Redox dynamics of 2D crystalline vanadium oxide phases on high-index anatase facets.

Vanadium oxides exist in a multitude of phases with varying structure and stoichiometry. This abundance of phases can be extended through the use of other oxides as supports, and through redox treatments. However, the combined effects of different supports and redox treatments can be difficult to identify, particularly when present as different terminating facets on nanoparticles. Here, we examine structural dynamics of 2D vanadium oxides supported on anatase TiO2 nanoparticles, correlated with changes in oxidation state, using in situ transmission electron microscopy imaging and electron energy loss spectroscopy. As the average oxidation state is reduced below V(IV), an ordered cubic V(II) phase is observed exclusively at the high-index {10l} facets of the support. This local accommodation of highly reduced states is necessary for explaining the observed range of average oxidation states. In turn, the findings show that oxidation states extending from V(V)-V(IV) to V(II) can be simultaneously stabilized by different supporting oxide surfaces during exposure to atmospheres with controlled redox potential.

Probing surface-sensitive redox properties of VOx/TiO2 catalyst nanoparticles.

Redox processes of oxide materials are fundamental in catalysis. These processes depend on the surface structure and stoichiometry of the oxide and are therefore expected to vary between surface facets. However, there is a lack of direct measurements of redox properties on the nanoscale for analysing the importance of such faceting effects in technical materials. Here, we address the facet-dependent redox properties of vanadium-oxide-covered anatase nanoparticles of relevance to, e.g., selective catalytic reduction of nitrogen oxides. The vanadium oxidation states at individual nanoscale facets are resolved in situ under catalytically relevant conditions by combining transmission electron microscopy imaging and electron energy loss spectroscopy. The measurements reveal that vanadium on {001} facets consistently retain higher oxidation states than on {10l} facets. Insight into such structure-sensitivity of surface redox processes opens prospects of tailoring oxide nanoparticles with enhanced catalytic functionalities.

The role of platinum on the NOx storage and desorption behavior of ceria: an online FT-IR study combined with in situ Raman and UV-vis spectroscopy.

The role of platinum on the room temperature NOx storage mechanism and the NOx desorption behavior of ceria was investigated by combining online FT-IR gas-phase analysis with in situ Raman and UV-vis spectroscopy. The type of pretreatment, leading to the presence of different platinum states (Pt0, and mixed Pt0/Pt2+), is shown to have a major effect on the NOx storage and desorption properties. Upon loading of ceria with platinum (1 wt%), NOx storage capacities decrease except for reductively pretreated Pt/CeO2, enabling new reaction pathways via activation of gas-phase oxygen. In the absence of oxygen, NO is reduced by metallic platinum leading to N2O and N2 formation. In situ Raman spectra provide mechanistic information, by monitoring changes in ceria surface and subsurface oxygen, as well as PtOx during NOx storage. In the presence of gas-phase oxygen, NOx storage is related to the consumption of (sub)surface oxygen and PtOx, and proposed to involve NO2 or [NO + O2] intermediates reacting with surface oxygen. The NOx desorption behavior is shown to be strongly related to the stored NOx species. Oxidative pretreatment of ceria resulted in the largest amount of stored nitrates, consistent with NOx being mostly desorbed at elevated temperatures, i.e., within 300-500 °C. Reductive pretreatment and/or addition of platinum significantly increased the fraction of stored nitrite, thereby shifting the main NOx desorption temperature to values <300 °C. Storage and subsequent desorption of NOx in PtOx/CeO2 was associated with PtOx reduction and reoxidation, as monitored by in situ UV-vis and Raman spectra. Through detailed analysis we were able to elucidate the influence of platinum on NOx storage/desorption and demonstrate the participation of different platinum states in room temperature NOx storage, with each platinum state opening a distinct new reaction pathway.

The reaction mechanism for the SCR process on monomer V(5+) sites and the effect of modified Brønsted acidity.

The energetics, structures and activity of a monomeric VO3H/TiO2(001) catalyst are investigated for the selective catalytic reduction (SCR) reaction by the use of density functional theory (DFT). Furthermore we study the influences of a dopant substitute in the TiO2 support and its effects on the known properties of the SCR system such as Brønsted acidity and reducibility of vanadium. We find for the reduction part of the SCR mechanism that it involves two Ti-O-V oxygen sites. One is a hydroxyl possessing Brønsted acidity which contributes to the formation of NH4(+), while the other accepts a proton which charge stabilizes the reduced active site. In the reduction the proton is donated to the latter due to a reaction between NH3 and NO that forms a H2NNO molecule which decomposes into N2(g) and H2O(g). A dopant substitution of 10 different dopants: Si, Ge, Se, Zr, Sn, Te, Hf, V, Mo and W at each of the sites, which participate in the reaction, modifies the energetics and therefore the SCR activity. We find that Brønsted acidity is a descriptor for the SCR activity at low temperatures. Based on this descriptor we find that Zr, Hf and Sn have a positive effect as they decrease the activation energy for the SCR reaction.

Monitoring catalysts at work in their final form: spectroscopic investigations on a monolithic catalyst.

A monolithic vanadia-titania based catalyst has been subjected to studies with in situ FTIR spectroscopy coupled with mass spectrometry, during the SCR (Selective Catalytic Reduction) reaction. A device based on a transmission reactor cell for monolithic samples was constructed, dedicated to the study of surface species during reaction. After analysing the steady state SCR activity under industrially relevant conditions, NH(3) chemisorption phenomena as a function of temperature and the subsequent SCR reaction of NO + O(2) with chemisorbed ammonia and ammonium ion species were also investigated. The observations reported here serve as a demonstration of the great potential for the application of operando spectroscopy on monolithic systems. This cross disciplinary approach aims to identify reaction pathways, active sites, intermediate- and spectator-species for catalytic reactions under truly industrial conditions in a shaped monolithic catalyst based on monitoring chemical profiles along its channels. In particular, by demonstrating the feasibility of the approach using the technically challenging operando transmission FTIR spectroscopy methodology, we foresee easy future adaption of this approach with other surface or bulk sensitive techniques, e.g. Raman and UV-vis spectroscopy.

