Heterogeneous catalysis is governed by a concerted and intricate interplay of several processes that take place at multiple time and length scales. The chemical environment that exists during catalyst operation as well as the kinetics of structural changes create configurations of typically unknown local geometries and chemistry. While surface reactions that break and make chemical bonds occur at time scales of picoseconds, they produce species such as carbon, hydrogen, or oxygen, that interact with the material initially placed in the reactor and may induce its local restructuring along with changes in the reactivity in the scale of hours, days, or even longer. Indeed, the catalytic performance of a material, i.e., its ability to more or less selectively convert reactants, typically changes in the beginning of the reaction during the so called induction period. During this induction period, the activity and/or the selectivity can drastically change until the surface chemistry reaches a steady state under the applied reaction conditions. At the steady state, a constant catalytic performance is observed with respect to time. At longer reaction times, the catalytic performance might change again, e.g., the activity might decrease. This latter process is known as deactivation. The period of time during which the catalytic process is operated is referred to as time on stream (t OS ).