Precise kinetic measurements and spatial uniformity of catalytic beds

Sufficient uniformity of the gas phase and surfaces within a catalytic bed is an essential prerequisite for precise kinetic characterization of heterogeneous catalysts. In addition to uniformity, the gas transport regime must be mathematically well-defined and allow for controlled alterations of the catalyst state. The Thin-Zone Temporal Analysis of Products (TZTR) reactor satisfies these requirements for industrially-relevant powdered catalysts. Numerical simulations of pulse-response experiments were used to chart the conditions of spatial uniformity within the TZTR for very fast (DaII up to 100) irreversible reactions, such as strong chemisorption or oxidation reactions. The simulations demonstrated that even when an initial catalyst state is so reactive that uniformity cannot be readily achieved, a series of reactant pulses eventually creates a catalyst state which is spatially uniform and at the same time reactive enough for kinetic characterization. This point on the scale of possible catalyst states is attained when the total amount of pulsed molecules exceeds the initial number of active sites in the sample, i.e. the theoretical monolayer coverage. Simulations also indicated that the rate at which the catalyst bed becomes more uniform from one pulse to the next is sensitive to reaction order: the desired uniformity is established for a second order adsorption after smaller amount of pulses than for a first order adsorption. Thus, a wider range of spatially uniform, yet kinetically-relevant catalyst states can be characterized for a second order adsorption. These results not only expand the range of validity for the TZTR approach, but also set new bounds for generally achievable precise kinetic measurements.