Precursors metal oxide catalysts

Skeletal Spectra of Precursors for Metal Oxide Catalysts 129... 

Complex precipitates, such as layered double hydroxides, are frequently prepared as precursors of oxide catalysts. Hydrotalcite is a hydrated hydroxy-carbonate mineral with the formula Mg,sAl2(0H)i6C03-4H20. It represents the better known and most popular member of a family of layered double metal hydroxide com-... 

Two main preparation methods have been used to synthesize Mo/V/Te/(Nb)/0 catalysts active in propane ammoxidation (a) the dry-up and (b) the hydrothermal synthesis. The dry-up method involves mixing aqueous slurries of metal oxide precursors followed by a gradual evaporation of the combined aqueous slurry. Solvent evaporation leads to nucleation and growth of precursor metal oxide phases, which require further heat treatment to obtain active catalysts. 

Although hydrotalcites are relahvely stable (up to circa 500 °C), they are also of potential applicahon as precursors of mixed metal oxide catalysts, for example Reference [66]. Dehydrahon-rehydration equilibria account for the switching between hydrotalcites and mixed/supported metal oxides, which is somehmes termed the memory effect [67-69]. Recent advances have seen attempts to prepare highly dispersed LDH systems, such as those dispersed within mesoporous carbon [70]. Owing to widespread interest in their application, hydrotalcite catalysts have been the subject of a number of reviews, for example References [71-75]. Other layered-based systems have also attracted attention for application in catalysis, for example Reference [76]. 

Alcohols, such as methanol and ethanol, has been used as solvents for the metal acetylacetonate systems, usually under reflux conditions. For many of these metal acetylacetonates, these alcohols are good solvents and show only a low affinity for most supports. However, as we discussed earlier, some metal acetylacetonates form p-alkoxy dimers when catalytic amounts of OH are present. The unexpected formation of these dimeric precursors may frustrate attempts to form a supported metal oxide catalysts with isolated metal cations decorating the surface. 

Venable, Margaret Hamm, Syntheses, structures and supported interactions of potential metal oxide catalyst precursors , MS thesis, Georgia Institute of Technology, (1990). 

The deposition-precipitation (DP) method has been scarcely used to prepare platinum supported catalysts [1-5], while it is the preferred method to obtain active gold ones [6-7]. Initially, the deposition-precipitation technique has been developed by Geus and Hermans for the production of highly loaded and highly dispersed metal/oxide catalysts [8]. This technique involves the precipitation of the active phase precursor at fte surface of Ae support and its subsequent deposition whereas nucleation in the solution itself should be avoided. 

Additionally, several reports of homogeneous water-oxidation catalysts pointed to the possibility that the complexes are not the catalysts and are acting as precursors to the respective metal oxides, which are ultimately the actual water-oxidation catalysts. Metal oxides have been known to catalyse water oxidation, but the most active were often oxides of rare- and precious-metal ions, operating best in highly basic conditions. " The past decade has seen major advances in the development of highly active metal-oxide catalysts based on cost-effective, abundant and nontoxic elements that can act under near-neutral pH conditions. These metal oxides can... 

Hydrothermal synthesis has also been used to prepare mixed-metal oxide catalysts. The group of Maier presented already in 1998 the first hydrothermal high-throughput preparation method for such catalytic materials [93]. Corma et al. used a hydrothermal treatment of sol-gel synthesized Ti-silicalite catalyst precursors to accelerate the crystallization [94]. 

Titanium oxide monolayer on y-AljOj is a potential support for noble metals [1-4]. Many studies have shown that two-dimensional transition metal oxide overlayers are formed when one metal oxide (Vj05, Nb205, MoOj, etc.) is deposited on an oxide support (AljOj, TiO, etc.) [5-7]. The influence of the molecular structures of surface metal oxide species on the catalytic properties of supported metal oxide catalyst has been examined [8-9]. It has been demonstrated that the formation and location of the surface metal oxide species are controlled by the surface hydroxyl chemistry. Moreover, thin-layer oxide catalysts have been synthesized on alumina by impregnation technique with alkoxide precursor [10]. It has been found for titanium oxide, by using Raman spectroscopy, that a monolayer structure is formed for titanium contents below 17% and that polymeric titanium oxide surface species only posses Ti-O-Ti bonds and not Ti=0 bonds. Titanium is typically ionic in its oxy-compounds, and while it can exist in lower oxidation states, the ionic form TF is generally observed in octahedral coordination [11-12]. However, there is no information available about the Ti coordination and structure of this oxide in a supported monolayer. In this work we have studied the structural evolution of the titanium oxy-hydroxide overlayer obtained from alkoxide precursor, during calcination. 

In industrial applications activated carbons are used as supports for precious metal and metal oxide catalysts. These catalysts can be prepared by adsorption from solution, impregnation, precipitation and other techniques as gas phase deposition [4]. The most common way of preparation is adsorption and impregnation by bringing the activated carbon in contact with a solution of the desired metal compound or with a solution of a metal precursor, in most times a complex salt. Then the impregnated activated carbon is dried. Reduction is carried out when the metal precursor has to be transformed in the metal. Important quality criteria are ... 

The basic principle behind supported metal oxide catalyst preparation involves deposition followed by activation. According to The International Union of Pure and Applied Chemistry (lUPAC) recommendations, deposition is defined as the application of the catalytic component on to [sic] a separately produced support, and activation as the transformation of the precursor to the active phase, usually entailing the calcination of the composite material [17]. There are several well-studied deposition steps, with impregnation being the most common method for preparing industrial supported metal oxides. 

Transition metal nitrate hydrates are industrially favored precursors for the preparation of supported metal (oxide) catalysts because of their high solubility and facile nitrate removal. The final phase and particle size depend on the experimental conditions, as reported for both supported and unsupported metal nitrates [1-3]. Several authors report that decreasing the water partial pressure during the decomposition of unsupported nickel nitrate hexahydrate, via vacuum or a high gas flow, increases the final NiO surface area [3, 4], The low water partial pressure results in dehydration of the nickel nitrate hydrate to anhydrous nickel nitrate followed by decomposition to NiO. Decomposition at higher particle pressures, however, occitrred through the formation of intermediate nickel hydroxynitrates prior to decomposition to NiO. Thus, NiO obtained via intermediate nickel hydroxynitrate species showed a poorer siuface area (1 m /g) compared to NiO obtained via anhydrous nickel nitrate species (10 mVg) [4]. 

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