Determination of kinetics and controlling regimes for H2 oxidation on Pt/Al2O3 monolithic catalyst using high space velocity experiments

Recently developed criteria [S.Y. Joshi, M.P. Harold, V. Balakotaiah, Chemical Engineering Science 65 (2010) 1729–1747] are used to characterize the various controlling regimes (kinetic, pore diffusion and mass transfer controlled) during H2 oxidation on Pt/Al2O3 monolithic catalyst. The hydrogen conversion was measured over a wide range of temperatures and space velocities. Experiments at typical space velocities (104–105 h−1) revealed that complete conversion can be achieved at ambient temperature, a result of fast catalytic kinetics. The high activity complicates the analysis of the relative importance of reaction, washcoat diffusion and mass transfer. High space velocity operation with accompanying theoretical analysis enabled an efficient determination of the intrinsic catalytic kinetics, quantification of the heat and mass transport coefficients and determination of the resistances due to reaction, washcoat diffusion and external mass transport processes. Specifically, we investigated the effects of catalyst aging, space velocity and catalyst temperature on the regime transition. The analysis reveals that washcoat diffusion regime is dominant over a wide range of temperatures for the aged catalyst whereas the monolith transitions to a mass transfer controlled regime above 90 °C for the high dispersion fresh catalyst. The analysis also reveals that the aging of the catalyst due to sintering of Pt crystallites not only reduces the pre-exponential factor but also the observed activation energy.

Graphical abstractFigure optionsDownload full-size imageDownload as PowerPoint slideResearch highlights▶ Method to identify controlling regimes for H2 oxidation on Pt/Al2O3 monolith. ▶ Qualitative difference between fresh and aged catalysts over 30–300 °C. ▶ Shift from washcoat/external to external transport control for fresh catalyst. ▶ Shift from washcoat to washcoat/external transport control for aged catalyst. ▶ Analysis suggests pore blockage by sintered Pt leads to diffusion limitations.