Operando catalysis using synchrotron methods

In 2002 Banares and co-workers [1,2] took the word “operando” to define what amounts to an experimental philosophy, one that goes somewhat beyond the term that had preceded it, namely “in situ”. The central difference between these two notions is that in situ only specifies a place, whereas operando implies a specific function whatever is “operando” is working. From this it is easily observed that whilst all operando experiments are in situ, not all in situ experiments are operando. Catalysis is founded upon chemical processes that are ideally arranged to form reaction cycles that complete molecular transformations, whilst the active elements within the catalyst are stable such that they may continue to facilitate the desired conversion. The range of chemical conversions that are desired to be achieved are incredibly diverse, as are the conditions or timescales in which they may be achieved. What operando study demands is that, whatever catalytic process is desired to be studied, every effort is made to parameterise a given experiment in a manner that respects, as closely as possible, that which might be experienced by the catalyst in a real application. Whether an ideal operando experiment has ever actually been realised – as the parameter space that should be ideally adhered to is considerable is debatable. However, far more important is the stimulus the operando philosophy has lent to research in the formulation of new experimental methods and approaches that pay much more attention to process conditions than had gone before. A clear requirement for the development of operando experimentation it that one is in possession of probes that can be applied under the conditions specified by the process: methods can address issues of structure (on a wide range of length and timescales), molecular function, and reactivity so that relevant and quantitative structure function relationships (QSARS) that define the catalysis may be established. Methods that make use of the scattering, absorption, or emission of X-rays are extremely good at interrogating the structure of materials, be it physical or electronic, on length scales from the Angstrom to those of laboratory scale reactors. X-rays also have an intrinsic capacity to penetrate matter that permits much flexibility to the design of suitable reactors within which they may be studied. Importantly, in their modern forms, they may also operate on kinetically relevant timescales. Methods founded upon X-rays make ideal companions to a variety of laboratory based methods that are generally applied to the study of catalytic systems. In parallel with the evolution of operando techniques, 3rd generation synchrotron sources have proliferated and the technology associated with them has advanced to such a degree that entirely new generations of experiments, have become possible since 2002. In this issue, therefore, we highlight some of the ways that these modern X-ray methods may be applied to furthering our understanding of how catalysts are synthesised, how they work, and how, when applied in tandem with other non X-ray techniques, they might shed light on fundamental aspects of behaviour that need to be understood in order to further catalyst and process design. X-ray absorption spectroscopy (XAFS Gibson et al, Kroner et al, Martin et al, Rochet et al, Ma et al, Brazier et al, Martin et al) is a well-established method for interrogating working catalysts to reveal aspects of the chemical state and local structure of active components and how they change. It is, as a result, the most widespread and commonly used X-ray method for the operando study of many types of catalysts operating under a range of conditions. Increasingly, as synchrotron technology has evolved in the 21st century, XAFS is commonly used in a time resolving fashion and in tandem with other techniques, such as infrared (Gibson et al, Kroner et al, and Martin et al) and Raman spectroscopies (Rochet et al), that are able to address other aspects of the system under study (surface molecular speciation for instance), and that are crucial to establishing QSARS. Since their inception [3,4] such combined methods have become increasingly commonplace and extended into areas beyond XAFS, [5–7]. This is particularly the case for infrared spectroscopy (in Diffuse reflectance (DRIFTS) mode) with dedicated resources for such measurements now available at numerous beamlines around the world. Lastly, the ongoing development of X-ray technology offers the possibility of studying single, nano sizes catalytic entities. In respect of XAFS Martin et al assess how far X-ray technology may have come, and how far it may still have to go, in achieving the tantalising goal of meaningful operando in study of the behaviour of single metal (Pd) nanoparticles. As had been pointed out before, [8] the operando method also requires that significant attention is paid to sample presentation and reactor design to be used in such studies. This important aspect is addressed by Marchionni and co-workers in their consideration of a cell designed to be compatible with operando DRIFTS, transmission and fluorescence XAS, and methods based on X-ray scattering such as XRD and total X-ray scattering/pair distribution function (PDF) analysis. Methods based upon X scattering, such as Bragg diffraction, have also benefitted enormously from the advances in X-ray source, insertion device, and detector technology