morphology and nanoscale porosity were controlled using micelles of the surfactants cetyltrimethylammonium bromide (CTAB), or hexadecylamine (HDA). The use of CTAB resulted in mesoporous powders, whereas HDA yielded clusters of nanotubes. As compared to materials of the same composition made without the use of a surfactant, the catalyst made with CTAB had 50% higher catalytic activity, and that made with HDA had 70% higher activity. As compared to titania-supported vanadia catalysts with equivalent vanadium loading and synthesized using wet impregnation, the co-deposited materials exhibited significantly higher (up to 3.8x) catalytic activity.

Thermal decomposition of precursors and physicochemical characteristics of titania supported vanadia catalysts

Titania supported vanadia catalysts (2.5, 5, and 11 wt.% V 2O 5) were prepared by a wet impregnation technique and their thermal behavior, morphology as well as redox properties were examined by thermal analysis methods thermogravimetry (TGA), differential scanning calorimetry (DSC), temperature programmed-evolved gas analysis with mass spectroscopy, (EGA-MS), scanning electron microscopy (SEM), and temperature programmed reduction (TPR). The two Eurocat samples EL10V1 and EL10V8 containing 1 and 8 wt.% V 2O 5 were also characterized using the same techniques. Thermal decomposition of vanadium oxide precursors (ammonium vanadyl oxalate) supported on TiO 2 as evidenced by thermal analysis, occurs in three successive steps, which are influenced by the surrounding atmosphere (oxidative, reductive, and inert). The presence of tower-like vanadia crystals in the sample with the highest vanadia loading (11 wt.% V 2O 5) was identified by SEM. The H 2–TPR experiments revealed that the reduction temperature is a factor of the vanadia loading and the type of support. Vanadia species supported on Norton titania are more reducible that those supported on Eurocat titania.

Possible effects of site isolation in antimony oxide-modified vanadia/titania catalysts for selective oxidation of o-xylene

Titania-supported vanadia catalysts were modified by addition of antimony oxide for application in o-xylene selective oxidation to phthalic anhydride. It was shown that active and selective catalysts can be prepared by ball-milling mixtures of powders of TiO2, V2O5and Sb2O3followed by calcination. X-ray photoelectron spectroscopy proves the formation of highly dispersed overlayers of vanadium oxide and antimony oxide, in which V5+is partially reduced to lower oxidation states and Sb3+is partially oxidized to Sb5+. Antimony oxide segregated into the outermost surface layers. It is therefore inferred that the presence of the antimony oxide modifier spatially separates V–O species and leads to “site isolation” which may be responsible for the positive effect of the modifier for the catalyst's selectivity.

The structure of vanadia ultrathin films grown on TiO 2 (1 1 0) in an oxygen ambient

Vanadium oxide has been deposited on TiO 2 (1 1 0) by means of e-beam metal evaporation in an oxygen atmosphere at room temperature, following a procedure that has already been adopted by several groups. After an electronic structure characterisation based on XPS, X-ray excited Auger electron spectroscopy and valence band spectra, we have determined the lattice structure of ultrathin vanadia films approximately 5 and 18 monolayers thick by means of angle-scanned 2π photoelectron (XPD) measurements. While the electronic structure of the layers is close to that expected for V 2O 3, as judged from electron spectroscopies, XPD shows in a clear and direct way that the vanadia ultrathin films, instead of having the bulk-like corundum structure typical of V 2O 3, are characterised by a rutile lattice isomorphic to the substrate. This important difference with respect to vanadia ultrathin films grown on other substrates (e.g. Al 2O 3 (0 0 0 1) and Pd(1 1 1)), where the corundum structure is retained by the overlayer, could be a key to understanding the catalytic specificity and selectivity of titania-supported vanadia catalysts in many important chemical reactions.

15N NMR Study of the Adsorption of NO and NH3on Titania-Supported Vanadia Catalysts

The adsorption of nitric oxide (NO) by ammonia over a series of V2O5/TiO2catalysts used for selective catalytic reduction (SCR) has been investigated by solid-state15N NMR. The chemistry and the surface acidity are found to vary dramatically as the weight loading of vanadia is increased from 0 to 10%. NO reacts over evacuated TiO2and V2O5/TiO2at room temperature to form nitrous oxide (N2O). Over the V2O5/TiO2catalysts N2is also produced upon room temperature adsorption. The chemical shift of the terminal nitrogen of N2O adsorbed on the catalyst is an indicator of the Lewis acid character of the surface. The Lewis acidity of the surface is found to decrease with the loading of V2O5on the titania support. Ammonia adsorbs in two forms on evacuated TiO2and V2O5/TiO2. These two forms differ in their surface mobility. The reaction of NO with NH3over the catalysts proceeds to a measurable degree at room temperature. N2is the predominant product; however, N2O is also produced by the partial reduction of NO. Following preadsorption of ammonia, the chemical shift of the terminal nitrogen of N2O is similar to that of samples with a high V2O5loading regardless of the actual vanadia loading. This indicates that ammonia is adsorbed predominately on the Lewis acid sites of the catalyst, blocking the access of N2O to these sites.

