= Heterogeneous catalysis
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In <heterogeneous catalysis>, the <catalyst> and <reactants> occupy different <thermodynamic phases>; a solid surface can furnish the catalytic route.
For example, specify dissolved <hydrogen peroxide> as the <reactant>, <water> as the <solvent>, and <solid> <manganese dioxide> as the <catalyst>. The <solid> and liquid <chemical solution> are separate <thermodynamic phases>. A dispersion that looks thoroughly mixed still contains those separate phases; mixing distributes contact, rather than making the <solid> molecularly dissolved. The <manganese dioxide peroxide contact demonstration> makes the arrival of the liquid at the <solid> tangible.
A useful surface-route picture is approach from the liquid, <adsorption> at a <catalytic surface site>, conversion through surface-bound <reaction intermediates>, and release of <reaction products>. <Chemistry 2e by Paul Flowers and others/12.7 Catalysis>[Chemistry 2e by Paul Flowers and others; §12.7, Heterogeneous Catalysts, steps 1–4: adsorption, activation, reaction and product desorption]{ref} This sequence organizes possible roles; it is not a verified sequence of <elementary reactions> for every <catalyst>, or for the particular <manganese dioxide> demonstration. The net <chemical equation> supplies neither the binding geometry nor the identity of the working <catalytic surface site>.
This contact requirement makes <surface area> and <diffusion> practically important. More exposed <solid> can provide more <catalytic surface sites>, whereas <diffusion> through the surrounding liquid can limit their supply of <reactants>. Changing stirring or particle size can therefore change the observed <reaction rate> without identifying the microscopic <rate constant>. A recoverable <solid> is attractive for reuse, but separation alone does not establish retained activity: compare with <catalyst deactivation>.
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