264187 Metal-Modified Carbides for Conversion of Alcohols to Syngas and Aldehydes
Metal-Modified Carbides for Conversion of Alcohols to Syngas and Aldehydes
Thomas G. Kelly and Jingguang G. Chen1*
1Center for Catalytic Science and Technology, Department of Chemical and Biomolecular Engineering, University of Delaware, Newark, Delaware 19716 (USA)
In the search for energy sources to replace fossil fuels, biorenewable fuels have become a distinct possibility. One class of biorenewable molecules are small-chain alcohols. These alcohols can be reformed to produce syngas or thermochemically converted to useful products. Recently, 3d/Pt(111) bimetallic surfaces have been shown to be active for methanol and ethanol reforming. The Ni/Pt(111) surface displayed higher reforming yield than both of the parent metals [1,2]. However, Pt-based bimetallics face the problems of Pt scarcity and poor overlayer stability. Metal carbides such as tungsten carbide (WC) and molybdenum carbide (Mo2C) have been shown to exhibit Pt-like properties . Also, WC can be used as a less expensive and more stable support in place of Pt, as was shown in using Ni/WC to replace Ni/Pt for ethanol decomposition . In the current study, we have extended our previous surface science studies for methanol on Pt-modified WC  to C1-C3 alcohols on carbides modified with Ni, Rh, Cu, and Au. Furthermore, an understanding of bond scission sequence should also provide insights into the utilization of carbides for catalytic conversion of alcohols.
Density Functional Theory (DFT) was used to calculate the binding energies of alcohols and relevant intermediates on model surfaces of WC and Mo2C, which showed the possibility to correlate reactivity with binding energy. Temperature-programmed desorption (TPD) was used on model WC and Mo2C surfaces 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 carbide and metal-modified carbide 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 since activity decreased with increasing Au coverage . Similar experiments have been conducted for ethanol and propanol decomposition on Rh/WC to investigate the pathway for 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. Lastly, the clean Mo2C surface was shown to completely decompose ethanol to its constituent elements with a small amount of ethylene and methane production. Ni-modified Mo2C produced CO as the major product, indicating C-C bond scission. Similarly to methanol on Au-modified WC, ethanol desorbed intact on Au-modified Mo2C. Interestingly, a pathway divergent from reforming was observed on Cu-modified Mo2C, which produced acetaldehyde. Apparently Cu was efficient at breaking the O-H and α C-H bonds but further dehydrogenation was not favored over desorption.
Of further interest is the bond scission sequence of additional C2 and C3 oxygenates on metal-modified WC. HREELS experiments on WC and Rh/WC were conducted for ethanol, acetaldehyde, propanol, and propanal to investigate the bond scission sequence in these oxygenates. 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.
The results of decomposition for C1-C3 alcohols on carbides hold promise for applications in heterogeneous reforming. On native carbides, C-O scission occurred to produce hydrocarbons. Ni and Rh supported on carbides have been shown to be efficient at cleaving C-H and C-C bonds of alcohols while leaving the C-O bond intact. Alternatively, modifying Mo2C with Cu resulted in acetaldehyde production from ethanol. The ability to selectively decompose alcohols by using various admetals demonstrates the versatility of metal-modified carbides. Future experiments will involve synthesis of supported metal-modified carbide catalysts and reactor testing for alcohol decomposition.
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