Catalysis and Photocatalysis by Nanoscale Au/TiO2: Perspectives for Renewable Energy

Nanoscale gold−titania (Au/TiO2) catalysts may provide the right combination of electronic structure, structural dynamics, and stability to facilitate wide ranging chemical transformations, including reactions for utilization of renewable energy sources. The Au/TiO2based systems have also emerged as promising photocatalysts capable of promoting light-induced production of hydrogen and other renewable hydrocarbon-based fuels. This Perspective summarizes some of the fundamental aspects and concepts built over the last 30 years that help explain the catalytic and photocatalytic performance of Au/TiO2 materials. The application of emerging operando methods, based on synchrotron experimental techniques, is also briefly highlighted within the context of key catalytic reactions that may have fundamental importance in renewable energy production and storage. For nearly all of human history, gold has been sought after for its natural beauty, immutability, and unique balance of malleability and durability. In contrast, gold had virtually no value in the chemical sciences until the remarkable discovery, first by Hutchings in 1985 and then by Haruta et al. in 1987, that, when subdivided to nanoscale dimensions, gold can catalytically oxidize CO to CO2 at subambient temperatures. The next 30 years saw an exciting period of scientific interest in gold that focused intently on uncovering the potential of nanoscale gold as a catalyst. Scientists from different branches of experimental and theoretical research combined their efforts to develop a deep fundamental understanding of the factors that control the unusual catalytic behavior of nanoscale gold and to advance formulations of supported Au-based catalysts. Beyond thermal catalysis, nanoscale gold has also been shown to facilitate photocatalytic processes in the ultraviolet (UV) and visible range, even for large band gap semiconductor supports. Thus, a new research field in catalysis science emerged: the “Catalysis and photocatalysis by nanoscale gold”. Developing a detailed scientific understanding for why gold morphs from inert to catalytic as individual particles approach the nanoscale is a formidable challenge because electronic and physical properties of the material cannot be independently controlled for systematic studies. In thermal catalysis, one must identify the effects of energy, geometry, and electronic structure on mechanistic pathways. In photocatalysis, knowledge of the optical response of the catalyst, the dynamics of charge photogeneration and separation, and the photophysical mechanisms of charge transfer must be untangled to provide fundamental insight into the overall chemistry. Moreover, thermal chemistry and thermal effects almost always accompany photochemistry. Distinguishing the thermally driven stages of a reaction from the photoinduced steps has been, and remains, one of the major challenges in this field. While isolated gold clusters are known to be reactive, the active gold particles are typically dispersed on a support, which Received: March 2, 2017 Accepted: April 10, 2017 Gold had virtually no value in the chemical sciences until the remarkable discovery that, when subdivided to nanoscale dimensions, gold can catalytically oxidize CO to CO2 at subambient temperatures. Distinguishing the thermally driven stages of a reaction from the photoinduced steps has been, and remains, one of the major challenges in this field. Pespec iv e http://pubs.acs.org/journal/aelccp © XXXX American Chemical Society 1223 DOI: 10.1021/acsenergylett.7b00189 ACS Energy Lett. 2017, 2, 1223−1231 adds an additional layer of complexity and richness to the chemistry. Titania-supported gold particles, Au/TiO2, have emerged as the most important model and practically applied gold-based catalyst. Many of the same properties that are responsible for the use of pure TiO2 as a heterogeneous catalyst, including its high stability, reducibility, and electronic structure, make the material an ideal support for Aubased catalysts. Au/TiO2 exhibits remarkable performance in the dark for catalytic oxidation of small molecules such as CO and H2, as well as in selective chemical transformations (hydrogenation, oxidation, etc.) of complex organic molecules. In oxidation reactions, the initial adsorption and activation of oxygen is characterized as oxygen atoms interacting with perimeter Au atoms and the coordinately unsaturated (cus) Ti site on TiO2 at the gold/titania interface. 