Atomic-Step Enriched Ruthenium-Iridium Nanocrystals Anchored Homogeneously on MOF-Derived Support for Efficient and Stable Oxygen Evolution in Acidic and Neutral Media

Achieving an efficient and stable oxygen evolution reaction (OER) in an acidic or neutral medium is of paramount importance for hydrogen production via proton exchange membrane water electrolysis (PEM-WE). Supported iridium-based nanoparticles (NPs) are the state-of-the-art OER catalysts for PEM-WE, but the nonhomogeneous dispersion of these NPs on the support together with their nonuniform sizes usually leads to catalyst migration and agglomeration under strongly corrosive and oxidative OER conditions, eventually causing the loss of active surface area and/or catalytic species and thereby the degradation of OER performance. Here, we design a catalyst comprising surface atomic-step enriched ruthenium-iridium (RuIr) nanocrystals homogeneously dispersed on a metal organic framework (MOF) derived carbon support (RuIr@CoNC), which shows outstanding catalytic performance for OER with high mass activities of 2041, 970 and 205 A g(RuIr)(-1) at an overpotential of 300 mV and can sustain continuous OER electrolysis up to 40, 45, and 90 h at 10 mA cm(-2) with minimal degradation in 0.5 M H2SO4 (pH = 0.3), 0.05 M H2SO4 (pH = 1), and PBS (pH = 7.2) electrolytes, respectively. Comprehensive experimental studies and density functional theory (DFT) calculations reveal that the good performance of RuIr@CoNC can be attributed, on one hand, to the presence of abundant atomic steps that maximize the exposure of catalytically active sites and lower the limiting potential of the rate-determining step of OER and, on the other hand, to the strong interaction between RuIr nanocrystals and the CoNC support that endows homogeneous dispersion and firm immobilization of RuIr catalysts on CoNC. The RuIr@CoNC catalysts also show outstanding performance in a single-cell PEM electrolyzer, and their large-quantity synthesis is demonstrated.

Revealing the Surface Chemistry for CO2 Hydrogenation on Cu/CeO2–x Using Near-Ambient-Pressure X-ray Photoelectron Spectroscopy

Young Jun Ko, Mo Li, Wen Luo, Emadeddin Oveisi, Thi Ha My Pham, Andreas Züttel

Catalytic reduction of CO2 to valuable products is an attractive route for CO2 recycling. CeO2-supported Cu catalysts have shown high activity and selectivity for the hydrogenation of CO2 to CO. To uncover the origin of their high performance, we prepared a practical and well-defined model of Cu/CeO2–x catalysts with Cu nanoparticles dispersed on a CeO2–x support. We studied the structure and catalytic activity of the practical catalyst and the evolution of the active phase and surface intermediates using near-ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS) over the model catalyst under CO2 hydrogenation conditions. For both model and practical catalysts, metallic copper and partially reduced ceria with oxygen vacancies were found to be active sites for the reduction of CO2 to CO. Over the model catalyst, partial covering of Cu by CeO2–x and diffusion of Cu into CeO2–x was observed, suggesting that a strong interaction between Cu and CeO2–x was favored during CO2 hydrogenation. The surface analysis results indicated that the CO2 hydrogenation proceeded through the following steps: activation of CO2 in the form of carbonate on the surface of CeO2–x, hydrogenation of carbonate to formate on the surface of Cu by the dissociated H2, and conversion of formate to CO. This work provides direct experimental evidence on the surface properties of Cu and ceria during the RWGS reaction and the activation and hydrogenation processes of CO2 over the Cu/CeO2–x surface for a high RWGS catalytic performance.

