(583cc) Exploring Structural and Compositional Changes of Metal Phosphide Nanoparticle Catalysts Under Pyrolysis Oil Upgrading Conditions

Advanced processes for pyrolysis oil upgrading may require high temperatures (350-500 °C) and low H₂ pressure (atmospheric). New catalysts that promote deoxygenation while maintaining stability under these conditions are needed to enable the production of “drop-in” ready feedstocks for renewable transportation fuels. Metal phosphides are promising candidates for this purpose, but are less commonly studied than traditional hydrotreating catalysts, such as transition metal sulfides and noble metals. However, changes in metal phosphides under deoxygenation conditions by (i) oxidation to form metal oxide or oxyphosphide phases,  (ii) reduction by H₂ leading to loss of phosphorus, and (iii) carbon deposition at the catalyst surface, impact the long-term stability and catalytic activity of these materials. Consequently, the structure and composition of metal phosphide catalysts is dependent on reaction conditions, and a better understanding of the relationship between these conditions, catalyst structure and composition, and catalytic performance is critical for the development of efficient and selective deoxygenation catalysts.

This presentation will focus on our recent work on the synthesis and characterization of supported metal phosphide catalysts. We have applied the versatility of solution synthesis methods to develop well-defined metal phosphide and mixed-metal phosphide nanoparticles with controlled size and composition. These tailor-made metal phosphide nanoparticles were dispersed on silica for characterization under conditions relevant for the deoxygenation of pyrolysis vapors (300-500 °C, atmospheric H₂ pressure). Dynamic structural characterization (XRD, TEM) of the metal phosphides under reaction conditions will be presented, including in-situ experiments in reactive gases to identify compositional changes due to oxidation or reduction. Hydrogen activation and surface acidity, which are hallmarks of bifunctional metal phosphides, were analyzed following exposure of the catalysts to different pre-treatment and reaction conditions. Additionally, the effects of pre-treatment conditions on the fate of the organic molecules required for stabilization of colloidal nanoparticles during synthesis was investigated by TGA and FTIR spectroscopy, and will be discussed with regard to the choice of stabilizing molecule and pre-treatment conditions.