DE10205873A1 - Au catalysts supported by metal oxides, processes for their production and their use - Google Patents

Abstract

The invention relates to metal oxide-supported Au catalysts with a narrow Au cluster size distribution and a high degree of dispersion of the Au clusters, obtainable by drying moist Au / metal oxide catalyst precursors obtained in a known manner by coprecipitation, deposition precipitation or impregnation, and drying the precursors in reduced the gas phase with H¶2¶ or CO at temperatures of 250 ° C. DOLLAR A The catalysts of the invention are suitable for selective CO oxidation in reformer gases (PROX), for low-temperature water gas conversion (WGS), for methanol synthesis, for epoxidation of olefins or for the total oxidation of CO, hydrocarbons or halogenated gases (VOC).

Description

The present invention relates to metal oxide-supported Au catalysts narrow Au cluster size distribution and high degree of dispersion of the Au clusters, a process for their production and their use, especially for selective CO oxidation in reformer gases (PROX) and at Low temperature water gas conversion (WGS).

Technical background

The use of metal oxide carriers has been used for around 10 years Gold catalysts discussed with increasing interest and their suitability for a Many different ones, especially at low temperatures carried out, catalytic reactions examined. As technical processes at where such gold catalysts can be used are here in particular mentions the low temperature water gas conversion (also as "Water-Gas Shift Reaction" or "WGS"), the selective CO oxidation in reformer gases (also as "Preferential Oxidation" or "PROX" referred to), both for the purification of fuel gases in the Fuel cell technology is important, as well as methanol synthesis or epoxidation of olefins, especially propene to propylene oxide, all under reducing conditions, or those on the oxidative side, like the total oxidation of VOCs (Volatile Organic Compounds), like CO, Hydrocarbons or halogenated hydrocarbons, in the area of Environmental technology.

From previous studies it is known that significant conversions are only possible with oxide-supported Au catalysts whose Au clusters have a diameter of less than about 5 nm, which corresponds to a degree of dispersion of ≥ 20%. The choice of the oxidic carrier plays an important role especially in those reactions where oxygen species influence the reaction either directly (oxidation with O ₂ ) or indirectly (as with WGS). The positive, co-catalytic effect of easily reducible oxide supports, such as Fe ₂ O ₃ (Fe ₃ O ₄ ), Co ₃ O ₄ , Ni ₂ O ₃ (NiO), Mn ₃ O ₄ or TiO ₂ , which promote the transport of activated Controlling oxygen accelerating to the gold clusters has also been recognized in various reactions.

State of the art

Essentially three methods are used to produce metal oxide supported Au catalysts (Au / MeO _x ). In coprecipitation (CP), water-soluble metal salts and a water-soluble Au compound, usually HAuCl ₄ , are simultaneously reacted with a base in an aqueous medium, the precipitate formed is dried and subjected to calcination in air at temperatures of usually 300 to 400 ° C. In the case of deposition precipitation (also referred to as "deposition precipitation" or "DP"), the carrier component is first deposited in the same way as with CP catalysts, but without the addition of a gold compound. Rather, the gold is deposited on the precalcined metal oxide support. In the impregnation, powdered metal oxides are suspended in water and reacted with a water-soluble Au compound in an aqueous medium with a base. This is also followed by calcination in air at usually 300 to 400 ° C.

A summary of the activity, selectivity and long-term stability of various metal oxide-supported gold catalysts for the PROX reaction in H ₂ -rich gases can be found in Catalysis Letters Vol. 76, No. 3-4 (2001) 143-150. It is mentioned here that calcination in air at 400 ° C. gives gold catalysts with the greatest activity and selectivity for the PROX reaction. A lower calcination temperature of 300 ° C was used only for the Au / Mg (OH) ₂ system, since treatment at 400 ° C gave the corresponding Au / MgO catalyst.

Hyperfine Interactions 126 (2000) 95-99 describes investigations of nano-sized Au catalysts supported on Mg (OH) ₂ and TiO ₂ using ¹⁹⁷ Au-Mössbauer spectroscopy. The catalyst samples were prepared by precipitation precipitation and calcination in air at 473-573K for 4 hours. From this work it can be concluded that Au (+) presumably shows a higher catalytic activity for the oxidation of CO than Au (0).

Catalysis Letters Vol. 77, No. 1-3 (2001) 87-95 describes activity studies of nano-sized Au / CeO ₂ catalysts for low-temperature water gas conversion. It is concluded that the activity strongly depends on the presence of nano-sized CeO ₂ particles. The catalyst samples were produced by coprecipitation, deposition coprecipitation or gelation / coprecipitation with subsequent calcination in air at 400 ° C., 650 ° C. or 800 ° C. The gold particles for the CP catalysts had an average size of 8 nm, while the gold particles for the DP catalysts had a size of at least 4.5 nm.

