Another drawback that may arise from spatially localizing a catalyst is mass transport. The heterogenization of a previously homogeneous catalyst introduces transport from the bulk solution to the catalyst site as a fundamental step in order for catalysis to proceed. Often, this is the source of reduced activity, when observed. Should this step prove limiting, it may be counterproductive to spatially control a catalyst. However, an effective way to circumvent this limitation is the implementation of continuous flow processing. Instead of relying solely on diffusion, the introduction of fluid transport may help overcome a mass transport limitation. Further, conducting a reaction in flow provides numerous additional parameters such as flow rate and residence time, giving it a wider space compared to a batch process for optimization. Mass transport limitations may also be exploited to avoid unwanted background reactions. This would greatly depend on the pervasiveness of such mass transport limitations, as well as the competition between diffusive and kinetic phenomena. There are also limitations and potential drawbacks when temporally controlling a catalyst. First, it must be ensured that the mode of temporal control, photochemical, electrochemical, or simply chemical, is compatible with other reagents in the reaction medium. Understandably, an applied potential or light source that switches a catalyst between active states may have undesired consequences on other species in solution. One method to circumvent this incompatibility would be to spatially separate species. For example, if a catalyst is to be switched electrochemically, immobilizing it onto the electrode surface may help prevent some unwanted redox reactions with other species. However, if the other species are free to diffuse, they may still be decomposed by an applied potential. Further, compartmentalization of the incompatible species could also help. Thus, great care must be taken to ensure other species in an integrated system are compatible with the means of temporal control.