(739a) Metal-Modified Tungsten Carbide (WC) for Catalytic and Electrocatalytic Conversion of Alcohols

Authors: 
Kelly, T. G., University of Delaware
Chen, J. G., University of Delaware


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.

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