A modification of the coprecipitation method is described in DE 198 36 585 C1 for the production of Au / Fe ₂ O ₃ catalysts. Here, an iron hydroxide gel is first precipitated, gold is deposited on the still moist hydroxide gel, the reaction product formed is dried and subjected to calcination at temperatures between 350 and 700 ° C. Using this method, Au clusters with a diameter of less than 4.5 nm can be generated.

The coprecipitation method leads to relatively large Au clusters in all cases in which a metal oxide carrier precursor salt which is easily oxidizable by Au (3+) is used, such as the nitrates of cobalt, manganese and cerium or the salts of divalent iron. Evidence of the light Au coagulation in an aqueous suspension can be provided by subsequent reduction of the Au complex deposited on a support or precursor with selectively acting H ₂ O ₂ . If, for example, a gold complex applied by impregnation onto a TiO ₂ support is treated with H ₂ O ₂ in the cold, the formation of large Au clusters becomes visible not only by coloring the suspension, but also in the X-ray diffraction (XRD) spectra of the dried powder; the subsequent calcination does not change the size of the Au clusters. While the usual calcination in air at 300 to 400 ° C without prior wet chemical reduction results in Au cluster sizes between 2 and 3 nm, when using a previous wet chemical reduction, Au crystallites of greater than approx. 8 nm are consistently measured. Corresponding to this Au crystallite size, the catalytic activity in such preparations is very low.

Another practical inadequacy of oxide-borne So far, Au catalysts have failed to succeed in very small nano-Au clusters of <3 nm on pre-calcined (i.e. dimensionally stabilized) oxide supports to be separated, since the subsequent calcination at temperatures of 400 ° C or above to a sharp increase in gold and possibly the oxide carrier crystallite also leads. Generally everyone leads Au impregnation on coarsely crystalline oxides or oxide precursors in the subsequent Calcination tends to lead to larger Au clusters.

The necessity of the previously usual calcinations in an oxygen-containing atmosphere, usually air, before the catalysts are used for catalytic reactions, arises from the relatively high thermal stability of the Au ₂ O ₃ precursor or corresponding organic gold complexes on the surface of the metal oxide support. So z. B. in combined TGA / XRD measurements showed that the gold (3 +) - oxide only changes between 220 and 290 ° C depending on the heating rate to metallic gold.

Aim of the invention

The present invention has for its object, metal oxide-supported Au catalysts with increased activity and selectivity, especially for PROX and WGS applications at low temperature, and more adequate Provide long-term stability and a process for their production.

Summary of the invention

The above object is achieved by Au catalysts supported by metal oxides according to claim 1 and a method for their production according to claim 6 solved. Advantageous or preferred embodiments of the Subject of the invention are specified in the subclaims.

The invention accordingly relates to metal oxide-supported Au catalysts with a narrow Au cluster size distribution and a high degree of dispersion of the Au clusters, obtainable by drying the moist Au / metal oxide catalyst precursors obtained in a known manner by coprecipitation, deposition precipitation or impregnation and drying the dried ones Precursors in the gas phase reduced with H ₂ or CO at temperatures of ≤ 250 ° C.

The invention further relates to a process for the preparation of such Au catalysts, in which one dries moist Au / metal oxide catalyst precursors obtained by coprecipitation, deposition precipitation or impregnation in a known manner, and the dried precursors in the gas phase with H ₂ or CO Temperatures reduced to ≤ 250 ° C.

Finally, the invention also relates to the use of the above described Au catalysts for selective CO oxidation in Reformer gases (PROX), for low-temperature water gas conversion (WGS), for Methanol synthesis, for the epoxidation of olefins or for total oxidation of CO, hydrocarbons or halogenated gases (VOC).

