Glycolaldehyde As a Probe Molecule for Biomass-Derivatives: Reaction of C-OH and C=O Functional Groups on Monolayer Ni Surfaces

Global energy demand growth and green house effect are two of the main drivers for the development of replacing fossil fuels. Biomass-derived molecules are a promising class of alternative resources because they offer the advantages of being widely available, renewable, and potentially carbon-neutral. Such molecules generally contain more oxygen atoms than are found in petroleum-based feedstocks. Previously, small alcohols and polyols [1-2] were used as representatives of oxygenates to be studied with gradually increasing the complexity of the molecular structure. The result revealed the possibility of oxygenates reforming to H₂ and CO (syngas) by selectively controlling the C-H, C-C, C-O, and O-H bond scissions. In this work, glycolaldehyde (HOCH₂CH=O), which contains both –OH and –C=O functionalities similar to many biomass derived molecules, was studied as the probe molecule for biomass conversion to syngas. The current study of glycolaldehyde activity utilized density functional theory (DFT) prediction, as well as experimental verification using temperature programmed desorption (TPD) and high resolution electron energy loss spectroscopy (HREELS).

Bimetallic catalysts are known to often exhibit unique properties different from either of the parent metals, and show potential for biomass conversion. The Ni/Pt(111) bimetallic system has been extensively investigated and was therefore studied for glycolaldehyde reactions. As established in previous studies of alcohols and polyols [1-2], enhanced catalytic conversion of these molecules on the bimetallic surfaces could be correlated to the binding energies and the surface d-band center with respect to the Fermi level. The binding energy of glycolaldehyde was found to increase as the surface d-band center approached the Fermi level, with the NiPtPt(111) configuration exhibiting the highest binding energy and thus predicted to present the highest activity. In order to verify the DFT prediction, glycolaldehyde TPD experiments were performed on Pt(111), NiPtPt(111), PtNiPt(111) surfaces and a thick Ni(111) film. H₂ and CO were observed as the reaction products, which confirmed the prediction to produce syngas from glycolaldehyde. The activity of glycolaldehyde on each surface was quantified from the TPD peak areas. Among the four surfaces, the NiPtPt(111) surface showed the highest activity, consistent with the DFT prediction. HREELS experiments of glycolaldehyde were employed to show the intermediates in the reactions. At 300K, the C-C peak disappeared and a CO peak was observed, demonstrating the C-C bond cleavage to produce CO, which was consistent with the TPD result.

However, the favorable NiPtPt(111) bimetallic structure is not stable at high temperature [3]; the top monolayer Ni atoms tends to diffuse into the Pt bulk and stay beneath of the Pt surface. The resulting PtNiPt(111) subsurface configuration showed significantly lower reforming activity. Since tungsten monocarbide (WC) [4-6] has been shown to possess similar electronic properties to Pt(111), Ni-modified WC surfaces were proposed to replace NiPtPt(111) in the glycolaldehyde study. Parallel DFT glycolaldehyde binding energy on the one-monolayer Ni-modified WC (1ML NiWC) surface was calculated and found to be similar to that on NiPtPt(111). TPD experiments of glycolaldehyde were performed on WC and Ni-modified WC surfaces. On clean WC surface, ethylene was observed as the product, which was consistent with the HREELS result. On Ni-modified WC surface, H₂ and CO were produced, exhibiting the same chemistry with NiPtPt(111) configuration. A similar glycolaldehyde reforming activity was also found on the 1ML NiWC surface after the quantification of reaction activity. These results suggested that Ni monolayer catalysts supported on WC may be preferable to Ni/Pt bimetallics as active and selective catalysts for biomass reforming with higher stability and lower cost.

References

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