(700b) Catalytic Reformation of Glycolaldehyde, Glyceraldehyde and Glucose On Pd(111): A Mechanistic Study
The environmentally motivated shift towards a carbon neutral, sustainable hydrogen economy has spotlighted the need for advances in biomass reforming science. Utilization of biomass and biomass derivatives for hydrogen production can provide hydrogen fuel with a considerably smaller carbon footprint than the ubiquitous natural gas reforming process. However, the catalytic conversion of complex biomass feed molecules to hydrogen is not well understood, therefore there is a distinct need to elucidate the fundamental science for larger oxygenate bond scission. With a more comprehensive understanding of the catalytic bond scission of biomass derived oxygenates, extensions can be made to functionally similar molecules and catalytic chemistry predicted. An approach for bond scission characterization of the cellulosic monomer D-glucose and two simpler functional analogues on a Pd(111) catalyst is presented.
Characterization difficulties arising from the cellulosic sugars’ low vapor pressures have so far precluded their use in ultra high vacuum (UHV) surface science studies that have proven so successful in mapping out reaction mechanisms. Demonstration of the successful implementation of glucose in a UHV surface science study shows that this is no longer the case. A reproducible molecular glucose flux is verified using mass spectrometry and its adsorption on the catalyst surface is corroborated using temperature programmed desorption (TPD). Adsorption of glucose, glyceraldehyde and glycolaldehyde on the Pd(111) surface is characterized using high resolution electron energy loss spectroscopy (HREELS) in conjunction with TPD. Density functional theory calculations provide additional insights into the favored adsorption configurations. Results indicate an initial open ring, η1-adsorption through the aldehyde oxygen of glucose followed by transition into the energetically favored η2 di-sigma bonding configuration. This is observed similarly with glyceraldehyde and glycolaldehyde molecules pointing to initial chemistry being dominated by the aldehyde functionality.
Evolution of the energy losses in the HREELS data with increasing temperature outlines the sequence of bond scission events. Following transition into the η2 bonding configuration, the smaller molecules see O-H cleavage and subsequent adsorption through the alcohol oxygen. Continuing the sequence, C-C scission ultimately results in the production of CO and H2 products. These and additional similarities in adsorption mechanics and reaction chemistry between D-glucose and the simpler glyceraldehyde and glycolaldehyde molecules provide support for the use of functionally similar model compounds to predict the chemistry of complex biomass derived oxygenates in future studies.