(691f) Experimental and Theoretical Studies of Surface Chemistry of Tungsten Carbide and Platinum-Modified Tungsten Carbide

The motivation of this research is to assess the feasibility of tungsten carbides as alternative electrocatalysts in direct methanol fuel cells (DMFC). DMFCs directly convert chemical energy into electrical energy by the electro-oxidation of methanol. Compared to internal combustion engines, DMFCs offer several attractive features, such as high theoretical efficiency, low operating temperature, and minimum vibration and noise.1 Current fuel cells require the use of Pt/Ru bimetallic anode catalysts which suffer from CO poisoning and high cost2. The commercialization of such technologies requires less expensive and more CO tolerant materials. Transition metal carbides often show catalytic properties similar to the Pt-group metals and recent research suggests that tungsten carbides may be an appropriate substitute3.

Temperature programmed desorption (TPD) was used to study the reactions of CO, H2, CH4 and CH3OH on polycrystalline platinum, tungsten monocarbide (WC), and platinum-modified WC foils4. Polycrystalline foils were chosen as test materials since their morphological complexity better approaches that of real commercial catalysts. Results from TPD studies show that all tested surfaces are active to methanol decomposition, with WC being the most active. Also, the addition of submonolayer coverages of Pt can eliminate the production of CH4, which is an undesired reaction occurring over WC.

The electrochemical properties of WC and Pt-modified WC foils were studied to determine the applicability and feasibility of tungsten carbides as alternative DMFC electrocatalysts4. Samples were evaluated using cyclic voltammetry (CV) and chronoamperometry (CA) in an in-situ electrochemical half-cell that allowed examination by X-ray photoelectron spectroscopy (XPS) before and after electrochemical measurements. A synergistic effect was observed on Pt-modified WC as evidenced by an increase in the onset of metal oxidation potentials compared to WC in an acidic environment at anodic potentials5. Additionally, both WC and Pt/WC displayed two oxidation features when exposed to methanol in an electrochemical environment which were attributed to methanol oxidation at intermediate potentials and metal oxidation at high potentials. CA measurements indicated that both WC and Pt/WC showed higher methanol turnover frequencies when compared to Pt in an acidic environment.

The ultrahigh vacuum (UHV) work in this study was supported by DFT calculations. The binding energies of H, CO, CH3OH, and CH3O were calculated on various single crystal surfaces of platinum, tungsten carbide, and platinum-modified tungsten carbide. Results show that for H, CO, and CH3OH the binding energy increases as the surface d-band center moves closer to the Fermi level. The methoxy (CH3O) intermediate does not follow this trend and binds much more strongly to WC than to any other surfaces. Using Sabatier’s Principle, the high binding energy of CH3O on WC may explain the high methanol decomposition activity on this surface, as evidenced in both UHV and electrochemical studies. Combining the current work with our recent electrochemical studies, our results indicate that WC and Pt-modified WC are promising alternative electrocatalysts for direct methanol fuel cells.

(1) Schlapbach, L.; Zuttel, A. Nature 2001, 414, 353.

(2) Carrette, L.; Friedrich, K. A.; Stimming, U. Fuel Cells 2001, 1, 5.

(3) Hwu, H.; Chen, J. G. Chemical Reviews 2005, 105, 185.

(4) Weigert, E.; Stottlemyer, A.; Zellner, M.; Chen, J. Journal of Physical Chemistry C 2007, 111, 14617.

(5) Weigert, E. C.; Zellner, M. B.; Stottlemyer, A. L.; Chen, J. G. Topics in Catalysis 2007, 49, 349.