Detailed description of the invention

According to the invention, it has surprisingly been found that it is possible to fix Au clusters in crystallite sizes on the surfaces of the metal oxide supports which are at least about 0.5 nm smaller in diameter than those after the usual calcination in air at 300 to 400 ° C obtained when the pre-dried Au ₂ O ₃ / MeO _x catalyst precursors with H ₂ or CO are subjected to a reduction treatment at temperatures of ≤ 250 ° C. For example, the reduction of an Au / TiO ₂ catalyst precursor with a relatively high Au loading of 4.5% by weight with H ₂ at 200 ° C. leads to Au nanoclusters with a diameter of approximately 1.7 nm, whereas the calcination of this catalyst precursor at 400 ° C. without such a reduction treatment gives Au clusters with a diameter of approximately 2.6 nm. In addition, the Au cluster size distribution for the Au catalysts according to the invention is much narrower (± 0.5 nm) than for the catalysts conventionally produced by calcination (± 0.9 nm). Accordingly, the catalysts according to the invention have an increased degree of dispersion of the Au clusters of at least approximately 55%, compared to approximately 32% in the case of conventionally calcined Au catalysts. This means that the Au catalysts according to the invention have a catalytic activity which is increased by a factor of almost two, as can be demonstrated by the PROX reaction at 80 ° C.

The influence of the temperature in the reduction treatment used according to the invention is shown in FIG. 1. Fig. 1 shows the melting limit of nano-Au surfaces, depending on the temperature and the Au diameter (XRD, TEM) of Au (4.5 wt .-%) / TiO ₂ catalysts, after reduction with H ₂ at various temperatures. It is known from earlier studies that Au particles in the nano-size range of less than about 5 nm begin to melt on the surface far below the melting temperature of solid gold (1064 ° C), and that the melting limit decreases the smaller the Au clusters become , If one or more other Au nanoclusters are in the immediate vicinity of a superficially melting Au cluster, these (Ostwald ripening) coagulate to form larger particles. If one extrapolates the relationships between the size of an Au cluster and its melting temperature down to temperatures of <450 ° C, it results that, for example, at 400 ° C the melting limit with a size of approx. 2.6 ± 0.2 nm and at 300 ° C of about 2.4 ± 2 nm (see solid line in Fig. 1). Without being bound by any theory, all of the even smaller Au clusters are obviously in an unstable solid / liquid state and, depending on the type of carrier oxide, are partially mobile on its surface, so that they easily form larger clusters with other neighboring particles can converge. This interpretation explains in a semi-quantitative manner why, after calcination above 300 ° C, on the one hand Au particle sizes of <1.7 nm are only retained at very high dilution (<0.2% by weight Au), as well as the observation, why carriers with a high specific surface area with the same Au loading provide smaller Au clusters than coarsely crystalline carrier powder.

The full squares in Figure 1 represent the mean values of the Au diameter of the gold reflections [111] and [200] obtained from XRD difference spectra. The circular symbols (full: XRD; empty: TEM) represent the reference samples calcined in air. It can be seen from FIG. 1 that if the temperature during the reduction treatment in the gas phase does not exceed 250 ° C., the Au clusters remain unmelted and in a size range below 2 nm or even at approximately 1.6 nm (reduction temperature at 100 ° C).

Known metal oxide supports are suitable for the Au catalysts according to the invention. However, the metal oxide is preferably selected from the group consisting of Fe ₂ O ₃ , TiO ₂ , Al ₂ O ₃ , CeO ₂ , Ce (Zr) O ₂ , SiO ₂ , ZrO ₂ , Co ₃ O ₄ , NiO, MnO _x and Fe ₂ O ₃ - γ-Al ₂ O ₃ . If TiO ₂ , Al ₂ O ₃ , CeO ₂ , Ce (Zr) O ₂ and ZrO _{2 are used} as metal oxide supports, however, the reduction treatment is preferably carried out at temperatures of 200 200 ° C. In the case of very easily reducible oxide supports, such as Fe ₂ O ₃ or Fe ₂ O ₃ -γ-Al ₂ O ₃ , the gas phase reduction is preferably carried out at temperatures at which the outer oxide layers are reduced at most, ie at temperatures of ≤ 140 ° C. , In the case of metal oxide carriers, such as Co ₃ O ₄ (cobalt spinel) or NiO, which are more easily reducible thermodynamically, the reduction treatment is preferably carried out at temperatures of 80 80 ° C.

The Au loading of the catalysts of the invention can be done in one wide range and is preferably 0.5-8 wt .-% Au, further preferably 2-5 wt% Au.

Furthermore, it is desirable that the catalysts of the invention have the highest possible specific surface area, preferably of at least about 20 m ² / g, more preferably at least about 50 m ² / g according to the BET method. Furthermore, the Au clusters in the catalysts according to the invention have the highest possible degree of dispersion, so that the Au clusters preferably have a diameter of less than about 6 nm, more preferably less than about 4 nm, most preferably 1-3 nm , A high specific surface area and a high degree of dispersion of the Au clusters are particularly advantageous from a kinetic point of view, since the step determining the reaction rate takes place at the gold-metal oxide interface during the CO oxidation. Therefore, with the same Au coating, the degree of dispersion of the gold is very important with regard to the CO conversion rate.

