Role of the Support in Gold-Containing Nanoparticles as Heterogeneous Catalysts

Cite this: Chem. Rev. 2020, 120, 8, 3890–3938
Publication Date (Web):March 30, 2020
Copyright © 2020 American Chemical Society
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In this review, we discuss selected examples from recent literature on the role of the support on directing the nanostructures of Au-based monometallic and bimetallic nanoparticles. The role of support is then discussed in relation to the catalytic properties of Au-based monometallic and bimetallic nanoparticles using different gas phase and liquid phase reactions. The reactions discussed include CO oxidation, aerobic oxidation of monohydric and polyhydric alcohols, selective hydrogenation of alkynes, hydrogenation of nitroaromatics, CO2 hydrogenation, C–C coupling, and methane oxidation. Only studies where the role of support has been explicitly studied in detail have been selected for discussion. However, the role of support is also examined using examples of reactions involving unsupported metal nanoparticles (i.e., colloidal nanoparticles). It is clear that the support functionality can play a crucial role in tuning the catalytic activity that is observed and that advanced theory and characterization add greatly to our understanding of these fascinating catalysts.

1. Introduction

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Heterogeneous catalysis, where the reactants are in a different phase (typically gas or liquid) from the catalyst (typically solid), plays a central role in the modern-day production of chemicals and fuels. (1) In 2016, the total catalyst market size was reported to be somewhere around $23 billion and is expected to reach $40 billion by 2022, with an annual growth rate of 4.8%. (2) Supported metal catalysts are an important class of catalysts widely used in industry for several reactions including oxidation, (de)hydrogenation, hydrogenation, hydrotreating, deNOx reactions, ammonia synthesis, and Fischer–Tropsch synthesis. In these catalysts, the metal component is often expensive and is used in very small amounts (typically <5%). (1) Smaller metal particles tend to have more active sites exposed compared to larger metallic particles. Hence, metal particle size is a critical structural parameter that determines the catalytic activity of supported metal catalysts. (3) Common catalysts contain very small metal particles, typically in the nm range, which are dispersed onto a high surface area refractory support. Even before the explosion of nanoparticle synthesis methodologies in the recent nanotechnology era, small metal nanoparticles (NPs), dispersed on a solid support material were already being used as catalysts. (4)
Colloidal gold NPs are among the earliest nanomaterials to be produced and exploited in a technological application. For instance, gold colloids were used to introduce the dichroic behavior in the now famous Lycurgus Cup, which dates back to the Romans in the fourth century AD. (5) Since then, gold and other metal NPs have been used to a generate range of colors in glassware and windows. Another important milestone in nanotechnology was the synthesis of stable colloidal Au NPs by Faraday in the mid-19th century. (6) Despite its long history of use in metallurgy and glass technology, because gold was considered to be the archetypal unreactive noble metal, it was incorrectly assumed to be a poor candidate for a catalyst material. This perception changed dramatically after two seminal discoveries in the 1980s. Haruta found that supported gold NPs display unparalleled catalytic activity for low temperature CO oxidation. (7) Hutchings predicted and demonstrated that supported gold is the catalyst of choice for producing vinyl chloride monomer via the acetylene hydrochlorination reaction. (8) Since then, gold catalysis has become the subject of received intense attention from both the academic and industrial research communities. (9−11) It has become clear that the identity of the support material and the gold–support interfacial sites generated often play a crucial role in determining the catalytic behavior of supported gold NPs. (10,12)
In the field of supported metal catalysts, support materials were often considered to be inert and their primary role was to enhance the stability of the small metal particles via anchoring. In the 1970s, Tauster introduced the term strong metal–support interaction (SMSI) to explain the unexpected H2 chemisorption properties of noble metal NPs supported on TiO2. (13,14) After this report, the potential of the support playing a more active role has been investigated at a fundamental level, mostly using model catalysts. The role of support in practical catalysts is rather more complex and is still not clearly understood in many cases. Supports can play different roles during a catalytic reaction either directly or indirectly. These include providing specific defects sites onto which the metal NPs can be anchored and even stabilized in the case of metastable particles. The support can also enable electron transfer to or from the metal particles and provide additional functionality such as acidity or basicity to the overall supported metal catalyst. All of these factors can dramatically affect the catalytic properties of supported metal materials, and this is especially true for supported Au-based nanoparticle catalysts.
Because of the recent advances in spectroscopic and microscopic methods to characterize supported metal catalysts, a number of articles have now been published on the role of support on the catalytic properties of supported gold-based catalysts, including both monometallic and bimetallic catalysts. In this review, we discuss selected articles in which the role of support on the catalytic properties has been specifically explored. In the first part of this review, we discuss the role of support material in influencing the structural properties of Au-based NPs. Following that, each subsequent section reviews the role of the support on the catalytic properties of Au-based NPs for specific reactions, including CO oxidation, liquid phase alcohol oxidation, hydrogenation/hydrogenolysis, and C–C coupling reactions. The final section of this review briefly discusses the catalytic properties of unsupported gold-based colloids. We have primarily covered selected articles published over the last nine years on this topic, and we conclude the review with a summary and future outlook for this particular line of catalyst research.

