The mechanism of CO2 reduction over Pd/Al2O3: a combined steady state isotope transient kinetic analysis (SSITKA) and operando FTIR investigation

2017 Spring Symposium

Xiang Wang, Hui Shi and János Szanyi, Institute for Integrated Catalysis, Pacific Northwest National Laboratory, Richland, WA

Abstract – Understanding the critical steps involved in the heterogeneous catalytic CO2 reduction has attracted a lot of attention recently. In order to fully understand the mechanism of this reaction the determination of both the rate-determining steps and reaction intermediates are vital. Steady-State Isotopic Transient Kinetic Analysis (SSITKA) is one of the most powerful techniques used to investigate the elementary steps under steady-state reaction conditions. This technique provides valuable information on mean resident lifetime of surface intermediates, surface concentrations of adsorbed reactant species and an upper bound of the turnover frequency. Coupling SSITKA with operando-FTIR spectroscopy allows us to discriminate between active and spectator species present on the catalytic surface under steady state reaction conditions.  In the present work operando SSITKA experiments coupled with transmission FTIR, mass spectrometry (MS) and gas chromatography (GC) were performed to probe both the chemical nature and kinetics of reactive intermediates over a Pd-Al2O3 catalysts and provide a clear mechanistic picture of the CO2 hydrogenation reaction by revealing the rate-determining steps for CH4 and CO production.

Figure 1 shows normalized real-time signals for the decay and increase of methane (a) and carbon-monoxide (b) in the effluent at 533 K reaction temperature after the feed gas was switched at 0 s from CO2/H2/Ar mixture to 13CO2/H2 mixture.  With increasing temperature, the decay of CH4 and CO get faster.  By integration under the decay curves , the mean surface-residence times CH4 and  CO), the abundance of adsorbed surface intermediates leading to CH4 and CO products  CH4 and  CO) at 533-573 K were calculated. At low temperature, CO2 methanation is slower than the reverse water-gas shift reaction, but became faster as the temperature was increased over 563 K.  The similar apparent activation energies obtained for the hydrogenation of adsorbed CO and for the formation of CH4 indicates that the hydrogenation of CO is the rate-determining step during the CO2 methanation reaction. Moreover, the similar apparent activation energies estimated for the consumption of adsorbed formates (FTIR) and for the formation of CO (MS), indicates that the H-assisted decomposition of formates is the rate determining step in the reverse water gas shift reaction.  The rate-determining step for CO formation is the conversion of adsorbed formate, while that for CH4 formation is the hydrogenation of adsorbed carbonyl. The balance of the hydrogenation kinetics between adsorbed formates and carbonyls governs the selectivities to CH4 and CO. We applied this knowledge to design catalysts and achieved high selectivities to desired products. 

Figure 1. Normalized response of (a) CH4 and 13CH4 products and (b) CO and 13CO products as functions of time.

Biography – Dr. Szanyi`s research is focused on surface science, spectroscopy and kinetic studies on heterogeneous catalytic reaction systems aimed at understanding structure-reactivity relationships. In particular, he is interested in understanding the mechanistic consequences of very high (atomic) metal dispersion on different support materials. Using a series of ensemble averaged spectroscopy methods he investigates the fundamental properties of metal atoms and small metal clusters prepared under well controlled UHV conditions. These results provide information on the energetics of the interactions between highly dispersed metals and selected probe molecules. Applying in situ RAIR spectroscopy they study the binding configurations of adsorbates to metals, and identify surface species present on the metal and support materials under elevated reactant pressures. Simultaneously, they are conducting detailed kinetics and operando spectroscopy measurements on model high surface area supported metal catalysts using flow reactors and SSITKA/FTIR/MS techniques. These measurements provide detailed kinetic information together with surface speciation that allow them to greatly enhance our mechanistic understanding of heterogeneous catalytic systems, in particular the reduction of CO2. Dr Szanyi is also involved in research related to the fundamental understanding of automotive emission control catalysis, conducting research in selective catalytic reduction of NOx on zeolite-based catalysts, low temperature NO and CO oxidation on metal oxides, and low temperatures NOx and HC storage in zeolites.