According to a preferred embodiment, a calcination in air at temperatures of at least about 300 ° C., preferably 300 to 400 ° C., is carried out according to the invention after the reduction treatment. It has been shown that only a relatively small enlargement of the Au clusters occurs with such a re-calcination (see also diamonds in FIG. 1; full: XRD; empty: TEM). This applies not only to the Au / TiO ₂ catalysts according to the invention, but also to the other systems, such as Au / Fe ₂ O ₃ and Au / CO ₃ O ₄ . With a predried (200 ° C) Au / Co ₃ O ₄ catalyst precursor after reduction treatment at 70 ° C, a catalyst with highly dispersed (75%) gold and a very narrow Au cluster size distribution of 1.3 ± 0.4 is obtained nm (measured by TEM). If such a catalyst is re-calcined at 300 ° C for stabilization, the Au clusters only grow together very moderately. The degree of Au dispersion obtained here is still approximately 60% with an average size of 1.4 ± 0.6 nm (see also Example 6 below). It has thus been shown that the degree of Au dispersion of the catalysts reduced according to the invention and subsequently post-calcined at temperatures of 300 to 400 ° C. is in any case higher than that of the conventional Au catalysts which have only been calcined. The increased catalytic activity of the catalysts according to the invention is also of particular relevance for technical processes which take place at temperatures higher than 200 ° C., such as the (CO + H ₂ ) reactions, the selective oxidation of propene to propylene oxide (for example with Au / TiO ₂ ) or the oxidative removal of organic trace gases from the exhaust air (for example with Au / Co ₃ O ₄ ).

The Au catalysts according to the invention are generally suitable for selective CO oxidation in reformer gases (PROX), in which Low temperature water gas conversion (WGS), in methanol synthesis, for the epoxidation of Olfefins or for the total oxidation of CO, hydrocarbons or halogenated hydrocarbons (VOC). In particular, the Catalysts according to the invention used for selective Low temperature CO oxidation in reformate hydrogen for PEM fuel cells or for Low temperature water gas conversion (WGS), especially at temperatures of ≤ 150 ° C.

In catalytic PROX activity tests (see also the following examples), an Au / CeO ₂ catalyst was found to be particularly active, in which the Au precursor (Au ₂ O ₃ ) was applied to a commercial cerium dioxide powder with a high specific surface area and the dried catalyst precursor was reduced according to the invention with hydrogen. In this context, it should be noted that with all small-crystalline (diameter <approx. 15 nm) CeO ₂ carriers, there is no full reduction of the trivalent gold to the metal as in the Au / TiO ₂ system in TGA experiments and subsequent XRD measurements (no Au (O) reflex) could be detected. Instead, a reversible reduction of the CeO ₂ surface groups and (from the weight difference after their reoxidation) an irreversible reduction of the trivalent gold was only observed to the level of the monovalent gold. It can therefore be assumed that the exceptional activity of this catalyst treated in this way on the very fast tandem effect of a redox system Au ⁽⁺⁾ / Ce ⁽³⁺⁾ ↔ (CO) _ad -Au ⁽⁰⁾ / Ce ⁽⁴⁺⁾ during the oxygen activation by CeO _{2 is} due.

embodiments

In the following examples with different oxide-supported gold catalysts (supports: TiO ₂ , Fe ₂ O ₃ , Fe ₂ O ₃ -γ-Al ₂ O ₃ , Co ₃ O ₄ , CeO ₂ and Ce (Zr) O ₂ ), the advantage is compared the dry-reductive treatment according to the invention compared to conventional calcination can be seen. The PROX reaction in the idealized reformate (1.0% CO, 1.0% O ₂ , 75% H ₂ , rest N ₂ ) served as a test for the catalytic activity at 80 ° C. in a microreactor under differential reaction conditions (weighed in powdered catalysts after dilution with α-Al ₂ O ₃ , 50 to 100 mg, gas flow 50 to 100 Nml / min.). The measured reaction rates (normalized to the gold mass) are summarized in Table 1 below together with some physical data determined by means of XRD, TEM, ICP-Au analysis and BET.

