Characterization of Active Species in Cu-Beta Zeolite by Temperature-Programmed Reduction Mass Spectrometry (TPR-MS)

摘要

The number and type of copper species present in an under-exchanged Cu-Beta zeolite catalyst were characterized using temperature-programmed reduction mass spectrometry (TPR-MS). Both H2 consumption and H2O evolution were tracked, yielding additional insights into the nature of Cu species. Prior to the TPR-MS tests, catalyst samples were subjected to various high-temperature aging treatments in the presence of O2 in He, in order to assist with interrogating and resolving different types of sites. Absence of reducible species in the precursor material was confirmed by subjecting the H-form of the same zeolite to the TPR. Upon aging at temperatures below 700 °C, the TPR pattern showed several distinct peaks of different intensities. Aging at 800 °C led to shifting and broadening of these H2 consumption peaks, as well as to some reduction of the integral amount of the reducible sites. The 900 °C exposure resulted in a drastic change of the TPR pattern, with several new well-resolved H2 consumption peaks, indicative of drastic changes in the catalyst structure. The integral amount of H2 consumed quantitatively matched the amount of Cu species present in the catalyst, assuming the Cu~(2+) → Cu° stoichiometry. The combination of experimental findings in this work lends support to a hypothesis that different TPR peaks are indicative of different types of sites rather than of step-wise reduction of copper species.