•Combined Cell for performing combined FTIR and gravimetric studies at the solid-gas interface•Quantification of absorption bands through determination of molar absorption coefficients.•Example application for oxygenates adsorbed on alumina and cobalt alumina catalysts•Good consistency and reproducibility for ratio of molar absorption coefficients CH2/CH3Absorption coefficients for the asymmetric stretching modes of CH3 and CH2 groups formed by adsorbing alkyl chained species from the vapour phase onto two different adsorbents; a γ-alumina support material and a supported metal catalyst have been determined using a custom made thermogravimetric-infrared cell. Results show that despite variations in the individually calculated absorption coefficients (ca. ± 20%), the ratio of the absorption coefficients (CH2:CH3) remained consistent despite employing adsorbates of varying chain length and functionality, and despite the choice of adsorbents which exhibited different surface areas and light scattering characteristics. The use of this absorption coefficient ratio has been shown to be applicable in the quantification of the average chain length of multiple adsorbed species of differing chain length. The potential for applying this to scenarios where reactions on surfaces are monitored is discussed.Download high-res image (154KB)Download full-size image

•Copper promoted the photocatalytic performance of titania for nitrate reduction.•XPS, TPR and FTIR confirm that copper was present as Cu2O.•Addition of activated carbon reduced reaction rates but improved selectivity to N2.•System is stable with no detected loss of copper species.Cu2O/TiO2 (1–10 wt% Cu2O) and 2.5% Cu2O/TiO2-AC (2.5–20 wt% AC) photocatalyst composites were synthesised by an ethanol reduction method. The materials were characterised by a number of techniques which confirmed the presence of Cu2O in contact with the TiO2. Pure TiO2 alone was not active for the simultaneous photocatalytic removal of nitrate and oxalic acid under conditions employed, however, photocatalytic activity was observed for TiO2 and TiO2/AC in the presence of Cu2O. This may have resulted from suppression of charge recombination via creation of a p-n heterojunction between Cu2O and TiO2. Within the series, 2.5% Cu2O/TiO2 exhibited the best photocatalytic performance with 57.6 and 99.8% removal of nitrate and oxalic acid, respectively, with selectivities of 45.7, 12.4 and 41.9% to NH4+, NO2− and N2, respectively after 3 h. For the carbon containing photocatalysts, 2.5% Cu2O/TiO2-2·5AC displayed the highest activity with 42.5 and 96.6% removal of nitrate and oxalic acid, respectively, with 32.7, 11.6 and 55.7% selectivities to NH4+, NO2− and N2, respectively. The highest AC loading tested resulted in selectivity to NH4+ of 21.6 with no NO2− detected, together with an improved N2 selectivity (78.4%) albeit at lower (12.7%) nitrate conversion. Data suggests that Cu2O/TiO2 can be used in the photocatalytic reduction of nitrate and improved selectivity towards N2 can be attained by influencing factors which control the relative rate of oxalic acid consumption.Download high-res image (102KB)Download full-size image

Rechargeable aluminum batteries (RABs) are amongst the most promising post-lithium energy storage systems (ESS) with a substantially higher specific volumetric capacity (8046 mA h cm−3), higher safety and lower cost. The development of such efficient and low cost ESSs is essential in order to meet the future energy storage demands of modern society. In recent years, a number of research articles have been reported on the evolution of cathode materials for RABs, which makes a critical review timely in order to provide inspiration for future research. This article highlights the cathode materials developed specifically for RABs, in detail, the development of carbon-based cathode materials, and then that of transition metal oxide (TMO), sulfide and chloride based cathode materials and then finally, a few other cathode materials are also discussed. Accordingly, future perspectives and opportunities are highlighted.

