Metal-Modified Tungsten Carbide (WC) for Catalytic and Electrocatalytic Conversion of Alcohols

Direct alcohol fuel cells (DAFCs) utilizing methanol or ethanol have been proposed as an alternative to hydrogen fuel cells.  Alcohols are easier to transport and more readily obtained from biorenewable sources than other fuels.  The anode material most widely studied for DAFCs is currently a PtRu alloy; however, Pt is scarce and expensive.  Additionally, Pt is not active towards scission of the C-C bond in ethanol.  Recently, researchers reported using a Pt/Rh/SnO₂ alloy that was more active towards ethanol electrooxidation than Pt, and Rh was determined to be crucial for effective C-C bond cleavage [1].  Tungsten monocarbide (WC) has been shown to have Pt-like properties and is active towards methanol electrooxidation [2,3].  Also, metal-modified WC can be used in place of bimetallic catalysts that are less stable and more expensive, as was shown recently in using Ni/WC to replace Ni/Pt for ethanol decomposition [4].  In the current study, we have extended our previous surface science studies for methanol on Pt-modified WC [5] to C1-C3 alcohols on WC modified with Ni, Rh, and Au.  Furthermore, an understanding of the bond scission sequence of alcohols on metal-modified WC should also provide insights into the utilization of carbides for catalytic reforming reactions of alcohols.

Density Functional Theory (DFT) was used to calculate the binding energies of alcohols and relevant intermediates on WC and metal-modified WC, which showed the possibility to correlate reactivity with binding energy. Temperature-programmed desorption (TPD) was used with a WC polycrystalline foil modified with different metals to quantitatively determine the activity of the surfaces and their selectivity towards C1-C3 alcohol decomposition.  High resolution electron energy loss spectroscopy (HREELS) identified different reaction intermediates on WC and metal-modified WC surfaces.  The first step in alcohol decomposition on these surfaces was O-H bond cleavage to form an adsorbed alkoxy species.  In the case of methanol, the predominant reaction product on clean WC was methane; adding small amounts of Ni or Rh promoted the C-H bond scission and shifted the selectivity towards CO and H₂.  Au simply acted as a site blocker with decreasing activity with increasing Au coverage [6].  Similar experiments have been conducted for ethanol and propanol decomposition on Rh/WC to investigate the problem of C-C bond scission.  The selectivity on clean WC was for C-O bond scission to produce ethylene.  Adding Rh led to C-H and C-C bond cleavage; HREELS showed that the C-C bond in ethanol is broken by 200 K. 

Of further interest is the bond scission sequence of ethanol and propanol on these surfaces.  On most transition metals, after initial O-H bond scission, these alcohols decompose through aldehyde intermediates by cleavage of the α C-H bond.  However, on Rh(111), the β C-H bond breaks to form an oxametallacycle intermediate that rapidly decomposes [7-9].  HREELS experiments on WC and Rh/WC were conducted for ethanol, acetaldehyde, propanol, and propanal to investigate the reaction pathways.  On WC, ethanol followed a decomposition pathway similar to acetaldehyde; the spectra suggested that both molecules formed a di-σ species that facilitates C-O bond cleavage.  This behavior was mirrored by propanol and propanal on WC.  On Rh/WC, ethanol and acetaldehyde again followed similar pathways, except that the pathway in this case is through C-C bond scission instead of C-O scission.  For both molecules, the C-C bond was broken by 200 K.  A slightly different result was observed for propanol and propanal on Rh/WC.  By 200 K, the C-C bond was broken in propanal, but the propanol spectra still showed peaks due to propoxy.  The difference in spectra between propanol and propanal suggested that the molecules may have followed different decomposition pathways.

The results of alcohol decomposition for C1-C3 alcohols on WC and metal-modified WC hold promise for applications in heterogeneous reforming and electrochemical applications.  Futhermore, butanol may be an attractive prospect since it can be produced through biomass fermentation.  Future work will focus on electrochemical experiments on metal-modified WC for alcohol electrooxidation in an acidic medium.

References

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4.      Ren, H., et. al. ACS Catalysis 1 (2011) 390.

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6.      Kelly, T.G., et. al. J. Phys. Chem. C. 115 (2011) 6644.

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8.      Brown, N.F. and Barteau, M.A., Langmuir 8 (1992) 862.

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