Strain-Dependent Activity-Stability Relations in RuO\u3csub\u3e2\u3c/sub\u3e and IrO\u3csub\u3e2\u3c/sub\u3e Oxygen Evolution Catalysts

Strain engineering is an effective strategy in modulating activity of electrocatalysts, but the effect of strain on electrochemical stability of catalysts remains poorly understood. In this work, we combine ab initio thermodynamics and molecular dynamics simulations to examine the role of compressive and tensile strain in the interplay between activity and stability of metal oxides considering RuO2 and IrO2 as exemplary catalysts. We reveal that although compressive strain leads to improved activity via the adsorbate-evolving mechanism of the oxygen evolution reaction, even small strains should substantially destabilize these catalysts promoting dissolution of transition metals. In contrast, our results show that the metal oxides requiring tensile strain to promote their catalytic activity may also benefit from enhanced stability. Importantly, we also find that the detrimental effect of strain on electrochemical stability of atomically flat surfaces could be even greater than that of surface defects

Anisotropic strain variations during the confined growth of Au nanowires

The electrochemical growth of Au nanowires in a template of nano-porous anodic aluminum oxide was investigated in situ by means of grazing-incidence transmission small- and wide-angle x-ray scattering (GTSAXS and GTWAXS), x-ray fluorescence (XRF) and 2-dimensional surface optical reflectance (2D-SOR). The XRF and the overall intensity of the GTWAXS patterns as a function of time were used to monitor the progress of the electrodeposition. Furthermore, we extracted powder diffraction patterns in the direction of growth and in the direction of confinement to follow the evolution of the direction-dependent strain. Quite rapidly after the beginning of the electrodeposition, the strain became tensile in the vertical direction and compressive in the horizontal direction, which showed that the lattice deformation of the nanostructures can be artificially varied by an appropriate choice of the deposition time. By alternating sequences of electrodeposition to sequences of rest, we observed fluctuations of the lattice parameter in the direction of growth, attributed to stress caused by electromigration.. Furthermore, the porous domain size calculated from the GTSAXS patterns was used to monitor how homogeneously the pores were filled.Comment: Short communication manuscript. Four figure

Coverage dependence of adsorption-site geometry in the Cs/Ru(0001) system: A low-energy electron-diffraction analysis

The ordered overlayer structures formed by Cs adsorbed on a Ru(0001) surface were analyzed by use of low-energy electron diffraction (LEED). The phase diagram reflects the dominance of dipole-dipole repulsions between the adparticles and comprises quasiliquid configurations characterized by diffraction rings up to a coverage Θ=0.17, followed by a (2×2) structure with maximum intensity of the diffraction spots at Θ=0.23. Beyond Θ=0.25, a series of structures with rotated unit cells is identified which are followed by a (√3 × √3 )R30° structure around Θ=0.33 (≊completion of the first monolayer). In the (2×2) phase the Cs atoms are located in on-top sites with a Ru-Cs bond length of 3.25±0.08 Å, corresponding to a hard-sphere radius of 1.9 Å for the Cs atom. In the (√3 × √3 )R30° structure, on the other hand, the adatoms occupy threefold hollow hcp sites with Ru-Cs bond lengths of 3.52±0.02 Å, corresponding to a Cs hard-sphere radius of about 2.2 Å. The increase in bond length and effective radius of the adparticle is paralleled by the transition of the character of bonding from more ‘‘ionic’’ at Θ=0.25 (large dipole moment) to more ‘‘metallic’’ at Θ=0.33 (dipole moment reduced by about 30%). The associated change of the type of adsorption site (from on-top to hollow) is qualitatively rationalized by a model according to which inherently less favorable sites may become preferred due to improved effective screening of the dipole-dipole repulsion by the location of substrate atoms in the region between neighboring adatoms

In situ\textit{In situ} hydride breathing during the template-assisted electrodeposition of Pd nanowires

We investigated the structural evolution of electrochemically fabricated Pd nanowires $\textit{in situ}$ by means of grazing-incidence transmission small- and wide-angle x-ray scattering (GTSAXS and GTWAXS), x-ray fluorescence (XRF) and 2-dimensional surface optical reflectance (2D-SOR). This shows how electrodeposition and the hydrogen evolution reaction (HER) compete and interact during Pd electrodepositon. During the bottom-up growth of the nanowires, we show that $\beta$-phase Pd hydride is formed. Suspending the electrodeposition then leads to a phase transition from $\beta$- to $\alpha$-phase Pd hydride. Additionally, we find that grain coalescence later hinders the incorporation of hydrogen in the Pd unit cell. GTSAXS and 2D-SOR provide complementary information on the volume fraction of the pores occupied by Pd, while XRF was used to monitor the amount of Pd electrodeposited.Comment: 17 pages, 11 figures, 4 appendice

Rate-Determining Step or Rate-Determining Configuration? The Deacon Reaction over RuO₂(110) Studied by DFT-Based KMC Simulations

Ab initio kinetic Monte Carlo (KMC) is successfully applied to simulate the experimentally observed promoting effect of O₂ on the HCl oxidation reaction (Deacon process) catalyzed by RuO₂(110). Density functional theory (DFT) calculations provide, in addition to the adsorption energies of reaction intermediates and activation energies, also interaction energies between the adsorbates within the cluster expansion approach. KMC simulations with this extended set of energy parameters were analyzed employing the concept of “degree of rate control”. In contrast to previous propositions, our simulations indicate that neither the dissociative O₂ adsorption (the sterically hindered first reaction step) nor the associative desorption of chlorine (the step with the highest activation energy) are rate determining under typical Deacon conditions. Instead, the hydrogen transfer in the water formation determines the rate of the overall reaction. These hydrogen transfer processes are not highly activated but turn out to be strongly configuration controlled

Adsorption characteristics of CO and N2 on RuO2(110)

Low-energy electron diffraction and density-functional theory calculations are used to examine the adsorption properties of CO and N-2 On RuO2(110). Both molecules adsorb over the coordinatively unsaturated Ru sites (cus-Ru atoms) with their molecular axes normal to the surface plane. The chemisorption mechanism is well described within a donor-acceptor model, i.e., the Blyholder model. Since N-2 is not reacting with lattice oxygen of RuO2, quite in contrast to CO, N-2 may serve as a chemical, nondestructive probe to titrate but also to selectively block the cus-Ru atoms; recently, the cus-Ru atoms were shown to be the active centers for the chemisorption of molecules