Selective Catalytic Reduction of Nitric Oxide over Vanadia/Titania Catalysts: Temperature-Programmed Desorption and Isotopically Labeled Oxygen-Exchange Studies

Temperature-programmed desorption (TPD) and isotopically labeled oxygen-exchange studies were performed over titania-supported vanadia catalysts and bare titania support. The catalysts were characterized using BET surface area analysis, X-ray diffraction, laser Raman spectroscopy, scanning electron microscopy, X-ray photoelectron spectroscopy, and thermal analysis techniques. The TPD experiments were performed over both the catalyst and the bare support using NO and NH[sub 3] as adsorbates. The exchange reactions performed were: [[sup 14]N[sup 16]O] [yields] [[sup 15]N[sup 18]O] and [[sup 16]O[sub 2]] [yields] [[sup 18]O[sub 2]]. Transient experiments which involved step changes such as [NH[sub 3] + O[sub 2]] [yields] [O[sub 2]] were also performed to observed the transient behavior of various products. These results were combined with the earlier work, and conclusions were drawn regarding the adsorption/desorption and exchange characteristics of these catalysts in the SCR reaction.

Chapter 8 Electrical conductivity characterization of eurocat titania-supported vanadia catalysts

Two Eurocat V 2O 5/TiO 2 catalysts (EL10V1 and EL10V8), were characterized by electrical conductivity measurement at 300°C. It was demonstrated that vanadium is partially dissolved as V 5+ ions into the lattice of TiO 2 during the preparative calcination. The concentration of dissolved V 5+ ions increases with the amount of deposited vanadia and concordant results indicate that 2.5 times more V 5+ ions are dissolved in the sample supporting 8 wt.-%/V 2O 5 than in that supporting only 1 wt.-% V 2O 5. The major part of V 2O 5 is present as deposited particles. There is an indirect evidence (i) for the absence of a regular and continuous V 2O 5 surface layer model and (ii) for an increase in the mean particle size as the percentage of deposited V 2O 5 increases. At a higher temperature (450°C), equal to the final calcination temperature during catalyst preparation, anionic vacancies were detected at the surface of TiO 2 and are in equilibrium with gaseous oxygen. Their concentration in TiO 2 increases with the content of deposited V 2O 5. Consequently, TiO 5 appears as a non-inert support and should intervene in the catalytic performance of the solid. Etching in boiling isobutanol selectively removes V 2O 5-supported particles but does not destroy the solid solution of V 5+ in TiO 2. A schematic representation is proposed for the two catalysts tested: this consists of a solid solution (V 5+ inTiO 5) supporting V 2O 5 particles. The solubility of V 5+ and the mean particle size (with the assumption of homodispersed particles) both increase with the percentage of deposited vanadia (2–3 nm and 15–20 nm for V 2O 5 particles in EL10V1 and EL10V8 samples, respectively).

Chapter 3.6 Characterization of titania-supported V 2O 5 catalysts by reduction and subsequent oxygen adsorption

The aim of this study was to test the suitability of oxygen uptake measurements for the characterization of titania-supported vanadia catalysts. Before oxygen uptake measurements, the two catalysts (EL10V1 and EL10V8) were reduced in flowing hydrogen (623 K, 2 and 6 h). The subsequent oxygen adsorption at different temperatures was determined by using a static (microbalance) and a dynamic (GC pulse experiments) method. There was only a significant influence of reduction time on oxygen uptake for catalyst EL10V8. Considering the oxygen uptake of the two catalysts (μmol O 2/μmol V 2O 5), EL10V1 shows a higher degree of reduction for all temperatures, which is explained by the higher dispersion of the vanadia leading to stronger support interaction and easier reducibility of these species. The static and the dynamic method are both suitable for oxygen uptake measurements for characterizing supported vanadia catalysts. However, it seems unlikely that the reduction and oxidation processes are confined to the outermost surface layer of vanadia. The following reference conditions are proposed: reduction of the catalyst for 6 h at 623 K and subsequent exposure to oxygen at the same temperature.

