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Why are homogeneous catalysts usually in the same gas or liquid phase as the reactants?
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catalysis
Nanoparticles of a gold-palladium (yellow-blue) alloy supported on acid-treated carbon (gray) directly catalyzing hydrogen peroxide formation from hydrogen (white) and oxygen (red) while shutting off unwanted hydrogen peroxide decomposition.
Homogeneous catalysis is defined by the catalyst and the reacting substances being present together in a single state of matter, typically as a gas or a liquid.12
They are classified this way because the catalyst is molecularly dispersed within the same phase as the reactants. By existing in the same phase, the catalyst and reactants can interact uniformly throughout the mixture, allowing the catalyst to facilitate the reaction at a molecular level. This contrasts with heterogeneous catalysis, where the catalyst and reactants are separated by a distinct phase boundary, such as a solid catalyst acting upon liquid or gaseous reactants.21