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

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

IR spectroscopy in catalysis

Ceria has been the subject of thorough investigations, mainly because of its use as an active component of catalytic converters for the treatment of exhaust gases. However, ceria-based catalysts have also been developed for different applications in organic chemistry. The redox and acid-base properties of ceria, either alone or in the presence of transition metals, are important parameters that allow to activate complex organic molecules and to selectively orient their transformation. Pure ceria is used in several organic reactions, such as the dehydration of alcohols, the alkylation of aromatic compounds, ketone formation, and aldolization, and in redox reactions. Ceria-supported metal catalysts allow the hydrogenation of many unsaturated compounds. They can also be used for coupling or ring-opening reactions. Cerium atoms can be added as dopants to catalytic system or impregnated onto zeolites and mesoporous catalyst materials to improve their performances. This Review demonstrates that the exceptional surface (and sometimes bulk) properties of ceria make cerium-based catalysts very effective for a broad range of organic reactions.

https://www.onlinelibrary.wiley.com/doi/pdf/10.1002/cssc.201000054

Ceria‐Based Solid Catalysts for Organic Chemistry

Supported metal catalysts: some unsolved problems

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

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.

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

Hysteresis during ammonia synthesis over promoted ruthenium catalysts supported on carbon-covered alumina☆

TiO2 supported ceria catalysts with 20 wt% CeO2 were prepared by impregnating rutile and anatase polymorphs in aqueous solution of ammonium ceric nitrate [(NH4)2Ce(NO3)6], characterized by BET surface area measurement and low temperature oxygen chemisorption (LTOC) at −78°C and evaluated for ammoxidation of β- and γ-picolines. Anatase supported catalyst has exhibited higher ammoxidation activities than rutile supported one and pure ceria. Turnover frequencies calculated from ammoxidation rates and irreversible O2 uptakes in respect to a particular picoline isomer were almost equal for pure supports on one hand and for supported ceria catalysts on the other. The two structural isomers, β- and γ-picolines seem to have exhibited marked steric effect in their reactivity in catalysis on the catalysts.

Highly active titania supported ceria catalysts for ammoxidation of picolines

V2O5/CeF3 catalysts with 2.4–15.2 wt.% of V2O5 were prepared by wet impregnation method. The catalysts were characterized by BET surface area and pore size distribution, powder X-ray diffraction and FT-IR. It was found that the formation of Cerium Vanadate (CeVO4) occurs during the stages of the catalyst preparation. The X-ray diffraction studies of the catalysts revealed the presence of crystalline CeVO4 phase. The lines due to CeVO4 phase were found to increase in intensity with the increase in amount (wt.%) of V2O5 in the catalysts. The FT-IR investigation also showed the formation of CeVO4 phase with increasing loadings of V2O5. The shift of pore-size distribution to lower pore range and increased pore volume confirmed the formation of CeVO4 structure in the catalysts. The catalytic activity for the ammoxidation of 3-picoline to nicotinonitrile on V2O5/CeF3 catalysts as a function of V2O5 loading on CeF3 has been studied. The continuous increase of CeVO4 with V2O5 loadings is responsible for the higher ammoxidation activity of the V2O5/CeF3 catalysts.

S. Khaja Masthan

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

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

Hysteresis during ammonia synthesis over promoted ruthenium catalysts supported on carbon-covered alumina☆

Highly active titania supported ceria catalysts for ammoxidation of picolines

Cerium fluoride supported V2O5 catalysts: physico-chemical characterization and 3-picoline ammoxidation activity