Microkinetics of steam methane reforming on platinum and rhodium metal surfaces

The elementary reaction steps in methane steam reforming were investigated for Pt surfaces by DFT calculations and compared to Rh. Dissociative CH4 dissociation is rate controlling for all catalytically active surfaces. Rh is more active than Pt. The reaction mechanism to form CO depends on the metal and surface topology. [Display omitted] ► Elementary reaction steps of SMR for Pt(111), Pt(533) and Pt(210) calculated by DFT. ► The rate-controlling step is dissociative methane adsorption. ► Rhodium is more active than platinum in methane steam reforming. ► The reaction mechanism toward CO depends on metal and surface topology. ►An alcoholate intermediate is implicated for Pt(210) in contrast to Rh(211). We have investigated the most important elementary reaction steps in the steam methane reforming (SMR) process for planar and stepped Pt surfaces (dissociative CH4 adsorption, CHads–Oads recombination, H2O activation) and compared activation barriers for Rh surfaces. Compared to Rh, the lower reactivity of Pt results in (i) higher barriers for dissociative CH4 adsorption and (ii) endothermic formation of OHads and Oads. Microkinetic simulations show that Rh nanoparticle catalysts will be more active than Pt ones. The rate-controlling step is dissociative CH4 adsorption occurring on low-coordinated surface atoms (edges, corners, step-edges). The stepped surfaces are much more reactive than planar surfaces of the corresponding metals. For stepped Pt surfaces, CO formation via recombination of Cads+OHads is favored because of the low Oads coverage. At higher temperatures, deactivation may occur due to poisoning by carbonaceous species because the rate of OHads/Oads formation becomes too low compared to the rate of CHads formation. This occurs at lower temperature for Pt than for Rh because of the lower Pt–O bond energy.