Heterogeneous catalysis

Surface science experiments in general and soft X-ray techniques in particular have been extremely successful in elucidating reaction mechanisms in heterogeneous gas-phase catalysis (see e.g. [1] and references therein). However, the behaviour of catalysts at high pressures, under which they are operated in industrial processes (typically 1-50 bar), may be different from that under the ultra-high vacuum (UHV) conditions in which they are traditionally studied. Even seemingly well-understood reaction mechanisms such as that of CO oxidation on transition metal surfaces have been scrutinised recently after highly active surface oxide phases have been discovered which only exist at higher pressures ^[2,3]. Slow processes, such as oxidation or carbidisation, can dramatically affect the nature and, hence, the catalytic activity of the surface involved in the steady-state reaction ^[4,5,6]. Although the pressure range covered by the VERSOX Ambient Pressure X-ray Photoelectron Spectroscopy (AP-XPS) endstation (section 4) is still lower than that of industrial processes, this is only by 1-2 orders of magnitude, whereas UHV experiment s differ from realistic pressure conditions by at least a factor 10¹⁰.

Importantly, the 10-100 mbar pressure regime, which will be accessible with the proposed instrument, ensures laminar gas-flow conditions – as opposed to molecular flow at lower pressure – and allows reaction temperatures similar to those in industrial processes. This will enable a major step up towards bridging the “pressure gap” between fundamental catalysis research and industrial processes within the catalysis and energy sector and is critical for the design and development of next-generation clean and sustainable catalytic technologies. Notably in heterogeneous catalysis soft X-ray techniques can elucidate surface chemical processes in-situ, via time-resolved (on the sub-second timescale) X-ray photoelectron spectroscopy (XPS). Many groups world-wide have exploited time-resolved synchrotron XPS to gain valuable insight into the operation of commercial catalysts for pollution abatement and selective hydrocarbon oxidation/hydrogenation ^[1]. However, the constraint of ultra-high vacuum operation has greatly restricted the phenomena amenable to study. Hydrogenation reactions, in particular, usually require pressures at least in the mbar range. Currently there are only two synchrotron facilities with soft X-ray beamlines (BESSY and the ALS) capable of performing ambient/high-pressure measurements in the range offered by VERSOX. Both are heavily over-subscribed by the international community with a large contingent of UK researchers. The ability to determine molecular orientation of adsorbed reactants, intermediates and products via high-resolution X-ray Absorption Spectroscopy in particular focusing on Near-Edge Fine Structure (NEXAFS) under reaction conditions will provide additional invaluable insight into the structural constraints and steric effects of catalytic reactions. The other VERSOX end-station, offering laterally-resolved XPS/ XAS, will also accelerate the ability to screen combinatorial arrays of multi-metallic and mixed metal oxide libraries. While such libraries can enable rapid optimisation of complex catalyst formulations, there are few methods to efficiently screen the surface chemistry of individual library members necessary to evaluate e.g. resistance to carbon deposition. Spatially-resolved spectroscopy at VERSOX would thus provide a unique opportunity to quickly identify and quantify the nature of such deposits, and thus develop core science discoveries into patented technologies for commercialisation.

Within bimetallic single crystal and nanoparticulate catalysts the surface composition can change when they are exposed to reactive environments (temperature, ambient gas pressure). This phenomenon is driven by the change in the gas phase chemical potential and can, therefore only be studied under ambient conditions.

Phenomena such as sulfiding/sulfation and oxidation/reduction can be studied under conditions of steady-state turnover. Processes affected are e.g. selective alcohol oxidation, trans-esterification of algal/plant oils to biodiesel, and automotive pollution abatement over metal and metal oxide single crystals and nanoparticles. For example, it is thought that, during selective oxidation, the surface (and bulk) of Pd nanoparticles is carbidised and therefore shows relatively low reactivity. A major effect of alloying with Au may be to stop this process occurring ^[7], leaving the metal available for reaction. Recently, Schlögl et al. could show, using the high-pressure XPS instrument at BESSY, that dissolved carbon leads to dramatic changes in the selectivity of a Pd model catalyst in ethane synthesis ^[4]

The direct reduction of CO₂ to fuel over semiconductor nanoparticles is at the very heart of green technologies. The sticking probability of CO₂ at higher temperatures is too low to get sufficient signal under UHV conditions. Therefore the relevant catalyst systems can only be studied using ambient-pressure X-ray photoelectron spectroscopy.

