Interpretation of H<sub>2</sub>-TPR from Cu-CHA Using First-Principles Calculations
Joachim D. Bjerregaard, Joonsoo Han, Derek Creaser, Louise Olsson, Henrik Grönbeck
Abstract
High Resolution Image Download MS PowerPoint Slide 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 H 2 temperature-programmed reduction (H 2 -TPR) of Cu-chabazite (Cu-CHA), which is a material used for ammonia assisted selective catalytic reduction of NO x (NH 3 -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 Z 2 Cu, where Z represents an Al site. Here, we analyze H 2 -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 H 2 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 Z 2 CuOCu and ZCuOH, whereas the peaks at higher temperatures can be assigned to paired Z 2 Cu and Z 2 CuHOOHCu species (360 °C) or paired Z 2 Cu and Z 2 CuOOCu (500 °C). The results are in good agreement with the experiments and facilitate the interpretation of future TPR experiments.