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In Situ and Operando Spectroscopy: a Powerful Approach Towards https://doi.org/10.1595/205651318X15234323420569 Johnson Matthey Technol. Rev., 2018, 62, (3), 316–331 www.technology.matthey.com In Situ and Operando Spectroscopy: A Powerful Approach Towards Understanding Catalysts Revealing reaction mechanisms to find the correlation between structure and surface composition Liliana Lukashuk§ of new catalysts or the optimisation of existing Institute of Materials Chemistry, Technische catalysts is not yet widely employed in the Universität Wien, Getreidemarkt 9/BC/01, chemical industry. In this article, examples of 1060 Vienna, Austria using in situ or operando spectroscopy for studying the surface and bulk of catalysts under process §Present address: Johnson Matthey, PO Box 1, conditions are discussed, with an overview of Belasis Avenue, Billingham, Cleveland, TS23 applying in situ X-ray absorption spectroscopy 1LB, UK (XAS), in situ infrared (IR) spectroscopy and in situ near-ambient pressure X-ray photoelectron Karin Foettinger* spectroscopy (NAP-XPS) for monitoring the bulk Institute of Materials Chemistry, Technische and surface composition of PdZn/ZnO and Pd2Ga/ Universität Wien, Getreidemarkt 9/BC/01, Ga2O3 methanol steam reforming catalysts. 1060 Vienna, Austria 1. Approaches for Catalyst *Email: [email protected] Development 1.1 Discovery of a ‘Lucky Catalyst’ The improvement of catalytic processes is via High-Throughput Synthesis strongly related to the better performance of catalysts (higher conversion, selectivity, yield Research and development of more efficient and and stability). Additionally, the desired catalysts sustainable catalysts, either the development of should meet the requirements of being low cost novel catalysts or optimisation of existing ones, is as well as environmentally and user-friendly. a vital part of industrial and academic research. All these requirements can only be met by One of the most common approaches for catalyst catalyst development and optimisation following development is the synthesis of a large number of new approaches in design and synthesis. This catalysts, followed by screening of their catalytic article discusses three major approaches in the activity (known as high-throughput synthesis) design and development of catalysts: (a) high- (1, 2). A high-throughput synthesis approach is throughput synthesis; (b) reaction kinetic studies; the most common method of catalyst discovery (c) in situ and operando spectroscopy for studying and optimisation and can be strongly boosted catalysts under process conditions. In contrast to by the combinatorial design of experiments approaches based on high-throughput synthesis and using robotic equipment to synthesise and and reaction kinetic studies, an emerging approach test catalysts (3–8). One of numerous catalysts of studying catalysts under process conditions prepared using high-throughput synthesis might using in situ and operando spectroscopy and be very active for a particular reaction (a ‘lucky transferring the gained knowledge to design catalyst’). However, employing such an approach 316 © 2018 Johnson Matthey https://doi.org/10.1595/205651318X15234323420569 Johnson Matthey Technol. Rev., 2018, 62, (3) alone without understanding activity-controlling Later works of Mars and van Krevelen on reducible factors often does not lead to a gain in knowledge oxides in 1954 suggested another mechanism that or deep understanding of factors driving the activity today is called the Mars-van-Krevelen type redox and selectivity of catalysts (for example the nature mechanism. According to the Mars-van-Krevelen of active sites and reaction mechanisms). mechanism, one reactant forms a chemical bond Understanding the structure-performance with the catalytic surface; the other reactant relationships of catalysts is a crucial step for the reacts directly from the gas phase. A vacancy that successful development of the next generation of is formed when the reaction product desorbs was more efficient catalysts and covers different aspects. thought to be filled with an atom from the bulk Some research lines focus on the preparation (14). However, nowadays it is generally accepted of catalysts by varying certain parameters that the vacancy created by the reaction is filled (for example preparation methods, elemental with the first reactant again. composition or loading) and then correlate catalytic The type of mechanism depends on the type properties of the synthesised materials with their of catalyst. The Mars-van-Krevelen type redox structure, reducibility or tendency to adsorb or mechanism typically applies to reducible metal desorb the educts and products for a particular oxide catalysts, where lattice oxygen is involved in reaction. Combining knowledge of a catalyst’s