Selective n-propanol formation from CO2 over degradation-resistant activated PdCu alloy foam electrocatalysts

Abstract

We present a novel, foam-type, high surface area electrocatalyst for the CO₂ reduction reaction (CO₂RR) that is not only highly selective toward n-propanol (PrOH) formation (FE_PrOH = 13.7%, j_PrOH = −1.15 mA cm⁻²) at relatively low overpotentials (−0.65 V vs. RHE) but also demonstrates an excellent long-term stability during CO₂ electrolysis experiments of 102 h in duration. A dynamic hydrogen bubble template approach is applied to electrodeposit a binary PdCu alloy foam yielding a nominal bulk composition of 9 at% Pd and 91 at% Cu (denoted as Pd₉Cu₉₁). The material is further modified by means of thermal annealing (12 h at 200 °C in air), which completely transforms the as-prepared metallic Pd₉Cu₉₁ alloy foam into its oxidic state. The ultimate catalyst activation is achieved by subsequent reduction (at −0.65 V vs. RHE for 45 min) of the oxidic precursors (composite of Cu₂O, CuO, and CuPdO₃) into metallic state, as indicated by operando Raman spectroscopy. Identical location scanning electron microscopy (IL-SEM) analysis, carried out prior to and after the activation treatments, demonstrates significant morphological alterations of the Pd₉Cu₉₁ foam on the nm length scale, which go along with a phase segregation into nm-range Pd-rich and Cu-rich domains that helps to increase the PrOH selectivity. Time-dependent ICP-MS analyses of the electrolyte solution, carried out during the catalyst activation, demonstrate preferential (rapid) Cu dissolution followed by (slow) Cu redeposition on the catalyst surface. These processes are found to be superimposed on the actual oxide reduction. A two-fold selectivity of PrOH was observed over ethanol (EtOH). The excellent long-term stability of the activated Pd₉Cu₉₁ foam catalyst is rationalized by the full suppression of the C1 hydrocarbon (methane) pathway. The improved product selectivity towards the highly valuable C3 alcohol is rationalized by an efficient and concerted spillover of chemisorbed carbon monoxide (*CO) and atomic hydrogen (*H) species from the Pd-rich domains to the activated Cu-rich domains of the oxide-derived Pd₉Cu₉₁ foam catalyst where the C–C coupling and subsequent hydrogenation processes take place to form the targeted oxygenate product.