2. The Role of the Support during Catalyst Synthesis

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The role of support in Au catalysts can be first seen during the catalyst preparation, which has been a challenging task partly due to a relatively low melting temperature of Au (1064 °C) compared to that of other precious metals such as Pd (1555 °C) and Pt (1768 °C). Historically, the discovery that Au-based catalysts can be effective in many industrially important reactions is owed to the successful development of various preparation methods for Au catalysts pioneered by Haruta. (7) The development and the choice of synthesis method are often dictated by the physical and chemical nature of the support materials, including surface area, surface acid/base properties, possible phase transformations, and the presence of defects. Because Au catalysts are well-known to be sensitive to the synthesis methods, clarifying the role played by the support materials for Au catalysts in the preparation process is the first step toward developing an overall understanding of their role in any specific catalytic processes.
This section aims to provide a brief overview of common supported catalyst preparation methods for Au catalysts. In some cases, the support materials interact with Au species during the catalyst preparation process to facilitate a high dispersion of Au. They can also serve as anchor sites to stabilize the catalytically active species. These will be mainly discussed in the section 2.1, Preparing Au Catalysts on Oxides and Other Conventional Supports. In other cases, support materials carry specific morphologies and functionalities that need to be preserved or achieved during the catalyst synthesis procedure. These will be discussed in the section 2.2, Preparing Au Catalysts on “Engineered” Nanostructured Support Materials. The following discussion will focus on supported monometallic Au catalysts. Publications on nanoporous Au and Au alloys will be excluded from this section.

2.1. Preparing Au Catalysts on Oxides and Other Conventional Supports

Early attempts at preparing Au catalysts on refractory supports was achieved via conventional impregnation methods using chloroauric acid (HAuCl4) as the gold precursor. Relatively large Au particles (i.e., >30 nm) were usually generated after the calcination step, and the resultant catalysts were ultimately not active. (15) Later research concluded that the ineffectiveness of the impregnation method was due to (i) the weak interaction between the AuCl4 ion with the oxide surface in an acidic (pH ∼ 1) environment, (ii) particle growth during the calcination step when the chlorine content is high in the catalyst (16) possibly due to a weak bonding between the cationic Au (a soft acid) and the chlorine ion (a hard base), and (iii) chlorine poisoning of the active sites. (16,17) Therefore, alternative methods for preparing Au catalysts were developed to counter these adverse effects, and these are described in this section. An alternative method using sol-immobilization is discussed subsequently in the review as this is a means of fine-tuning the nature of the nanoparticle prior to being supported.
Deposition–precipitation (DP) is one of the main methods for preparing supported Au NPs. Haruta and co-workers (18,19) first succeeded in making active Au catalysts using this method in the early 1990s. In a typical synthesis procedure for Au/TiO2, preformed TiO2 particles are first mixed with an aqueous solution of HAuCl4. The pH of the mixture is then adjusted using a base (e.g., NaOH) to a pH between 6 and 10 and aged at 70 °C for an hour. The suspension is then washed, dried, and calcined at 400 °C for 4 h. Similar methods have been used for depositing Au on other oxide supports such as Fe2O3, (20) Al2O3, (17) and MgO. (19) A variant of the DP method utilizing urea instead of a strong base like NaOH for preparing Au catalysts was first attempted by Dekker et al. (21) and then later by Zanella et al. (22) In this method, the hydrolysis of urea (CO(NH2)2) permits gradual and homogeneous control of the OH concentration and avoids local increases in pH and subsequent precipitation of Au(OH)3 away from the support. The DP method is also effective on other types of supports, such as activated carbon, (23) nanodiamond, (24) phosphates, (25,26) hydrotalcite type layered double hydroxides, (27,28) g-C3N4, (29) and hydroxyapatite (HAP). (30) The DP method can even be used to prepare single-atom dispersed Au catalysts. (31) By using UV irradiation instead of a heat treatment, Flytzani-Stephanopoulos and co-workers (32) prepared a Au/TiO2 material with atomically dispersed Au species that was shown to be active for the low-temperature water–gas shift (LT-WGS) reaction.
During DP synthesis, a key parameter is the surface charge state of the support material, which depends on the pH value and the isoelectric point (IEP) of the oxide. (33,34) For TiO2 (IEP ∼ 4.5–6.3), the surface will be positively charged (terminated by –OH2+) at pH values lower than the IEP and negatively charged (terminated by O) at higher pH values. In conjunction with this, the nature of the Au species generated also depends on the pH value, as well as the concentration of gold and chlorine present, the ionic strength of the solution, and the reaction temperature. According to Moreau et al., (35) the hydrolysis reactions of AuCl4 gives rise to a complex equilibrium of different gold chloro-hydroxy species at a given pH value (Figure 1a). At pH < 2, AuCl4 is the dominant Au containing species; at pH > 8, the dominant species will be Au(OH)4; for pH values between 2 and 8, the major species present are charged AuClx(OH)4–x anions or neutral AuClx(OH)3-x(H2O) species. Therefore, in the DP process, electrostatic interaction between the oxide surface and Au species occurs at lower pH levels, which explains a higher Au uptake under those conditions. However, the Cl content remains high, which could result in large Au particles during the calcination step. The optimum pH for TiO2 usually lies around 6–8, where electrostatic adsorption should not take place. Using X-ray absorption spectroscopy, Louis et al. (36) proposed that in a DP method utilizing NaOH, Au species were grafted onto the OH groups associated with the TiO2 support, forming Ti–O–Au(OH)3 metal complexes. This explains the relatively low Au uptake (i.e., 60% at pH = 6) (37) from the Au precursor solution, which is one of the main limitations of the DP method. The DP method utilizing urea allows a higher Au loading be achieved (i.e., 8%) through the precipitation of nitrogen-containing amorphous compounds or the adsorption of an ammino-hydroxo-aquo cationic gold complex. (38,39) At pH values >9, the Au loading is limited by the increasing solubility of the Au hydroxide species. (38)

