Design Scientific Progress
The Design theme of the UK Catalysis has undertaken a wide range of fundamental projects related to the understanding of catalytic processes, design of better catalysts and in particular developing the use of large facilities such as Diamond light source and ISIS Neutron and muon source for Catalysis Research. All the projects have involved teams from multiple institutions and disciplines bringing together expertise from around the UK and internationally which is core to the Hub ethos and one of the strengths of the funding mechanism from the Hub grant. Highlights of some of the projects are described:
The upgrading of methane into C2-C6 hydrocarbons could prove a valuable alternative to oil as a source of platform chemical. A combination of operando X-ray methods and fluorescence lifetime imaging microscopy has been brought to bear on the well-known Mo-ZSM-5 zeolite to provide a detailed understanding of how the Mo site evolves during the catalytic conversion of methane and how this influences the reaction products. The multi-institutional collaboration with the Central Laser Facility, the European Synchrotron Radiation Facility and Utrecht University has proved to be essential for this work.
I. Lezcano-González, R. Oord, M. Rovezzi, P. Glatzel, S. W. Botchway, B. M. Weckhuysen, A. M. Beale, Angew. Chem. Int. Ed.2016, 55, 5215.
The uses of in situ and operando spectroscopic methods are important for investigating and developing improved catalyst materials. This project aimed to develop a combined XAFS/DRIFTS system where the local structure and oxidation state information provided by X-ray Absorption fine structure (XAFS) and surface sensitive information obtained from FTIR. This powerful combination of techniques has been demonstrated by the study of the restructuring of a bimetallic AuPd/Al2O3 catalyst during CO oxidation the structural chaged affect the activity and are shown in the Figure. This powerful combination of techniques is now available for use by the wider UK Catalysis Hub network, and has already been used by several groups to study a variety of catalytic systems.
This project is aimed at the developments of spatially resolved XAFS techniques for gas phase investigation as well as high temperature/pressure batch reactor that allows for operando XAFS investigations. The technique (SpaciFB, Figure 1) has already undergone three phases of testing. 1) investigation of the hydrogen effect promotion of the CO oxidation. 2) The first ever operando structure-activity investigations of Non-Thermal Plasma enhanced catalysts. 3) The spatially resolved investigation of kinetic oscillations during CO oxidation. The results of phase 1 & 2 are being summarized in two high impact publications (preparation).
The aim of the project was to characterise molecular adsorbates on high surface area metals using diffraction to probe the short-range order using pair distribution functions. Three systems were investigated, Pd/H2, Ni/benzene and Cu/Formic acid. Looking at the Cu-formate system, it was expected that it would form adsorbed formate on the copper surface, but further reaction appeared to give a bulk copper formate phase, which was refined in collaboration with modellers in the Hub. The nickel/benzene system has not yet been resolved. Modelling of the system is ongoing. The relationship between ISIS and the hub has led to fruitful collaborations beyond the initial project, with many successful beamtime applications and development of new users within ISIS. Many publications not directly associated with the project have resulted, including the publication of the special PCCP edition.
This project is jointly funded by the themes of Design and Energy, and in collaboration between academia and industry. The project aims to investigate the hydrocarbon species formed in zeolite catalysts during the conversion of alcohols to hydrocarbons, by examining catalyst samples removed from a working reactor at various stages during the catalyst lifetime. Inelastic neutron scattering gives a vibrational spectrum of the hydrocarbon species over the full frequency range, complemented by EPR spectroscopy which detects radical species formed in the working catalysts. (“An Assessment of Hydrocarbon Species in the Methanol to Hydrocarbon Reaction over ZSM-5”, Suwardiyanto, R.F.Howe, E.K.Gibson, C.R.A.Catlow, J.McGregor, P.Collier, S.F.Parker, D.Lennon, Faraday Discussions of the Chemical Society, in press).
