In the industrial and biological spheres, the significant role of catalysts in augmenting chemical conversions, with respect to both rate and selectivity, cannot be overestimated. Yet the one-dimensional free-energy vs reaction coordinate diagrams presented are frequently either opaque or misleading. They obscure the significance of the potential energy surfaces that control reaction mechanisms, and provide no hints of constraints under which catalytic processes operate. No wonder students carry away impressions that catalysts are mysterious or magical! Here we argue that one may describe, in general terms, how catalysts modify reaction mechanisms by entering and leaving a conversion sequence, using potential energy surfaces that are conceptually correct. Thermochemical and kinetic constraints providing bounds that limit catalytic processes are formulated. 3-D diagrams are proposed. These clearly illustrate the basic principle that catalysts initially associate with the substrates and thus directly participate in modified conversion pathways, but are regenerated in the final step, thereby providing for a turnover number greater than unity.
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