Figure 1

Figure 1. Relative calculated equilibrium concentration of gold complexes ([Cl] = 2.5 × 10–3 M) as a function of the pH of the solution. Reproduced with permission from ref (35). Copyright 2005 Elsevier.

Another significant limitation of the DP method is that it does not work well for oxides having a low IEP, such as SiO2 (IEP < 2–4). (34) Anion adsorption (AA) methods have also been attempted by many researchers. In an AA process, the surface of the support is tuned to be positively charged, on which the negatively charged gold chloro-hydroxy species can gradually electrostatically adsorb. The AA process usually takes a long time to complete (typically ∼16 h). This AA method was used by Zenella et al., (22) and Au particles smaller than 4 nm supported on TiO2 were achieved. Lessard et al. (40) used this approach and made active gold catalysts for low-temperature water–gas shift reactions on La2O3 and La2O2SO4. Because of the high IEPs of these supports, the pH of the mixture was tuned so that the main species adsorbed was Cl-free (i.e., Au(OH)4).
The Au uptake and highest Au loading attainable depends on the nature of the support. By carefully tuning the HAuCl4 concentration and the pH of the solution, Pitchon, Petit, and co-workers (41,42) achieved 100% Au uptake and a 2% final loading on an Al2O3 support. Furthermore, no gold was lost during the filtration and washing steps. It was also suggested that there is some kind of anion exchange process taking place between the Au species with the surface hydroxyl groups associated with the Al2O3 support. A similar method was later used by the same group to disperse Au onto layered double hydroxides. (43) Nguyen et al. (44) reported a better thermal stability of the Au/γ-Al2O3 catalysts prepared by the AA method compared to those prepared by conventional deposition precipitation (DP).
Single-atom Au catalysts can be also prepared using the AA route. Wang et al. (45,46) reported atomically dispersed gold on ZnZrOx, while Qiao et al. prepared single-Au atom catalysts on Co3O4, (47) CeO2, (48) and FeOx. (49) The metal loading of these catalysts were usually kept at a very low level of around 0.05 at %.
For a SiO2 support that has a low IEP, cationic adsorption is one possible strategy to prepare active Au catalysts. Trichlorobis(ethylenediamine)gold(III) (Au(en)2Cl3) can serve as the cationic precursor, and Au/SiO2 catalysts have been prepared by Zanella et al. (36) and later by Dai and Overbury (50−52) using this cationic absorption approach. As mentioned earlier, in the DP urea or ammonia process, the cationic adsorption of amino-hydroxo-aquo cationic Au complexes were thought to be taking place. (39,53)
Preparing Au/SiO2 with the more readily available HAuCl4 precursor can be done by adjusting the surface functionality of the SiO2. Amine functional groups can provide positively charged aminium ions in an acidic solution, and therefore AuCl4 anions can be electrostatically adsorbed. In a 2009 study by Liu et al., (54) commercial SiO2 supports were refluxed with APTES (H2N(CH2)3Si(OEt)3) in ethanol for 24 h, so that the amine functional group can be grafted onto the support before adding HAuCl4. This effectively shifts the IEP of the SiO2 to a higher value. (55) In a more recent study, branched polyethylenimine was used to functionalize the SiO2 to anchor glutathione-protected Au clusters. (56) It should also be noted that Au catalysts on silica supports can also be prepared using a double-support strategy, which can in some sense be considered as functionalizing the SiO2 surface with another oxide. For instance, Au catalysts on TiO2, (57) CoOx, (58) and FeOx-modified (59) SiO2 supports have been reported.
Functionalizing activated carbon surfaces by acid washing has been studied by Willock and co-workers. (60) It was found that washing the carbon support with nitric or hydrochloric acid can almost exclusively generate surface hydroxyl groups, which can better assist the nucleation of Au particles compared to a carbon surface covered with ketone groups.