9−11 Scientists also established that these same interfacial sites can promote the activation of C−H and C−O bonds, leading to the selective deoxygenation of organic acids, an important step in the utilization of renewable energy sources such as biomass. Beyond thermal chemistry, Au/TiO2 systems have emerged as promising photocatalysts capable of promoting catalytic hydrogen production from renewables. Photocatalytic reduction of CO2 into renewable hydrocarbon-based fuels over Au/TiO2-based catalysts has also shown promise in the development of artificial photosynthesis strategies. In this Perspective, we highlight some of the fundamental aspects and concepts associated with Au/TiO2 catalysts with the goal of providing a brief introduction to the topic that may motivate further exploration in this exciting field. In particular, we highlight the synergy between “descriptors” (energetic, geometric, and electronic) that are responsible for the unique catalytic activity of gold nanoparticles. This discussion provides a platform for introducing advanced approaches for structural and compositional manipulation of nanoscale gold that are designed to provide the right combination of properties for achieving high catalytic activity and selectivity. Equally important to materials development is the challenge of characterizing catalyst structure (geometrical and electronic) in real time and under practical operational conditions. These aspects of Au/TiO2 catalysis research are discussed within the context of key catalytic reactions that may have fundamental importance in renewable energy production and storage. Reactivity of Gold: Relativistic Ef fects, Nobleness, and the d-Band Model. Catalysis hinges on the propensity for a surfacebinding site or group of sites acting in concert to alter the electronic structure of reactants to weaken (activate) specific bonds and produce intermediates that are poised to transform into more stable products. The products must then exit the site of reaction in a way that returns the catalyst to its original state. The first requirement, activation, does not occur on macroscopic gold because of the well-known relativistic effects for this heavy element, which lead to unusually large sd hybridization, and low energy. The reactivity of gold and other transition metals has been described by a simple model presented by Hammer and Nørskov that relates the nobleness of metals to two characteristics: (1) the degree of filling of the antibonding adsorbate−metal d states and (2) the degree of overlap between the electronic states of interacting atoms or molecules and the gold d states (Figure 1). These two factors determine the strength of the adsorbate−metal bonding and the activation energy for adsorbate dissociation. Under typical conditions, gold exhibits both a filled antibonding adsorbate−metal d state and the largest coupling matrix element. The coexistence of the two properties renders bulk gold the most noble metal. For example, clean Au(110)-(1×2) does not dissociatively adsorb or activate hydrogen or oxygen; thus, this surface is completely inactive as a catalyst for hydrogen oxidation. However, the character of the d states depends on the width of the band, which changes with coordination number. As the coordination number decreases in late transition metals, the d states shift toward the Fermi level. When the d band is higher in energy than the Fermi level, the antibonding orbital for most molecule−surface interactions remains empty, which leads to strong bonding interactions. Thus, adsorbates may become activated at the surface of very small metal particles or particles on substrates that affect the overall electronic structure. For example, when subdivided to nanoparticles of ∼3 nm in diameter, goldwhen supported on TiO2catalyzes the H2 + O2 → H2O reaction even at 200 K with a very small apparent activation energy of 0.22 eV. Beyond particle size, however, the catalytic activity of TiO2/Au systems depends on a balance between numerous other critical factors. Nanoscaling Ef fects on Gold Reactivity. Following the pioneering studies of Haruta et al. and Goodman and Lai, three different but related types of gold catalyst systems have been the focus of intense research efforts: (1) isolated model systems, gold clusters and bulk gold single crystals; (2) planar systems, more complex model catalysts composed of size-selected or well-characterized gold nanoparticles deposited onto singlecrystal oxide supports; and, (3) high surface area systems, more realistic (actual) catalysts composed of gold nanoparticles, often having inexact sizes and structures, deposited on high surface area (powdered or three-dimensional) oxide supports. Use of these systems enabled scientists to clearly demonstrate how the catalytic activity of Au nanoparticles depends on particle size and shape (Figure 2). One reason for this dependence, beyond how size affects the band structure of the material, is related to the fraction of low-coordinated (LC) corner and edge atoms at the surface of the particles, as effectively illustrated in Figure 2B. A key result from the work cited above is that structural, dynamic, electronic, and environmental effects influence the catalytic activity and selectivity of supported gold nanoparticles. Various models have been proposed to explain the unique behavior of finely dispersed gold. Table 1 highlights the accumulated fundamental knowledge about the nature of these Figure 1. Schematic illustration of the formation of a chemical bond between an adsorbate valence level (dark blue) and the s (light blue) and d (red) states of a transition-metal surface. The bond is