•Pd4S based catalyst shows both high activity and stability.•Excellent performance retained at pressures of industrial relevance.•80–85% alkene selectivity obtained from mixed alkyne/alkene C2 and C3 feeds.•High selectivity attributed to ‘site isolation’ of Pd atoms in Pd4S structure.The Pd4S phase of palladium sulfide is known to be a highly selective alkyne hydrogenation catalyst at atmospheric pressure. Results presented here demonstrate that high selectivity can be retained at the elevated pressures required in industrial application. For example, in a mixed acetylene/ethylene feed, 100% conversion of acetylene was attained with a selectivity to ethylene in excess of 80% at 18 bar pressure. Similarly, almost 85% selectivity can be obtained with mixed C3 feeds containing methyl acetylene, propadiene, propylene and propane at 18 bar pressure. Using a low loaded sample (0.1 wt% Pd) it was possible to estimate the TOF to be 27 s−1. High selectivity was related to the crystal structure of Pd4S with the unique spatial arrangement thought to favour Pd atoms acting in isolation from one another. Based on these results, it is proposed that this catalyst could be a potential replacement for PdAg alloys currently used by industry.Download high-res image (89KB)Download full-size image

•Quantitative FTIR analysis of Fischer-Tropsch catalysts under reaction conditions.•Location of adsorbed species on either catalyst metal or support is distinguished.•Quantification of amount and chain length of hydrocarbon species is determined.•Short chained species can migrate to the support via the gas phase.•Spillover of hydrocarbon species to support can occur over large (>100 nm) distances.In-situ FTIR spectroscopy was employed to investigate the location, chain length and quantity of hydrocarbon species adsorbed on supported-cobalt Fischer-Tropsch catalysts. The length of the hydrocarbon units observed was quantified using an appropriately determined absorption coefficient ratio. The individual amounts of CH2 and CH3 groups were calculated with absorption coefficients derived specifically for adsorbed hydrocarbon species, unlike previous studies, which employ absorption coefficients derived from liquid phase hydrocarbons. Results show that it is possible for reaction products to re-adsorb from the gas phase onto the support as well as spillover to the support from the active metal cobalt. Qualification and quantification of the chain length of these re-adsorbed species has shown that the support material (γ-alumina) selectively re-adsorbs shorter chain length species from the gas phase with a different functional group to the majority of observable species on a Co/Al2O3 catalyst. Comparison of Co/Al2O3 with Co/SiO2, which utilises a more inert support relative to γ-alumina, shows that longer chained species are located on the cobalt metal itself during reaction and can be transported to the γ-alumina support via a process of spillover.Download high-res image (70KB)Download full-size image

•Combined quantitative FTIR and on-line GC analysis of Fischer-Tropsch catalysts.•Various reaction conditions and Fischer-Tropsch active metals investigated.•Spectroscopically observed species ascribed to adsorbed reaction products.•Species located on γ-alumina support are thought to be oxygenated products.•Spectroscopically observed species do not directly link to reaction products.A method for the quantification of the chain length of surface hydrocarbon species on Fischer-Tropsch catalysts by in-situ FTIR was combined with online gas chromatography in order to investigate any potential relationship between adsorbed hydrocarbons species and those detected as reaction products. By varying the reaction conditions and the active metal, changes to the chain lengths of surface species and the product distribution were monitored. Results show that the species observed spectroscopically are predominantly adsorbed reaction products which do not directly relate to the detected reaction products. Hydrocarbon species which can be transported to the support material (γ-alumina) may be ascribed to oxygenated products which are not produced in detectable amounts by GC at low conversion but are selectively adsorbed from the gas phase.Download high-res image (71KB)Download full-size image

The structural stability of a Cu-based metal–organic framework (MOF), subject to different conditions including exposure to ambient air and liquid water, was investigated. A detailed characterization of the substrate was performed using FTIR, XRD, SEM, N2 adsorption, and TGA. Cu3(btc)2 was found to be stable after exposure to ambient air for short periods but undergoes irreversible changes during long-term exposure. These changes are not only manifested in terms of structural modifications as determined by XRD and FTIR data but also suggested by an altered morphology as observed by electron microscopy. Slow hydrolysis reactions initially involving a weakening of the metal–ligand bonds are identified as the main mechanism for the irreversible degradation of the Cu3(btc)2. The length of exposure and the amount of water were found to be the key parameters that determine the stability of the MOF.