Another way of achieving high dispersion of Au and an intimate contact with the support is to prepare both components cooperatively. The most common approach is the coprecipitation (CP) method, developed by Haruta and co-workers (18,61) for Au catalysts. They reported that certain oxide supports (e.g., α-Fe2O3, Co3O4, and NiO) can be precipitated out from the solution largely simultaneously with Au, therefore ensuring good mixing. Haruta’s original method involves quickly pouring an aqueous solution of HAuCl4 and the corresponding metal nitrate precursors into an aqueous solution of sodium carbonate (Na2CO3). The resultant precipitates (usually hydroxides) will then be subjected to washing, vacuum drying, and calcination, typically at 400 °C to form the final catalyst. In this CP process, through a quick mixing of the acidified precursor solution into the basic Na2CO3 solution (pH = 8), the AuCl4 will undergo the hydrolysis process described earlier and release Cl, which can be further removed during the washing step. The final Au/metal oxide catalyst is generated during the calcination step. As confirmed by XRD and TEM characterization, at Au loadings of 5–10 at % with respect to the support transition metal, Au NPs below 10 nm in size are generally formed. (18)
Andreeva et al. (62) and Hutchings and co-workers (63,64) later reported slightly different CP methods for preparing such metal oxide supported Au catalysts. The modification comes from varying the sequence of mixing the acid and base precursors as compared to Haruta’s original method. (18,61) In this case, the Na2CO3 solution was added gradually into an aqueous solution of metal nitrates and HAuCl4 until a pH of 8–9 was attained, followed by the usual washing, drying, and calcination steps. The resultant catalysts were shown to be active for low-temperature water–gas shift, (62) CO oxidation at ambient temperature, (63) and the direct synthesis of hydrogen peroxide. (64)
With the advances of aberration-corrected scanning transmission electron microscopy (AC-STEM), Herzing et al. (65) reported that Au subnanometer clusters and isolated atomic Au species also exist in the catalysts prepared by the CP method. Furthermore, it was proposed that the subnanometer clusters might be responsible for the high activities observed in CO oxidation reactions rather than the Au NPs. Using cyanide leaching and in situ electron microscopy, Allard et al. (66) demonstrated that a significant amount of atomically dispersed Au can be trapped inside the CP generated support materials, which can subsequently diffuse outward to the surface during subsequent heat treatment (Figure 2). This is not too surprising considering the nature of the CP method. He et al. (67) later studied Au on Fe2O3e prepared by the above two CP methods and found that a larger fraction of Au can be trapped into the support when the acidic and basic solutions are mixed quickly (i.e., via Haruta’s original method). The dynamic evolution of Au species during the heat treatment (e.g., via diffusion and aggregation of Au on the surface and via outward diffusion of trapped internal Au species) determines the final population distribution of Au species on the oxide surface as well as the catalytic activity of the material in the CO oxidation reaction.

Figure 2

Figure 2. High angle annular dark field images of the leached catalyst showing changes as a result of in situ heating. Very little change was seen with time at 250 °C. (a) Starting image recorded after a few minutes at temperature. Only slight changes were seen after 2 min at 500 °C (b), but an additional 5 min at 500 °C showed void shrinkage, coalescence and growth of Au NPs, and the diffusion of Au species to the surface to form discrete nanocrystals (arrowed). Reproduced with permission from ref (66). Copyright 2009 Oxford Academic.

