Tuning/exploiting Strong Metal-Support Interaction (SMSI) in Heterogeneous Catalysis
Introduction
The application of catalytic technologies, on a major scale in the 20th century, to a huge number of industrial synthetic processes - ranging from petrochemicals to pharmaceuticals, as well as to diverse applications, such as, the treatment of waste, the sensing of crucial physiological metabolites and the generation of green energy, has undoubtedly been a major driving force underlying both social change and economic progress [1].
Given the now ubiquitous presence of catalysts within the industrial base of the developed economies, it should be of no great surprise to find catalysts occupying an ever more important role in the energy economy. The drive for ever increasing efficiency in energy harvesting, storage and conversion; especially with respect to the development of fuel cells, solar cells, and bio-energy, has highlighted the need for ever more versatile and robust functional materials. To anyone who has ever paid more than a cursory glance at the history of chemical reactions in the energy economy the central importance played by catalysts will be obvious.
Recently a wide range of supports, in addition to those involving the oxides of transition or non-transition metals, have been investigated and shown to have the potential to augment the catalytic potential of platinum and other catalytically active metals. The catalyst support should ideally embody such properties as mechanical strength, a resistance to dissolution or disintegration and the ability to act as a carrier upon which the catalytic particles can be distributed as uniformly as possible – thus maximizing the surface area exposed to the reactants. Thus, supports can be formed as powders, granules, pellets, wires, or as extruded structures for specific applications.
The interaction between metals and oxide supports, so-called metal-support interactions, are of great importance in heterogeneous catalysis [2], [3] and electrocatalysis [4]. The term strong metal-support interaction (SMSI) was first used by Tauster et al., in 1978 [5]. They discovered that the chemisorption of H2 and CO of metal clusters on TiO2 would decrease significantly, when the material was chemically reduced at high temperatures. Now, the term of SMSI has been generally used to summarize changes in catalytic activity and selectivity when group VIII metals supported on reducible oxides are treated by a high-temperature reduction process [3], [6]. SMSI commonly appears at the metal-support interface where the support is reducible, e.g. with TiO2, CeO2, Nb2O5, [7] and V2O3 [5], [6], [8]. SMSI plays a critical role in determining catalytic activity and stability. Gold particles supported on metal oxides, such as TiO2, Fe2O3, and CeO2 possess significant oxidation activity when compared to unsupported gold particles [9]. SMSI was originally explained as an electron transfer between the support and the metal [10], or by the formation of intermediate phases [11]. Current thinking with respect to the fundamental concepts underlying strong metal-support interactions additionally attaches importance to interfacial and transport phenomena, together with charge redistribution during metal-support interface formation. SMSI gives rise to three major effects (electronic, geometric and bifunctional), as shown in Scheme 1. The change in the electronic properties of the metal catalyst originates from the strong interactions between the cluster and the oxide support [12], [13]. The geometric effect originates from the physical covering of a thin layer of a reducible oxide species blocking active catalytic sites on the metal's surface. [3], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23] The bifunctional effect provides dual active sites at the perimeter between the metal and the support leading to significantly improved catalytic activity/selectivity. The reaction species can migrate or spillover either from metal or support to the boundary or edge where the chemical reactions occur. Recognition of the importance of SMSI has attracted many researchers in diverse heterogeneous catalytic applications such as chemical synthesis [24] water splitting [25] and environmental engineering [26], [27], [28], [29], [30].
In heterogeneous catalysis, the size of the catalytic materials has to be in the nanosized region in order to have the highest surface area upon which the reaction will take place. With such nanosized catalysts, formed as supported nanoparticles, the interactions between the catalytic nanoparticles and the support are significantly enhanced. Understanding and ‘tuning’ that interaction is crucial for enhancing the performance and durability of catalytic materials. Much progress in the understanding of metal-support interaction has been made thanks to modern surface science techniques [31], [32] and advanced computational modeling methods [33], [34]. A useful representative model system consists of an ultrathin layer of metal or nanometer scale metal cluster supported on well-defined single crystal surfaces. By utilizing this model we can simplify a variety of issues related to interface interactions [35]. Such a computational simulation also contributes to our understanding of many aspects of the nature of metal/oxide interfaces [33].
This review, focused on Strong Metal-Support Interactions (SMSI) in heterogeneous catalysis, commences by examining the current state of knowledge with regard to the electronic, geometric and bifunctional effects that underlie the activity, selectivity and stability of heterogeneous catalysts. Prior to examining contemporary practical applications of SMSI in various reactions such as CO oxidation, methanol oxidation, oxygen reduction reaction, hydrogenation, and in applications such as fuel cells and heterogeneous catalysts, consideration will be given to material characterization by various techniques, including spectroscopic techniques, such as X-ray Photoelectron Spectroscopy (XPS), X-ray Absorption Spectroscopy (XAS), Electron Energy Loss Spectroscopy (EELS), and microscopic techniques, such as Transmission Electron Microscopy (TEM), and Scanning Probe Microscopy (SPM). The remainder of the paper looks to the future of SMSI, specifically by examining the way we can manipulate catalytic properties by synthesizing rationally designed materials; such design considerations embody the totality of our current understanding of the electronic/structural nature of the catalytic materials. In fact, the incorporation of computation into an experimental design will play a pivotal role not only in enabling us to understand reaction mechanisms, but also in the search for new catalysts. Theoretical computations provide us with deep insights; especially with respect to reaction mechanisms, cluster growth and interfacial characteristics that cannot be gleaned from experimentation due to limitations imposed by instrumental sensitivity or extreme conditions. However, it is regrettable that the paper cannot include a more rigorous treatment of theoretical investigation approaches due to the article length.
Section snippets
Mechanism of SMSI
The excellent review article by Fu and Wagner [31] provides an extensive discussion focusing on the thermodynamics and kinetics of interfacial phenomena that occur at metal-support interfaces. Interfacial phenomena involving chemical reactions, transport phenomena or charge redistribution during metal-support interface formation define the underlying fundamental concepts of strong metal-support interactions. The properties of metals and their supports are affected by strong metal-support
Characterization
To achieve rational design and fabrication of nanostructured catalysts with performance require more understanding about the nature of metal-support interface and interaction. Advanced analytical techniques have been applied to probe fundamental heterogeneous catalytic mechanisms. The electronic properties of the metal-oxide interface depend strongly on the chemical bonding between the metal and the oxide. However, such bonding depends also on the surface composition and the atomic structure
Oxidative and Reductive Treatment Approach
Contacting a metal with a support may result in
Conclusions
The investigation of SMSI is a fast-growing multifaceted area of research that requires an understanding of how electronic, geometric and bifunctional effects impact on the catalytic activity, selectivity and stability of various reaction systems, such as heterogeneous catalysis and electrocatalysis. This review in addition to summarizing and tabulating the various protocols reported in literature for the manipulation of the interactions between metals and supports also gives an insight into
Acknowledgements
This work is supported by, the Ministry of Science and Technology (MOST) (104-3113-E-011-002, 104-2923-M-011 -002 -MY3, 104-2911-I-011 -505 -MY2), and the Top University Projects of Ministry of Education (MOE) (100H451401). The authors would like to acknowledge the facility supports from the National Synchrotron Radiation Research Center (NSRRC) and National Taiwan University of Science and Technology (NTUST).
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These authors equally contributed to this work.