Deactivation Mechanisms for Pd/Ceria during the Water–Gas-Shift Reaction

The mechanism responsible for the irreversible deactivation of ceria-supported precious metals for the water–gas-shift reaction has been investigated through accelerated aging tests. It is shown that deactivation of Pd/ceria occurs more rapidly at 673 than 523 K when operating with an integral reactor in 25 Torr each of CO and H2O. By heating a fresh catalyst in H2, H2O, CO, or CO2, it was shown that deactivation occurs due to the presence of CO. Measurements of metal dispersion by CO adsorption and by X-ray diffraction show that deactivation on Pt/ceria and Pd/ceria catalysts in our studies was due to a loss of metal surface area. Finally, water–gas-shift rates on a series Pd/ceria catalysts with ceria crystallite sizes ranging from 7.2 to 40 nm and Pd loadings of either 1 or 6 wt% demonstrated that rates were strictly proportional to the Pd surface area. Based on these observations, ceria–supported precious metals could be active and stable for the water–gas-shift reaction if steps are taken to avoid metal particle size growth.