Stanislaw Dzwigaj

Role of silanol groups in the incorporation of V in β zeolite

Aqueous solutions of ammonium metavanadate of increasing concentrations have been contacted with a b zeolite before . . sample Alb ,S irAls 11 and after sample Sib ,S irAl) 1000 dealumination in a 13 N HNO solution. UV-visible and 3 51 V NMR characterizations of the V-loaded zeolites show that mainly octahedral V species are formed in Alb whereas, up . . to a loading of 1.75 V atoms per unit cell 2.3 V wt.% , only pseudotetrahedral V species nonhydroxylated SiO V5O and 3 . . . hydroxylated SiO HO V5O are observed in Sib. XRD and FT-IR results indicate that the V atoms present in Sib are 2 incorporated in the framework in a pseudotetrahedral environment. These V species are resistant to washing either with y1 . water or with an aqueous solution of NH OAc 1 mol l , 12 h . In contrast, the octahedral V species present in VAlb 4 . . whatever the V loading and in VSib at loadings higher than 1.75 V per unit cell are eliminated upon washing showing that these species are loosely bound to the zeolite. As demonstrated by FT-IR, the incorporation of V in the Si b framework involves silanol groups formed upon dealumination. This incorporation generates new Bronsted acidic sites. New FT-IR ¨ y1. bands 3650, 3620, 980, 950 cm are formed whose attribution is proposed. The maximum amount of V atoms . incorporated in Sib 1.75 per unit cell is lower than the potential amount of framework vacant T-sites generated by . dealuminating the b framework 5.3 per unit cell . However, this V amount is 20 times larger than that usually found in V-loaded zeolite prepared by hydrothermal synthesis. Therefore, this study confirms that dealuminating a zeolite is an attractive method to generate silanol groups which can be used to react with vanadium complexes and allow the incorporation in the zeolitic walls of high amounts of V atoms resistant to washing treatments. q 2000 Elsevier Science B.V. All rights reserved.

Effect of calcination/rehydration treatments on the environment of V in β zeolite

A dealuminated zeolite β has been contacted with solutions of ammonium metavanadate to incorporate vanadium as tetrahedral V species which are not removed under treatment with aqueous solution of ammonium acetate. The effects of calcination and rehydration on the environment of the V species have been studied by diffuse reflectance UV-visible and 51V NMR. The reducibility of the V species has been followed by TPR and ESR. The dehydroxylation/rehydroxylation process occurring in the β framework upon dehydration/rehydration has been monitored by 29Si MAS NMR and FT-IR spectroscopy.

Singlet Oxygen-Trapping Reaction as a Method of 1O2 Detection: Role of Some Reducing Agents

The production of singlet oxygen by H2O2 disproportionation and via the oxidation of H2O2 by NaOCl in a neutral medium was monitored by spin trapping with 2,2,6,6 tetramethyl-4-piperidone (TMPone). The singlet oxygen formed in both reactions oxidized 2,2,6,6 tetramethyl-4-piperidone to give nitroxide radicals. However the production of nitroxide radicals was relatively small considering the concentrations of H2O2 and NaOCl used in the reaction systems. Addition of electron donating agents: ascorbate, Fe2+ and desferrioxamine leads to an increase in the production of nitroxide radicals. We assumed that a very slow step of the reaction sequence, the homolytic breaking of the O-O bond of N-hydroperoxide (formed as an intermediate product during the reaction of 1O2 with TMPone) could be responsible for the relatively small production of nitroxide radicals. Electron donating agents added to the reaction system probably raise the rate of the hydroperoxide decomposition by allowing a more rapid heterolytic cleavage of the O-O bond leading to a greater production of nitroxide radicals. The largest effect was observed in the presence of desferrioxamine. Its participation in this process is proved by the concomitant appearance of desferrioxamine nitroxide radicals. The results obtained demonstrate that the method proposed by several authors and tested in this study to detect singlet oxygen is not convenient for precise quantitative studies. The reactivity of TMPone towards O2.-/HO2. and .OH has been also investigated. It has been found that both O2.-/HO2. and .OH radicals formed in a phosphate buffer solution (pH 7.4, 37 degrees C), respectively by a xanthine-oxidase/hypoxanthine system and via H2O2 UV irradiation, do not oxidize 2,2,6,6 tetramethyl-4-piperidone to nitroxide radicals.