Molybdenum carbide on sulfated zirconia was prepared by impregnation of MoO3 on sulfated zirconia to give a loading of 5 wt% followed by carburisation at 923 K in a mixture of CH4/H2 (4:1). The resulting catalyst was characterised by N2 adsorption–desorption, CO chemisorption, FTIR of pyridine adsorption, XRD, TPR, TGA, Raman and SEM–EDX. This combination of characterisation studies suggests formation of a well-dispersed Mo2C phase over tetragonal zirconia. When employed in the hydroconversion of n-heptane, high temperature and low space velocity lead to substantial cracking. However, under some conditions, an increase in the research octane number (RON) from 0 (n-heptane) to ca. 50 was attained. Between 723 and 873 K, n-heptane is mainly cracked to iso-pentane and ethane. The product distribution as a function of conversion suggests that the reaction did not simply follow a consecutive reaction pathway, but that other parallel routes were involved.

Pd doped Cu catalysts have been prepared by co-impregnation, sequential impregnation and a colloidal approach. In each case, the Cu:Pd ratio was optimised leading to catalyst activity which exceeded that offered by monometallic Cu at low temperature (393 K and below) but with a product selectivity which suggests the reaction is still taking place on a Cu surface (i.e., high ethylene selectivity). Pd is therefore thought to influence hydrogen dissociation rates and enhance spillover onto Cu sites. Catalytic testing under more demanding conditions showed differences between the preparation methods. In general, the most active of the samples appeared to be the least selective and vice-versa. Under optimised conditions, a 50:1 Cu:Pd ratio prepared by sequential impregnation showed an ethylene selectivity of 80% at 98% conversion at only 353 K. Further testing under competitive conditions suggested good ethylene selectivity could be retained under industrially relevant conditions in the absence of CO.

Pd/TiO2 catalysts were modified by exposure to triphenylphosphine to assess the impact of the presence of the ligand on the selective removal of acetylene from ethylene rich feeds. The ligand modified metal created a catalyst which showed significantly improved selectivity as a result of a decrease in the extent of over-hydrogenation of ethylene to ethane. The physical presence of the ligand modifier is thought to create sites which permit adsorption of acetylene but hinder access of ethylene to metal sites. In addition, the rate of acetylene hydrogenation appeared to be enhanced in the presence of the modifier consistent with a promotional effect. A potential contribution to enhanced selectivity due to suppression of sub-surface hydrogen formation/diffusion cannot be discounted.

Recent advances with Pd containing catalysts for the selective hydrogenation of acetylene are described. The overview classifies enhancement of catalytic properties for monometallic and bimetallic Pd catalysts. Activity/selectivity of Pd catalysts can be modified by controlling particle shape/morphology or immobilisation on a support which interacts strongly with Pd particles. In both cases enhanced ethylene selectivity is generally associated with modifying ethylene adsorption strength and/or changes to hydride formation. Inorganic and organic selectivity modifiers (i.e., species adsorbed onto Pd particle surface) have also been shown to enhance ethylene selectivity. Inorganic modifiers such as TiO2 change Pd ensemble size and modify ethylene adsorption strength whereas organic modifiers such as diphenylsulfide are thought to create a surface template effect which favours acetylene adsorption with respect to ethylene. A number of metals and synthetic approaches have been explored to prepare Pd bimetallic catalysts. Examples where enhanced selectivity is observed are generally associated with decreased Pd ensemble size and/or hindering of the ease with which an unselective hydride phase is formed for Pd. A final class of bimetallic catalysts are discussed where Pd is not thought to be the primary reaction site but merely acts as a site where hydrogen dissociation and spillover occurs onto a second metal (Cu or Au) where the reaction takes place more selectively.

