The Influence Of Support Crystalline Structure On The Dispersion Behavior Of Supported Metal Oxides

Spontaneous dispersion of salts and metal oxides on the surfaces of supports to form a submonolayer or monolayer has been widely accepted and becomes an important guideline for the effective design of superior catalysts.In this work,the dispersion properties and catalytic performance of metal oxides supported on TiO₂ supports with rutile?r-TiO₂?and anatase?a-TiO₂?crystalline structures have been explored based on the monolayer dispersion theory.Tetragonal rutile SnO₂ has the same crystalline structure with rutile r-Ti O₂.A series of n%SnO₂/r-TiO₂,n%SnO₂/a-TiO₂ and n%SnO₂/P25 catalysts with different SnO₂ loadings have been prepared by physical-mixing method,and the monolayer dispersion capacities of SnO₂ supported on the three types of TiO₂ supports with different crystalline structures have been quantified by XRD extrapolation method,which are 0.559 mmol/?100 m² r-TiO₂?,0.229 mmol/?100 m² a-TiO₂?and 0.305 mmol/?100 m² P25?,respectively.The results indicate that if a support and a supported metal oxide active phase have same crystalline structure,it is favorable for the dispersion of the supported active phase,thus getting higher dispersion capacity.Similar to the famous “like-dissolves-like” rule for solutions,this phenomenon is found for the first time by us,which is named “like-disperses-like” behavior.To confirm this,a series of n%SnO₂/r-TiO₂ and n%SnO₂/a-TiO₂ catalysts with different SnO₂ loading have also been prepared by deposition-precipitation method.With this method,the monolayer dispersion capacities of SnO₂ supported on r-TiO₂ and a-TiO₂ are 0.610 mmol/?100 m² r-TiO₂?and 0.232 mmol/?100 m² a-TiO₂?,respectively.Within the experimental error,the dispersion capacities measured on the catalysts prepared with differet methods are the same,which has verified the existence of “like-disperses-like” behavior.H₂-TPR results have testified that the interaction between SnO₂ and TiO₂ supports with different crystalline structures are different,hence leading to different SnO₂ reduction behaviors.In detail,SnO₂ supported on r-TiO₂ is reduced in one-step,while that on a-TiO₂ is reduced in two steps.As a consequence,the CO oxidation activity of SnO₂/r-TiO₂ is higher than that of SnO₂/a-TiO₂.To explore further the the universality of the “like-disperses-like” behavior,rutile RuO₂,which possesses the identical tetragonal rutile structure with r-TiO₂,has been supported on both r-TiO₂ and a-TiO₂ supports to investigate its dispersion property.A series of n%RuO₂/a-TiO₂ and n%RuO₂/r-TiO₂ catalysts with different RuO₂ loadings have been prepared by impregnation method.The monolayer dispersion capacities of RuO₂ supported on r-TiO₂ and a-TiO₂ have been quantified by XRD and Raman techniques,which are 0.112 mmol/?100 m² r-TiO₂?and 0.557 mmol/?100 m² a-TiO₂?in sequence.These results have confirmed the existence of “like-disperses-like” behavior,which could provide novel thoughts for people to design catalysts with high performance.Based on the “like-disperses-like” phenomenon,RuO₂/a-TiO₂ and RuO₂/r-TiO₂ catalysts with a 2 wt% Ru loading have been prepared by impregnation method and used for CO oxidation.H₂-TPD,H₂-TPR,XPS and Raman results have testified that there is a strong interaction between RuO₂ and rutile r-TiO₂ due to their high degree of crystal structure matching.Apparently,this leads to a higher Ru/RuO₂ dispersion on the r-TiO₂ support and induces the formation of more abundant active surface oxgen species.As a consequence,RuO₂/r-TiO₂ displays a higher activity than RuO₂/a-TiO₂ for CO oxidation.