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Catalysts 2012, 2(1), 24-37; doi:10.3390/catal2010024
Abstract: Gold has been considered as an active catalyst only when suitable techniques of preparation provided high metal dispersion. A comprehensive survey of the different methods now available for preparing active gold catalysts is reported with particular attention to the role of the supporting material in determining catalyst characteristics.
Gold catalysts have now reached an established important role in the field of catalysis since Haruta  and Hutchings  disclosed the peculiarity of the activity of this metal in CO oxidation and ethylene hydrochlorination 20 years ago. The use of gold catalysts has been enormously expanded [3,4] but, since the beginning of its story, the use of gold for manufacturing new catalytic systems was affected by the high variation of the catalytic results depending on the preparation method employed related to the support used . In fact many studies stated the importance of the combination of these parameters in determining the morphology of the gold particles and metal-support interaction, both able to profoundly modify the activity and/or the selectivity of the whole catalyst. Therefore a lot of attention has been paid to the preparation of gold catalysts in order to assess as much as possible the relation between support/preparation method and characteristics of the produced materials.
The aim of this paper is to give the reader a survey of the most recent techniques related to any single support in order to provide a rapid guide in the choice of suitable preparation method depending on the desired application.
All the general methods for preparing metal supported catalysts have been tested also in the case of gold. However, gold presents peculiar characteristics, namely a low melting point and low affinity for oxides, which contrast the production of gold particles with a catalytic relevant activity, i.e., with nanometric (1-9 nm) diameter. In this paper, attention will be paid both to the most commonly used preparation methods, i.e., impregnation, deposition-precipitation, coprecipitation and to special techniques such as vapor-phase deposition, sol immobilization and grinding.
2. Results and Discussion
In this method the support is contacted with a solution of the metal precursor, and then it is aged, dried and calcined. Depending on the volume of solution with respect to that of the support, two types of impregnation can be distinguished: the so-called “incipient wetness” impregnation if the solution volume does not exceed the pore volume of the support and the “wet” impregnation when an excess of solution is used. The characteristics of the catalyst strictly depend on the post-treatment conditions (rate of heating, time, final temperature, atmosphere) and, obviously, on the type of supporting materials. In fact during the calcination step sintering of the precursor and reaction between the metal precursor and the support might occur. Moreover the use of different conditions can lead to different metal-support interaction, of fundamental importance for the catalytic applications. It should be noted that in the case of gold, the oxide spontaneously decompose to the metal at T > 200 °C. As the effect of temperature is normally detrimental from the metal dispersion point of view, sometimes a reduction step with H2 or, under basic conditions with HCHO or HCOOH can be used instead of calcination [11,12,13,14].
Conventional impregnation with chloroauric acid (HAuCl4) has shown to produce much lesser active gold catalysts than deposition-precipitation (DP) or coprecipitation (CP). However, the simplicity of the methodology and the convenience of using chloroauric acid as the gold source, make impregnation attractive for industrial scale-up purposes, and because of that, much research has been dedicated to improving the preparation method. The effect of the precursor solution aging in the impregnation preparation of Au/γ-Al2O3 for CO oxidation was studied . Solutions of HAuCl4 were aged in pH ranging from 5 to 11 for 15–720 min. UV/vis spectroscopy showed that during aging, hydrolysis of the AuCl4− complex gave rise to several species. It was found that Au particle size and loading were influenced by the precursor speciation. A significant improvement in the impregnation method has been reported using a two-step procedure . Impregnation of alumina with chloroauric acid was followed by washing off the excess of Au precursor and the solid is treated with a strong base to convert the chloride to an absorbed hydroxide. Drying and calcination at 400 °C yielded a catalyst with Au particles having an average diameter of 2.4 nm. The activity of the catalyst is comparable to catalysts prepared by deposition precipitation and is stable to hydrothermal sintering. Quite recently  it was also disclosed that in addition to the pH effect, a relevant role is played also by Cl− concentration in the solution. In fact HAuCl4 dissolved in 2 M HCl, after reduction by H2 at 250 °C, produced on Al2O3 metallic particles of 1–2 nm mean diameter. It should be noted that, in contrast to gas phase oxidation, the catalytic behavior in liquid phase oxidation appeared to be not affected by the presence of chloride. A detailed study on the ion exchange between Al2O3 and HAuCl4 has also been reported as a function of pH which in turn can modify both the species in the solution and the charge on the support surface [18,19,20]. It should be noted that this ion adsorption strictly depends on the isoelectric point (IEP) of the supporting material. Oxides with IEP around 7 (TiO2, CeO2, ZrO2, Fe2O3) produced very active species, whereas acidic support (SiO2) or basic (MgO) usually appeared less active. In this regard, the amphoteric character of Al2O3 (IEP 8–9) is more sensitive to pH variation.