Use of titania based photocatalysts for the selective reduction of nitrates to nitrogen through photocatalytic methods has attracted some interest in recent years and the need to operate this process remotely and without energy sources will ensure that a level of interest will remain in attempting to improve this process. There remain numerous avenues that have not yet been investigated in the effort to optimise a catalyst for this reaction. Work on metal containing systems offer the potential to; improve activity through offering a route to improved electron-hole separation, manipulate selectivity by controlling particle size which affects the flat band potential under Fermi equilibration conditions and for operation in the visible region by manipulating particle shape and using SPR properties. Opportunities to further manipulate all three of these situations by using bimetallic particles using core-shell, true alloy and other forms, remains open for exploration. The introduction of hetero-species into bulk titania in order to reduce the band gap and permit operation within the visible region. The key issues in the case of the reaction involved would be to retain the oxidation potential of the valance band but to sacrifice the reduction potential of the conduction band thus permitting nitrate reduction but avoiding competition for electrons by protons in solution. In a more general approach, the key challenge should be to obtain a route to uniform bulk doping without the need to use complex ion implantation routes. Composites offer the opportunity to manipulate adsorption capacity although as yet, attempts to improve nitrate adsorption with respect to other organic anions has not been attempted. They may also be used to improve electron transfer properties and to enhance charge-carrier lifetimes.

The addition of Au as a promoter/modifier for alumina supported Co catalyst has been studied by combined in-situ high temperature, high pressure Fourier transform infrared (FTIR) and on-line gas chromatography. The combination of these tools permitted the state of the active catalyst surface to be monitored while following the elution of reaction products during the first 5–7 h on stream of the catalyst. The catalysts under study were a 10%Co/Al2O3 and a 2.5%Au/10%Co/Al2O3. Samples were characterised before use using Raman and temperature programmed reduction (TPR). During the initial stages of reaction, hydrocarbons were built up on the surface of the catalyst as monitored by FTIR and the nature and amount of these species were assessed in terms of CH2/CH3 ratio and the density of these alkyl fragments by employing absorption coefficients for the individual components. The nature and reducibility of the Co particles were modified by the presence of Au while the later also shifted the CO/H2 balance by acting as an effective water-gas shift catalyst during the early stages of reaction. This characteristic was lost during reaction as a consequence of redistribution of the two metallic phases.

The selective hydrogenation of acetylene from ethylene rich streams was conducted at high pressure and in the presence of CO over two 1 wt% loaded Pd/TiO2 catalysts with differing dispersions. Although, the more poorly dispersed sample did not result in high acetylene conversion only a small proportion of the total available ethylene was hydrogenated to ethane. The more highly dispersed sample was able to remove acetylene to a level below the detection limit but this was at the expense of significant proportion (ca. 30%) of the available ethylene. Modification of the catalysts by exposure to triphenyl phosphine or diphenyl sulfide and subsequent reduction at 393 K led to improved performance with increased conversion of acetylene and decreased propensity to hydrogenate ethylene resulting in an overall net gain in ethylene. The higher dispersed sample which had been ligand modified provided the best results overall and in particular for the diphenyl sulfide treated sample which was able to completely eliminate acetylene and still obtain a net gain in ethylene. The differences observed are thought to be due to the creation of appropriate active ensembles of Pd atoms which are able to accommodate acetylene but have limited ability to adsorb ethylene. Sub-surface hydrogen formation was suppressed, but not eliminated, by exposure to modifier.