characterized by the degree to which the antibonding state between the adsorbate state and the metal d states is occupied. For details, see ref 13 and 109. Copyright 2005 Springer. ACS Energy Letters Perspective DOI: 10.1021/acsenergylett.7b00189 ACS Energy Lett. 2017, 2, 1223−1231 1224 nanoscaling effects, along with key references. It is worth noting that there are many other references that are also important to this field, but an exhaustive list cannot be provided in this short Perspective. (For a more comprehensive review of the field, please see ref 4.) As can be deduced from Table 1, the complexity of multiple, often synergistic, nanoscaling effects that influence the catalytic behavior of gold necessitates application of combined advanced experimental and theoretical approaches before a complete understanding of the chemistry will be possible. Selective Catalytic Processes by Au/TiO2: Relation to Renewable Energy Production. As researchers continue to uncover fundamental insight into the structure and functionality of Au-based catalysts, others have forged into the use of Au-based catalysts for energy-related applications. Researchers have now effectively demonstrated the potential activity and selectivity of Au-based systems in catalytic conversion of renewable chemicals, including biomass-derived substances, into a diverse array of valued compounds such as fuels, fine chemicals, polymers, and a variety of commodities. Carbohydrates, lignin, vegetable oils, and many other substances extracted from biomass can serve as renewable feedstock and potentially replace petroleum-derived chemicals. A variety of catalytic processes based on basic chemical reactions such as oxidation, reduction, hydrogenation, isomerization, etc. have been studied on both model and real gold catalysts. Below, we briefly highlight the potential of Au/TiO2-based systems as catalysts for selective oxidation and selective reduction reactions with compounds derived from renewable sources. Selective Catalytic Oxidation. The ability of supported nanoscale gold to adsorb and activate oxidant molecules such as molecular O2, water, and H2O2, and thus to promote chemical transformations with high selectivity under mild conditions, make gold systems ideal catalysts for a variety of oxidation reactions. More specifically, these include but are not limited to (a) selective oxidation−dehydrogenation reactions of primary alcohols, aldehydes, and carboxylic acids; (b) selective oxidation of sugars (glucose, arabinose, galactose, and other sugars); and (c) selective oxidation of secondary and poly alcohols. Each of these oxidation reactions has potential for enabling utilization of biomass-derived compounds and other energyrelated applications. For the simple reaction of CO oxidation, both model and real gold catalysts exhibit significant structural sensitivity. The measured turnover frequency for CO2 production (molecules of CO2 formed per Au atom per second) showed a volcano relationship with a maximum at Au particle sizes of 2−3 nm (Figure 2A). Analogous relationships have also been found for oxidation of much more complex molecules like D-glucose and L-arabinose. This chemistry is driven by LC atoms on small Au nanoparticles (Au-NPs) that serve as sites for adsorption and activation of reductant molecules, e.g. CO, H2, 17,49 arabinose, etc. As shown in Figure 2C, the relative number of LC atoms is size and shape dependent. Moreover, activation and scission of bonds such as H−H, C−H, and C−O may occur at such sites. Experiments have shown that the adsorption and activation of O2 on Au/TiO2 is strongly dependent on the temperature. Thus, the CO oxidation mechanism and the active sites differ for the low-temperature (<320 K) and high-temperature regions (>320 K). Some theoretical (density functional theory) studies propose that at low temperature, O2 activation occurs at Au−Ti dual perimeter sites via formation of a Au−O−O−Ti intermediate (Figure 3a), whereas other studies find an adsorption configuration with O2 directly binding at the Au structure (Figure 3b). At higher temperatures (∼420 K), O2 can dissociate at the perimeter sites forming O adatoms that can readily interact with bound hydrocarbon intermediates either via nucleophilic attack on CO and CC bonds or via activation of weakly acidic C−H or O−H bonds. The Au−O−O−Ti intermediate is very similar in character to Figure 2. (a) Comparison of the catalytic activity versus Au particle size dependence for oxidation of CO to CO2 on Au/TiO2 catalysts prepared by various chemical ways. For details, see ref 19. Copyright 2011 American Chemical Society. (b) Gold particle shapes with lowest energy found in the simulations for different particle sizes. (c) When the particle becomes sufficiently small, the fractions of sixand seven-coordinated corner and edge atoms become comparable to the fraction of surface atoms. Reprinted with permission from ref 20. Copyright 2011 Elsevier B.V. Table 1. Fundamental Aspects of Nanoscaling nature of nanoscaling effect references A. Predominantly geometric effects a. fraction of LC sites, relation to