Interpretation of H₂-TPR from Cu-CHA Using First-Principles Calculations

Temperature-programmed reduction and oxidation are used to obtain information on the presence and abundance of different species in complex catalytic materials. The interpretation of the temperature-programmed reaction profiles is, however, often challenging. One example is H₂ temperature-programmed reduction (H₂-TPR) of Cu-chabazite (Cu-CHA), which is a material used for ammonia assisted selective catalytic reduction of NO_x (NH₃-SCR). The TPR profiles of Cu-CHA consist generally of three main peaks. A peak at 220 °C is commonly assigned to ZCuOH, whereas peaks at 360 and 500 °C generally are assigned to Z₂Cu, where Z represents an Al site. Here, we analyze H₂-TPR over Cu-CHA by density functional theory calculations, microkinetic modeling, and TPR measurements of samples pretreated to have a dominant Cu species. We find that H₂ can react with Cu ions in oxidation state +2, whereas adsorption on Cu ions in +1 is endothermic. Kinetic modeling of the TPR profiles suggests that the 220 °C peak can be assigned to Z₂CuOCu and ZCuOH, whereas the peaks at higher temperatures can be assigned to paired Z₂Cu and Z₂CuHOOHCu species (360 °C) or paired Z₂Cu and Z₂CuOOCu (500 °C). The results are in good agreement with the experiments and facilitate the interpretation of future TPR experiments.