(468d) Dilute Alloys of Palladium in Indium Supported on ?-Al2O3 for the Selective Hydrogenation of CO2 to Methanol

Methanol synthesis from CO₂ has attracted growing attention as a method to utilize the increasing amount of CO₂ being emitted into the atmosphere. Recently In₂O₃ has emerged as a highly selective catalyst for CO₂ hydrogenation to methanol. Density functional theory predictions suggested that methanol would be the most favorable product on defective In₂O₃(110) surfaces with oxygen vacancies.¹ Pérez-Ramírez and coworkers experimentally demonstrated this to be true and additionally showed that the addition of Pd via co-precipitation improved the rate, enabling 9.7% conversion to be achieved with 80% methanol selectivity.²

In this work we synthesized dilute alloys of Pd in In supported on γ-Al₂O₃ with varying Pd:In ratios using the galvanic replacement method to achieve distinct Pd surface structures: isolated atoms, small clusters, and extended ensembles. The structure and composition of all catalysts were studied by DRIFTS of adsorbed CO, XPS, ICP, STEM, and XAS. The catalysts were tested with 3:1 H₂:CO₂ at a pressure of 30 bar and temperature of 240-300°C. Pd-In alloys containing Pd clusters demonstrated a substantial improvement in methanol selectivity (>80%) over monometallic Pd (<10%) at 0.5% conversion. In and Pd-In with isolated Pd possessed similar methanol selectivities (~65%), though Pd demonstrated a small promotional effect on the rate (~2x). The rate improved further with small and larger Pd ensembles (~7x), though utilization of the Pd was most efficient with small Pd ensembles, demonstrating 2 times the specific rate (per Pd) of isolated Pd and 40 times the specific rate of extended Pd.

References:

1) Ye, J.; Liu, C.; Ge, Q. The Journal of Physical Chemistry C 2012, 116(14), 7817–7825

2) Frei, M. S.; Mondelli, C.; García-Muelas, R.; Kley, K. S.; Puértolas, B.; López, N.; Safonova, O. V.; Stewart, J. A.; Ferré, D. C.; Pérez-Ramírez, J. Nature Communications 2019, 10(1).