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Steady-State Catalytic Decomposition of Aspartic Acid on Cu(111)
Yun, Yongju,Kondratyuk, Petro,Gellman, Andrew J. American Chemical Society 2019 JOURNAL OF PHYSICAL CHEMISTRY C - Vol.123 No.13
<P>Decomposition of tartaric acid (TA, HO<SUB>2</SUB>CCH(OH)CH(OH)CO<SUB>2</SUB>H) and aspartic acid (Asp, HO<SUB>2</SUB>CCH(NH<SUB>2</SUB>)CH<SUB>2</SUB>CO<SUB>2</SUB>H) on Cu surfaces occurs via a vacancy-mediated surface explosion mechanism with nonlinear kinetics: <I>r</I><SUB>e</SUB> = <I>k</I><SUB>e</SUB>θ(1 - θ)<SUP>2</SUP>, where θ is the adsorbate coverage, and (1 - θ) is the vacancy coverage. During temperature-programmed reaction experiments on naturally chiral Cu(<I>hkl</I>)<SUP>R&S</SUP> surfaces, these kinetics, coupled with the chirality of the adsorbates, lead to highly enantiospecific decomposition rates. Herein, it is demonstrated that, in the presence of a thermal molecular beam with a constant Asp flux, <I>F</I><SUB>Asp</SUB>, the decomposition of Asp on Cu(111) occurs catalytically and in steady-state to turnover numbers >40 without contamination or deactivation of the surface. Moreover, the decomposition rates at a given (<I>T</I>, <I>F</I><SUB>Asp</SUB>) manifest the two different steady-states predicted by the nonlinearity of the decomposition kinetics. The unusual ability to catalytically decompose Asp on Cu, coupled with the ability to distinguish between <SMALL>D</SMALL>- and <SUP>13</SUP>C-<SMALL>L</SMALL>-Asp using mass spectrometry, opens an avenue for the future study of enantioselective catalysis on Cu(<I>hkl</I>)<SUP>R&S</SUP> surfaces.</P> [FIG OMISSION]</BR>
Kondratyuk, Petro,Karagoz, Burcu,Yun, Yongju,Gellman, Andrew J. American Chemical Society 2019 The Journal of Physical Chemistry Part C Vol.123 No.31
<P>Surface explosion reactions have highly nonlinear reaction kinetics that exhibit autoacceleration under isothermal conditions. These can lead to phenomena such as oscillatory surface reaction rates and to highly enantiospecific reactions of chiral adsorbates on chiral surfaces. Tartaric acid (TA) decomposes on Cu surfaces by an explosion mechanism that is propagated by vacancies, empty adsorption sites that self-replicate autocatalytically during TA decomposition. Surface explosion kinetics result from chain-branching steps in which one vacancy decomposes an adsorbate to yield two vacancies. In the absence of vacancies, surface explosions cannot occur; they require some initiation step that creates vacancies. By comparison with the chain-branching explosion step, little is known about the processes that initiate or nucleate surface explosion reactions. Time-resolved XPS measurements during the early stages of explosion initiation of TA/Cu(<I>hkl</I>) reveal a process that involves direct loss of TA from the surface to create the initial vacancies. In the presence of a gas phase flux to the surface, such vacancy nuclei can be repopulated to suppress the onset of explosion. Measurements on 18 different Cu(<I>hkl</I>) surface orientations demonstrate that the kinetics of the initiation process are structure-insensitive. This implies that the highly enantiospecific TA decomposition kinetics observed on chiral Cu(<I>hkl</I>) surfaces must arise from the structure sensitivity of the chain-branching explosion kinetics.</P> [FIG OMISSION]</BR>
Surface and Internal Reactions of ZnO Nanowires: Etching and Bulk Defect Passivation by H Atoms
Kim, Wooseok,Kwak, Geunjae,Jung, Minbok,Jo, Sam K.,Miller, James B.,Gellman, Andrew J.,Yong, Kijung American Chemical Society 2012 The Journal of Physical Chemistry Part C Vol.116 No.30
<P>Reactions of ZnO nanowires (NWs) with atomic hydrogen were investigated with temperature-programmed desorption (TPD) mass spectrometry, scanning electron microscopy, and photoluminescence (PL) spectroscopy. During TPD, molecular H<SUB>2</SUB>, H<SUB>2</SUB>O, and atomic Zn desorbed from ZnO NWs pretreated with atomic H at 220 K. Three distinct H<SUB>2</SUB> TPD peaks, two from surface H states and one from a bulk H state, were identified. The TPD assignment of the bulk H state was corroborated by significantly suppressed emission at 564 nm and enhanced emission at 375 nm in PL experiments. Etching of ZnO NWs by atomic H was confirmed by desorption of molecular H<SUB>2</SUB>O and atomic Zn in TPD and by electron microscopic images of H-treated ZnO NWs. A mechanistic model for underlying H/ZnO NW reactions is proposed and discussed.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/jpccck/2012/jpccck.2012.116.issue-30/jp304191m/production/images/medium/jp-2012-04191m_0006.gif'></P>