Variable-temperature kinetic analysis of reaction profiles for the rapid assessment of heterogeneous catalysts
Variable-temperature reaction profiles, measured in a packed bed reactor (PBR) with a constant temperature heating rate, show the conversion vs. the reactor temperature. Reaction profiles that are kinetically-limited can be simulated using just three equations: a power rate law, Arrhenius equation, and the PBR design equation. Numerical solutions are derived for transport-limited reaction profiles with the inclusion of the internal effectiveness factor and the rate of external mass transport. Simulations demonstrated that the position and shape of a reaction profile is unique for each rate law and set of kinetic parameters.
Analysis of experimental variable-temperature reaction profiles is demonstrated for a typical supported catalyst, 2 wt% Pd/Al2O3, for the oxidation of H2, C3H8, and CO by oxygen. These reactions were shown to be kinetically-limited and free from gradients in temperature and pressure. Quantitative information about the rate law and reaction constant were reliably extracted by curve-fitting as few as two adjustable parameters to reaction profiles. The reaction orders and observed activation energy were rapidly extracted from a single reaction profile recorded in less than an hour (with appropriate data truncation, when necessary), and they are at least as accurate as the same parameters obtained from much more time-intensive conventional kinetic analysis. More precise curve-fitting parameters can be obtained by global curve-fitting of a series of reaction profiles recorded with different volumetric flow rates.
In situ x-ray absorption spectroscopy (XAS) and operando infrared (IR) spectroscopy coupled with variable-temperature kinetic analysis was used to probe the active phase of the catalyst during CO oxidation. Quantitative analysis of reaction profiles predicted that the active phase was Pd(0), which was confirmed by XAS and IR. Sudden deviation from ideal kinetic behavior is observed for CO oxidation catalyzed by either Pd or Pt nanoparticles on alumina. The abrupt increase in activity results from changes in the surface chemistry when CO no longer poisons the surface. IR spectroscopy for the Pd catalyst during CO oxidation captured the transient phase of the catalyst in which metallic interactions are substantially reduced. For CO oxidation catalyzed by Pt, the abrupt increases occur more readily for catalysts that are susceptible to internal and/or external mass transport limitations.