Applying low-temperature titration for determination of metallic sites on active oxide supported catalysts

Abstract

Chemisorption–titration, as a surface sensitive analysis method, is widely applied for direct measurement of the specific surface area of metallic clusters on heterogeneous catalysts. However, on active oxide supports, the product yield of this method is strongly affected by the exchange of oxygen between the support and the titration gases. In this study, Pt- and Cu-based catalysts are used as the source for metallic site characterization through isothermal titration experiments within a wide temperature range. Pt/Fe₂O₃ catalysts were characterized using CO oxidation, and Cu/CeO₂ catalysts were characterized using N₂O decomposition. The titration products are CO₂ and N₂, respectively. Detailed measurements require instant online mass spectroscopy characterization and temperature control from subzero to room temperatures; for this purpose an online micro reactor is developed and applied for quantitative product calibration. For both series of catalysts, MS signals are also measured on pure metallic powder and calibrated using a commercial chemisorption measurement system. The temperature dependence of both total titration products from the supported samples reveals that for each catalyst, there are narrow subzero temperature windows in which the titration products are only related with metallic sites. The results provide an approach towards a one-step chemisorption–titration procedure for measurement of dispersion on active oxide supported catalysts, which is a possible benchmark that bridges fundamental model catalysts and industrial applied catalysts in performance evaluation.