nondiffusioncontrolled

Nondiffusioncontrolled refers to a regime in which the overall rate of a process is governed primarily by intrinsic chemical or interfacial kinetics rather than by the diffusion of reactants to the reactive site. In contrast, diffusion-controlled processes are limited by mass transport, such that the rate is determined by how fast species can diffuse to the reaction zone.

This term is commonly used in electrochemistry and catalysis. In electrode reactions, nondiffusioncontrolled (kinetically controlled) behavior occurs when the interfacial charge-transfer steps or surface reactions are slower than the rate at which reactants are supplied by diffusion. The observed current in such cases follows kinetic rate laws and shows sensitivity to temperature and catalyst properties, rather than being limited by diffusion to the electrode surface. Mixed or partially diffusion-controlled regimes can arise when both mass transport and surface kinetics contribute to the overall rate.

Characterization and analysis often involve distinguishing kinetic control from diffusion control. Techniques such as rotating disk electrode experiments help identify diffusion limitations through limiting currents and Levich behavior. In nondiffusioncontrolled systems, current–potential relationships reflect rate constants, activation energies, and surface coverages, and may be described by appropriate kinetic models or Arrhenius-type analyses. In many practical situations, the observed process transitions from diffusion-controlled at low overpotentials to nondiffusioncontrolled as overpotential increases, or under conditions where convection alters mass transport.

See also: diffusion-controlled, mixed control, reaction kinetics, mass transfer, Koutecky-Levich analysis.