ACS2021

Nitrous Oxide Abatement over Fe-Zeolite

Bryan Bromley

The use of zeolites for catalytic reactions is a field in continuous development. Since it was demonstrated that metal-zeolites are efficient catalysts for NOx abatement, decomposition of nitrous oxide (N2O) over Fe-containing zeolites has attracted great interest by the scientific community mainly due: N2O is the third most important gas contributing to global warming, its concentration in atmosphere is still on the rise and direct N2O decomposition to N2 and O2 is a sustainable alternative. N2O interaction with Fe-zeolite leads to the formation of "special" adsorbed oxygen (often called α-oxygen) with great potential for selective oxidation. Although the focus of a number of studies, the nature of the actives sites for the formation of this adsorbed oxygen is still unclear. The main objectives of this work are: 1) study the decomposition mechanism of N2O over isomorpously substituted Fe-ZSM-5 zeolite aiming on catalyst optimization and 2) optimization and further use of structured Fe-zeolite in novel micro-structured reactor. A number of refined transient kinetics methods were intensively used such as transient response, Temperature Programmed Desorption (TPD) and Temporal Analysis of Product (TAP). Density Functional Theory (DFT) and in situ IR spectroscopy studies were conducted in collaboration and our results support the following: In chapter 3, quantitative measurements have been carrier out to analyze the effect of temperature on the activation (under helium flow) and the irreversible deactivations that can happen during the pretreatment. The results obtained indicate that high temperature treatment (1273 K) results in the formation of a high amount of α-sites. The addition of a very low fraction of oxygen (2%) in the feed demonstrates that the catalyst is sensitive to its presence. Certain sites can be irreversibly destroyed, a result ascribed to the oxidation of active Fe(II) to Fe2O3 clusters which contain inactive Fe(III). Catalyst treatment at high temperature (1273 K) in He for up to 3 hours, also reduced irreversibly the concentration of α-sites. TPD measurements have shown desorption of oxygen at 3 different temperatures. Desorption at the lowest temperature (650 K) was assign to the α-oxygen recombination. The oxygen which desorbs at 700 K was associated to the oxygen from the deactivated sites. Oxygen which desorbs at the highest temperature (730 K) is originated from the surface NOx that desorbed as NO and O2. After a comparison of the concentration of the molecules accumulated, we propose that NO2 is the adsorbed specie. The mechanism of N2O decomposition on the binuclear oxo-hydroxo bridged extraframework iron core site [FeII(μ-O)(μ-OH)FeII]+ inside the ZSM-5 zeolite has been studied by combining theoretical and experimental approaches (see chapter 4). Rate parameters computed using standard statistical mechanics and transition state theory reveal that elementary catalytic steps involved into N2O decomposition are strongly dependent on the temperature. This theoretical result was contrasted with the experimentally observed steady state kinetics of the N2O decomposition and TPD experiments. As predicted by the theoretical study, the switch of the reaction order with respect to N2O pressure (from zero to one) occurs at around 800 K which confirms a change of the rate determining step from the α-oxygen recombination to α-oxygen formation. The proposed mechanism was also confirmed by O2-TD which confirmed the viability of the binuclear complex. The α-oxygen recombination at low temperature (

EPFL2010

Computational Modeling of the Oxygen Evolution Reaction at Semiconductor-Water Interfaces: A Path Towards Breaking Linear Scaling Relationships

Patrick Gono

Photoelectrochemical water splitting is a promising source of clean, renewable fuel in the form of hydrogen. Despite extensive research endeavors, the widespread adoption of this technology is impeded due to suboptimal catalysts for the oxygen evolution reaction (OER) taking place on the anode. Studies have shown that linear scaling relationships exist between the binding energies of the OER intermediates. These relationships are responsible for the emergence of an overpotential which limits the catalytic efficiency of electrochemical cells. This work is devoted to the study of the OER, the emergence of linear scaling relationships, and possible avenues for overcoming them. We employ a selection of ab-initio computational free energy methods based on density functional theory in order to investigate the OER at atomistic model semiconductor interfaces. An approach based on thermodynamic integration and \textit{ab-initio} molecular dynamics simulations is used to isolate the effect of the solvent on the OER free energy steps. The observed effect is twofold. First, the presence of explicit water affects the equilibrium configurations of the reaction intermediates. Second, explicit water modifies the OER free energy steps by up to 0.5 eV. This effect is rationalized in terms of electrostatic interactions at the interface. Next, we verify the existence of the linear scaling relationships across a set of semiconductor and metal interfaces. We establish the robustness of the scaling relationships with respect to the adopted energy functional. Two potential approaches of breaking the linear scaling are considered. First, a bifunctional mechanism that involves two close but functionally different active sites is investigated. The performance of the proposed mechanism is studied on a set of model interfaces, and the results indicate that specific combinations of catalysts may overcome the limitations imposed by the linear scaling relationship. We were able to establish general trends governing the efficiency of the bifunctional mechanism. In particular, a correlation between the valence band maximum and the hydrogen binding energy may be used as a descriptor in future searches for suitable hydrogen acceptors within the bifunctional scheme. Next, a detailed study of NiOOH/FeOOH catalysts identifies features most favorable to the bifunctional mechanism. Overpotentials as low as 0.13 eV are found for specific configurations. These results support the bifunctional mechanism as a means to break the linear scaling relationships. Second, hole polarons at the TiO

_2

surface induced by electronegative adsorbates in the OER intermediates are studied. The computational hydrogen electrode method in conjunction with a hybrid density functional is employed to quantify the effect of the surface polarons on the OER free energy steps. We find that the hole bipolarons reduce the overpotential of the reaction-determining step leading to good agreement with experiment. The stability of the polarons is further confirmed at the hydrated surface through a free energy study involving the explicit treatment of the solvent. Finally, the polaron energy level is aligned with respect to the redox levels in water to better understand the role of the polarons in the OER. Since the occurrence of surface hole polarons is unrelated to the scaling relationships, it offers an additional handle in the search for improved catalysts.

EPFL2020