Influence of the Molecular Structure on the Electrocatalytic Hydrogenation of Carbonyl Groups and H₂ Evolution on Pd

Abstract

We investigated the electrocatalytic hydrogenation (ECH) of model aldehydes and ketones over carbon-supported Pd in the aqueous phase. We propose reaction mechanisms based on kinetic measurements and on spectroscopic and electrochemical characterization of the working catalyst. The reaction rates of ECH and of the H 2 evolution reaction (HER) vary with the applied electric potential following trends that strongly depend on the organic substrate. The intrinsic rates of hydrogenation and H 2 evolution are influenced, in opposing ways, by the sorption of the reacting organic substrate. Strong interactions, that is, higher standard free energies of adsorption of the organic compound, induce high hydrogenation rates. The fast hydrogenation kinetics produces a hydrogen-depleted environment that kinetically hinders the HER and the bulk phase transition of Pd to a H-rich bulk Pd hydride, which is triggered by the applied potential in the absence of reacting organic compounds. As a consequence of strong organic−metal interactions, hydrogenation dominates at low overpotential. However, the coverages of organic substrates on the metal surface decrease, and the rates of H 2 evolution surpass those of hydrogenation with increasingly negative electric potential. We determined the range of electric potential favoring hydrogenation on Pd and quantitatively deconvoluted the effects of the sorption of the organic compound, and of the rates of proton-coupled electron transfers, on the kinetics of both ECH and HER. The results indicate that electrocatalysis offers hydrogenation pathways for polar molecules which are different and, in some cases, faster than those dominating in the absence of an external electric potential.

Preparation, Quantification, and Reaction of Pd Hydrides on Pd/Al2O3 in Liquid Environment

“…33 This interpretation is in line with the electrocatalytic observations of the high reactivity of aldehydes including furfural revealing that bulk hydrides are consumed in the presence of such reactants. 34 However, we believe that in our experiment catalytic activity toward furfural hydrogenation is not sustained by the adsorbates remaining on the catalyst and contributing to PdC y , rather the catalyst deactivates in a short time. We think that the change in Pd speciation is too abrupt at the point of addition of furfural and that probably the experimental conditions in terms of furfural/Pd ratio are not optimal to achieve stable activity, but this needs to be confirmed with further measurements.…”

Role of Hydride Formation in Electrocatalysis for Sustainable Chemical Transformations

“…This could arise from stronger binding of benzaldehyde, limiting the formation of H ads and sufficient local driving force for H intercalation, or more facile H-addition during ECH. Further study across a range of organics indicate that higher hydrogenation rates are coupled to less H abs within the PdH x lattice, suggested to correlate with lesser H ads being present on the surface as well (Figure B) . While the ECH discussion of hydrides remains focused on Pd, we note that more abundant materials like Ni are capable of organic ECH, albeit at low Faradaic efficiencies (FEs) for benzaldehyde reduction .…”

“…Further study across a range of organics indicate that higher hydrogenation rates are coupled to less H abs within the PdH x lattice, suggested to correlate with lesser H ads being present on the surface as well (Figure 2B). 43 While the ECH discussion of hydrides remains focused on Pd, we note that more abundant materials like Ni are capable of organic ECH, albeit at low Faradaic efficiencies (FEs) for benzaldehyde reduction. 44 In fact, the notable cathodic potentials required for Ni ECH are in fact commensurate with those expected for hydride formation (about −0.3 V vs RHE), which may suggest hydride formation leads to a more active electronic structure.…”

“…The open symbol represents the [H]/[Pd] ratio in solely acetate buffer, and the filled symbols represent it with added organic. Reprinted from ref . Copyright 2022 American Chemical Society.…”

In-situ Synthesis of Nitrogen-doped Graphene Layer Encapsulated Palladium Nanoparticles for Highly Selective Hydrogenation of Vanillin

“…关于催化醛类的还原, 已报道了 Ni [9] 、 Cu [10] 、 Co [11] 、 Pt [12] 、Au [13] 、Ru [14] 、Ir [15] 、Ru [16] 和 Pd [17][18][19][20] 众所周知, 载体的性质影响金属的状态、试剂的吸 附性质和传质过程, 合适的载体可以提升加氢反应的活 性. 现已有碳 [21] 、 聚合物 [22] 、 Al 2 O 3 [23] 、 介孔二氧化硅 [24] 、 沸石 [25] 、碳纳米管 [26] 、金属-有机框架材料(MOF) [27] 、 石墨烯 [28][29][30] 等载体应用于负载 Pd 纳米颗粒.…”