Dr David Watson

The bi-reforming of methane (BRM) has the advantage of utilising greenhouse gases and producing H2 rich syngas. In this work Ni stabilised in a pyrochlore-double perovskite structure is reported as a viable catalyst for both Dry Reforming of Methane (DRM) and BRM. A 10 wt.% Ni-doped La2Zr2O7 pyrochlore catalyst was synthesised, characterised and tested under both reaction conditions and its performance was compared to a supported Ni/La2Zr2O7. In particular the effect of steam addition is investigated revealing that steam increases the H2 content in the syngas but limits reactants conversions. The effect of temperature, space velocity and time on stream was studied under BRM conditions and brought out the performance of the material in terms of activity and stability. No deactivation was observed, in fact the addition of steam helped to mitigate carbon deposition. Small and well dispersed Ni clusters, possibly resulting from the progressive exsolution of Ni from the mixed oxide structure could explain the enhanced performance of the catalyst.

Enantioselective heterogeneous hydrogenation of C=O bonds is of great potential importance in the synthesis of chirally pure products for the pharmaceutical and fine chemical industries. One of the most widely studied examples of such a reaction is the hydrogenation of β-ketoesters and β-diketoesters over Ni-based catalysts in the presence of a chiral modifier. Here we use scanning transmission X-ray microscopy combined with near-edge X-ray absorption fine structure spectroscopy (STXM/NEXAFS) to investigate the adsorption of the chiral modifier, namely (R,R)-tartaric acid, onto individual nickel nanoparticles. The C K-edge spectra strongly suggests that tartaric acid deposited onto the nanoparticle surfaces from aqueous solutions undergoes a keto-enol tautomerisation. Furthermore, we are able to interrogate the Ni L2,3-edge resonances of individual metal nanoparticles which, combined with X-ray diffraction (XRD) patterns showed them to consist of a pure nickel phase rather than the more thermodynamically stable bulk nickel oxide. Importantly, there appears to be no “particle size effect” on the adsorption mode of the tartaric acid in the particle size range ~90 - ~300 nm.

Background and hypothesis: Humic acid (HA) is of considerable environmental significance, being a major component of soil, as well as being considered for application in other technological areas. However, its structure and colloidal properties continue to be the subject of debate, largely owing to its molecular complexity and association with other humic substances and mineral matter. As a class, HA is considered to comprise supramolecular assemblies of heterogeneous species, and herein we consider a simple route for the separation of some HA sub-fractions. Experiments: A commercial HA sample from Sigma-Aldrich has been fractionated into two soluble (S1, S2) and two insoluble (I1, I2) fractions by successive dissolution in deionized water at near-neutral pH. These sub-fractions have been characterized by solution and solid-state approaches. Findings: Using this simple approach, the HA has been shown to contain non-covalently bonded species with different polarity and water solubility. The soluble and insoluble fractions have very different chemical structures, as revealed particularly by their solid-state properties (13C NMR and IR spectroscopy, and TGA); in particular, S1 and S2 are characterized by higher carbonyl and aromatic contents, compared with I1 and I2. As shown by solution SAXS measurements and AFM, the soluble fractions behave as hydrophilic colloidal aggregates of at least 50 nm diameter.

Plasma processing, as a commercial and large-scale technology, was used to functionalize few-layer graphene (FLG) and multi-walled carbon nanotubes (MWCNT) in this work. The successful functionalities of FLG and MWCNT have been confirmed by elemental microanalysis, X-ray photoelectron spectroscopy, acid-base titration and zeta potential measurements. With the assistance of poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT/PSS), a water-dispersible and conductive polymer, a composite of functionalized FLG and MWCNT was fabricated into large-size flexible films and also interdigitated microelectrodes for microsupercapacitor application via simple and scalable techniques (i.e. doctor blading and laser-etching). When normalised by volume and area, the device made from FLG(NH3)-MWCNT(Acid) (19.9 F cm-3 at 5 mV s-1 and 12.2 F cm-3 at 200 mV s-1) and FLG-MWCNT(Acid) (19.5 mF cm-2 at 5 mV s-1 and 12.8 mF cm-2 at 200 mV s-1) show the best performing composites, respectively, indicating how effective functionalization of FLG and MWCNT is for the enhancement of electrochemical capacitance. In-situ Raman microscopy confirmed the reversible pseudo-capacitive behaviour of electrode materials and the stable electrochemical performance of the devices. The facile techniques used in this work and the good device performance show their great potential for wearable applications.

