A Theoretical and Experimental Study of Reaction Pathways for the Interaction of CO2 with Alkali-Modified Surfaces

Abstract

Previous work has shown that CO{sub 2} adsorption on bulk potassium leads to the formation of oxalate ions below room temperature. These C{sub 2}O{sub 4} groups decompose in vacuo to carbonates, CO{sub 3}, and CO around 300 K and the carbonates further to oxides and carbon dioxide above 700 K. The present work addresses reaction routes by calculations of adsorption coordinates in combination with spectroscopic data from time-resolved in situ infrared spectroscopy and analyses of desorbing species. Oxalate intermediates are formed by reactions between surface CO{sub 2} ions and adsorbed neutral molecules. Weakly adsorbed CO{sub 2} alone cannot follow this route. The energetics of oxalate formation is compared with CO{sub 2} adsorption on clean and oxidized potassium. Rearrangements between almost isoenergetic oxalates are discussed in connection with reaction routes and branching ratios. CO is released from one oxalate isomer, C{sub 2}O{sub 4}{sup 2-} - CO{sub 3}{sup 2-} + CO, with only a small activation barrier and bound as an entity in a K:CO complex at multilayer coverages. Desorption is only observed after annealing to around 650 K. Finally, CO and CO{sub 2} release as a result of carbonate decomposition and the reverse formation of CO{sub 3} are discussed, although thesemore » reactions are substrate dependent and not merely a rearrangement of an alkali compound. 40 refs., 9 figs.« less