Base-free CO2 hydrogenation to formic acid over Pd supported on defective carbon nitride modified by microwave and acid treatments

Abstract

Graphitic carbon nitride (g-C3N4) is synthesized via pyrolyzing urea by hybrid microwave annealing (HMA) to support Pd catalyst for direct CO2 hydrogenation to formic acid under a base-free condition. The HMA produces fragmented C3N4 sheets with abundant defects along their edges, because of the high annealing temperature (>900 °C) in an extremely short time (<5 min). The acid treatment of C3N4 further reduces its grain sizes forming additional defect sites to anchor small Pd nanoparticles. The modified C3N4 provides ample CO2 adsorption sites relative to C3N4 pyrolyzed by the conventional furnace annealing at 550 °C for 4 h. The Pd catalyst supported on the defective C3N4 exhibits the benchmark performance ever reported for base-free CO2-to-HCOOH conversion in terms of both palladium time yield based on total Pd, and turnover frequency based on exposed surface Pd atoms. The enhanced performance is explained by interplay between small Pd nanoparticles and the abundant CO2 supply from nearby defect sites along the edges of the modified C3N4 sheets.