Dissociative Adsorption Of H2 Research Articles

Dissociative Adsorption of Hydrogen Molecules at Al2O3 Inclusions in Steels and Its Implications for Gaseous Hydrogen Embrittlement of Pipelines

Hydrogen embrittlement (HE) of steel pipelines in high-pressure gaseous environments is a potential threat to the pipeline integrity. The occurrence of gaseous HE is subjected to associative adsorption of hydrogen molecules (H2) at specific “active sites”, such as grain boundaries and dislocations on the steel surface, to generate hydrogen atoms (H). Non-metallic inclusions are another type of metallurgical defect potentially serving as “active sites” to cause the dissociative adsorption of H2. Al2O3 is a common inclusion contained in pipeline steels. In this work, the dissociative adsorption of hydrogen at the α-Al2O3(0001)/α-Fe(111) interface on the Fe011¯ plane was studied by density functional theory calculations. The impact of gas components of O2 and CH4 on the dissociative adsorption of hydrogen was determined. The occurrence of dissociative adsorption of hydrogen at the Al2O3 inclusion/Fe interface is favored under conditions relevant to pipeline operation. Thermodynamic feasibility was observed for Fe and O atoms, but not for Al atoms. H atoms can form more stable adsorption configurations on the Fe side of the interface, while it is less likely for H atoms to adsorb on the Al2O3 side. There is a greater tendency for the occurrence of dissociative adsorption of O2 and CH4 than of H2, due to the more favorable energetics of the former. In particular, the dissociative adsorption of O2 is preferential over that of CH4. The Al-terminated interface exhibits a higher H binding energy compared to the O-terminated interface, indicating a preference for hydrogen accumulation at the Al-terminated interface.

Boosting selective hydrogenation of α,β-unsaturated aldehydes through constructing independent Pt and Fe active sites on support

The alloy catalysts usually exhibited excellent catalytic synergy owing to the electronic and ensemble effect. However, the process of alloying makes the displacement of the d-band center from the Fermi level, decreasing the adsorption strength of reactant molecules. Herein, we constructed the spatial separation catalyst in which the independent Pt and FeOx nanoparticles were uniformly distributed on SBA-15 support (denoted as Pt = Fe/SBA-15), which exhibits remarkable efficiency in the hydrogenation of α,β-unsaturated aldehydes, i.e. with a 4 fold higher than the corresponding alloy catalyst in reaction rate, and a 4 fold increase in selectivity for cinnamalcohol production compared to the single Pt catalyst. It is found that Pt primarily catalyzes the H2 dissociative adsorption into H*, while FeOx mainly catalyzes the reaction between H* and aldehydes, forming alcohols. Assuming that the rate-determining-step is the H-spillover from Pt to Fe, the two cascade reactions are interconnected. To confirm this mechanism, we employ a physical mixture catalyst of Pt/SBA-15 and FeOx/SBA-15, where its catalytic activity lies between Pt = Fe/SBA-15 and alloy materials. The catalysts’ structures are well characterized by STEM and EDS mapping analysis, and the mechanism is supported by experimental data and DFT calculations. The research we conducted offers a new perspective into the cooperative catalysis, achieved by coupling independent active centers in thermos-hydrogenation reactions.

Actuating Liquid Crystals Rapidly and Reversibly by Using Chemical Catalysis.

Microtubules and catalytic motor proteins underlie the microscale actuation of living materials, and they have been used in reconstituted systems to harness chemical energy to drive new states of organization of soft matter (e.g., liquid crystals (LCs)). Such materials, however, are fragile and challenging to translate to technological contexts. Rapid (sub-second) and reversible changes in the orientations of LCs at room temperature using reactions between gaseous hydrogen and oxygen that are catalyzed by Pd/Au surfaces are reported. Surface chemical analysis and computational chemistry studies confirm that dissociative adsorption of H2 on the Pd/Au films reduces preadsorbed O and generates 1 ML of adsorbed H, driving nitrile-containing LCs from a perpendicular to a planar orientation. Subsequent exposure to O2 leads to oxidation of the adsorbed H, reformation of adsorbed O on the Pd/Au surface, and a return of the LC to its initial orientation. The roles of surface composition and reaction kinetics in determining the LC dynamics are described along with a proof-of-concept demonstration of microactuation of beads. These results provide fresh ideas for utilizing chemical energy and catalysis to reversibly actuate functional LCs on the microscale.