Operando and in situ Raman studies of alumina-supported vanadium phosphate catalysts in propane ammoxidation reaction: activity, selectivity and active phase formation.

Alumina-supported VPO materials are efficient catalysts for acrylonitrile production by the propane ammoxidation reaction. In order to understand the structure-activity relationship and the nature of active sites, operando Raman-GC analyses follow the states of vanadium and phosphorous species on an alumina support during ammoxidation. These oxides were supported on γ-alumina by incipient wetness impregnation at a total V + P loading of two monolayers, which led to incipient formation of nanoscaled VPO crystallites possessing a high surface-to-volume ratio. Since catalysis occurs at the catalyst surface, which is related to the surface and outermost layers, this approach allows studying surface species phase transformations near the surface, and relates changes in activity and selectivity to variations in composition and structure. Dispersed surface V(5+) species appear selective to acetonitrile and V(4+) species would promote selectivity to acrylonitrile. This study suggests that V(3+) is probably involved in redox processes during propane ammoxidation and that the balance between these vanadium species would be determined by activation process.

Preparation, properties, and reactivities of unprecedented oxo-sulfido Nb(IV) aqua ions and crystal structure of (Me2NH2)6[Nb5(mu3-S)2(mu3-O)2(mu2-O)2(NCS)14].3.5H2O.

By treatment of Zn-reduced ethanolic solutions of NbCl5 with HCl in the presence of sulfide followed by cation-exchange chromatography, two oxo-sulfido niobium aqua ions, the red [Nb4(mu4-S)(mu2-O)5(H2O)10]4+ and the yellow-brown [Nb5(mu3-S)2(mu3-O)2(mu2-O)2(H2O)14]8+, were isolated. Both readily form their respective thiocyanate complexes, for which the structure for the former has been previously reported. Brown crystals of (Me2NH2)6[Nb5S2O4(NCS)14].3.5H2O (1) were isolated in the case of the latter, and the structure was determined by X-ray crystallography (space group: a = 15.4018(5) A, b = 21.1932(8) A, c = 22.0487(8) A, alpha=gamma = 90 degrees , beta = 103.4590(10) degrees , and R(1) = 0.0659). An unprecedented pentanuclear Nb5S2O48+ core is revealed in which short Nb-Nb distances (2.7995(8)-2.9111(8) A) are consistent with metal-metal bonding. A stopped-flow kinetic study of the 1:1 equilibration of NCS- with [Nb4(mu4-S)(mu2-O)5(H2O)10]4+ has been carried out. Equilibration rate constants are independent of [H(+)] in the range investigated (0.5-2.0 M) and at 25 degrees C; kf= 9.5 M(-1) s(-1), kaq = 2.6 x 10(-2) s(-1), and K = 365 M1). Conditions with first NCS- and then [Nb4(mu4-S)(mu2-O)5(H2O)10]4+ in excess revealed a statistical factor of 4, suggesting the presence of four kinetically equivalent Nb atoms. Attempts to study the 1:1 substitution of NCS- with [Nb5(mu3-S)2(mu3-O)2(mu2-O)2(H2O)14]8+ showed signs of saturation kinetics. Quantum chemical calculations using the density functional theory (DFT) approach were performed on both the Nb4O5S4+ and Nb5O4S28+ naked clusters. The highest occupied and lowest unoccupied molecular orbitals have dominant Nb(4d) character. The HOMO for Nb4O5S4+ is a nondegenerate fully filled MO, whereas for Nb5O4S28+, it is a nondegenerate partially filled MO with one unpaired electron. EPR spectroscopy on [Nb5(mu3-S)2(mu3-O)2(mu2-O)2(H2O)14]8+ shows that the molecule has total anisotropy (C2v), with all three tensors, gx= 2.399, gy= 1.975, and gz= 1.531, resolved. No hyperfine interaction expected from the nuclear moment of I = 9/2 for 93Nb was observed.

Crystal structure and spectroscopic characterization of K8(VO)2O(SO4)6.

Red and yellow dichroistic crystals of a vanadium(V) compound, potassium (mu-oxo, di-mu-sulfato)bis(oxodisulfatovanadate), K(8)(VO)(2)O(SO(4))(6), have been obtained from the ternary catalytic model melt system K(2)S(2)O(7)[bond]K(2)SO(4)[bond]V(2)O(5). By slow cooling of the melt from 420 to 355 degrees C, crystal growth occurred, using solid V(2)O(5) crystals present in the melt as nucleation promoter. The compound crystallizes in the monoclinic space group P2(l) with a = 13.60(9) A, b = 13.93(9) A, c = 14.05(9) A, beta = 90.286(10) degrees, and Z = 2. It contains two VO(6) octahedra linked together by a mu-oxo and two mu-sulfato bridges. Furthermore, each octahedron has two monodentate sulfate ligands, making the dimeric entity coordinatively saturated. IR spectroscopy shows bands arising from V[bond]O[bond]V and V[double bond]O stretches as well as splitting of sulfate bands due to the different degrees of freedom present for different conformations of sulfate ligands. The coordination of vanadium in K(8)(VO)(2)O(SO(4))(6) is discussed in relation to the reaction mechanism of SO(2) oxidation catalysis.