Alan J. McCue

Recent advances in selective acetylene hydrogenation using palladium containing catalysts

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.

Sulfur as a catalyst promoter or selectivity modifier in heterogeneous catalysis

For most heterogeneous catalysts adsorbed sulfur and sulphur-containing molecules are generally regarded as a poison. However, a growing number of reports from academia and industry demonstrate that sulfur may act as an activity promoter or selectivity modifier in heterogeneous catalysis. Reports from a variety of areas such as Fischer–Tropsch synthesis, catalytic reforming, regio and chemoselective hydrogenation as well as CO oxidation, hydrocarbon oxidation and NOx reduction are discussed. Based on the literature cited, comments on the levels of sulfur necessary to observe a promotional effect are suggested/proposed, as well as some of the techniques which can be applied to understand such effects.

Optimisation of preparation method for Pd doped Cu/Al2O3 catalysts for selective acetylene hydrogenation

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.

Triphenylphosphine: a ligand for heterogeneous catalysis too? Selectivity enhancement in acetylene hydrogenation over modified Pd/TiO2 catalyst

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.

Photocatalytic Activity of Doped and Undoped Titanium Dioxide 32 Nanoparticles Synthesised by Flame Spray Pyrolysis

*Email: felicity.sartain@bio-nano-consulting.com The photocatalytic activities of a series of titanium dioxide (TiO2) based nanoparticles, synthesised via flame spray pyrolysis (FSP), have been investigated and compared with the commercially available Evonik Aeroxide TiO2 P 25 (P 25). The effects of metal ions aluminium, tin and platinum, respectively, on the physical and chemical properties of the TiO2 nanoparticles are reported. The set of six samples were characterised by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), inductively coupled plasma-mass spectrometry (ICP-MS) and ultraviolet-visible (UV-vis) diffuse reflectance spectroscopy. Specific surface areas were determined using nitrogen adsorption and desorption measurements. Subsequent photocatalytic studies of the degradation of methyl orange (MO) dye under UV irradiation demonstrated that addition of Al and Sn had a negative effect on catalytic performance, whereas the addition of ≤0.7 at% Pt to each sample enhanced photocatalytic activity. Most interestingly, the Pt-doped composite samples (TiO2-Sn/Pt and TiO2-Al/Pt) both showed a significantly higher rate of degradation of MO, when compared to P 25. All Pt-doped samples show an increased visible photon absorption capacity. The relationships between the physical and chemical characteristics are discussed in relation to photocatalytic performance.

Gold modified cobalt-based Fischer-Tropsch catalysts for conversion of synthesis gas to liquid fuels

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.

Quantification of hydrocarbon species on surfaces by combined microbalance-FTIR

Absorption 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.

Confirmation of Chirality in Homogeneous and Heterogeneous Salen‐Based Catalysts

The extent to which diastereomeric excess (de) of epoxide products is influenced by the structure of the chiral olefin substrate has been assessed for two enantiomers of limonene and two enantiomers of pinene and using a number of homogeneous and heterogeneous [Mn(salen)]‐based catalysts, based on both chiral and achiral structures. No evidence was obtained to suggest that double asymmetric induction took place, but instead results indicate that the stereoselectivity of the epoxide product was governed wholly by the substrate. Results call into question the use of such substrates alone in drawing conclusions regarding the nature of presumed stereogenic centers in such catalysts.

Rapid-Scan Operando Infrared Spectroscopy

Novel methodology, referred to as rapid‐scan operando, has been demonstrated to be a highly powerful tool for studying reaction mechanisms in heterogeneous catalysis, because it combines rapid scan for IR monitoring in the milliseconds time frame and operando methodology. As proof of concept, the NOx–CO and NOx–H2 reactions were studied as model catalyzed reactions with practical interest for removal of NOx in diesel exhausts. Rapid‐scan operando experiments confirmed state‐of‐the‐art knowledge concerning the reaction mechanisms and, more importantly, allowed us to elucidate, for the first time, the different roles of the surface hydroxy groups depending on the reductant used (CO or H2). Moreover, this new tool was used to distinguish the behavior of carbonates and nitrites under reaction conditions that could not be monitored by conventional IR spectroscopy approaches owing to overlap of their absorbance bands.

Carbon Capture by Metal Oxides: Unleashing the Potential of the (111) Facet

Solid metal oxides for carbon capture exhibit reduced adsorption capacity following high-temperature exposure, due to surface area reduction by sintering. Furthermore, only low-coordinate corner/edge sites on the thermodynamically stable (100) facet display favorable binding toward CO2, providing inherently low capacity. The (111) facet, however, exhibits a high concentration of low-coordinate sites. In this work, MgO(111) nanosheets displayed high capacity for CO2, as well as a ∼65% increase in capacity despite a ∼30% reduction in surface area following sintering (0.77 mmol g-1 @ 227 m2 g-1 vs 1.28 mmol g-1 @ 154 m2 g-1). These results, unique to MgO(111), suggest intrinsic differences in the effects of sintering on basic site retention. Spectroscopic and computational investigations provided a new structure-activity insight: the importance of high-temperature activation to unleash the capacity of the polar (111) facet of MgO. In summary, we present the first example of a faceted sorbent for carbon capture and challenge the assumption that sintering is necessarily a negative process; here we leverage high-temperature conditions for facet-dependent surface activation.