A carbided molybdena on sulfated zirconia support has been prepared by in situ exposure to a methane/hydrogen mixture at 650 °C and the characteristics established using N2 adsorption and X-ray diffraction. The activity of the catalyst in the hydroconversion of C6 to C9n-alkanes was investigated in the temperature range 350–450 °C at 1 atm pressure. Activity and selectivity were found to show strong dependence on the choice of n-alkane with reaction rates found to be higher for the shorter alkanes. n-Hexane and n-heptane were hydroisomerised to the corresponding C6 and C7 isomers with high selectivity while the higher alkanes, especially nonane, produced mainly hydrocracking products. The catalyst offers a potential system for the upgrading of light naphtha range paraffins.

A low loading of Rh (0.5 wt%) was added to a MoO3/ZrO2 sample with the objective of lowering the temperature at which the molybdena phase could be transformed in the presence of CH4/H2 to produce an active carbide phase for the hydroisomerisation reaction of linear alkanes. The presence of Rh reduced the reduction temperature of the supported molybdena in hydrogen alone and reduced the temperature required for the onset of carburisation in hydrogen/methane. Pre-treatment cycles of reduction and oxidation further enhanced the extent of rhodium–molybdena interactions and further lowered the temperature required to form the carbidic phase. Although the presence of rhodium facilitated the formation of an active molybdenum carbide phase, the products formed in the hydroisomerisation reactions of both hexane and octane were mainly hydrogenolysis products due to the high activity of rhodium for these reactions under the conditions employed. The distribution of the fragments were different for sample carbided after calcination and sample carbided after a reduction–oxidation cycle suggesting a surface composition which was significantly different for the samples prepared by the two different routes.

Modification of Pd/TiO2 catalyst by adsorption of triphenylphosphine and phenyl sulfide leads to markedly enhanced selectivity for acetylene hydrogenation in the presence of ethylene and excess hydrogen. Similar selectivities were maintained in cases where ligand decomposition took place and sulfur was retained on the catalyst surface.

Preparation of india−titania mixed oxide via sol−gel procedures leads to the formation of high surface area materials in which the india is highly dispersed throughout the parent oxide. Although the resultant mixed oxide did not exhibit textural properties which are significantly different from a sol−gel-derived single oxide titania, the surface properties and adsorptive characteristics were notably altered. In particular, the modified oxide exhibited greater propensity to adsorb water, with a portion being dissociatively adsorbed, possibly facilitated by the much greater interaction observed for the modified oxide (favored by an enhanced acidity of the surface caused by the presence of india). Water was able to displace carbon dioxide adsorbed as carbonate from the mixed-oxide surface. Results are also discussed in terms of the potential use of this material as a support for gold catalysts.

The transesterification of two vegetable oils containing different quantities of free fatty acid have been compared over a series of BaO/Al2O3 catalysts with a range of baria loadings/dispersions. Dispersion of baria on the alumina was determined by pulse chemisorption of carbon dioxide. Limited agreement was found between the numbers of exposed sites for CO2 adsorption and the reaction rate and the rates measured were different for the two oils. The latter was unexpected as the rate determining step appears to involve only the activated adsorption of methanol, consistent with the change in rate measured when methanol was replaced by ethanol. Differences between the behaviour of the two oils and the lack of correlation between rates and available basic sites can both be accounted for by the strong dissociative adsorption of the free fatty acid which results in a less active catalyst for the transesterification of the triglyceride. Higher dispersed samples show less sensitivity to free fatty acid and give the highest rate per exposed surface site.

Sulphonato−salen complexes containing Mn(III), Co(III), and Fe(III) metal centers intercalated into Zn/Al layered double-hydroxide (LDH) host have been synthesized and tested as catalysts for the epoxidation of cyclohexene and dicyclopentadiene using pivaldehyde and molecular oxygen at atmospheric pressure and room temperature. Oxidation of cyclohexene gave cyclohexene oxide and 2-cyclohexen-1-one as reaction products, whereas dicyclopentadiene was transformed to the corresponding monoepoxide. Selectivity was found to depend on the central metal ion. Activity (TOF) increased according to LDH−[Fe(Cl)(salen)] < LDH−[Co(Cl)(salen)] < LDH−[Mn(Cl)(salen)] in both case of cyclohexene and dicyclopentadiene epoxidation. The structures of the metal(III) sulfonato−salen complexes were modeled by density functional theory calculations in order to compare differences in dimensions and geometries with differences in gallery height and catalytic behavior.