the reaction cycle and generation of vacancies is a performance with its structure adds substantially crucial reaction step. The Eley-Rideal and Langmuir- to the understanding of catalytic systems and the Hinshelwood mechanisms typically describe non- potential to transfer knowledge to similar processes reducible oxides and metal nanoparticle catalysts, or products. Yet this approach provides rather a where a network of elementary reaction steps involves macroscopic picture of the structure-performance adsorption of reactants on the catalyst surface, a relationships of catalysts, omitting the detailed catalytic reaction and desorption of products. microscopic insights into the activity-controlling As is demonstrated in the review of Rob Berger factors driving a particular reaction. et al., in the chemical industry knowledge of the reaction kinetics is of primary importance for: 1.2 Reaction Kinetic Studies (a) catalyst development; (b) process development; (c) process optimisation; and (d) mechanistic Another line of research focuses on microscopic research (12). 29% of the utilisation of kinetic data insights and explores the nature of the reaction in industry is dedicated to catalyst development sites of a catalyst and the mechanisms of reactions (12). For mechanistic studies and advanced (9, 10). The commonly used methodology is based catalyst development, a precise description of the on a detailed study of reaction kinetics (reaction reaction rate is required at the level of elementary rates and rate laws). It takes its roots back to steps. This calls for advanced kinetic studies such 1921 when the mechanisms of the catalytic action as fast transient kinetic investigations (employing of platinum in the reactions of carbon monoxide a temporal analysis of product reactor) (15) and oxidation and hydrogen oxidation were described advanced kinetic modelling studies (16–18). with the help of kinetics by Irving Langmuir and presented at a Discussion of the Faraday Society 1.3 The In Situ and Operando Approach held in September 1921 at the Institution of Electrical Engineers in London, UK (11). Langmuir’s Although studying the kinetics of reactions is a work was later elaborated by Cyril Hinshelwood. powerful tool for identifying steps of the reaction The combined work of Irving Langmuir and Cyril cycle and for obtaining hints about active sites, it Hinshelwood is known today as the Langmuir- does not allow a complete picture of the reaction Hinshelwood mechanism and is one of the mechanism to be obtained. Often several reaction most common reaction mechanisms describing mechanisms can run in parallel; therefore, an numerous catalytic reactions (12). According to exact identification of the reaction mechanism is the Langmuir-Hinshelwood mechanism, a catalytic not feasible. Moreover, for heterogeneous catalytic act is a reaction between two molecules, adsorbed reactions, the identification of active sites is rather on neighbouring sites (12). vague especially for solid catalysts because of their Some years later, in 1938, Dan Eley and Eric Rideal complexity. Often solid catalysts are multicomponent proposed the Eley-Rideal mechanism, in which a and multielement materials. Furthermore, the catalytic act is a reaction between an adsorbed structures of solid catalysts are usually not well molecule and a molecule from the gas phase (13). defined; thus, several sites might serve as active 317 © 2018 Johnson Matthey https://doi.org/10.1595/205651318X15234323420569 Johnson Matthey Technol. Rev., 2018, 62, (3) sites and as a result, different reaction pathways ‘before-reaction part’ or ‘after-reaction part’ “is like might simultaneously run on different reaction sites studying a life with access only to the prenatal and or in different temperature regimes. postmortem states” (23). Moreover, exposing the The complementary methodology based on the catalyst to ambient conditions might again change use of in situ and operando techniques for studying the catalyst’s structural and electronic properties the catalyst under reaction conditions can overcome and lead to data misinterpretation (Figure 1). some of the limitations of the traditional kinetic A more informative way is to monitor the catalyst methodology and provide a broader insight into directly under reaction conditions, in situ (meaning the complex network of reaction pathways, thereby on site, in position, undisturbed) (23). Watching strongly contributing to understanding the structure- the catalyst under reaction conditions by physical- performance relationships of catalysts, the nature chemical methods and simultaneously recording of active sites, reaction mechanisms, the structure catalytic data using a mass spectrometer or a of working catalysts and, based on the knowledge gas chromatograph
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