(5co) Controlling the Bond Scission Sequence of Methanol Decomposition as An Example of Rational Catalyst Design

The so called ?Holy Grail? of heterogeneous catalysis is a fundamental understanding of catalyzed chemical transformations which span multidimensional scales of both length and time, enabling rational catalyst design. Such an undertaking is realizable only with an atomic level understanding of bond formation and destruction with respect to intrinsic properties of the metal catalyst. In this study, we investigate the bond scission sequence of methanol on bimetallic transition metal catalysts and transition metal carbide catalysts, with the simplicity of methanol allowing us to follow the different reaction pathways both experimentally and with density functional theory (DFT) modeling. Additionally, methanol is of interest both as a hydrogen carrier for reforming to hydrogen and CO and as a fuel in direct methanol fuel cells (DMFC). In order to bridge the so-called ?materials gap? and ?pressure gap? this work adopted three parallel research approaches: (1) ultra-high vacuum (UHV) studies including temperature programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS); (2) electrochemical studies including cyclic voltammetry (CV) and chronoamperometry (CA); (3) DFT studies including thermodynamic and kinetic calculations.

The Ni/Pt bimetallic system was studied as an example for using methanol as a hydrogen source. There are two well characterized surface structures for the Ni/Pt system [1]: the surface configuration, in which the Ni atoms reside primarily on the surface of the Pt bulk, and the subsurface configuration, in which the second atomic layer is enriched in Ni atoms and the surface is enriched in Pt atoms. These configurations are denoted NiPtPt and PtNiPt, respectively. TPD studies revealed that the NiPtPt surface was more active to methanol reforming than the Pt or PtNiPt surfaces. HREELS confirmed the presence of strongly bound reaction intermediates [2], including aldehyde-like species [3], and suggested that the first decomposition step was likely O-H bond scission. Thus, the binding energies of the deprotonated reaction intermediates are likely important parameters in controlling the decomposition of methanol.

Recent studies have suggested that tungsten monocarbide (WC) may behave similarly to Pt for the electrooxidation of methanol [4, 5]. TPD was used to quantify the activity and selectivity of methanol decomposition for WC and Pt-modified WC (Pt/WC) as compared to Pt [5]. WC appeared to be more active than Pt, but C-O bond scission on WC resulted in gas phase methane, an undesired reaction for DMFC. When Pt was added to WC by physical vapor deposition, the methane reaction pathway was eliminated, suggesting that Pt synergistically modifies WC to improve the selectivity toward C-H bond scission to produce hydrogen and CO. Additionally, TPD confirmed WC and Pt/WC to be more CO tolerant than Pt [6]. DFT calculations suggested that the bond scission sequence of methanol could be controlled using monolayer coverage of Pt on WC and that the resulting mechanism was different for Pt/WC as compared to either parent surface. HREELS results verified that surface intermediates were different on Pt/WC as compared to Pt or WC.

Both studies demonstrated that the bond scission sequence of methanol can be controlled using either bimetallic or carbide catalysts. Such success was only possible using a methodology that combines both calculations to predict catalytic properties and experiments to fine-tune theoretical predictions.

[1] J. G. Chen, C. A. Menning, M. B. Zellner, Surface Science Reports 2008, 63, 201.

[2] A. L. Stottlemyer, H. Ren, J. G. Chen, Surface Science 2009, submitted.

[3] O. Skoplyak, C. A. Menning, M. A. Barteau, J. G. Chen, Journal of Chemical Physics 2007, 127, 114707.

[4] H. Hwu, J. G. Chen, Chemical Reviews 2005, 105, 185.

[5] E. C. Weigert, A. L. Stottlemyer, M. B. Zellner, J. G. Chen, Journal of Physical Chemistry C 2007, 111, 14617.

[6] Z. J. Mellinger, E. C. Weigert, A. L. Stottlemyer, J. G. Chen, Electrochemical and Solid-State Letters 2008, 11, B63.