The ammonia SCR reaction has been studied using catalytic and infrared spectroscopic methods and the results compared for three different iron containing catalysts supported on an alumina, a titania and a beta zeolite. The order of SCR performance was Fe/zeolite ≫ Fe/TiO2 > Fe/Al2O3 which was in the same order as the capacity to adsorb ammonia. No evidence was found to suggest that the presence of either Brønsted or Lewis acidity is key to the activation of ammonia. NO was readily oxidised to NO2 over the Fe containing samples as indicated by the formation of nitrate even at low temperatures. The presence of water led to reduced propensity to form nitrates but did not affect the selectivity to N2O.

By conducting the catalytic hydrogenation over water miscible Pd/carbon nanofibre catalysts, selective ring hydrogenation of aromatic acids can be performed in aqueous solution without the need to provide protection for the external acid function.

•CuPd bimetallic catalysts prepared with a Cu rich surface.•Evidence of hydrogen dissociation on Pd and spillover onto Cu surface.•Selective hydrogenation of acetylene at industrially applicable temperatures.•80% ethylene selectivity at >99% conversion at only 363 K.•Ready replacement for industrial PdAg catalysts.A Cu/alumina catalyst has been modified by addition of various quantities of Pd. Characterisation suggests formation of a bimetallic with the surface dominated by Cu. Optimisation of the Cu:Pd ratio (50:1) resulted in a catalyst which combined the properties of Cu and Pd (high ethylene selectivity and activity at low temperature). Using a 3:1 H2:acetylene feed, >99% acetylene conversion and >70% ethylene selectivity was attained at 373 K, representing a considerable reduction in the temperature necessary to achieve equivalent activity/selectivity over monometallic Cu. Increasing the H2:acetylene ratio to 10 resulted in >99% acetylene conversion at 363 K as well as enhanced ethylene selectivity (>80%). Enhanced activity of Cu at low temperature is attributed to H2 dissociation on Pd with spillover onto Cu sites where the reaction takes place. These bimetallic CuPd catalysts offer sufficient activity at low temperature and could represent a replacement to the current industrial PdAg catalysts.Download high-res image (202KB)Download full-size image

The photocatalytic degradation of nitrates in aqueous solution has been examined using different Au/TiO2 photocatalysts and using oxalic acid as hole scavenger. Although complete elimination of oxalic acid was possible under conditions employed, complete nitrate removal was not achieved and the extent of conversion showed a dependence on hole scavenger concentration. The reacting stoichiometry was greater than predicted from the corresponding redox equations and suggests competitive reaction between protons and nitrate for conduction band electrons. The different photocatalysts exhibited different temperature sensitivities with the Hombikat supported Au showing the greatest apparent activation energy.Graphical abstractAu promotes titania the simultaneous removal of nitrate and oxalic acid from aqueous solution.Download high-res image (98KB)Download full-size imageHighlights► Au/TiO2 is active for the simultaneous photocatalytic degradation of nitrate and carboxylic acids from aqueous solution. ► Nitrate is reduced in competition with protons for photogenerated electrons at the Au surface. ► At low reaction temperatures, CO2 generated as a product inhibits the reaction and the strength of adsorption depends on the characteristics of the titania support.

