Deactivation of the high-temperature water-gas shift catalyst in nonisothermal conditions

Abstract

The water-gas shift reaction over a commercial iron oxide/chromium oxide catalyst was investigated to determine the kinetics of the catalyst deactivation and the physical properties of the catalyst during the deactivation. A nonisothermal fixed bed reactor was used to collect data for the kinetics of catalyst deactivation. The laboratory scale reactor operated close to industrial practice at 575–723 K. The decline of the catalyst activity was found to be due to a sintering process. No carbon formation was detected according to scanning electron microscopy and X-ray photoelectron spectroscopy analyses. The decay of the catalyst activity was quite rapid during the first 150 h of operation and less rapid between 150 and 600 h. The decay of the catalyst activity was linked to a decrease of the surface area and to an increase of the mean pore size of the catalyst. The kinetic and deactivation parameters were determined by nonlinear regression using the temperature and carbon monoxide concentration profiles in the catalyst bed. The level of catalyst activity as a function of the catalyst age was best described by a hyperbolic model for the frequency factor k' o= A o/(1+ at) n where A o=6.60·10 6 (dm 3) 1.03 mol −0.03 kg −1s −1, a=0.042 h −1 and n= 1 3 . The time dependence of the surface area ( S) of the catalyst was also described best by a hyperbolic model S= S o/ (1 + bt) m where S o=53.2 m 2/g, b=8.70 h −1 and m= 0.048. Calculation of the order of the sintering kinetics from the k' o vs. S relationship showed that two different processes of the activity decay are involved: a fast initial activity decay with sintering order of 3 and a slow activity decay with a high order of sintering.