The impregnation method using as-precipitated wet metal hydroxides as supports and Au phosphine complexes as gold source has been studied [21,22,23,24]. As-precipitated hydroxide was impregnated with an acetone solution of Au(PPh3)(NO3), probably because a high concentration of OH at the surface is needed for a suitable interaction between the support and the organo-gold complex. This method has also been named as liquid-grafting (LG). Vacuum drying at room temperature and calcination in air followed. Changes in the activities of both Au/Fe oxide and Au/Ti oxide catalysts have been attributed mainly to changes of the Au particle size distribution occurring during calcination.
The interaction between the charge that the surface assumes in solution and ionic gold species, can be fruitfully used for preparing Au supported catalyst. Any surface is characterized by a pH called PZC (point of zero charge) or IEP (isoelectric point) at which the surface is neutral. At pH below the IEP the surface is negatively charged while at pH over the IEP, the surface is positively charged. The value for some oxides is reported in the literature . Thus cationic complex such as Au(ethylendiamine)2Cl3 can interact with negatively charged carbon surface in aqueous solution  providing Au well dispersed on the support surface. In contrast, using HAuCl4, largely aggregated Au metallic particles were obtained principally due to the redox properties of activated carbon  (Figure 1). In fact, the higher variety of activated carbon functionalities than those present on oxides can promote other electrostatic interactions.
An aqueous solution of HAuCl4 and a water soluble metal salt, generally nitrates, are poured in an alkaline solution. After precipitation the hydroxides or carbonates are filtered, washed, dried and then calcined. This technique can be applied to salts of metals in the first row of the transition series in groups 4–12 and also to Al and Mg, which can be precipitated as hydroxides or hydrated oxides.
Na2CO3 or K2CO3 are widely used to adjust the pH during the coprecipitation process. In contrast to NaOH, carbonates improve the stability of pH. The use of urea as a neutralized was also introduced . Since precipitating gold hydroxide Au(OH)3 is rapidly transformed into soluble Au(OH)4− by increasing the pH, the most efficient range for the precipitation is 7–10. Bond and Thompson [3,4] suggested ammonium carbonate or bicarbonate as more suitable bases because their ions readily decompose during calcination. Most authors indicate that after co-precipitation the solid is washed until no chloride is detected in the washing solution (i.e., reaction with silver nitrate).
Using this method Au on α-Fe2O3 [29,30,31,32,33,34,35,36,37,38,39,40], MnOx , ZnO [42,43,44,45] can be prepared with good dispersion. Addition of magnesium citrate during or after the precipitation was shown to be beneficial in some cases such as for Au/TiO2 . The effect of digesting time, calcination temperature and relative ratio of the precursors were studied concerning CO oxidation in the presence of O2 or H2 (PROX), but no general trends have been depicted.
This technique consists of the precipitation of a metal hydroxide or carbonate on the particles of a powder support via the reaction of a base with the precursor of the metal, and was the first efficient method reported to produce highly active gold supported nanoparticles . Rapid nucleation and growth in the solution leads to large crystallites unable to enter into the support pores, leading to a heterogeneous distribution of the metal. To produce good precipitation distributions, an effective mixing and a very slow addition of the base solution must be accomplished. Urea was found to be the best base and is now widely used in many co-precipitation preparations . It is usually added at room temperature and, by rising the temperature to 90 °C, it slowly hydrolyses generating ammonium hydroxide homogeneously through the solution. The rate of precipitation is generally higher than that of hydrolysis and, in this way the pH of the solution remains practically constant. After the deposition-precipitation step, the product is filtered, washed, dried and calcined as in the coprecipitation procedure. The optimum pH range for precipitation that also assures an efficient metal utilization (>90%) is primarily dictated by the isoelectric point (IEP) of the supporting material. This leads to the main constraint of this method, that is, the inapplicability of the so called acidic oxide (IEP < 5) such as SiO2 (IEP = 2). This method offers the advantage of locating all the active metal onto the support surface, so that no precious metal is wasted in the bulk of the support as in coprecipitation. It is also capable of producing very narrow particle size distributions. However the result is very sensitive to the nature of the support. For example although this method is well established for the preparation of Au on non-acidic oxides, the use of this method applied to active carbon failed in producing highly dispersed Au nanoparticles (Figure 2).