Pd/TiO2 catalysts have been exposed to diphenyl sulphide to assess the impact of the modifier on the selective removal of acetylene from ethylene rich feeds. The modified catalysts show a significantly reduced rate of ethylene hydrogenation without noticeably impacting on the rate of acetylene hydrogenation. Even after reduction at 393 K, which led to the loss of the phenyl ligands, the sulphur retained by the Pd was still capable of maintaining this enhanced selectivity of the catalyst to ethylene as opposed to ethane. Results are interpreted in terms of a templating effect on the active Pd surface as a result of the original ligand adsorption pattern that creates appropriate ensembles which allow alkyne adsorption and reaction but severely limits adsorption and reaction of the alkene. A potential contribution to this enhanced selectivity by suppression of subsurface hydrogen formation cannot be discounted.Graphical abstractDiphenylsulfide and its sulfur residue was found to modify Pd/TiO2 catalysts leading to significantly enhanced selectivity in acetylene hydrogenation in the presence of ethylene.Download high-res image (58KB)Download full-size imageHighlights► Diphenyl sulphide-modified Pd/TiO2 catalysts enhances C2H2 hydrogenation selectivity. ► Sulphur residue after ligand decomposition retains improved selectivity. ► Selectivity is based on ensembles which accommodate C2H2 but not C2H4.

Two Pd/Al2O3 catalysts of different loadings and dispersions were modified by the addition of various amounts of Bi and studied in the hydrogenation of 1-hexyne and 2-hexyne and in the consecutive reactions of the products formed. Catalyst behaviour was compared with a commercial Lindlar catalyst and Pb-free Pd/CaCO3. Results are consistent with a preference of Bi to occupy step and edge sites while leaving the terraces and extended facets relatively unaffected. Results show that while Bi had little effect on the rate of the 1-hexyne hydrogenation, the rate of subsequent reactions of the 1-hexene formed were suppressed. In this context, Bi was a more effective modifier than Pb. This situation was reversed when reactions were performed using 2-hexyne. Results are discussed in terms of the reaction mechanism and key intermediates in the process and the roles of bismuth, relative to lead, in creating an appropriate ensemble of surface Pd atoms to permit control of selectivity.Bismuth atoms preferentially occupying step sites on a Pd surface, forcing reaction onto the open terrace sites and modifying selectivity in hydrogenation reactions.Download high-res image (40KB)Download full-size image

The hydrogenation of aromatic acids and other substituted phenyls has been studied for two palladium-based catalysts; one supported on carbon nanofibres (CNFs) and the other on a steam-activated carbon. The reactions were conducted in both aqueous solution and aprotic organic solvents. The major product over both catalysts was the same irrespective of the substrate indicating that support characteristics and Pd dispersion play at most only a minor role in defining the reaction pathway. The key factor in determining the major reaction product resulting from either preferential hydrogenation of the aromatic ring, or reaction of the external functional group, was the extent to which the external group interacted with water molecules which acted in some cases to protect the external function from interaction with the metal surface and induced selective reduction of the aromatic ring.Enhanced ring relative to functional group hydrogenation was induced using water as a solvent due to solvation of the polar functional group, which orientates the latter away from the surface.Download high-res image (56KB)Download full-size image

Modification of Pd/TiO2 catalyst by adsorption of triphenylphosphine and phenyl sulfide leads to markedly enhanced selectivity for acetylene hydrogenation in the presence of ethylene and excess hydrogen. Similar selectivities were maintained in cases where ligand decomposition took place and sulfur was retained on the catalyst surface.

Pd/TiO2 catalysts were modified by exposure to triphenylphosphine to assess the impact of the presence of the ligand on the selective removal of acetylene from ethylene rich feeds. The ligand modified metal created a catalyst which showed significantly improved selectivity as a result of a decrease in the extent of over-hydrogenation of ethylene to ethane. The physical presence of the ligand modifier is thought to create sites which permit adsorption of acetylene but hinder access of ethylene to metal sites. In addition, the rate of acetylene hydrogenation appeared to be enhanced in the presence of the modifier consistent with a promotional effect. A potential contribution to enhanced selectivity due to suppression of sub-surface hydrogen formation/diffusion cannot be discounted.

