Activation and nickel surface area

As an example, the free energy of nickel oxide dissolved in magnesia can be expressed by  

For practical purposes, support interaction and crystal size effects mean that reduced catalysts will be oxidised when exposed to steam containing small amounts of H2 or to mixtures of steam and reducing 

The reduction of pure nickel oxide by hydrogen starts at temperatures of 200—250 C. Supported catalysts require higher temperatures to show a reasonable reduction rate. This may be ascribed to interaction with the support as indicated above. The addition of small amoimts of platinum, palladium or copper to the catalyst may enhance the activation rate, probably by providing sites for the dissociation of hydrogen [389]. 

When the formation of nickel aluminium spinel has taken place, temperatures above 800 C [389] may be required for complete reduction. Even without the presence of a spinel phase, alumina-supported nickel catalysts may show less reducibility, probably due to penetration of aluminium ions in the nickel oxide surface layers during the impregnation [364], These efforts may be aggravated by oxide additions (La203, MgO) [366], Counter diffusion of nickel and aluminium appear to be rate-determining for reduction of nickel present in spinel phases [389], 

Magnesia dissolved in the nickel oxide phase, even in small amounts, drastically influences the reduction rate of nickel oxide. The diffusion of nickel ions in magnesia has a high activation energy (180 kJ/mol), probably because the nickel ion must diffuse from a preferred octahedral position through a tetrahedral position in the magnesia lattice [389]. 

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