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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