Magnetic resonance studies of ammonia adsorption and decomposition on titania-supported vanadia catalysts

In-situ proton and deuterion single-resonance NMR techniques were used to study the adsorption and decomposition of ammonia as a function of temperature on a series of V{sub 2}O{sub 5}/TiO{sub 2} catalysts with the weight loading of vanadia ranging from 0 to 6.1 wt % (approximately one monolayer). After analysis of magnetic resonance line shapes, several different molecular species were found to be present on the catalyst surfaces. At low adsorption temperatures on the low-loading samples, the predominant adsorbed species was intact, coordinated ammonia rotating about its C{sub 3} axis. As the weight loading or adsorption temperature was increased, the adsorbed, intact ammonia exhibited an increased H-N-H angle; the total number of protons associated with intact ammonia per gram of sample decreased. At the point where intact ammonia was no longer observed, a new species was observed and assigned to NH{sub 2} moieties. On all samples, ammonium ions were observed at low adsorption temperatures (100{degrees}C) but were found to be easily removed by heating. Their concentration was greater on the higher weight loading samples. The formation of short-lived VOH groups from the protons of dissociated NH{sub 3} was inferred on the higher weight loading samples. Multiple-quantum spin-counting experiments were done tomore » further investigate the nature of the adsorbed species and their groupings on the surfaces. 45 refs., 12 figs., 2 tabs.« less

Infrared and Raman study of the adsorption of nitrogen oxides on titania-supported vanadia catalysts

Infrared and Raman spectra are reported of vanadia–anatase catalysts with three vanadia loadings exposed to nitric oxide and nitrogen dioxide at ambient temperature. Bridging, bidentate and monodentate nitrato species were identified. The surface species were less readily formed from nitric oxide than nitrogen dioxide and a smaller variety of adsorbed products was formed on catalysts with high rather than low vanadia loading. Reduced catalysts were reoxidised by nitrogen dioxide but not nitric oxide. The latter generated mono- and di-nitrosyl vanadium complexes on adsorption. Nitrous oxide failed to give adsorbed species on the catalyst surfaces.

Infrared study of the reaction between nitrogen oxides and ammonia on titania-supported vanadia catalysts

Infrared spectroscopy has been used to study sequential adsorption of ammonia and nitric oxide and of ammonia and nitrogen dioxide on three anatase-supported vanadia catalysts with different vanadia loadings. Facile reaction occurred between ammonium ions at Bronsted-acidic surface sites on vanadia and nitrato species formed by the adsorption of nitrogen dioxide or nitric oxide. Nitric oxide was less reactive because of depletion of surface oxygen during reaction. Pre-reduction of catalyst in hydrogen not only removed surface oxygen but also destroyed catalytically active Bronsted-acidic sites. A reaction mechanism involving an intermediate V—O—NN species is tentatively proposed.

Infrared and Raman study of the surface acidity of titania-supported vanadia catalysts

Infrared and Raman spectra are reported of ammonia and pyridine adsorbed on calcined (in oxygen at 723 K) vanadia catalysts supported on anatase. Infrared results are also given for catalysts which had been reduced in hydrogen at 723 K before adsorption of the bases. Both Lewis- and Bronsted-acidic surface sites exist on oxidised vanadia/anatase with vanadia loadings above and below the theroretical value for monolayer coverage of the anatase. Only Lewis-acidic sites were detected on reduced catalysts. Reduction increased the OH bond strength in surface hydroxyl groups which became unable to donate protons to the bases. Both exposed Ti and V ions acted as Lewis-acidic sites. Ammonia was dissociatively adsorbed at VO sites on vanadia at single-layer coverage on the support.

Raman spectroscopic study of titania-supported vanadia catalysts

Raman spectra are reported of four catalysts containing different vanadia loadings and prepared by calcination of anatase impregnated with ammonium metavanadate from an aqueous solution of oxalic acid. The spectra of vanadia components at loadings higher than that required for theoretical monolayer coverage of the anatase support surface resembled the spectrum of vanadium(V) oxide. Different spectra for loadings less than that for monolayer coverage are interpreted in terms of a bonded vanadia layer in which each exposed vanadium ion is coordinatively unsaturated after high-temperature thermal treatment. Adsorption of H2O, which is shown to be reversible, leads to a weakening of the V—O bond.