https://dspace.library.uu.nl/bitstream/handle/1874/26040/Catal.Today-Weckhuysen-2003.pdf?sequence=1

Chemistry, spectroscopy and the role of supported vanadium oxides in heterogeneous catalysis

Metal-mediated reductive hydrodehalogenation of organic halides.

Metal nanoparticles have attracted much attention over the last decade owing to their unique properties as compared to their bulk metal equivalents, including a large surface-to-volume ratio and tunable shapes. To control the properties of nanoparticles with particular respect to shape, size and dispersity is imperative, as these will determine the activity in the desired application. Supported metal nanoparticles are widely employed in catalysis. Recent advances in controlling the shape and size of nanoparticles have opened the possibility to optimise the particle geometry for enhanced catalytic activity, providing the optimum size and surface properties for specific applications. This Review describes the state of the art with respect to the preparation and use of supported metal nanoparticles in catalysis. The main groups of such nanoparticles (noble and transition metal nanoparticles) are highlighted and future prospects are discussed.

Sustainable Preparation of Supported Metal Nanoparticles and Their Applications in Catalysis

Vanadium oxide monolayer catalysts Preparation, characterization and catalytic activity

Due to the drive for better environmental protection and energy conversion efficiency, on-site generation of CO x -free hydrogen from ammonia decomposition for fuel cell applications has attracted much attention. The development of high performance solid catalysts is essential for the supply of such hydrogen from ammonia. In this mini-review, we provide a summary of the reaction kinetics of catalytic ammonia decomposition. Comparisons are then made among the catalysts that have different active components, supports, and promoters. According to the works reported in the literature and our recent research results, Ru is the most active catalyst, carbon nanotubes (CNTs) are the most effective support, and KOH is the best promoter. An increase in Ru dispersion results in better catalytic performance. Both support basicity and conductivity are important criteria for a NH 3 decomposition catalyst of high efficiency; and it seems possible to generate novel advanced support, such as oxide-CNTs nanocomposite materials, that bears such characteristics. Also, proper removal of the electron-withdrawing entities that originate from the precursors of active component, support or promoter can be effective in enhancing the catalytic activity of a Ru catalyst.

A mini-review on ammonia decomposition catalysts for on-site generation of hydrogen for fuel cell applications

In an earlier communication the ammoxidation activity of V2O5/TiO2 catalysts with V2O5 loadings in the range 0.4–9.9 mol% was correlated to the average oxidation number of vanadium in the catalysts. In the present work, these catalysts were characterised by SEM, FTIR, ESR, DTA techniques and chemisorption of NH3 and O2. The scanning electron micrographs of the catalysts indicate that deposition of vanadium is taking place inside the mesopores of titania (anatase) up to 3.4 mol% V2O5 corresponding to a monolayer coverage. Beyond this loading neddle-like and bulk structures of vanadia appear probably on the external surface of the catalysts. The bands at 1010–1020 cm−1 appearing in the FTIR spectra of fresh catalysts are characteristic of highly dispersed monomeric VOx units and two-dimensional structures. The FTIR spectra of the used catalysts are altogether different from those of the fresh catalysts suggesting that the active phase has been drastically modified during the course of the reaction. The ESR spectrum of 0.4 mol% V2O5 shows an eightfold well resolved hyperfine structure indicating that V4+ is in diluted conditions on anatase surface. As V2O5 content increases the hyperfine structure of ESR spectrum gets progressively smeared out due to strong coupling between V4+ dipoles. The results indicate that vanadium is in a highly dispersed distorted octahedral or square pyramidal geometry at 3.4 mol% corresponding to a monolayer coverage. The DTA curves contain endothermic peaks at 100–150°C and 630–675°C corresponding to desorption of adsorbed water and melting of vanadia particles and loss of oxygen from vanadia. Chemisorption of NH3 and O2 is observed to exhibit maximum at the monolayer V2O5 loading just as the ammoxidation activity of the catalysts.

Ammoxidation of 3-picoline over V2O5/TiO2 (anatase) system. II. Characterisation of the catalysts by DTA, SEM, FTIR, ESR and oxygen and ammonia chemisorption

Single step vapour phase hydrogenation of phenol over Pt-M/C (M = Cr, V and Zr) alloy catalysts was investigated at atmospheric pressure and 200°C. The alloying element shows a significant effect on the product selectivities. While Pt-Cr/C and Pt-V/C catalysts are highly selective for hydrogenation of phenol to cyclohexanone, the Pt-Zr/C catalysts are predominantly selective to cyclohexanol.

