= Catalytic surface site
{title2=$\ast$}
A <catalytic surface site> is a location on a <solid> surface capable of participating in a catalytic route. The schematic notation $\ast$ denotes a vacant site; $\mathrm A\ast$ denotes A bound there. A minimal illustrative <catalytic cycle> is
$$
\mathrm A+\ast\rightleftharpoons\mathrm A\ast,
\qquad
\mathrm A\ast\rightleftharpoons\mathrm B+\ast.
$$
The first step represents <adsorption>; the second combines conversion and release. Cancelling $\ast$ and $\mathrm A\ast$ gives $\mathrm A\rightleftharpoons\mathrm B$. The site is regenerated even though its occupancy changes during the cycle. This is an original bookkeeping model, not a measured <reaction mechanism> for <manganese dioxide>.
If the only site states are vacant and occupied by A, the site inventory obeys $n_\ast+n_{\mathrm A\ast}=n_{\mathrm{sites}}$. A blocked or inaccessible part of a particle does not contribute available <catalytic surface sites> merely because it contributes <mass>. Different surface structures can have different abilities to bind and convert <reactants>; equal geometric <surface area> need not mean equal catalytic activity.
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