Although the activities of some Au / MeO _x catalysts prepared by the method according to the invention and presented in the examples do not or only slightly exceed the activity of the Au / Fe ₂ O ₃ catalyst powder (example 7) described in DE 198 36 585 C1, it should be noted that it turns out to be difficult to apply the method used there to the production of conditioned porous pellets or similar 3D structures. In addition, the reproducibility in the production by impregnation is generally much higher than in the coprecipitations or in the "precipitations-deposition" method, where the deviations can be up to 25%. As Example 8 shows, a uniform pre-impregnation of the porous matrix with the ferrihydrite precursor is not successful, so that when Au is subsequently coated, a large part of the Au clusters is fixed to the catalytically far inactive Al ₂ O ₃ surface, which reduces the Overall activity leads. Examples 9 and 10, on the other hand, show that the porous pellets are uniformly coated with the oxidic active component via molten salt and calcination, and that the subsequent Au impregnation on the relatively low-surface active oxide (α-Fe ₂ O ₃ ) leads to comparable activity by reducing the precursor after the treatment described in this invention under mild conditions in the gas phase (Example 11). Such or another type of pre-fixation (impregnation and precalcination) of an oxidic active component within a porous matrix is also known today for other oxide-matrix composites (such as Ce (Zr) O ₂ -Al ₂ O ₃ or TiO ₂ -SiO ₂ ) of the technique.

example 1 Au / TiO ₂ (Au impregnation, calcined)

3 g of a TiO ₂ powder from Sachtleben Chemie pre-calcined at 670 ° C. (0.5 h) are dispersed in 300 ml of deionized water at 60 ° C. Within about 3 minutes. 10 ml of a 0.17 M aqueous HAuCl ₄ solution are then added successively under pH control (5.1-5.4, buffered by a 0.15 M sodium carbonate solution). The suspension is then stirred at 60 ° C. for half an hour. After several washes with warm water (approx. 40 ° C) and filtration to remove sodium and chloride ions, the last filter cake is dried under vacuum at room temperature (RT) for approx. 15 h. The light beige powder is finally calcined in air at 400 ° C for 1/2 h (color: purple). The Au loading of this reference catalyst is 4.48% by weight Au with an average Au cluster size of 2.8 nm according to XRD or 2.6 ± 0.9 nm according to TEM (statistics: 800 clusters counted). The BET surface area is 65 m ² / g, the crystallite size of the support (100% anatase) is 21.8 nm (XRD); see also table 1.

Example 2 Au / TiO ₂ (Au impregnation, wet reduced and calcined)

The first procedure is as described in Example 1, only the Suspension before the last filtration with an approx. 10% hydrogen peroxide solution added at RT, which quickly turns blue. After drying like in example 1 the XRD shows clear Au reflections, from whose width one can see the Scherr equation can determine a gold crystallite size of 14.5 nm. This does not change even after calcinations below 400 ° C more. The result of the kinetic measurements shows accordingly lower Au dispersion a much lower rate than with the reference Example 1.

Example 3 Au / TiO ₂ (Au impregnation, reduced with H ₂ ); inventively

Here, the powder according to Example 1 is brought to 200 ° C. under N ₂ after drying and, after a holding time of about 1/2 hour at this temperature, is treated with 10% H ₂ in N _{2 for} half an hour. The XRD difference spectra of this catalyst show very broad Au [111] and Au [200] reflections whose evaluation (Scherr equation) leads to an average d (Au) of 1.8 nm.

A mean is obtained from TEM images (2500 clusters counted) geometric diameter of the Au clusters also 1.8 nm Size distribution is very narrow at ± 0.5 nm. Corresponding to the high Au dispersion the activity of this preparation is very high; about twice as high as that the calcined reference according to Example 1.

Example 4 Au / Co ₃ O ₄ (Au deposition by coprecipitation, calcined)

34.4 g of cobalt nitrate hexahydrate and 1.06 g of HAuCl ₄ .3H ₂ O are dissolved in 120 ml of water and added to 60 ° C warm water (150 ml) with vigorous stirring and simultaneous buffering with 1 M Na ₂ CO ₃ solution was added dropwise at a constant pH of 8.0 for 1/2 hour. The resulting suspension is then stirred for a further 1/2 h, then cooled and filtered. After repeated washes, the filter cake is dried at 80 ° C. for 12 hours and then calcined at 400 ° C. for half an hour. The Au / Co ₃ O ₄ catalyst thus obtained shows very weak activity, largely due to the fact that the Au dispersion is very low (d (Au) ~ 9 nm). In analogy to example 2, this is due to the slight agglomeration of the metallic gold, which is formed during the precipitation by the in situ reduction of the Au (3 +) complex by the easily oxidizable divalent cobalt ion.

