Reduction Deposition Precipitation

For the study of the electrocatalytic reduction of oxygen and oxidation of methanol, our approach to the preparation of catalysts by two-phase protocol " provides a better controllability over size, composition or surface properties in comparison with traditional approaches such as coprecipitation, deposition-precipitation, and impregnation. " The electrocatalytic activities were studied in both acidic and alkaline electrolytes. This chapter summarizes some of these recent results, which have provided us with further information for assessing gold-based alloy catalysts for fuel cell reactions. 

Pd on ceria catalysts prepared by various methods were shown to exhibit varying characteristics depending on the preparation methods.625 Specifically, Pd on ceria samples prepared by the deposition-precipitation method are highly active in comparison with the catalyst prepared by the conventional impregnation method. Cationic palladium species are present in the former samples after reduction with hydrogen at 300°C, suggesting that the active species are produced by strong... 

Carbides and nitrides can be prepared in many ways (chemical vapour deposition, physical vapour deposition, precipitation of salts containing metal, carbon and oxygen followed by reduction and annealing, reaction of a metal or its oxides with a gas or with solid carbon). Carbides and nitrides are often nonstoichiometric with complex phase diagrams.4-9 The compounds sometimes contain multiple phases and impurities, notably oxygen. This can lead to even more complex compounds, like oxycarbides, carbonitrides or oxycarbonitrides. 

Highly active Au catalysts can be prepared by an appropriate selection of preparation methods such as co-precipitation (CP), deposition-precipitation (DP), deposition-reduction (DR) and solid grinding (SG) with dimethyl Au(III) acetylacetonate, depending on the kind of support materials and reactions targeted. 

DP, deposition-precipitation IMP, impregnation with chloroauric acid, unless specified LTR, low-temperature reduction under H2 at 473 K HTR, high-temperature reduction at 773 K C, air treatment at 673 K. 

Effecting deposition-precipitation by decreasing the pH level is interesting with metal ions present in the stable state in aqueous solution as anions [35]. With silica no interaction is observed, which has led to the development of the electrochemical reduction procedure. To apply metal ions, such as, molybdenum or vanadium, on alumina, a homogeneous decrease in pH level is interesting. The pH level has been decreased by injection of nitric acid or perchloric acid and electro-chemically. However, the rate of crystallization of the hydrated oxides of vanadium(V) and molybdenum(VI) was observed to be fairly low. To prevent dissolution of the alumina supports the pH cannot be decreased to levels below about 3, at which the crystallization of the hydrated metal oxides does not proceed rapidly. 

Electrochemical procedures can thus be used in the production of solid catalysts for the reduction of higher valent metal ions, usually present as oxyanions, to a lower valent state, where they are less acidic. Also, the above deposition-precipitation method can be extremely well controlled by electrochemical means. 

Considering now the chemical or physico-chemical interactions between the supported phase and the support, it is clear that a change of reactivity will be observed if the supported phase reacts chemically with the carrier for example, nickel can combine with silica to make some hydroxysilicate compound when the deposition-precipitation method is used for preparation. The reactivity of the hydroxysilicate with hydrogen during activation by reduction is very different (actually lower) compared to that of nickel oxide. But careful analysis of the various examples mentioned in literature shows that quite different situations may exist. 

Fig. 9.11. Left structures resulting from deposition-precipitation of Fe(II) by different procedures. Right temperature-programmed reduction of Fe oxide deposited on silica by hydrolysis of urea (U 90(1)), injection at 90°C (190(1)), and injection at 45°C. For comparison bulk Fe oxide is included.

The principal sources of sulfate in formation waters are dissolved marine sulfate, sulfate derived from the dissolution of evaporites, and sulfate formed by the oxidation of sulfides. Sulfate is destroyed by reduction to hydrogen sulfide. The value of 5 " S in gypsum is only — 1.6%o heavier than sulfate in solutions from which it precipitates. The isotopic composition of sulfur in gypsum in Phanerozoic deposits precipitated from seawater during evaporation thus tracks the secular changes in the isotopic composition of sulfur in seawater ( 10% to 30%o). 

In Ni/Al203 catalysts prepared by impregnation there is a weaker interaction between the impregnating salt and the support than there is when the catalyst is prepared by deposition-precipitation. Calcination of the former catalysts gave large nickel oxide crystallites and, on reduction, the supported metal was rather easily sintered. Catalysts prepared by precipitation-deposition, on the other hand, were resistant to sintering because of the strong interaction... 

Ni/Si02 catalysts (18 wt. % Ni), prepared by incipient wetness impregnation [9] and deposition-precipitation [10-12], were reduced by TPR up to 700°C (INi) and 900°C (DPNi) with a 5 % H2/Ar mixture, i.e., up to the minimum temperature required to obtain full nickel reduction [9,10]. For both catalysts, porous silica Spherosil XOA400 (Rh6ne Poulenc, A 400 m. g i) was used as a support. 

In the preparation of Ni/Hp catalysts by the deposition-precipitation method (DP), nickel hydrosilicates are formed mainly but not exclusively in the external surface of the Hp zeolite. The strong metal-support interaction induced by the DP preparation method prevents the Ni metal particles from sintering during the activation of the catalysts (calcination and reduction) and a homogeneous distribution of small nickel particles is obtained. The catalyst prepared by DP showed better catalytic activity in the hydrogenation of naphthalene than the catalyst prepared by cationic competitive exchange. 

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