Sulfuric Acid Electrolyte Impacts Palladium Chemistry at Reductive Potentials

Abstract

We report herein that sulfuric acid electrolyte affects the kinetics of hydrogen sorption and desorption, the amount of absorbed hydrogen, and the electrocatalytic activity of palladium using X-ray photoelectron spectroscopy (XPS) and in situ temperature-programmed desorption (TPD). This study utilized a custom TPD instrument tailored to the in situ characterization of palladium films electrochemically saturated with absorbed hydrogen (PdH x for x > 0.6), and we present the first experimental data for the formation of sulfates on palladium hydride at E < 0 V (vs RHE). The surface adsorption of sulfates leads to a retardation of hydrogen sorption and desorption kinetics by increasing the desorption energy of absorbed hydrogen by 92 kJ mol–1 relative to a clean palladium surface. Although previous studies have reported that the SO4 2– anions of H2SO4 only adsorb on palladium under oxidative potentials E > 0.5 V (vs RHE), the differing chemistry of hydrogen-free and hydrided palladium was not considered. The sulfate adsorbates affect the reaction chemistry through both a competitive binding of surface sites and by altering the electronic structure of surface atoms. The influence of the electrolyte is evidenced by a sluggish kinetics of H absorption and desorption in H2SO4 electrolyte relative to HCl, concomitant with a lower hydrogen uptake in H2SO4 (x = 0.64 ± 0.01) versus HCl (x = 0.697 ± 0.007). The findings of this study emphasize the critical and often overlooked role of the electrolyte utilized in electrochemical studies of the palladium–hydrogen system.

Critical role of hydrogen sorption kinetics in electrocatalytic CO2 reduction revealed by on-chip in situ transport investigations

“…The formate mechanism involves a proton-coupled electron transfer (PCET) process, during which the proton transfer and electron transfer occur in a same elementary step, and the M−H bond is formed on the surface 51 , 52 . Although H adsorption on the surface is thermodynamically more favorable (by 0.33 eV) compared to subsurface H absorption (which leads to hydride formation), H could diffuse into the subsurface and then bulk fcc Pd lattice at more negative potentials 18 , 53 , 54 . The overall hydrogen sorption process within Pd and Pd 4 Ag systems can be described by 55 : where the diffusion between H ads and H subsurface follows the equilibrium that is determined by the chemical potentials of H atoms in each phase ( μ Hads vs. μ Hsubsurface ).…”

Physical Separation of H2 Activation from Hydrogenation Chemistry Reveals the Specific Role of Secondary Metal Catalysts

“…TPD samples were prepared by electrochemically loading hydrogen into the catalyst‐coated membrane in 0.1 M HCl at a potential of −0.4 V (vs. Ag/AgCl) until a total charge of 10 C was passed. HCl was chosen over H 2 SO 4 because SO 4 2− has been demonstrated to strongly adsorb on palladium and slow H desorption kinetics [26] . This electrochemical protocol was designed to saturate the sample with absorbed hydrogen (H abs ).…”

“…HCl was chosen over H 2 SO 4 because SO 4 2À has been demonstrated to strongly adsorb on palladium and slow Hd esorption kinetics. [26] This electrochemical protocol was designed to saturate the sample with absorbed hydrogen (H abs ). Thes amples were immediately quenched in liquid nitrogen for 30 safter loading to suppress immediate desorption of H abs before transfer into the TPD chamber.Hydrogen desorption was then monitored by the atm-MS while the temperature was increased at arate of 10 Kmin À1 (Figure 5).…”

“…Hydrogen desorption kinetics were measured using an ex‐situ temperature‐programmed desorption (TPD) instrument developed by our group [26] to investigate the relationship between metal‐hydrogen binding energy and hydrogen flux. TPD samples were prepared by electrochemically loading hydrogen into the catalyst‐coated membrane in 0.1 M HCl at a potential of −0.4 V (vs. Ag/AgCl) until a total charge of 10 C was passed.…”

Improving the Hydrogen Oxidation Reaction Rate of Ru by Active Hydrogen in the Ultrathin Pd Interlayer

“…It is noteworthy that hydrogen, due to its extremely small atomic radius, can easily penetrate metal layers. , For example, Sorok et al reported that hydrogen can pass through a 7 nm Ru metal layer . On the other hand, hydrogen can also pass through metal layers or be directly stored in the Pd lattice. − Utilizing these unique properties of hydrogen is a promising approach to solve scientific problems faced by the HOR. , In this paper, we used Ru monolayer grown on Au@Pd core–shell nanoparticles as a HOR catalyst under alkaline conditions, which exhibited high activity comparable to Pt as well as excellent CO tolerance. Furthermore, the activity of Ru-based catalysts with triple-layered core–shell nanostructures is negatively correlated with the thickness of the intermediate Pd layer (the catalyst is denoted as Au@Pd x @Ru, where x represents Pd layer thicknesses of 0.8, 3.5, and 6.3 nm).…”