Example 5 Au / Co ₃ O ₄ (Au impregnation, calcined)

The precipitation according to Example 4 is repeated, but without the addition of a Gold compound. After drying, it is calcined at 400 ° C. for 2 hours. The received one Cobalt spinel (3.0 g) is then dispersed in 300 ml of water and opened 60 ° C warmed up. Covering with 6 ml of 0.14 M gold chloride solution Buffering with a soda solution (pH between 7.0 and 7.5) and others Operations are then carried out in the same way as shown in Example 1. The dried powder is finally calcined at 400 ° C. for 1/2 hour.

Example 6 Au / Co ₃ O ₄ (Au impregnation, reduced with H ₂ at 70 ° C, post-calcined at 300 ° C); inventively

Example 5 is repeated, but with the difference that after predrying under vacuum, post-drying at approx. 200 ° C in air. The mixture is then cooled to 70 ° C. and, after flushing with nitrogen, the sample is treated with 10% H ₂ in N _{2 for} two hours. Subsequently (mandatory for applications at higher temperatures, see below) the air is calcined again at 300 ° C for 1/2 h. The initial activity of the dry / reductively treated Au / Co ₃ O ₄ catalyst is significantly higher than that of the only calcined reference according to Example 5. Both this catalyst and that according to Example 5 are less suitable for the PROX reaction because Deactivate both quickly due to the strong formation of carbonate. On the other hand, very high activity is expected in applications in oxidizing atmospheres in the field of environmental chemistry, such as for the elimination of hydrocarbon from air.

Example 7 Au / Fe ₂ O ₃ (Au deposition precipitation on ferrihydrite, calcined)

80 ml of a solution of 0.08 M iron nitrate and at the same time a 0.94 M solution of Na ₂ CO ₃ are added dropwise to a template of hot water (320 ml) while stirring vigorously, so that the pH at 8.0 ± 0.2 can be maintained. After this ferrihydrite precipitation has ended, the suspension is cooled to 60.degree. Within the next 5 minutes, the suspension is then mixed with a HAuCl ₄ solution while buffering with dilute soda solution at pH ~ 8.0 and then stirred at 60 ° C. for 1/2 h. The further operations are largely identical to Example 1, with the difference that the drying takes place in air at 80 ° C. for 12 hours. The catalyst precursor (Au ₂ O ₃ / ferrihydrite) is finally converted to the end product Au / α-Fe ₂ O ₃ by calcination (1/2 h) in air at 400 ° C.

Example 8 Au / Fe ₂ O ₃ -γ-Al ₂ O ₃ (deposition precipitation on ferrihydrite fixed by neutralization in pellets, calcined)

200 grams of γ-Al ₂ O ₃ "Raschigring" pellets (length 4.7 mm, diameter 4.9 mm, wall thickness 1.5 mm) are evacuated and mixed with 210 ml of concentrated (2.4 M) iron nitrate solution and approx. Allow to stand for 12 hours under normal pressure. The pellets are then freed from excess solution by sieving. A solution of 67 g of soda in 300 ml of water is prepared and about half of it is quickly poured over the pellets with intensive stirring. The pH value initially drops to 7.5. The remaining soda solution is added successively within the next 5 hours, taking care that the pH remains at approximately 8.0. The formation of the hydrated iron oxide (ferrihydrite) within the pellets is then complete. After the pellets have been separated from the remaining suspension, they are immersed in 0.5 l of water and annealed at 80 ° C. for 1/2 h. This is followed by impregnation at 60 ° C with a HAuCl ₄ solution (4.04 g HAuCl ₄ .3H ₂ O in 10 ml water) within approx. 4 min. with stirring and simultaneous buffering with soda (pH: 6.3 to 7.3). The mixture is stirred for about 3 hours until the pH is constant. After cooling, the pellets are rinsed with deionized water overnight, then approx. Dried at 80 ° C for 10 hours and finally calcined at 400 ° C (1/2 h). A small part of the dark brown catalyst is pulverized for kinetic measurements. It turns out that the activity of the powdered sample only approx. 20% of the activity of the analog powder catalyst according to Example 7 is reached. As microscopic and SEM / EDX images of a micrograph show, this is due to an uneven distribution of the hematite within the pellets: while the gold is evenly distributed throughout, the center and the outer areas of the pellet rings remain free of the active oxide hematite.

