M. Rafiq H. Siddiqui

Graphene based metal and metal oxide nanocomposites: synthesis, properties and their applications

Graphene, an atomically thin two-dimensional carbonaceous material, has attracted tremendous attention in the scientific community, due to its exceptional electronic, electrical, and mechanical properties. Indeed, with the recent explosion of methods for a large-scale synthesis of graphene, the number of publications related to graphene and other graphene based materials has increased exponentially. Particularly the development of easy preparation methods for graphene like materials, such as highly reduced graphene oxide (HRG) via reduction of graphite oxide (GO), offers a wide range of possibilities for the preparation of graphene based inorganic nanocomposites by the incorporation of various functional nanomaterials for a variety of applications. In this review, we discuss the current development of graphene based metal and metal oxide nanocomposites, with a detailed account of their synthesis and properties. Specifically, much attention has been given to their wide range of applications in various fields, including electronics, electrochemical and electrical fields. Overall, by the inclusion of various references, this review covers in detail the aspects of graphene-based inorganic nanocomposites.

Identification of active phases in Au–Fe catalysts for low-temperature CO oxidation

In the light of a recent study which identified the beneficial influence of poorly crystallised ferrihydrite Fe5HO8·4H2O on the activity of CO conversion catalysts comprising gold nanoparticles on oxidic iron, we have investigated three series of ferrihydrite-rich samples prepared by coprecipitation. The samples were structurally and chemically characterised using X-ray diffraction and both 57Fe and 197Au Mossbauer spectroscopy, and tested for CO oxidation at room temperature using a microreactor with on-line GC. The highest activity, 100% conversion after 20 min on line, was observed in a dried sample that contained ferrihydrite and a non-crystalline and possibly hydrated gold oxyhydroxide phase, AuOOH·xH2O. The activity of the same materials after calcination, where the gold was transformed to 3–5 nm Au metal particles and the ferrihydrite to hematite, was less than ca. 7%. This is the first report of a synergistic interaction between AuOOH·xH2O and ferrihydrite resulting in an active catalyst for room temperature CO oxidation, and contrasts with previous work which has been interpreted in terms of the requirement for metallic Au nanoparticles.

Ambient temperature CO oxidation using copper manganese oxide catalysts prepared by coprecipitation: effect of ageing on catalyst performance

Copper manganese oxides are prepared using a coprecipitation procedure and studied for the oxidation of CO at ambient temperature. In particular, the effect of the ageing tune, i.e. the time that the precipitate remains in contact with the precipitating medium, is investigated. It is shown that this parameter is of crucial importance in controlling the catalytic performance and that catalysts which are aged for ⩽30 min or ⩾ 300 min give the best performance. Preliminary characterisation using powder X-ray diffraction indicates that a combination between CuO and copper manganese oxide may be responsible for the enhanced activity observed with these samples.

Hydration and acetoxylation of monoterpenes catalyzed by heteropoly acid

The liquid-phase hydration and acetoxylation of limonene (1), β-pinene (2) and α-pinene (3) catalyzed by dissolved or silica-supported heteropoly acid H3PW12O40 (PW) in acetic acid and acetic acid/water solutions have been studied. All three substrates give α-terpineol (4) as the main product along with α-terpenyl acetate (5). The reaction rate increases in the order: limonene<α-pinene<β-pinene. Synthetically useful homogeneous and heterogeneous acetoxylation and hydration of 1, 2 and 3 into 4 and 5 have been developed. At room temperature under optimized conditions, 2 and 3 form a mixture of 4 and 5 with 85% selectivity at 90% substrate conversion. 1 gives 4 and 5 with 85% selectivity at 50% conversion, with the main product being ester 5 in acetic acid (4/5≈30/70) and alcohol 4 in HOAc/H2O (90/10 v/v) solutions (4/5≈85/15). Virtually no oligomerization of monoterpenes occurs under the optimized conditions. The catalyst can be separated without neutralization and may be reused. The PW shows a much higher catalytic activity than conventional acid catalysts such as H2SO4 and Amberlyst-15.

High-activity Au/CuO–ZnO catalysts for the oxidation of carbon monoxide at ambient temperature

Ambient temperature CO oxidation has been studied with Au/ZnO, Au/CuO and Au/CuO–ZnO catalysts prepared by a coprecipitation method. The catalysts are highly active for CO oxidation and are capable of sustained activity at 20°C. The highest activity is observed with Au/ZnO and this catalyst is shown, using transmission electron microscopy, to be comprised of smaller Au metal particles (2–5 nm) and a higher Au dispersion when compared with the other catalysts.

Niobium peroxo compounds as catalysts for liquid-phase oxidation with hydrogen peroxide

Abstract The sodium tetraperoxo niobate, Na 3 [Nb(O 2 ) 4 ]·13H 2 O ( 1 ) was prepared by fusing Nb 2 O 5 with NaOH followed by reacting with aqueous H 2 O 2 and characterised by single-crystal X-ray analysis and TGA. 1 was tested for the epoxidation of cyclohexene and oxidation of alcohols with hydrogen peroxide as a homogeneous catalyst. The [Nb(O 2 ) 4 ] 3− anion was found to catalyse the epoxidation of cyclohexene with 30% aqueous H 2 O 2 in homogeneous solution giving cyclohexene oxide in a 73% selectivity and 7% cyclohexene conversion at 45°C. It also showed some catalytic activity for the homogeneous oxidation of alcohols (ethanol, isopropanol, 1- and 2-butanol) with H 2 O 2 . The [Nb(O 2 ) 4 ] 3− anion immobilised on a quaternary ammonium polystyrene resin was much less active in the above reactions than the homogeneous catalyst.