A reduction step is normally needed after DP deposition. Generally this step is carried out by calcination at T > 500 K and usually enlargement of Au nanoparticles is observed by increasing calcination temperature. Quite recently chemical reductions have been proposed with the advantage of obtaining smaller nanoparticles . It should be noted, however, that post-treatment at high temperature is expected to increase the support-metal interaction (SMSI) that can play a significant role in catalytic uses [49,50].
These methods could be referred as particular cases of impregnation and can be subdivided as:
a. Metal vapor deposition;
b. Solid grinding;
c. Metallic sol immobilization.
2.4.1. Metal Vapor Deposition
Dimethyl-Au(III)-acetyl acetonate can be vaporized in a vacuum system by heating and deposited on support . The method is highly efficient and does not suffer the limitation of DP preparation, i.e., it is applicable to any kind of support. Ligand exchange between Me2Au(acac) and the surface hydroxyl groups and adsorbed H2O molecules (Al2O3)  and hydrogen bonding between oxygen atoms of Me2Au(acac) and hydroxyl groups on the support surface (SiO2) [52,53] assured the immobilization of the gold precursor on the support surface. After deposition, the samples were calcined at 573 K to burn out the organic ligands in the gold precursor. The activity observed for prepared catalysts generally increased with their gold dispersion characterized by transmission electron microscopy. Finely dispersed Au/Al2O3 (with d(Au) < 5 nm) catalysts may be easily prepared by the CVD method . Au/Al2O3 samples prepared by the traditional DP method contained many large gold crystallites (d(Au) > 7 nm).
More complex, is the improved impregnation method reported by S.H. Wu et al. , namely solvated metal atom impregnation (SMAI). This method involves the preparation of an air-sensitive and thermally-unstable bis(toluene)Au(0) complex solution at −196° under dynamic vacuum. The mean diameter of Au particles prepared by SMAI, results in generally smaller particles than those prepared by conventional impregnation.
To overcome the limitation of using expensive organometallic precursors, two systems were set up, namely magnetron sputtering and solvated metal atoms dispersion SMAD, which use metallic gold as the starting material.
In the first system a high purity gold attached to a magnetron source was sputtered at an applied power of 14 W in an argon plasma. The sputtered species were deposited onto the support material as it was tumbled in a rotating stainless steel cup . The magnetron sputtering process entails depositing an atomic flux of gold atoms onto the support surface where the atoms nucleate and grow to form catalyst clusters; nanoparticles form because of low interfacial binding energy between the gold and the support and an insufficient concentration of deposited material to create a thin-film or coating on the substrate. The sputtering process is a line-of-sight technique so only surfaces directly exposed to the metal flux will capture atoms. Consequently catalysts are grown in “egg-shell” like configurations on the outside of a support material and not within a support material or in micropores. Gold loading and particle size are a function of deposition time, material volume, and exposed surface area . An interesting example is the deposition of gold on active carbon which produced a highly active catalyst for liquid phase oxidation where also atomic gold was detected . Nucleation sites likely appears to be functional groups and/or defect sites as represented in Figure 3, showing the Au nanoparticles distribution obtained using active carbon (high number of functionalities and high surface area) and graphite (low number of functionalities and low surface area).