CO2 utilisation is becoming an appealing topic in catalysis science due to the urgent need to deal with greenhouse gases (GHG) emissions. Herein, the dry reforming of methane (DRM) represents a viable route to convert CO2 and CH4 (two of the major GHG) into syngas, a highly valuable intermediate in chemical synthesis. Nickel-based catalysts are economically viable materials for this reaction, however they show inevitable signs of deactivation. In this work stabilisation of Ni in a pyrochlore-perovskite structure is reported as a viable method to prevent fast deactivation. Substitution of Zirconium by Ni at various loadings in the lanthanum zirconate pyrochlore La2Zr2O7 is investigated in terms of reactant conversions under various reaction conditions (temperature and space velocity). XRD analysis of the calcined and reduced catalysts showed the formation of crystalline phases corresponding to the pyrochlore structure La2Zr2-xNixO7-δ and an additional La2NiZrO6 perovskite phase at high Ni loadings. Carbon formation is limited using this formulation strategy and, as a consequence, our best catalyst shows excellent activity for DRM at temperatures as low as 600 °C and displays great stability over 350 hours of continuous operation. Exsolution of Ni from the oxide structure, leading to small and well dispersed Ni clusters, could explain the enhanced performance.

The chemoselective hydrogenation of crotonaldehyde to crotyl alcohol was studied by temperature programmed desorption/reaction, high resolution XPS and NEXAFS. The organic molecule adsorbed without decomposition, all three possible hydrogenation products were formed and desorbed, and the clean overall reaction led to no carbon deposition. Selectivities up to 95% were found under TPR conditions. The observed behavior corresponded well with selectivity trends previously reported for Ag/SiO2 catalysts and the present findings permit a rationalization of the catalytic performance in terms of pronounced coverage-dependent changes in adsorption geometries of the reactant and the products. Thus at low coverages the C=O bond in crotonaldehyde lay almost parallel to the metal surface whereas the C=C was appreciably tilted, favoring hydrogenation of the former and disfavoring hydrogenation of the latter. With increasing coverage of reactants, the C=C bond was forced almost parallel to the surface, rendering it vulnerable to hydrogenation, thus markedly decreasing selectivity towards formation of crotyl alcohol. Butanol formation was the result of an overall two-step process: crotonaldehyde → crotyl alcohol → butanol, further hydrogenation of the desired product crotyl alcohol being promoted at high hydrogen coverage due to the C=C bond in the unsaturated alcohol being driven from a tilted to a flat-lying geometry. Finally, an explanation is offered for the strikingly different behavior of Ag(111) and Cu(111) in the chemoselective hydrogenation of crotonaldehyde in terms of the different degrees of charge transfer from metal to C=O π bond, as suggested by C 1s XPS binding energies.

The essential role of oxygen in enabling heterogeneously catalyzed Glaser-Hay coupling of phenylacetylene on the Ag(100) was elucidated by STM, laboratory and synchrotron photoemission and DFT calculations. In the absence of co-adsorbed oxygen, phenylacetylene formed well-ordered dense overlayers which, with increasing temperature, desorbed without reaction. In striking contrast, even at 120 K, the presence of oxygen led to immediate and complete disruption of the organic layer due to abstraction of acetylenic hydrogen with formation of a disordered mixed layer containing immobile adsorbed phenylacetylide. At higher temperatures phenylacetylide underwent Glaser-Hay coupling to form highly ordered domains of diphenyldiacetylene that eventually desorbed without decomposition leaving the bare metal surface. DFT calculations showed that while acetylenic H abstraction was otherwise an endothermic process, oxygen adatoms triggered a reaction-initiating exothermic pathway leading to OH(a) + phenylacetylide, consistent with the experimental observations. Moreover, it was found that with a solution of phenylacetylene in nonane and in the presence of O2, Ag particles catalyzed Glaser-Hay coupling with high selectivity. Rigorous exclusion of oxygen from the reactor strongly suppressed the catalytic reaction. Interestingly, too much oxygen lowers the selectivity towards diphenyldiacetylene. Thus vacuum studies and theoretical calculations revealed the key role of oxygen in the reaction mechanism, subsequently borne out by catalytic studies with Ag particles that confirmed the presence of oxygen as a necessary and sufficient condition for the coupling reaction to occur. The direct relevance of model studies to mechanistic understanding of coupling reactions under conditions of practical catalysis was reaffirmed.

