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 [15]. 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 [16]. 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 [17] 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 [25]. Thus cationic complex such as Au(ethylendiamine)₂Cl₃ can interact with negatively charged carbon surface in aqueous solution [26] 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 [27] (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 [28]. 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 [41], 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 [46]. 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 [47]. 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 [28]. 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 [48]. 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: