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A systematic analysis has been carried out to distinguish "kinetic" and "transport" limited regimes for photocatalytic degradation in a TiO 2 slurry system using benzoic acid as a model component. Experiments were performed to investigate the effect of both monoparametric (e.g., catalyst loading, circulation rate, and initial concentration) and biparametric (e.g., catalyst loading and circulation rate, initial concentration of solute, and light intensity) on the degradation rate to differentiate the kinetic regime from the transport regime. It is shown that there is no external mass-transfer limitation at any catalyst loading, but at high catalyst loading, there exist both internal mass-transfer limitation and light limitation. The domain of kinetic limitation, the domain of internal mass-transfer limitation (particle agglomeration effect), and the domain of light transport limitation (shielding effect) were distinguished. Experimental results demonstrate a significant difference in the rate values between the "optimal catalyst loading" regime and the kinetic regime. The true kinetic dependences of the photocatalytic degradation rate on light intensity and the substance initial concentration were determined.

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In the development of catalytic materials, a set of standard conditions is needed where the kinetic performance of many samples can be compared. This can be challenging when a sample set covers a broad range of activity. Precise kinetic characterization requires uniformity in the gas and catalyst bed composition. This limits the range of convecting devices to low conversion (generally <20%). While steady-state kinetics offer a snapshot of conversion, yield and apparent rates of the slow reaction steps, transient techniques offer much greater detail of rate processes and hence more information as to why certain catalyst compositions offer better performance. In this work, transient experiments in two transport regimes are compared: an advecting differential plug flow reactor (PFR) and a pure-diffusion temporal analysis of products (TAP) reactor. The decomposition of ammonia was used as a model reaction to test three simple materials: polycrystalline iron, cobalt and a bimetallic preparation of the two. These materials presented a wide range of activity and it was not possible to capture transient information in the advecting device for all samples at the same conditions while ensuring uniformity. We push the boundary for the theoretical estimates of uniformity in the TAP device and find reliable kinetic measurement up to 90% conversion. However, what is more advantageous from this technique is the ability to observe the time-dependence of the reaction rate rather than just singular points of conversion and yield. For example, on the iron sample we observed reversible adsorption of ammonia and on cobalt materials we identify two routes for hydrogen production. From the time-dependence of reactants and product, the dynamic accumulation was calculated. This was used to understand the atomic distribution of H and N species regulated by the surface of different materials. When ammonia was pulsed at 550 °C, the surface hydrogen/nitrogen, (H/N), ratios that evolved for Fe, CoFe and Co were 2.4, 0.25 and 0.3 respectively. This indicates that iron will store a mixture of hydrogenated species while materials with cobalt will predominantly store NH and N. While much is already known about iron, cobalt and ammonia decomposition, the goal of this work was to demonstrate new tools for comparing materials over a wider window of conversion and with much greater kinetic detail. As such, this provides an approach for detailed kinetic discrimination of more complex industrial samples beyond conversion and yield.