Example 9 Au / Fe ₂ O ₃ -γ-Al ₂ O ₃ (via Fe nitrate melt, Au impregnation, calcined)

30 g of γ-Al ₂ O ₃ pellets (3.2 × 3.5 mm, approx. 100 m ² / g) are immersed in an Fe (NO ₃ ) ₃ melt at 70 ° C. with stirring and then removed from the excess melt separated by sieving. This is followed by thermal decomposition of the nitrate up to 300 ° C (+2 h holding time) in air to form a uniform layer of hematite containing Al ₂ O ₃ within the pellet matrix structure. After cooling, the pellets are immersed in preheated water (60 ° C.) and impregnated with stirring in the same way as in Example 1 with a HAuCl ₄ solution while keeping the pH constant (6.5 to 7.0) and approx. Stirred for 2 hours. After rinsing with deionate as in Example 8 and drying, the pellets are finally calcined at 350 ° C. in air (1/2 h). Part of the black pellets is ground for kinetic measurements. The content of α-Fe ₂ O ₃ is approx. 9.5%, that of gold 0.5% by weight. As in the previous example, the determination of the Au size via XRD or TEM proves difficult because of the amorphous Al ₂ O ₃ content and the low Au concentration. Due to the uniform distribution of the iron oxide layer in the interior of the pellets, a higher rate than in Example 8 is achieved by pre-impregnation with the melt.

Example 10 Au / Fe ₂ O ₃ -γ-Al ₂ O ₃ (via Fe nitrate melt, Au impregnation, reduced at 120 ° C); inventively

Analogous to Examples 3 and 5, the final calcination is dispensed with and the pellets produced according to Example 9 are dried at 200 ° C. in air. After cooling to 120 ° C. and intermediate rinsing with N ₂ , the pellets are reduced with 10% H ₂ in N _{2 for} half an hour. The activity (measured on the pulverized product) is also higher here than that of the catalyst conventionally conditioned in air at 300 to 400 ° C. according to Example 9. Here too it is not possible to measure the Au cluster size.

Example 11 Au / CeO ₂ (Au impregnation on coarse crystalline support, calcined)

120 ml of a 1 M solution of Ce (NO ₃ ) ₃ are neutralized with a H ₂ O ₂ -containing soda solution (1 M) at RT and constant pH around 7.0 to the hydrated CeO ₂ . After careful washing by repeated redispersion and filtration, part of the CeO ₂ powder which has subsequently been dried is calcined in air at 800 ° C. for 2 hours. The carrier obtained is relatively coarse-crystalline (21 nm) with a correspondingly small BET surface area of 23 m ² / g. A portion (2.5 g) of this powder is stirred into 150 ml of warm water (60 ° C.) and, as in Example 1, with 5 ml of a 0.09 M HAuCl ₄ solution with simultaneous buffering with a soda solution at pH values between 6.5 and 7.0 offset. After the usual further work-up and drying (RT under vacuum), this Au / CeO ₂ catalyst precursor is calcined in air for 1/2 h. It can be seen from Table 1 that the Au crystallites are relatively large and their size distribution is relatively wide. As already explained above, this should be due to the easy mobility and the confluence of melted gold ponds on the relatively "smooth" oxide surface during and after the thermal Au ₂ O ₃ decomposition above approx. 350 ° C. The activity of this catalyst is rather modest due to the low Au dispersion.

Example 12 Au / CeO ₂ (Au impregnation on coarse crystalline support, reduced with H ₂ ); inventively

The procedure here is as in Example 11, but with the difference that the Au / CeO ₂ precursor is dried with hydrogen for 3/4 h after drying at 200 ° C. and then with 5% O ₂ in N at the same temperature ₂ and then oxidized again with air to stabilize the cerium oxide carrier. In this way it is also possible to stabilize very small Au clusters on the support, which also shows the higher activity compared to example 11. Examples 11 and above all 12 also demonstrate that controlled Au impregnation can be used to deposit very small Au clusters (<5 nm) even on coarsely crystalline oxide substrates (<50 m ² / g).

Example 13 Au / Ce (Zr) O ₂ (Au impregnation, calcined)

Commercial Ce _0.7 Zr _0.3 O ₂ powder (Rhodia, "Ce-Zr 70-30") is calcined in advance at 400 ° C. for 2 hours and 3.0 g thereof in 300 ml of water at 60 ° C. under Dispersed stirring. The procedure is the same as in Example 11. After calcining the precursor at 400 ° C in air (1/2 h), a black powder with Au clusters around 3.5 nm (XRD) is obtained. Due to the poor contrast in the TEM (atomic weight of cerium is very high) and the small difference between d (Au) and d (oxide), the gold cluster size can only be determined imprecisely (~ 2.2 nm). Because of the higher Au dispersion compared to, for example, 11 and the high specific surface area of the support, the rate of CO oxidation is higher.

