Rational Catalyst Design for Higher Propene Partial Electro-oxidation Activity by Alloying Pd with Au

Abstract

Selective partial oxidation of hydrocarbons to oxygenates plays a large role in the chemical industry, while falling prices for electricity from renewable sources make electrification of such industrial chemical processes relevant. The oxidation of propene is an interesting model system as propene can be oxidized in two different positions, allowing for insights into the reaction mechanism. On Pd, a layer of adsorbates formed in situ governs the reaction by steering reactant adsorption to achieve high selectivity for allyl oxidation, albeit largely inhibiting the reaction rate. Through rational catalyst design, we demonstrate that alloying reactive Pd with inert Au influences the adsorbate layer formation, enhancing activity while maintaining high selectivity toward allyl oxidation. We obtain mechanistic insights with a combination of ab initio computational modeling and electrochemical measurements with ex situ product quantification and online mass spectrometry. Using a statistical approach, we explore the correlation of the Au:Pd ratio with Pd surface cluster size and density, which determine the properties of the adsorbate layer and thus the reaction outcome. We report an activity enhancement by a factor 2.4 with 10% Au in Pd and propose that (i) activity is maximized at potentials just before Pd cluster oxidation and (ii) the optimal catalyst surface contains approximately one Au every six Pd atoms, statistically most frequent at the nominal alloy composition Au14Pd86.

Dynamically Reversible Interconversion of Molecular Catalysts for Efficient Electrooxidation of Propylene into Propylene Glycol

“…The *OH binding energy ( E B (*OH)) serves as a key factor in determining the catalytic performance toward alkene electrooxidation. − For instance, Sargent and co-workers explored the doping effect for regulating the E B (*OH) to facilitate the transfer of *OH into ethylene, achieving efficient electrooxidation of ethylene into ethylene glycol . As for the electrooxidation of propylene into PG, the generation of *OH and the transfer of *OH into the CC bond in propylene are both affected by the E B (*OH). , The strong E B (*OH) favors the dissociation of H 2 O into *OH, whereas the transfer of *OH to propylene would be impeded. , On the contrary, the weak E B (*OH) is beneficial to the combination of *OH and propylene but detrimental for *OH generation. , As such, the catalytic performance for propylene electrooxidation would be restricted due to the scaling relationship of the E B (*OH) over the conventional catalysts.…”

Reaction Mechanism and Selectivity Tuning of Propene Oxidation at the Electrochemical Interface

“…5,10,16,21 Chorkendorff's group 5 have systematically investigated propene electrooxidation on Pd electrodes and proposed that the adsorbed organic species formed in situ determine the selectivity of allylic oxidation. Furthermore, Seger et al 20 have succeeded in the synthesis of a PdAu alloy to enhance the activity and selectivity of allyl oxidation. However, undesired byproducts, such as CO 2 and acetone, have also been observed.…”

“…However, the activity and selectivity of propene oxidation electrocatalysts cannot meet the application requirements of the electrolyzers yet. So far, a variety of catalyst materials, such as Pt, ,− Pd, ,,, Ag, Au, , and PdAu, have been explored for this reaction. The adsorbates of propene electrooxidation on Pt electrodes include C3, C2, and/or C1 oxygen-containing species, which favor overoxidation to CO 2 due to the high reactivity of Pt.…”

“…The noble-metal Pd has been regarded as the most promising electrocatalyst for propene oxidation because it can produce various valuable products including propene oxide, propene glycol, acrylic acid, and so on. ,,, Chorkendorff’s group have systematically investigated propene electrooxidation on Pd electrodes and proposed that the adsorbed organic species formed in situ determine the selectivity of allylic oxidation. Furthermore, Seger et al have succeeded in the synthesis of a PdAu alloy to enhance the activity and selectivity of allyl oxidation. However, undesired byproducts, such as CO 2 and acetone, have also been observed.…”

Bipyridine-Confined Silver Single-Atom Catalysts Facilitate In-Plane C–O Coupling for Propylene Electrooxidation

“…2,12−14 Some strategies such as facet engineering, doping modification, and dynamic ligand regulation have been applied to tune the energy barrier of the C−O coupling process during the propylene electro-oxidation. 2,12,15 Despite the efforts mentioned above, the catalytic performance of propylene electrooxidation is still unsatisfactory (yield of <23 g PG m −2 h −1 ). 1,2,13,15,16 To the best of our knowledge, the C−O coupling reactions in homogeneous synthesis typically rely on mononuclear metal complexes.…”