A mechanistic study on the CO2 activation over Pd-containing MCM-22 zeolite based on DRIFT analysis: Impact of counter cations in the zeolite framework

Zeolites have been playing an important role in the field of so-called "Carbon dioxide Capture, Utilization, and Storage (CCUS)". Catalytic activation of CO2 over zeolite-based catalysts has been extensively investigated, while many issues remain unsolved. In this study, the MWW-type zeolites impregnated with the Pd nanoparticles were used as the catalysts for CO2 activation. Furthermore, various amounts of charge balance cations (Na+ and H+) were added to the Pd-MWW with the intention of improving CO2 adsorption capacity. A fixed-bed reactor equipped with the Mass Spectrometer for effluent gas analysis was used to evaluate the steady-state conversion of the CO2 activation on these as-prepared catalysts from 50 to 650 degrees C. The compositions of the effluent gases and their associated quantities were monitored for each test. Finally, two infrared techniques (steady-state and dynamic measurements) were applied to identify the spectroscopic evidence of the heterogeneous catalysis reaction. In the first technique, the amount of chemisorbed CO2 was quantified by the infrared transmitted intensity across the samples from the vacuum to various levels of CO2 pressure at ambient temperature. The second technique investigates the hydrogenation mechanisms of adsorbates by the time-resolved FTIR with an in-situ diffusive reflectance airtight chamber. Both methods facilitated the infrared spectra of surface-adsorbed species during the hydrogenation. In combination with the temperature-programmed CO2 activation tests, this research prompted to reveal the kinetic mechanisms of CO2 activation.