(25f) First Principles Design of Metal-Modified Molybdenum Carbide Surfaces for Controlling the Activity and Selectivity of Ethylene Glycol Reactions

The declining supply of petroleum
resources, combining with the increasing energy demand by the rapidly
developing economies, as well as the political and environmental concerns about
fossil fuels lead to the imperative development of sustainable energy. Biomass,
because of its advantages of being widely available, renewable and potentially
CO₂-neutral, is regarded as an alternative energy source to fossil
fuels. The derivatives of biomass generally contain more oxygen atoms than
those are found in petroleum-based feedstocks, and therefore selective deoxygenation
and reforming reactions are considered as desirable reaction pathways of
biomass-derivatives to chemicals and fuels.  The design of effective catalysts
for biomass conversion requires the understanding and controlling of the bond
scission mechanisms in biomass-derived molecules.  Previously, a high yield of ethylene glycol (at 61%)¹ was achieved from a direct
conversion of cellulose using a Ni-promoted tungsten
carbide catalyst. After that, ethylene glycol, the smallest polyol with
the same atomic C/O ratio with C5 and C6 sugars, has been employed as a model
compound for biomass-derived molecules in several previous studies^2-4.  The activity and selectivity of the ethylene glycol deoxygenation
reaction to form ethylene and the reforming reaction to produce synthesis gas
(H₂ and CO) have been investigated in the current study. 

The reaction of ethylene glycol has
been studied on different 3d/Pt bimetallic surfaces^5,6 and the monolayer 3d metal on Pt (3d-Pt) surface is identified to show higher activity
than either of the parent metals.  However, the favorable
3d-Pt bimetallic structure for ethylene glycol reforming is not stable at high
temperatures due to the diffusion of surface 3d atoms into the Pt bulk^7,8.  Transition metal carbides
have been reported in previous studies^9-12 to exhibit similar electronic properties to Pt and are three
orders of magnitude cheaper than Pt.  Studies^2,13,14 have been carried out
to substitute the Pt bulk with tungsten monocarbide (WC) and confirmed the
feasibility of using WC to replace Pt as a less expensive and more thermally
stable^15,16 reforming catalyst for C2
oxygenates.  On the other hand, different from Pt, the clean WC surface favors
the deoxygenation reaction for C2 oxygenates². Molybdenum carbide
(Mo₂C) is of low cost, easy to synthesize, can be
of high surface area, and can serve as a diffusion barrier to metal adlayers
allowing for catalyst stability under working conditions.  In this work, clean
Mo₂C and different admetal (Ni, Au, Cu and Pt) modified Mo₂C
surfaces are chosen for the catalyst design for ethylene glycol reaction. 

The
searching of good catalysts for the reaction of ethylene glycol can be time and
effort consuming. While Sabatier's principle and the associated volcano curve
have shaped our thinking over the years toward catalysts of optimal properties,
no general methodology exists that can address materials' selection for the
harder problem of selectivity. The latter is controlled by competitive and
often convoluted pathways of the reaction network. In this work, a methodology
for catalyst design is introduced by constructing an ?extendable' microkinetic
model to predict not just the catalyst activity trend but also the selectivity.
Our computational work is followed by temperature program desorption (TPD)
experiments to verify the predicted trends on clean Mo₂C and admetal
(Ni, Au, Cu and Pt) modified Mo₂C surfaces as well as the
well-studied Ni/Pt(111) surfaces. High resolution electron energy loss
spectroscopy (HREELS) experiments are also performed to verify the selective
bond scission of ethylene glycol reaction pathways predicted from the
microkinetic modeling.

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