Selectivity dependence on the alloying element of carbon supported Pt-alloy catalysts in the hydrogenation of phenol†

γ-Alumina is modified by carbon coverage and the resulting carbon covered alumina is used in the preparation of supported Pd catalysts with varying Pd loadings. The prepared catalysts have been characterized by BET surface area, CO-chemisorption, X-ray diffraction and temperature programmed reduction (TPR). The catalytic activities of these catalysts have been tested for the selective hydrogenolysis of dichlorodifluoromethane to difluoromethane. TPR studies reveal that fresh Pd/CCA catalysts at lower Pd loading (up to 4 wt.%) exhibit features of Pd/Al 2 O 3 (decomposition of β-PdH x ) along with Pd/C (reaction of Cl − with hydrogen) and beyond 4 wt.%, only Pd/C features are observed. TPR of spent catalysts reveal the increase in the intensity of β-PdH x with increase in Pd loading indicating the reorganization of Pd particles during the course of hydrogenolysis reaction. TPR of CCA and active carbon supports showed that the nature of carbon is somewhat different in CCA and active carbon. XRD data has shown that some amount of uncovered alumina is converted into its hydroxide fluoride. High selectivity to HFC-32 (∼95%) over 4 wt.% Pd/CCA catalyst at a reaction temperature of 220 °C is attributed to the beneficial role of carbon coverage on γ-alumina.

Selective hydrogenolysis of dichlorodifluoromethane on carbon covered alumina supported palladium catalyst

Calcined hydrotalcites (CHTs) are studied for the tert-butylation of phenol using iso-butanol in the temperatures ranging from 350 to 500 °C The major products of this reaction on calcined magnesium–aluminium hydrotalcites (CMA-HTs) are O-tert-butyl phenol (tert-butyl phenyl ether, OTBP) and 2-tert-butyl phenol (o-tert-butyl phenol, 2TBP) with O-butenyl phenol (butenyl phenyl ether, OBP) and 2-butenyl phenol (o-butenyl phenol, 2BP) as useful by-products With a view to understand the reaction mechanism and the reaction path, tert-butylation of phenol is studied by changing both Mg[M(II)] and Al[M(III)] ions with Zn2+ and Cr3+, respectively Thus, Mg–Al (MA), Mg–Cr (MC) and Zn–Al (ZA) hydrotalcites (with M2+:M3+ ratio=2) are prepared and characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), atomic absorption spectroscopy (AAS), differential thermal analysis/thermo gravimetric analysis (DTA/TG), BET-surface area (BET-SA) and acidity–basicity measurements (temperature-programmed desorption (TPD) of NH3 and CO2) Mixed oxides obtained from CHTs are found to be more active over their individual oxides for the tert-butylation of phenol The butylation activities of the three different CHTs are in the order CMA-HT>CZA-HT>CMC-HT A different product distribution is obtained over calcined Mg–Cr and Zn–Al hydrotalcites for the tert-butylation of phenol showing the influence of acid–base properties on the activity of the catalysts A probable mechanism based on the experimental observations has been proposed in order to explain the formation of butenyl phenols

Calcined Mg–Al, Mg–Cr and Zn–Al hydrotalcite catalysts for tert-butylation of phenol with iso-butanol—a comparative study

A series of titania (anatase)-supported vanadia catalysts ranging in V 2 O 5 content from 0.4 to 9.9 mol% was prepared by wet impregnation technique, characterized by BET surface area measurement and X-ray diffraction, and evaluated for ammoxidation of 3-picoline. The average oxidation number of vanadium in the fresh and used catalysts was determined by titrimetric methods. The ammoxidation activity and the average oxidation number were observed to increase with vanadia loading up to 3.4 mol% in the catalyst which corresponds to a monolayer coverage. The phase transformation of anatase to rutile after the reaction was observed at a V 2 O 5 loading of 5.9 mol%. The slow decrease of ammoxidation activity beyond 3.4 mol% V 2 O 5 was attributed to the coverage of active monomeric VO x species on the support by bulk vanadia and by other oxides, and also to compound formation with ammonia.

P. Kanta Rao

Papers

Ammoxidation of 3-picoline over V2O5/TiO2 (anatase) system. II. Characterisation of the catalysts by DTA, SEM, FTIR, ESR and oxygen and ammonia chemisorption

Selectivity dependence on the alloying element of carbon supported Pt-alloy catalysts in the hydrogenation of phenol†

Selective hydrogenolysis of dichlorodifluoromethane on carbon covered alumina supported palladium catalyst

Calcined Mg–Al, Mg–Cr and Zn–Al hydrotalcite catalysts for tert-butylation of phenol with iso-butanol—a comparative study

Ammoxidation of 3-picoline over V2O5/TiO2 (anatase) system I. Relationship between ammoxidation activity and oxidation state of vanadium