“Fundamental assessment of catalytic systems for methane conversion and photocatalytic water splitting”
Saudi Arabia is not only a platform for 30% of oil reserve of the world but also that large quantity of solar energy is available in wide area. Collective efforts are undergoing at KAUST Catalysis Center (KCC) to address both conventional fossil fuel chemistries and future renewable solar fuel researches. At KCC, we study catalytic phenomena based on the concept, ‘catalysis by design’, which is the opposite of “black box screening”: Our ongoing efforts include to understand fundamental aspects of various catalytic systems in heterogeneous thermal-, electro- and photo-catalysis. The catalytic reactions are investigated using detailed kinetic analysis, spectroscopic techniques, and various in-operando characterizations to pin down key elementary steps involved in the catalytic cycles. This particular presentation will address two reactions: oxidative coupling of methane (OCM) and photocatalytic overall water splitting (OWS).
(OCM): Some catalysts, especially at high temperatures, are known to produce radical species into the gas phase, generating a complex heterogeneous-homogeneous reaction network. Such reactions are prevalent for catalytic combustion reactions of hydrocarbons, but it is interesting if partial oxidation products are targeted as a consequence of such radical chemistries. This contribution discusses our recent progress on selective catalytic generation of OH radicals from H2O and O2 mixture, which is utilized for hydrocarbon transformations, such as methane coupling and ethane dehydrogenation. Alkali metal catalysts (mainly Na), often at a molten state under reaction condition, are active for the selective generation of OH radical through a quasi-equilibrated formation of peroxide species, without activating hydrocarbons significantly on the surface. This OH radical reaction path makes it silent for competitive adsorption of hydrocarbons and their derivative products, beneficial to improve selectivities, such as CH4 vs. C2H4. Gas phase reaction selectivity generally reflects the C-H bond strengths of the hydrocarbons to abstract H from them and it enables to predict the targeted product selectivity and yield. As a consequence, ~30% carbon yield from CH4 to C2 was achieved using Na2WO4/SiO2 catalyst at ~850-900 °C. A perspective to produce other hydrocarbons via OH radical pathways is also discussed.
(OWS): Photocatalysis using powder suspension system is ill-defined in terms of chemical potentials and resultant kinetics. Our efforts include to achieve quantitative descriptions of the associated physical and chemical properties that determine which parameters are most influential to improving the overall photocatalytic performance, in contrast to arbitrarily ranking different photocatalyst materials. First, the quantifiable properties are identified. Second, each property is separately measured and/or calculated. Third, the obtained values of these properties are integrated into equations so that the kinetic/energetic bottlenecks of specific properties/processes can be identified. The specific properties can then be altered to further improve the overall efficiency. Accumulation of knowledge ranging from solid-state physics to electrochemistry and the use of a multidisciplinary approach to conduct measurements and modeling in a quantitative manner are required to fully understand and improve the efficiency of photocatalysis. For photocatalytic overall water splitting, it is critical to establish nanoscale surface modification that avoid unwanted back reaction from H2 and O2 to form H2O, which will also be discussed in this contribution.
Date(s) - Mar 09, 2018
10:00 am - 11:00 am