We have studied the proline-directed, Pd-catalyzed enantioselective hydrogenation of isophorone in the liquid state using a variety of methods. Our results unambiguously reveal the true reaction pathway and demonstrate that all earlier mechanistic hypotheses are wrong: although a proline/isophorone condensation product is formed, it is merely a spectator and not a key reaction intermediate in subsequent heterogeneous hydrogenation. Enantioselectivity is the result of kinetic resolutiona process that occurs homogeneously in solution and not at the metal surface. Racemic 3,3,5-trimethylcyclohexanone (TMCH) is produced by initial heterogeneous hydrogenation of isophorone; proline then reacts homogeneously, preferentially with one enantiomer of TMCH, leaving an excess of the other. Thus in complete contrast to the case of ketoester asymmetric hydrogenation, the metal surface is not involved in the crucial enantio-differentiation step. The mechanism we propose also explains why the maximum attainable yield of enantiopure TMCH cannot exceed 50%.

Molecular Operating Environment (MOE) software has great potential when combined with the Quantitative Structure-Property Relationship (QSPR) approach, and was proven to be useful to make good prediction models for series of polybenzoxazines [1–3]. However, the effect of heterogeneities in the crosslinked network to the prediction accuracy is yet to be tested. It was found that polybenzoxazines with polymerisable functional group (e.g. acetylene-based benzoxazines) form up to 40% higher char yield compared to their analogue polybenzoxazines due to the contribution of the polymerisable functional group (e.g. ethynyl triple bond) in the cross-linked network. In order to investigate the effect of the inconsistent cross-linking network, a data set consisting of thirty-three benzoxazines containing various structures of benzoxazines was subdivided into two smaller data sets based on their functional group, either benzoxazines with polymerisable functional group (acetylene-based benzoxazines set (Ace-M)) or non-polymerisable functional group (aniline-based benzoxazines (Ani-M)). Char yield predictions for the polybenzoxazines for these data sets (Ace-M and Ani-M) were compared with the larger thirty-three polybenzoxazines data set (GM) to investigate the effect of the inconsistency in crosslink network on the quality of prediction afforded by the model. Prediction performed by Ace-M and Ani-M were found to be more accurate when compared with the GM with total prediction error of 3.15% from both models compared to the GM (4.81%). Ace-M and Ani-M are each better at predicting the char yields of similar polybenzoxazines (i.e. one model is specific for a polymerisable functional group; the other for non-polymerisable functional group), but GM is more practical as it has greater ‘general’ utility and is applicable to numerous structures. The error shown by GM is considerably small and therefore it is still a good option for prediction and should not be underestimated.

The adsorption rates onto a range of platinum single-crystal surfaces of key species involved in the proline-directed heterogeneous enantioselective hydrogenation of isophorone were investigated by electrochemical means. Specifically, the uptakes of the prochiral reactant (isophorone), the chiral hydrogenation product (3,3,5-trimethylcyclohexanone), and the chiral directing agent ((R)- and (S)-proline) were examined. The effects of R,S chiral kink sites on the adsorption of (R,S)-proline were also studied. The reactant adsorbs approximately 105 times faster than the chiral modifier so that under conditions of competitive adsorption the latter is entirely excluded from the metal surface. Supplementary displacement and reaction rate measurements carried out with practical Pd/carbon catalysts show that under certain reaction conditions isophorone quickly displaces preadsorbed proline from the metal surface. Thus both kinetics and thermodynamics ensure that the chiral modifier can play no role in any surface-mediated process that leads to enantiodifferentiation. These results are fully consistent with the recent proposal1 that the crucial step leading to enantiodifferentiation occurs in the solution phase and not at the metal surface. In addition, it is found that there is no preferred diastereomeric interaction between (R,S)-proline and R,S step kink sites on Pt{643} and Pt{976}, implying that such sites do not play a role in determining the catalytic behavior of supported metal nanoparticles.

