Improving hydrothermal stability of carbon-supported metal catalysts for biomass conversions
Human civilization and modern development relies on energy, fuels, and chemicals. Since the mid-late 1700s fossil carbons have been the major source of our energy and chemicals. This continues to pose environmental concerns such as greenhouse gas emission and concerns about its unsustainable nature. As a result, developing alternative clean and renewable energy and chemicals is one of the most urgent challenges facing humanity. Biomass have demonstrated the potential to be upgraded to fuels and chemicals with several challenges to be addressed. Hydrothermal stability of catalyst is one of the challenges as many biomass conversion reactions are happening in aqueous phase.
In the first chapter, general concerns about fossil fuels were discussed, followed by an overview of biomass conversion to both fuels and chemicals. The challenges facing biomass conversion were discussed including hydrothermal stability of the catalyst. Hydrothermal stability of common catalyst supports was summarized and compared. We also demonstrated different deactivation mechanisms of catalyst during hydrothermal conditions. Several strategies to improve the hydrothermal stability of catalyst were also raised.
In the second chapter, we used the different pyrolysis temperature to tune the surface chemistry of the carbon coated SBA-15 support. Hydrothermal treatments and reactions were performed on the supported Pd catalyst to investigate the hydrothermal stability of the Pd particles. Through detailed 13C NMR, carbon surface chemistry was analyzed and compared in a systematic way. A better Pd stability was found on low-temperature synthesized carbon supports with more oxygen-functional groups. Leaching and sintering was the main reason for deactivation of the catalyst.
In the third chapter, a nitrogen-doped carbon coated SBA-15 supported Pd material was synthesized. The catalyst showed improved stability than the only carbon coated SBA-15 supported Pd catalyst with reduced sintering and leaching. 15N and 13C NMR revealed the nitrogen, oxygen and carbon surface chemistry of the catalyst. The mesoporous structure of SBA-15 was found collapsed during the hydrothermal treatment and reactions without diminishing the stability and activity of Pd particles. Further nitrogen-doped carbon coating on CMK-3 support showed the potential application to other materials. The improved stability of Pd nanoparticles on Pd 300NC materials was ascribed to the synergistic effect of oxygen and nitrogen heteroatoms as well as decorative carbon overlayer on Pd nanoparticles.
In the fourth chapter, carbon deposition and sintering was found to be a major deactivation mechanism on the carbon supported Pt catalyst. A new PANI XC72R was found to be able to stabilize Pt particles with minimum sintering. Besides, a simple regeneration method including low temperature air oxidation and H2 reduction was effective to fully recover the catalytic activity without affecting the carbon support over long time on stream. The regeneration helped to remove the carbon deposits on the catalyst and redisperse Pt particles. The same regeneration method was found effective on Ru catalysts.
Future directions will be focused on further improving the hydrothermal stability of the catalyst including fundamental understanding the deactivation mechanisms and developing novel stable catalyst materials.