Effect of the addition of propane and distortion of tetrahedral vanadium(V) species in VSiβ zeolites on the photodecomposition of NO

VxSiβ catalysts with x=0.05, 0.2 and 1.75 V wt%, prepared by a two-step post-synthesis method, have been characterised by physical techniques and their photocatalytic reactivity investigated in the decomposition of NO. XRD and diffuse reflectance UV-Vis spectroscopy show that vanadium is incorporated as tetrahedral Vv species, a result confirmed by XANES and EXAFS. Photoluminescence spectra of VxSiβ catalysts show well-resolved vibrational fine structures due to three different kinds of tetrahedral VV (α, β, γ) species, with different degrees of distortion. Their relative amounts depend on V content and calcination/hydration treatments. Calcined-hydrated V1.75Siβ, calcined V1.75Siβ and calcined V0.05Siβ exhibit mainly the α, γ and β kind of tetrahedral vanadium, respectively. These samples are active in the decomposition of NO under UV-irradiation, leading mainly to N2 and O2. The decomposition of NO is strongly enhanced in presence of propane, leading to higher yield of N2. The latter strongly depends on the distortion of tetrahedral V species. The highest activity is found for the catalyst with tetrahedral V of highest distortion, lowest V=O bond length and biggest lifetime of its excited triplet state.

Chapter 1 Applications of Photoluminescence Spectroscopy to the Investigation of Oxide‐Containing Catalysts in the Working State

Abstract This chapter is an update of an earlier version (Adv. Catal.44, 119 (1999)) devoted to the applications of photoluminescence (PL) spectroscopy to the characterization of solid surfaces in relation to adsorption, catalysis, and photocatalysis. This chapter concerns highly dispersed or bulk oxide‐containing catalysts, including a discussion of the effects of pressure and temperature on PL spectra of various materials. The aim of this chapter is to describe recent advances in the identification of catalytically active sites by PL spectroscopy and other spectroscopic techniques, such as IR, EPR, XAS (XANES and EXAFS), UV–vis, and NMR. One can thus obtain precise information about the nature and local environment of transition metal ions in zeolites and oxide and hydroxide ions on MgO surfaces. PL appears particularly useful in the case of zeolites and supported samples with low metal loadings (e.g., vanadium‐containing Siβ zeolite). Indeed, PL spectroscopy in static and dynamic modes allows one to distinguish, in VSiβ zeolite, three kinds of tetrahedral V(V) species, the relative amounts of which depend on vanadium content and calcination/rehydration treatments. PL, which is far more sensitive than UV–vis and 51V NMR spectroscopies, gives electronic and vibrational (fine structure and energy) information about each type of vanadium. Moreover, dynamic PL allows investigation of the chemical nature and properties of catalytically active surface sites in the working state and their role in photocatalytic reactions. This chapter provides a description of the advantages of PL techniques to characterize acidic and basic sites by means of probe molecules, as well as various materials used in catalysis and electroluminescence, including a consideration of future directions of research involving PL spectroscopy.

Low Temperature NO x Adsorption Study on Pd-Promoted Zeolites

In this study Pd-promoted zeolites (BEA, FER, MFI and MOR) are synthesized and investigated for potential use in the low-temperature NOx adsorption. The catalysts are characterized by BET, XRD, UV–Vis and XRF, while the adsorption/desorption characteristics of the samples are investigated by NOx adsorption/TPD, with and without water in the feed. The nature of the adsorbed NOx species has been analyzed by operando FT-IR spectroscopy. Under dry conditions at 50xa0°C all the investigated zeolite frameworks are able to store significant amounts of NOx, up to 1xa0mmol/gcat for the MOR sample; the presence of Pd has notxa0a significant impact on the amounts of the stored NOx. In fact, under dry conditions at 50xa0°Cxa0the zeolite frameworks oxidize NO to NO2, and store NOx from NO2 over the zeolitic support. FTIR data indicates that NOx are stored in the form of nitrosonium ion NO+ and nitrate ions; nitrosyl species formed on Al3+ and Pdn+ sites have also been observed, although to a lower extent. Evidences have also been provided for the existence of gaseous/weakly interacting species (mostly NO2) contained in the pores of the zeolites. Water inhibits NO oxidation over all investigated samples, resulting in a strong decrease in the NOx storage capacity. Still, appreciable amounts of NOx could be stored at 50xa0°Cxa0(up to 60xa0µmol/gcat for Pd/MFI) in the presence of water over the Pd-doped carriers.

State of cobalt in CoSiBEA zeolite and ITS role in NO SCR with ethanol

Abstract Co x SiBEA catalysts (x = 0.7 and 3.6 Co wt %) prepared by two-step postsynthesis are characterized by physical techniques and investigated in NO SCR with ethanol. Their catalytic properties depend on the nature and environment of Co species as shown by XRD, diffuse reflectance UV-vis, EPR and TPR.

14-P-35-Spectroscopic study of the nature of vanadyl groups: influence of the support (SiO2 and Alß and Siß zeolites)

Publisher Summary This chapter presents a spectroscopic study of the nature of vanadyl groups. Diffuse reflectance ultraviolet (UV)-visible and photoluminescence spectroscopies have been used to study the local environment of vanadium ions dispersed on the surface of amorphous (SiO2) and crystalline (nondealuminated Als and dealuminated Sis zeolites) supports. The molecular structure of vanadium (V) species depend strongly on the nature of the support. The application of photoluminescence spectroscopy helps to distinguish in as prepared VSiO2, VAls, and VSis materials one, two, and three kinds of tetrahedral vanadium (V) species, respectively.