Combined computational and experimental investigation of SOx adsorption on MgO

One of the main obstacles to the use of alkaline-earth-oxide-based NOx adsorbents for emissions control is the ready poisoning of these materials by SOx. To shed light on the mechanisms of poisoning, density functional theory (DFT) calculations and infrared spectroscopy experiments are used to study SOx adsorption on MgO as a model alkaline earth oxide. DFT plane-wave, pseudopotential results are presented for SOx adsorption on MgO(001) terraces and steps. Both SO2 and SO3 are found to exhibit both weakly bound physisorbed and strongly bound chemisorbed forms, with minimal activation barriers separating the two. Chemisorption is dominated by interactions between Lewis acidic sulfur and Lewis basic oxide anion sites to form surface sulfites and sulfates from SO2 and SO3, respectively. Within the generalized gradient approximation to DFT, the SO2 adsorption energy ranges from 25 kcal mol(-1) on terrace sites to 46 kcal mol(-1) on lower-coordinated step edge sites; analogous SO3 adsorption energies range from 49 to 76 kcal mol(-1). Surface sulfite is readily observed upon exposure of calcined MgO powder to SO2 and produces vibrational signatures consistent with that calculated for chemisorbed SO2. Sulfation is accomplished by exposure of the same powders to SO2 and O-2 at elevated temperatures. Comparisons to calculated vibrational spectra indicate that both chemisorbed SO2 and SO3 are present under these conditions; further heating induces formation of a bulklike sulfate that is difficult to remove. The results demonstrate the high reactivity of SOx toward metal oxides and illustrate the challenges in developing sulfur-resistant oxide-based NOx traps.