(44a) Controlling Bond Scission Sequence of Methanol Decomposition On Pt-Modified Tungsten Carbide

Recent studies have suggested that tungsten monocarbide (WC) may behave similarly to Pt for the electrooxidation of methanol [1, 2]. Temperature programmed desorption (TPD) was used to quantify the activity and selectivity of methanol decomposition for WC and Pt-modified WC (Pt/WC) as compared to Pt [2]. While WC appeared to be more active than Pt in ultra-high vacuum (UHV), C-O bond scission resulted in gas phase CH₄, an undesired reaction for DMFC. When Pt was added to WC by physical vapor deposition, the CH₄ reaction pathway was eliminated, suggesting that Pt synergistically modifies WC to improve the selectivity toward the C-H bond scission to produce hydrogen and CO. Additionally, TPD confirmed WC and Pt/WC to be more CO tolerant than Pt [3].

Density functional theory (DFT) was used to study the reaction network of methanol on Pt/WC(0001) as compared to Pt(100) and WC(0001). Results suggested that the bond scission sequence of CH₃OH could be controlled using submonolayer coverages of Pt on WC and that the resulting mechanism was different for Pt/WC as compared to either parent surface. C-H bond scission to form hydroxymethyl (CH₂OH*) was favored on Pt(100), likely eliminating CH₄ formation. On the other hand, O-H bond scission to form the methoxy intermediate (CH₃O*) was the first energetically favored step over WC(0001). Subsequent C-O bond scission was favored to form CH₃*, which likely reacted with a nearby proton to produce CH₄, a reaction product observed in TPD studies of CH₃OH on WC. DFT suggested the mechanism for 0.8 ML Pt/WC(0001) was unique, initially cleaving the O-H bond to form CH₃O*, similar to WC, but then cleaving the C-H bond to form CH₂O*.

High-resolution electron energy loss spectroscopy (HREELS) verified that surface intermediates were different on Pt/WC as compared to Pt or WC and that methanol decomposition occured at lower temperatures for the WC surfaces.

Overall, the combined experimental and theoretical results suggest that Pt-modified carbide surfaces can be used to control the bond scission sequence of small oxygenates and that WC is a more CO tolerant material than Pt; thus, WC and Pt/WC could be promising alternatives to Pt electrocatalysts for low temperature fuel cells.

[1] H. Hwu, J.G. Chen, Chemical Reviews 105 (2005) 185.

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

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