The oxidation of sulfur dioxide to sulfur trioxide in a gas-phase process uses a solid catalyst. This is an example of which type of catalysis?

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In catalysis, the important distinction is the phases of the catalyst and the reactants. When the catalyst is a solid and the reacting species are gases (or liquids) that are not in the same phase as the catalyst, the process is heterogeneous catalysis. In this example, the reaction occurs in the gas phase with a solid catalyst present, so the catalyst and the reactants are in different phases. That surface on the solid provides sites where SO2 and O2 can adsorb, react, and desorb as SO3, which is the hallmark of heterogeneous catalysis.

If both catalyst and reactants were in the same phase (for instance, both gases or both liquids), it would be homogeneous catalysis. Autocatalysis would involve the product acting as the catalyst for the reaction, which is not what's happening here. Activation of a catalyst isn’t a type of catalysis itself.