Surface properties of mixed-metal catalysts

GOODMAN Surface Properties of Mixed-Metal Catalysts... 

Bulk and supported mixed oxide compositions, from binary metal oxides to quaternary metal oxides, consist in general of large crystalline phases possessing low surface area values (typically from 1 to 10m g ). Examples of oxides of this type of catalytic relevance are V-Nb-0, Mo-Nb-O, Co-Ti-0, Ni-Ti-0, etc. The acid-base properties of mixed metal oxides have been found to change with the nature of the constituents and their relative concentrations, preparation (co-precipitation and sol-gel synthesis among are the most popular methods used), and pre-treatments procedures. Appropriately choosing the mentioned variables, mixed oxides can be used to prepare catalysts with the desired-acid-base characteristics. 

Nikolajenko, V., Bosacek, V., and Danes, V., Investigation of properties of the metallic nickel surface in mixed Ni-MgO catalysts. J. Catal. 2,127 (1963). 

In principle it would seem reasonable that the bulk structure and surface properties of a solid would influence the catalytic performance. Verification of this view and an assessment of its importance may be more significant than first appears since it incorporates an implication that catalytic preparation should be designed to achieve the bulk structure and surface properties that give the optimized catalytic performance. Materials which have been shown to catalyze the conversion of propylene to acrolein have included metal oxides, mixed oxides, and more lately the multicomponent catalysts. A consideration of all these solids would require the assessment of numerous data and speculation. However, the mixed oxide catalysts have been associated with many of the more recent investigations of the course of catalytic oxidation, and these catalysts therefore seem to be worthy of detailed consideration. 

In our relativistic density-functional study of mixed Pt-M nanoparticle surfaces is represented by a two-layer cluster with seven surface and three second-layer atoms, Ptio-nMn(7,3) [6]. The subnano cluster model does not simulate bulk surface properties because of its limited size and undercoordinated metal atoms. However, the model is suitable for simulating the properties of nanoscale particle catalysts, e.g., Pt-Ru alloy nanoparticles wife an fee surface. Catalytically much more active than bulk metal surfaces, these nanocrystals exhibit a transition from metallic to insulator properties [48]. The cluster model is also suitable for rough Pt-M electrode surfaces that exhibit a high surface density of reactive Pt-M sites [49]. 

Due to their high surface-to-volume ratio and size-dependent electronic properties nanostructured materials like NPs are good as catalysts. NPs of different sizes and structures can show significantly different catalytic activities and thus provides an opportunity to understand the structure-function relationship. NPs prepared usually in ensembles of NPs immobilized on an electrode. Thus the electrocatalytic property result of the average properties of the ensemble. Optimization of the catalyst requires increasing the number of sites available for the reaction to occur, shape and size effect of NP and composition of particles (in case of mixed metal... 

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