(Figure Presented) Tilt it a little: X-ray absorption spectroscopy and photoemission were used to elucidate the mechanism of promoter action in the Cu-catalyzed chemoselective hydrogenation of crotonaldehyde. Sulfur adatoms electronically perturb the C=O bond and tilt the C=C bond away from the surface, thus activating the former and rendering the latter inert to hydrogenation. Quantitative conversion of the reactant with 100% selectivity may be achieved

Sulfur adatoms strongly activate the otherwise inert Cu(111) surface towards chemoselective hydrogenation of crotonaldehyde by electronically perturbing and strongly tilting the reactant.

We have studied the proline-directed, Pd-catalyzed enantioselective hydrogenation of isophorone in the liquid state using a variety of methods. Our results unambiguously reveal the true reaction pathway and demonstrate that all earlier mechanistic hypotheses are wrong: although a proline/isophorone condensation product is formed, it is merely a spectator and not a key reaction intermediate in subsequent heterogeneous hydrogenation. Enantioselectivity is the result of kinetic resolution—a process that occurs homogeneously in solution and not at the metal surface. Racemic 3,3,5-trimethylcyclohexanone (TMCH) is produced by initial heterogeneous hydrogenation of isophorone; proline then reacts homogeneously, preferentially with one enantiomer of TMCH, leaving an excess of the other. Thus in complete contrast to the case of ketoester asymmetric hydrogenation, the metal surface is not involved in the crucial enantio-differentiation step. The mechanism we propose also explains why the maximum attainable yield of enantiopure TMCH cannot exceed 50%.

The development of nanostructured materials is an important step in tuning the reactivity and selectivity of modern catalysts and electrodes. Here, electrochemical reduction is used to create pores, wires and networks from lyotropic liquid crystalline phases. Using platinum single crystal substrates it is possible to control the internal structure of the pore as well as that of the interstitial surface. By varying specific deposition conditions the diameter, depth and frequency of the templated structures can be controlled. Results from techniques such as cyclic voltammetry (cv), small angle x-ray spectroscopy (SAXS) and scanning electron microscopy (SEM) are presented.

Chirality plays a vital role in our day-to-day lives; from the way food tastes and smells to the delicate interactions that help to treat all manner of diseases and ailments using pharmaceuticals. The worldwide market for single enantiomer pharmaceuticals is currently in excess of $200 billion and is rapidly rising. Current manufacturing techniques for the required enantiopure drugs are often slow, costly and wasteful. Many involve post-synthesis separation of the desired enantiomer from the reaction mixture. Any unwanted products are either discarded, with associated environmental impact, or reprocessed at additional cost. Enantioselective heterogeneous catalysis offers a direct synthetic route to chemicals of high optical purity. As with many heterogeneous catalytic systems, they often exhibit the additional benefit of being significantly more atom-efficient than comparable homogeneous routes, i.e. they do not require expensive and hard to handle “single-atom” donors. In this talk I will briefly highlight three successful examples of enantioselective heterogeneous catalysis that employ additional chiral modifiers or auxiliaries to impart chirality into an otherwise racemic system. They show how single-crystal based model systems studied under ultra high vacuum conditions may be used to modify and direct the reactions occurring at atmospheric and higher pressures using high surface area supported catalysts. The systems I will discuss show how both the chemical nature of the chiral modifier and reactant molecules and their molecular orientations and interactions at the catalyst surface greatly affect the reaction outcome.

The development of nanostructured materials is an important step in tuning the reactivity and selectivity of modern catalysts and electrodes. Here, electrochemical reduction is used to create a templated structure consisting of a regular array of pores from lyotropic liquid crystalline phases. Surfaces such as these exhibit high surface areas and a regular repeat pattern on the nanoscale show incredible promise for uses in heterogeneous catalysis, battery storage, sensors and molecular and chemical sieves and filters. By depositing structures onto a Pt(111) single crystal substrates we have shown that it is possible to control the internal structure of the pore as well as that of the interstitial surface. Results from techniques such as cyclic voltammetry (cv), Grazing Incidence Small Angle X-ray Scattering (GISAXS) and scanning tunnelling microscopy (STM) are presented. By varying specific deposition conditions the diameter, depth and frequency of the templated structures can be controlled and manipulated for a number of different applications.