Kudo et al. (68) further modified Haruta’s CP method by adding HAuCl4 approximately 1.5 h after mixing the metal nitrates with Na2CO3. They found that this procedure significantly increased the available Au sites for CO adsorption, probably because the support and Au no longer precipitate simultaneously, meaning that much less Au will be trapped inside the support. The method then becomes very similar to a deposition–precipitation method. Another modification of the conventional CP method was reported later by Zhang et al. (69) Here, an electrochemical approach was used to monitor the concentration of Cl ions in the solution mixture after coprecipitation, which is thought to affect the precipitation of the support and gold hydroxides. The most active catalysts can then be reliably reproduced when the [Cl] concentration lies in the 1–3 ppm range and the Au NPs are thought to be mainly sitting on the edge of nanosized Fe2O3 particles after calcination.
One recent example of Au catalysts prepared by coprecipitation is the Au/α-MoC catalysts reported by Ma and co-workers. (70) The catalysts were prepared by mixing aqueous solutions of (NH4)6Mo7O24·4H2O and HAuCl4, followed by washing, drying, and a 500 °C calcination step. AC-STEM characterization of their materials confirmed the formation of epitaxial Au rafts, 1–2 nm in diameter and 2–4 atomic layers thick, grown on an α-MoC support, which are highly active for the water–gas shift reaction even at room temperature.
Finally, heat treatment is usually needed when preparing Au catalysts in order to convert the precursors of Au and support oxides into their active forms. One of the key roles of the support is to stabilize the Au species and maintain their high dispersion. Recently, de Jongh and co-workers (71) showed that on a TiO2 support, Au particle agglomeration can be accelerated by the presence of water and/or the presence of Cl, and it is more pronounced in an oxidizing atmosphere. In contrast, Au on nonreducible supports such as SiO2 and Al2O3 are remarkably stable in a nonoxidizing atmosphere. In another study by Zhang and co-workers, (72) 2–5 nm Au particles epitaxially supported on MgGa2O4 spinel were shown to retain their original particle size even after heating above the melting temperature of bulk gold (1064 °C), demonstrating the potential efficacy of particle stabilization effects from the support lattice.

2.2. Preparing Au Catalysts on “Engineered” Nanostructured Support Materials

In many cases when synthesizing Au catalysts, efficiently and homogeneously dispersing the Au is the main concern during the preparation, especially when commercial metal oxide support materials are used. In other cases, however, creating and/or maintaining the special nanostructure of the support material can be equally important. Support materials with well-designed nanostructures and architectures can not only bring additional functionalities to the Au catalysts, but it also can serve as model catalysts for mechanistic investigations. (73) In this section, we will discuss the preparation of Au catalysts on a variety of support materials having specially designed nanostructures. These include (i) ordered porous materials, such as zeolites, mesoporous silica, and metal–organic frameworks (MOFs), (ii) Au catalysts with iron oxide heterostructures that allow magnetic separation, and (iii) Au catalysts with yolk–shell or core–shell nanostructures and so-called “inverse” oxide/metal catalysts.

2.2.1. Au on Mesoporous Supports

Mesoporous silica, zeolites, and other materials with controlled pore structures are widely used as catalysts and support materials. (74) Preparing Au catalysts on such mesoporous materials is potentially attractive for at least two reasons: First, it is harder for Au species in the mesoporous materials to migrate and agglomerate due to the geometric constraints imposed by the structure, thus resulting in Au catalysts having superior stability compared to those prepared on conventional high surface area metal oxide supports. For example, Dayte and co-workers (75) showed that the sintering of Au particles is dependent on pore size, pore wall thickness (which determines the pore wall strength), and pore connectivity. For instance, Au supported on materials with two-dimensional pore structures and lower connectivity (e.g., SBA-15) showed better stability compared to those supported on materials with 3-D pore structures and high interconnectivity (e.g., SBA-12), in which Au can migrate more easily between pores and even to the outer surface (Figure 3).

Figure 3

Figure 3. HAADF-STEM images of Au catalysts samples after reduction at 200 °C for 2 h in flowing hydrogen supported by (a) MCM-41, (b) SBA-15 with 4.8 nm thick pore walls, (c) SBA-15 with 2.6 nm-thick pore walls, (d) HMM-2, (e) SBA-12, and (f) SBA-11. Reproduced with permission from ref (75). Copyright 2005 American Chemical Society.