(715b) First Principles Insight into the High Activity and Selectivity of Pd3Cu Alloys for the Guerbet Reaction

The Guerbet and related condensation reactions can be used to convert biomass-derived short chain alcohols into higher branched alcohols.¹ A variety of transition metal-based heterogeneous catalysts (e.g. Pd-based catalyst) possess good activity for this reaction, but the suppression of undesired side reactions must be further improved.² A newly discovered Pd-Cu alloy catalyst, however, was experimentally found to combine the activity of Pd with the selectivity of less active Cu, resulting in a unique catalyst with exceptional reaction rate and low tendency for the undesired decarbonylation reaction.³ In this work, we present results from density functional theory simulations and micro-kinetic modeling to explain the origin of the remarkable properties of this bimetallic alloy catalyst.

The computational model was chosen to match the comprehensive experimental characterization showing a Cu-rich surface with a Pd-rich bulk. In agreement with experimental observations our microkinetic model predicts that the PdCu alloy with a Cu-rich surface has comparable activity with monometallic Pd catalysts for the desired (de)hydrogenation steps that covert alcohols to aldehyde and form saturated higher alcohol products. The high activity of the alloy can be attributed to the rise of the d-band center of the surface Cu atoms, making them nearly as active as surface Pd atoms.

While (de)hydrogenation steps are facile on both Pd sites and exposed Cu sites, the undesired decarbonylation reaction, however, requires a pair of adjacent Pd sites. The formation of a Cu-rich surface on the PdCu bulk alloy decreases the availability of these Pd ensembles, thus suppressing decarbonylation. Moreover, any produced CO will gradually block the unselective Pd sites under reaction conditions, but it does not affect the activity of surface Cu sites. This interpretation is consistent with experimental observations, showing that the ratio of decarbonylation to dehydrogenation decreases with increasing conversion and CO formation.

Finally, the effect of particle size was investigated by comparing the key reactions on terrace sites with those on stepped surfaces.⁴ Congruent with experimental data the (de)hydrogenation steps are favored on larger particles, while decarbonylation is favored on smaller particles. In addition, we propose that CO can induce re-segregation of Pd atoms to the undercoordinated step sites, which further enhances decarbonylation and amplifies the geometric effect.

In summary, we have used ab-initio methods to elucidate the fundamental characteristics of PdCu alloys that lead to their exceptional catalytic performance for the Guerbet reaction. The overall catalyst properties stem from synergistic alloying, segregation, coverage and particle size effects. We anticipate that other bimetallic alloys with tailored performance can be discovered based on the fundamental understanding contributed by this study.

Reference:

(1) Kozlowski, J. T.; Davis, R. J. ACS Catal. 2013, 3, 1588–1600.

(2) Sitthisa, S.; Resasco, D. E. Catal. Letters 2011, 141, 784–791.

(3) Goulas, K. A.; Sreekumar, S.; Song, Y.; Kharidehal, P.; Gunbas, G.; Dietrich, P. J.; Johnson, G. R.; Wang, Y. C.; Grippo, A. M.; Grabow, L. C.; Gokhale, A. A.; Toste, F. D. J. Am. Chem. Soc. 2016, 138, 6805–6812.

(4) Goulas, K. A.; Song, Y.; Johnson, G. R.; Chen, J. P.; Gokhale, A. A.; Grabow, L. C.; Toste, F. D. submitted.