In the SMAD process, evaporation at reduced pressure of relatively volatile metals by means of resistive heating or electron beam and subsequent co-condensation at low temperatures of these metals, with the vapor of organic solvent on the walls of a reactor cooled with liquid nitrogen. In the vapor phase, in excess of organic vapors, metal atoms are present which are then trapped in the frozen matrix of the solvent. When this frozen metal/organic mixture is allowed to warm, nucleation and growth take place and a colloidal dispersion of the metal was then obtained after melting. SMAD can be used to prepare supported Au catalyst simply by mixing SMAD with solid support and in such a way Au particles of 2 to 15 nm are deposited on oxide and polymers [59,60,61,62,63]. As in the case of Magnetron sputtered catalyst, AuNPs can be considered as “naked” NPs because no strong coordinating ligands are present. The main drawback in employing SMAD for making supported Au catalyst, is the poor reproducibility of the method due to the scarce control in the evaporation of bulk metal.
2.4.2. Solid Grinding
Dimethyl-Au(III)-acetyl acetonate was used as solid material thus avoiding any solvent in a procedure called solid grinding (SI) that was fruitfully applied for preparing gold on porous coordinated polymers  and then extended to other supports such as active carbons . The gold precursor is ground together with the support using different apparatus, such as mortar or ball milling, and then calcined in air at 300 °C. Similar interactions between Me2Au(acac) and the support surface as in the case of vapor deposited can be supposed. The advantage of this technique is the easy applicability to each kind of support.
2.4.3. Metallic Sol Immobilisation
The immobilisation of pre-formed metallic sols is also widely applied. The advantage of using this technique principally lies in its applicability regardless of the type of support employed and the possible control on particle size/distribution, obtaining normally highly dispersed metal catalyst [27,65]. The method is based on the preparation of Au nanoparticles and their subsequent immobilization on a support. Therefore generally no subsequent catalyst reduction is needed. Thus modifications on morphology and properties of the material which can occur during calcination are avoided. For catalytic applications particle size has to be ranged from 2 to 25 nm and sols have to show good stability [66,67,68,69]. Common procedures include: reduction of metal salts, photochemical or thermal decomposition, and reduction of organometallic complexes . For improving their resistance against coagulation, aqueous colloidal solution of metal particles have been stabilized by three methods: (a) surface potential and/or charge density are increased by the adsorption of surface active long-chain ions (i.e., surfactants); (b) van der Walls forces are reduced by adsorption of relatively rigid hydrophilic macromolecules (i.e., dextrin, starch); (c) besides these stabilizing effects depending on Coulomb or van der Waals forces, a third type of stabilization, “steric stabilization”, has been considered . A crucial point in this technique is represented by the immobilization step (Table 1 and Figure 4).
Normally this step is simply performed by dipping the support in the sol and metal particles resulted adsorbed from the solution. The kinetic of adsorption depends on sol stabilizer and on the IEP and surface area of the support . Adsorption can be therefore modulated by tuning the pH of the sol.
As shown in Table 1 and Figure 4, the influence of the support nature in obtaining good metal dispersion can be important. As a general trend, it has been shown that metal dispersion increases by increasing functionalities (compare for example Au(PVA) immobilized on carbon nanotubes in Figure 4(a) and on functionalized carbon nanotubes in Figure 4(b)).
Moreover, the nature of the protective agent is an additional factor influencing the metal dispersion. Figure 5 reports the different Au dispersion on activated carbon using different protective agent.
The stabilizer should be carefully checked depending on the nature of support employed. It has been reported that improving the surface functionalization of the supporting material, a decreasing of aggregation is obtained and higher metallic dispersion observed . The presence of the stabilizer should also affect the activity (Figure 6) and can have a positive effect on the durability of the catalyst (Figure 7).
However it has been shown that the protective agent can be removed by thermal decomposition or under milder conditions by solvent washing . This latter method has the advantage of limiting the coarsening of metallic nanoparticles that normally occurs during thermal treatment.
Gold is the most inert of all the metallic elements. However, at a nanoscale level, it presents peculiar properties as a catalytic material. Academic and industrial interests are recorded in the vast amount of literature and patents on this subject. However, much has not yet been understood and apparently similar preparations can lead to catalytic material showing activities of a different magnitude. A lot of parameters are indeed important in gold catalyst preparation (precursor, methodology, support, etc.) and the intricate evaluation of the catalytic activity/selectivity could be restrictive. The industrial use of gold catalysts will be boosted only when stable, long-life catalysts become available. Single cases have been solved but a more general approach is still needed.
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