Example 14 Au / Ce (Zr) O ₂ (Au impregnation, reduced with H ₂ ); inventively

As in Example 12, the only dried catalyst precursor becomes after Example 13 reduced in a hydrogen stream at 200 ° C. and then again oxidized. The activity measured afterwards is now approx. 70% above that of the conventional powder calcined at 400 ° C of the previous one Example. In the XRD, no Au reflex can be made visible, what means that here either amorphous Au (d (Au) <approx. 1.5 nm) or the gold in univalent state (Au (+)), which is represented by the tetravalent cerium is stabilized (see below).

Example 15 Au / CeO ₂ (Au impregnation, reduced with H ₂ ); inventively

Commercially available, pure CeO ₂ powder (Rhodia "HSA 15") is precalcined analogously to Example 13, impregnated with HAuCl ₄ at pH values between 6.5 and 7.0, washed, filtered and dried under vacuum at RT. After reduction with H ₂ at 200 ° C as described above, a dark brown-gray powder is obtained which has no Au reflections in the XRD. The measurement of the Au dispersion using TEM also fails because of the poor contrast between Au and the nanocrystalline cerium dioxide. The activity of this catalyst with measured rates of 27 ± 3.10 ^-3 moles CO / (g (Au) .s) exceeds all other previously measured catalysts by at least a factor of 3. Furthermore, the reproducibility of the production is excellent.

Claims (13)

1. Metal oxide-supported Au catalysts with a narrow Au cluster size distribution and high degree of dispersion of the Au clusters, obtainable by drying moist Au / metal oxide catalyst precursors obtained in a known manner by coprecipitation, deposition precipitation or impregnation, and the dried precursors in the Gas phase reduced with H ₂ or CO at temperatures of ≤ 250 ° C. 2. Metal oxide-supported Au catalysts according to claim 1, wherein the metal oxide is selected from the group consisting of Fe ₂ O ₃ , TiO ₂ , Al ₂ O ₃ , CeO ₂ , Ce (Zr) O ₂ , SiO ₂ , ZrO ₂ , Co ₃ O ₄ , NiO, MnO _x and Fe ₂ O ₃ -γ-Al ₂ O ₃ . 3. metal oxide-supported Au catalysts according to claim 1 and / or 2, containing 0.5-8 wt .-% Au, preferably 2-5 wt .-% Au. 4. Metal oxide-supported Au catalysts according to at least one of claims 1-3 with a BET specific surface area of at least about 20 m ² / g, preferably at least about 50 m ² / g. 5. Metal oxide supported Au catalysts according to at least one of the Claims 1-4, wherein the Au cluster is less than about a diameter 6 nm, preferably less than about 4 nm, more preferably 1-3 nm, exhibit. 6. A process for the preparation of metal oxide-supported Au catalysts according to at least one of claims 1-5, in which the moist Au / metal oxide catalyst precursor obtained in a known manner is dried by coprecipitation, deposition precipitation or impregnation and the dried precursors in the gas phase with H ₂ or CO reduced at temperatures of ≤ 250 ° C. 7. The method according to claim 6, wherein when using TiO ₂ , Al ₂ O ₃ , CeO ₂ , Ce (Zr) O ₂ and ZrO ₂ as the metal oxide carrier, the reduction treatment is carried out at temperatures of ≤ 200 ° C. 8. The method according to claim 6, wherein when using Fe ₂ O ₃ or Fe ₂ O ₃ -γ-Al ₂ O ₃ as the metal oxide carrier, the reduction treatment is carried out at temperatures of ≤ 140 ° C. 9. The method according to claim 6, wherein if Co ₃ O ₄ or NiO is used as the metal oxide support, the reduction treatment is carried out at temperatures of ≤ 80 ° C. 10. The method according to at least one of claims 6-9, wherein according to the reduction treatment calcination in air at temperatures of at least about 300 ° C, preferably 300 to 400 ° C, is carried out. 11. Use of the metal oxide-supported Au catalysts according to the Claims 1-5 or by the method according to claims 6-10 obtained metal oxide supported Au catalysts for selective CO oxidation in Reformer gases (PROX), for low-temperature water gas conversion (WGS), in methanol synthesis, for the epoxidation of olefins or for Total oxidation of CO, hydrocarbons or halogenated gases (VOC). 12. Use according to claim 11, wherein the metal oxide-supported Au catalysts for selective low-temperature CO oxidation in reformate Uses hydrogen for PEM fuel cells. 13. Use according to claim 11, wherein the metal oxide-supported Au Catalysts for low-temperature water gas conversion, in particular at temperatures of ≤ 150 ° C.