Pd doped Cu catalysts have been prepared by co-impregnation, sequential impregnation and a colloidal approach. In each case, the Cu:Pd ratio was optimised leading to catalyst activity which exceeded that offered by monometallic Cu at low temperature (393 K and below) but with a product selectivity which suggests the reaction is still taking place on a Cu surface (i.e., high ethylene selectivity). Pd is therefore thought to influence hydrogen dissociation rates and enhance spillover onto Cu sites. Catalytic testing under more demanding conditions showed differences between the preparation methods. In general, the most active of the samples appeared to be the least selective and vice-versa. Under optimised conditions, a 50:1 Cu:Pd ratio prepared by sequential impregnation showed an ethylene selectivity of 80% at 98% conversion at only 353 K. Further testing under competitive conditions suggested good ethylene selectivity could be retained under industrially relevant conditions in the absence of CO.

Use of titania based photocatalysts for the selective reduction of nitrates to nitrogen through photocatalytic methods has attracted some interest in recent years and the need to operate this process remotely and without energy sources will ensure that a level of interest will remain in attempting to improve this process. There remain numerous avenues that have not yet been investigated in the effort to optimise a catalyst for this reaction. Work on metal containing systems offer the potential to; improve activity through offering a route to improved electron-hole separation, manipulate selectivity by controlling particle size which affects the flat band potential under Fermi equilibration conditions and for operation in the visible region by manipulating particle shape and using SPR properties. Opportunities to further manipulate all three of these situations by using bimetallic particles using core-shell, true alloy and other forms, remains open for exploration. The introduction of hetero-species into bulk titania in order to reduce the band gap and permit operation within the visible region. The key issues in the case of the reaction involved would be to retain the oxidation potential of the valance band but to sacrifice the reduction potential of the conduction band thus permitting nitrate reduction but avoiding competition for electrons by protons in solution. In a more general approach, the key challenge should be to obtain a route to uniform bulk doping without the need to use complex ion implantation routes. Composites offer the opportunity to manipulate adsorption capacity although as yet, attempts to improve nitrate adsorption with respect to other organic anions has not been attempted. They may also be used to improve electron transfer properties and to enhance charge-carrier lifetimes.

The selective hydrogenation of acetylene from ethylene rich streams was conducted at high pressure and in the presence of CO over two 1 wt% loaded Pd/TiO2 catalysts with differing dispersions. Although, the more poorly dispersed sample did not result in high acetylene conversion only a small proportion of the total available ethylene was hydrogenated to ethane. The more highly dispersed sample was able to remove acetylene to a level below the detection limit but this was at the expense of significant proportion (ca. 30%) of the available ethylene. Modification of the catalysts by exposure to triphenyl phosphine or diphenyl sulfide and subsequent reduction at 393 K led to improved performance with increased conversion of acetylene and decreased propensity to hydrogenate ethylene resulting in an overall net gain in ethylene. The higher dispersed sample which had been ligand modified provided the best results overall and in particular for the diphenyl sulfide treated sample which was able to completely eliminate acetylene and still obtain a net gain in ethylene. The differences observed are thought to be due to the creation of appropriate active ensembles of Pd atoms which are able to accommodate acetylene but have limited ability to adsorb ethylene. Sub-surface hydrogen formation was suppressed, but not eliminated, by exposure to modifier.

Rechargeable aluminum batteries (RABs) are amongst the most promising post-lithium energy storage systems (ESS) with a substantially higher specific volumetric capacity (8046 mA h cm−3), higher safety and lower cost. The development of such efficient and low cost ESSs is essential in order to meet the future energy storage demands of modern society. In recent years, a number of research articles have been reported on the evolution of cathode materials for RABs, which makes a critical review timely in order to provide inspiration for future research. This article highlights the cathode materials developed specifically for RABs, in detail, the development of carbon-based cathode materials, and then that of transition metal oxide (TMO), sulfide and chloride based cathode materials and then finally, a few other cathode materials are also discussed. Accordingly, future perspectives and opportunities are highlighted.