Mechanism of photocatalytic CO2 methanation on ultrafine Rh nanoparticles.

Selective hydrogenation of CO2 to yield CH4 relies on the appropriate catalysts that can facilitate the cleavage of CO bonds and dissociative adsorption of H2. Ultrafine Rh nanoparticles loaded on silica nanospheres were used as a class of photocatalysts to significantly improve the selectivity and reaction rate of producing CH4 from the mixture of CO2 and H2 under the illumination of a broadband visible light source. The intense light scattering resonances in the silica nanospheres generate strong electric fields near the silica surface to enhance the light absorption power in the supported ultrafine Rh nanoparticles, promoting the efficiency of hot electron generation in the Rh nanoparticles. The interaction of the hot electrons with the adsorbate species on the Rh nanoparticle surface weakens the C-O bond to facilitate the deoxygenation of CO2, favoring the production of CH4 with a unity selectivity at a faster rate in the presence of surface adsorbed hydrogen (H*). The systematic studies on reaction kinetics and diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy under different conditions, including various temperatures, illumination powers, and feeding gas compositions, reveal the reaction mechanism responsible for CO2 methanation and the role of photoillumination.

Theoretical insight into the rearrangement of sulfur atoms on the Ni- and Cu-doped MoS2 S-edge induced by hydrogen adsorption under HDS reaction conditions.

Density functional theory (DFT) calculations and an atomistic thermodynamic approach were used to study the geometric rearrangement of sulfur atoms on the Ni- and Cu-doped MoS2 S-edge upon hydrogen adsorption. Under HDS conditions, thermodynamically stable hydrogenated structures were identified as SH groups on the undoped S-edge with 100% sulfur coverage, on the Ni-doped S-edge with 50% sulfur coverage and on the Cu-doped S-edge with 25% sulfur coverage. It was found that the rearrangement of the S atoms is essential to reach the most stable state at the edge for the undoped and Ni-doped S-edge. Hydrogen adsorption on the Ni-doped S-edge leads to the greatest amount of S rearrangement (ΔERearrang = 0.93 eV/H2). Our results suggest that under the reaction conditions, the H2 dissociative adsorption process is strongly coupled to the rearrangement of the sulfur atoms. By examining the differential hydrogen adsorption energy on the most stable edge structures, we found a plausible explanation for the trend in the hydrogenation activity of the doped edges. Our results suggest that Ni enhances the hydrogenation activity of the S-edge by decreasing the S-H bond strength, while Cu poisons it by increasing the S-H bond strength.

Hydrogen Reduction Reduction (HRR)