The main objectives of this exemplar project were to design monometallic and bimetallic active sites within microporous and hierarchical aluminophosphate (AlPO) architectures. Co(II) and Sn(IV) ions were implanted the framework. The local structure of the Sn and Co sites was probed using x-ray absorption spectroscopy (XANES and EXAFS) and this revealed multiple Sn environments that could be exploited for a range of catalytic applications. DRIFTS measurements identified the relative fractions of Lewis and Brønsted. It was found that Sn(IV) species were predominantly located in tetrahedral framework positions and served as the loci for the generation of strong Lewis acid sites. The incorporation of tetrahedral Co(II) sites, adjacent to the Sn(IV) sites, modulated the acidity of the bimetallic catalyst, which had a direct implication in the resulting catalytic performance.
This project seeks to understand the various catalytic roles that Fe and Cu species located in zeolite materials such as SSZ-13 have in the selective catalytic reduction (SCR) of NOx using sacrificial NH3. The project makes use of the consortia expertise in the preparation of zeolite materials (St. Andrews, Johnson Matthey Plc), the characterisation (UCL, PSI (Switzerland) and testing (UCL, Johnson Matthey Plc). Initial results have shed light on the significance of Cu-OH species in the catalytic reduction cycle and the role of CuOx species in NOx oxidation.
Modelling techniques were used to investigate the design of Hierarchically Structured Porous Catalysts with Superior Performance due to Nano-confinement and Multiscale Optimisation of the Pore space. Statistical mechanical simulations were carried out to evaluate the effect of grain boundaries in crystals on diffusion of gases that adsorb from a pore bordering such a crystal; a pore cut in a quartz crystal, and filled with CO2 vapour was used as a first step towards studying similar effects in zeolites. It was found that molecules tend to accumulate along corners, with longer residence times than along the walls, and similarly slower diffusion along the corners than along the walls.
Spatially controlled nanoparticle pairs to probe surface species diffusion in heterogeneous catalysis
Surface diffusion and migration of reacting and intermediate species is often overlooked/ignored and is difficult to probe in catalysis. However, in catalysts that contain sites with different activities, diffusion processes will likely be highly important in determining catalytic behavior. We are tackling this problem by preparing novel spatially controlled nanoparticle pair catalysts. The control of the diffusion length on the catalyst surface achieved by the preparation of these spatially controlled nanoparticle pair catalysts opens the door on a range of exciting new studies into surface diffusion phenomena. In particular, we intend to use them to study the impact of the separation distance on activity and selectivity initially in simple reactions such as total oxidation of ethylene.
In this project, we are investigating the mechanism of the water gas shift reaction (WGSR), on Pd surfaces. We thus employ density functional theory (DFT) and kinetic Monte Carlo (KMC) simulations to propose a probable mechanism. DFT calculations have yielded the adsorption energies of reactants, intermediates and products, whereas the climbing-image nudged elastic band (CI-NEB) method has been used to find out the minimum energy path for the steps of all relevant pathways. Vibrational frequency analyses are carried out to verify the transition states. The DFT derived energies and frequencies are further used as an input for graph theoretical KMC approach to determine the rate constants and perform simulations of the overall reaction mechanism. Our analysis so far indicates that the WGSR proceeds by splitting of water followed by an interaction of CO with adsorbed atomic O-species and the formation of a formate species leading to the evolution of CO2 and H2 gas.
The current project aims to clarify structure-property relationships in nanoparticles and their interactions with oxide surfaces such as ceria to design efficient catalysts and to directly relate theoretical findings with those of experimental studies. So far, we have investigated the interaction between gold atoms with pristine ceria as well as on surfaces with oxygen vacancies in great detail. Based on these findings we are now performing rigorous quantum chemical calculations to develop a deeper understanding of how gold nanoparticles interact with the preferred surface facets of ceria, which we believe will provide fundamental understanding at the molecular level to design novel ceria supported gold based catalyst for a range of oxidation processes.
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