In many cases oxide surfaces are superior to metal catalysts in terms of selective oxidation. The adsorption rates on oxide surfaces are, however, of ten relatively low compared with metals, due to their low surface energy and lack of electron density at the Fermi level. Thus elevated pressures are necessary for studying adsorption and surface reactivity. The use of high-pressure XPS it will enable the simultaneous identification of adsorbate chemical states, the active oxygen species and metal oxidation states, and surface segregation (in the case of mixed oxides). Amongst many others, one particularly interesting example is the behaviour of ferric molybdate, which is the industrial catalyst of choice for the selective oxidation of methanol to formaldehyde ^[8]. It is a mixed oxide with the cations normally in the highest oxidation state. The reactivity of this system is only partly understood, in particular the nature of the active site during the reaction eludes us. Another, famous, example is TiO₂, with its excellent photocatalytic properties ^[9]. Despite a massive body of work under UHV conditions, the behaviour and electronics of this material under reaction conditions is barely studied at a fundamental level.

Water is the “matrix of life”. Therefore practically all reactions involving bio-molecules or their building blocks take place in aqueous solution or involve other solvents. Solution-catalyst interfaces are far less well understood than the catalyst interfaces for gas-phase reactions because both solvent and reactants interact at the solid surface and make the adsorption complex more complicated in many ways. For example, the interaction of reactants with each other and/or the catalyst surface will depend on whether they are embedded in a hydrogen-bonded solvation shell or clathrate of solvent molecules. The application of conventional surface science techniques is very limited in this context as the coadsorption of the relevant surface species cannot be modelled under reaction conditions in UHV, where water only forms stable condensed layers below 150K. At these temperatures the kinetic barriers are too high for the key reaction steps, the exchange of molecules between surface layer and solution and chemical reactions between the surface species. Reactions in aqueous solution require temperatures near 300K where the vapour pressure is in the mbar range. First experiments using the ambient-pressure beamline at ALS showed that even the adsorption behaviour of pure water changes dramatically under ambient conditions compared to UHV ^[10]. Even more dramatic effects are found for organic adsorbates in the presence of water ^[11]. Clearly, the presence of solvents has a profound influence on any surface reaction, therefore relevant reaction mechanisms can only be found if bio-related reactants are studied under appropriate ambient pressure conditions.

By balancing the vapour pressure and temperature it is possible to keep thin films (sub-monolayer to several layers) of water or other solvents on a catalyst surface. This approach, in combination with ambient-pressure photoemission provides a new way to study the chemical composition and electronic structures of solid-liquid interfaces and, thus, opens access to a number of technologically important material systems which could not be studied by soft X-ray spectroscopies at all so far. These include electrochemical and biological interfaces and liquid-phase heterogeneous catalysts. Using VERSOX, adsorption, exchange, corrosion and diffusion processes near such solid surfaces can be studied at realistic reaction temperatures. Fundamental studies of new technologies, such as biodiesel production and the synthesis and/or enantioselection of fine chemical and pharmaceuticals, will benefit immensely from permitting the investigation of key chemical processes such as competitive solvent adsorption, gas-liquid phase reactions and catalytic deactivation. Many of the most important commercial chemical syntheses (e.g. cross-couplings to produce liquid-crystals and medicinal drugs) involve volatile organics with vapour pressures between 10-100 mbar; VERSOX offers an unprecedented opportunity to understand the solid catalyst-liquid interface, and there by increase reaction efficiency. In addition to chemical research, medical research and new energy technologies will also benefit from knowledge created by the proposed beamline. For example, VERSOX will enable studies of

• adsorption and surface-reactions of bio-molecules in their native aqueous environment, relating to bio-mineralisation, bio-compatibility of implants, and drug discovery;
• key processes at solid-solution interfaces such as the charge-transfer interaction between the adsorbed dye and the electrolyte in a dye-sensitised solar cell;
• photocatalytic splitting of water using or ganometallic dye complexes on oxide surfaces;
• adsorption, dissociation and recombination processes on fuel cell electrodes.