Super-enantiospecific Autocatalytic Decomposition of Tartaric Acid and Aspartic Acid on Cu Surfaces
Autocatalytic reaction mechanisms are observed in a range of important chemical processes including catalysis, radical-mediated explosions, and biosynthesis. Because of their complexity, the microscopic details of autocatalytic reaction mechanisms have been difficult to study on surfaces and heterogeneous catalysts. Autocatalytic decomposition reactions of tartaric acid (TA) enantiomers adsorbed on Cu(110) offer molecular-level insight into these processes, which until now, were largely a matter of speculation. The decomposition of TA/Cu(110) is initiated by a slow, irreversible process that forms vacancies in the adsorbed TA layer, followed by a vacancy-mediated, explosive decomposition process that yields CO2 and small hydrocarbon products. Initiation of the explosive decomposition of TA/Cu(110) was studied by measurement of the reaction kinetics, time-resolved low energy electron diffraction (LEED), and time-resolved scanning tunneling microscopy (STM). Initiation results in a decrease in the local coverage of TA and a concomitant increase in the areal vacancy concentration. Once the vacancy concentration reaches a critical value, the explosive, autocatalytic decomposition step dominates the subsequent TA decomposition rate.
Aspartic acid is an excellent probe molecule for investigating the surface chemistry of autocatalytic reactions . Because a wide range of isotopically labeled varieties of aspartic acid are commercially available, we have been able to conduct a detailed investigation of its autocatalytic reaction mechanism. Experimental data obtained for variable initial coverage, variable heating rate and isothermal TPRS experiments, while monitoring CO2 desorption is in excellent agreement with a rate law which explicitly accounts for an initiation step and an explosion step which is second order in vacancy concentration
Autocatalytic surface explosion mechanisms can be exploited to attain extremely high enantiospecificities in the case of TA decomposition on naturally chiral Cu(hkl)R&S surfaces. Interaction energies of chiral molecules with naturally chiral surfaces are small and typically lead to modest enantioselectivities. However, the highly non-linear kinetics of autocatalytic reaction mechanism amplifies these small differences to result in high enantiospecificities. The observed phenomenon has the characteristics of autocatalytic processes that have been postulated to lead to biomolecular homochirality in life on Earth; processes with relatively small differences in reaction energetics that, nonetheless, lead to extremely high enantioselectivity.