Structure, Chemistry and Activity of Well-Defined Cu-ZSM-5 Catalysts in The Selective Reduction of NOX

Abstract Three types of copper species have been identified in a series of Cu/ZSM-5 catalysts which have been studied for the selective catalytic reduction (SCR) of nitric oxide by hydrocarbon in the presence of excess oxygen. Electron spectroscopy and X-ray absorption spectroscopy have been used to show the presence of isolated copper species, (referred to as Type I), and small copper/oxygen aggregates, (Type 11) in catalysts prepared by ion exchange. Larger, copper oxide like aggregates (Type III) predominate in catalysts prepared by impregnation. Interconversion between Cu(I) and Cu(II) is facile for both Types I and II. and the oxidation state observed depends on the environment. Disruption of Type III has been observed with the proton form of the zeolite, and results in the formation of Type I and II species which gradually migrate into the zeolite channels. Interestingly. a physical mixture of Cu/Na-ZSM-5 and H-ZSM-5 is much more active than the individual components. The activity is attributed to migration of copper ions into the proton form of the zeolite. In general the turnover number for NO reduction increases with copper dispersion, however the catalytic reaction may be favoured by the inter convertibility of copper species I and II, and by their interaction with the acidic centres of the zeolite.

Probing the conformational properties of guest molecules in solid inclusion compounds via EXAFS spectroscopy: bromine K-edge EXAFS studies of the bromocyclohexane/thiourea and trans-1-bromo-2-chlorocyclohexane/thiourea inclusion compounds

Thiourea inclusion compounds comprise a thiourea host structure containing one-dimensional (1D) tunnels within which guest molecules are accommodated. Bromine K-edge EXAFS spectroscopy has been used to investigate local structrural properties of bromocyclohexane and trans-1-bromo-2-chlorocyclohexane guest molecules included within the thiourea tunnel structure.For the trans-1-bromo-2-chlorocyclohexane/thiourea inclusion compound, the intramolecular Br⋯Cl distance (ca. 4.50 A) determined from Br K-edge EXAFS spectra recorded between 295 and 80 K indicates that the trans-1-bromo-2-chlorocyclohexane guest molecules are in the diaxial conformation, and the intramolecular Br⋯C(3,5) distance (ca. 3.27 A) further confirms that the bromine substituent is in an axial position with respect to the cyclohexane ring. Furthermore, an intermolecular Br⋯Br distance is observed for this inclusion compound at 80 K, suggesting that there is head-to-head ordering of the guest molecules within the thiourea tunnel structure at this temperature.For the bromocyclohexane/thiourea inclusion compound, the intramolecular Br⋯C(3,5) distance (ca. 3.27 A) indicates that the major conformation of the bromocyclohexane guest molecules is the axial conformation. An intermolecular Br⋯Br distance is evident from the EXAFS data (at both 295 and 80 K), suggesting that there is head-to-head ordering of the bromocyclohexane guest molecules in this inclusion compound.

Effect of Al and Ca addition on the copper catalysed formation of silanes from Si and CH3Cl

Abstract The effect of Al and Ca addition on formation of silanes in the Rochow Direct Process is described and discussed. The catalysts are evaluated in a stirred-bed microreactor using on-line gas chromatographic analysis. The formation of silanes from the reaction of Si and CH 3 Cl is studied using a copper catalyst in the form of copper chloride. Initial tests using a typical silicon source showed that the reactor could be used to follow the consumption of Si with reproducibility of ±2%, and similar reproducibility was observed for product selectivity. The reactivity of a pure silicon and an industrial silicon sample were compared using the reactor and both samples showed identical reactivity and selectivity with respect to the amount of silicon consumed. Addition of Al metal (Cu:Al mole ratio=1.50, 0.30, 0.15) was found to significantly enhance reactivity and, to a much lesser extent, enhanced selectivity to dichlorodimethylsilane. Addition of CaCl 2 (Cu:Ca mole ratio=0.530, 0.105, 0.053) significantly decreased reactivity but had no marked effect on selectivity. Modification of the CuCl catalyst is reported in which CuCl is reacted with silanes and a form of CuCl was synthesised with significantly increased reactivity but with high selectivity to light products.

Enantioselection using modified zeolite catalysts

Abstract The proton form of zeolite Y was modified with R -1,3-dithiane-1-oxide at a loading of one molecule per supercage to create a chiral acid catalyst. The enantiomeric discrimination of this catalyst was demonstrated using the dehydration of the separate enantiomers of butan-2-ol and over the temperature range investigated the S -enantiomer was always more reactive. This catalyst system was then studied using computational simulation methods. The lowest energy structures for the enantiomers of butan-2-ol docked into a model of the modified zeolite were calculated and it was found that the binding energy for the S -enantiomer is 64.7 kJ mol − and that for the R -enantiomer is 48.3 kJ mol − . This difference in the adsorption of the two enantiomers is considered to be the origin of the enhanced reactivity of the S -enantiomer.