2012 Spring Symposium
Yadan Tang, Charles A. Roberts, Israel Wachs
Department of Chemical Engineering
Abstract — Measured trends in catalytic reactivity over varying metal catalysts have been used to facilitate the optimization of bimetallic catalysts. An important example of such a trend is the Sachtler-Fahrenfort volcano curve, in which reactivity of metal surfaces for formic acid decomposition is plotted against the stability of intermediates, i.e. the bulk heat of formation of the formate on a specific metal surface. It is questionable, however, to correlate a bulk property with catalytic reactivity, a process that occurs exclusively at the surface. The current study investigates the correlation between formic acid decomposition and reactivity of bulk metal catalysts (i.e. Fe, Ru, Pd, Pt, Au, Ag, Ni, Co, and Cu) using modern techniques such as in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and temperature programmed surface reaction (TPSR) spectroscopy. In situ DRIFTS monitors the formate structure on the surface of bulk metal catalysts during the adsorption and decomposition of formic acid. By utilizing a temperature ramping procedure, in situ DRIFTS also provides insights into thermal stability of adsorbed formates. TPSR spectroscopy detects the temperature at which the peak activity for decomposition of the adsorbed formates occurs, therefore providing a measure of the reactivity of the metal surface. In situ DRIFTS and TPSR spectroscopy experiments agree with the previous reported finding that the decomposition of HCOOH proceeds via two steps: 1) formation of surface adsorbed formate (HCOO-M) intermediates; and 2) decomposition of formate intermediates into gas phase products such as CO, CO2, H2 and H2O. The formate structure on various metal catalysts are identified and assigned based on a previous study on formic acid via high resolution electron energy loss spectroscopy (HREELs). The current study finds that the formate species on Fe, Ru, Pd, Pt and Au are bridged; on Co and Ni are monodentate; and on Cu and Ag are converted from monodentate to bridged at higher temperature in agreement with HREELs work on both Cu(100) and Ag(110). The TPSR decomposition temperatures, Tp, were plotted versus the bulk heat of formation of formates reported by Sachtler and Farenfort. Rather than a volcano trend, the plot is observed to contain two distinct linear relationships indicating that trends in reactivity of metals should be evaluated based on surface properties rather than bulk.
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Speaker’s Biography — Yadan Tang is a graduate student in Chemistry at Lehigh University, advised by Professor Israel Wachs. She received her B.S. in Material Science and Engineering Department at East China Univ. of Science and Technology in 2006. She received her M.S. in Chemistry Department at Lehigh Univ in 2010. Since joined in Wachs group in 2011, she has been involved in formic acid decomposition on bulk metal catalyst and supported metal oxides on zeolite.