The year 2023 marks the bicentennial of the discovery of heterogeneous H2 catalysis. Nowadays, the hydrogen oxidation reaction (HOR), which converts H2 to protons (H+), is one of the most important transformations in energy research. Meanwhile, hydrogenation reactions, which transfer net H-atoms (H·) to organic substrates from H2, play pivotal roles in various chemical industries. In comparison, it is surprising to see that a heterogenous catalytic reaction that directly converts H2 to hydrides (H–) still remains elusive. Hydride transfer is a critical elementary reaction step that spans biological catalysis, organic synthesis and energy conversion. Conventionally, hydride transfer reactions are performed using (bio)molecular hydride reagents under homogeneous conditions. Here we report a conceptually distinct heterogeneous hydride transfer reaction via the net electrocatalytic hydrogen reduction reaction (HRR), which reduces H2 to hydrides. The reaction proceeds by H2 dissociative adsorption on a metal electrode to form surface M−H species, which are then negatively polarized to drive hydride transfer to molecular hydride acceptors with up to 91% Faradaic efficiency. The hydride transfer reactivity of surface M−H species is highly tunable, and, depending on the electrode potential, the thermodynamic hydricity of Pt−H on the same Pt electrode can continuously span a range of >40 kcal/mol.In addition to this electrochemical transformation, we further developed an alternative strategy to achieve heterogeneous hydride transfer on dispersed metal catalysts under non-electrochemical conditions. In the presence of a base, deprotonation of surface M-H builds up electron density at the metal surface and thereby causes spontaneous polarization which can then drive hydride transfer from metal nanoparticles. Based on this, HRR can be performed under standard hydrogenation conditions with the addition of a base using dispersed metal catalysts, and the surface hydricity is now controlled by the solution basicity. We further demonstrate that this non-electrochemical HRR method can be applied to NADH regeneration which is closely related to industrial biocatalysis. The discovery of HRR reveals the fundamental hydride transfer reactivity of surface M-H species as well as its polarization-dependent nature. Both the electrochemical and non-electrochemical HRR methods establish sustainable strategies for accessing reactive hydrides directly from H2. Figure 1

Introducing the embedded random phase approximation: H2 dissociative adsorption on Cu(111) as an exemplar

The random phase approximation (RPA) as a means of treating electron correlation recently has been shown to outperform standard density functional theory (DFT) approximations in a variety of cases. However, the computational cost of the RPA is substantially more than DFT, especially when aiming to study extended surfaces. Properly accounting for sufficient surface ensemble size, Brillouin zone sampling, and vacuum separation of periodic images in standard periodic-planewave-based DFT code raises the cost to achieve converged results. Here, we show that sub-system embedding schemes enable use of the RPA for modeling heterogeneous reactions at reduced computational cost. We explore two different embedded RPA (emb-RPA) approaches, periodic emb-RPA and cluster emb-RPA. We use the (experimentally and theoretically) well-studied H2 dissociative adsorption on Cu(111) as our exemplar, and first perform full periodic RPA calculations as a benchmark. The full RPA results match well the semi-empirical barrier fit to experimental observables and others derived from high-level computations, e.g., from recent embedded n-electron valence second order perturbation theory [Zhao et al., J. Chem. Theory Comput. 16(11), 7078–7088 (2020)] and quantum Monte Carlo [Doblhoff-Dier et al., J. Chem. Theory Comput. 13(7), 3208–3219 (2017)] simulations. Among the two emb-RPA approaches tested, the cluster emb-RPA accurately reproduces the energy profile (maximum error of 50 meV along the reaction pathway) while reducing the computational cost by approximately two orders of magnitude. We therefore expect that the embedded cluster approach will enable wider RPA implementation in heterogeneous catalysis.

Design and modeling of the electrostatically controlled nanowire FET for ppt-level hydrogen sensing

We present the design of a H2 gas sensor based on palladium (Pd) decorated silicon-on-insulator (SOI) nanowire field effect transistor (FET) with a standard SOI complementary metal-oxide-semiconductor fabrication process, where a top Pd layer plays a dual role of a catalyst and a surrounding metal gate. A numerical study was conducted based on a simplified steady-state model to describe the sensing mechanism of H2 in dry air at 300 K. The simulation is based on the model of dissociative H2 adsorption on the Pd surface and the formation of a dipole layer at the Pd/SiO2 interface. The H atoms induced dipoles lead to a potential drop which exponentially increases the FET drain current and consequently, the sensor response. The FET drain current is controlled by its back-gate bias and by varying the H2 concentrations; it is shown that the drain current response reaches 1.8 × 108% for 0.8% H2 in air and a superior sensitivity of 4.58 × 104%/ppm in the sub-threshold operation regime. The sensor exhibits an outstanding theoretical detection limit of 50 ppt (response of 1%) and an upper dynamic range limit of 7000 ppm which allow for timely and accurate detection of H2 gas presence. The power consumption ranges between ∼10 fW (dry air) to ∼20 nW (0.8% H2 in dry air) and therefore paves the way for a very large-scale integration commercial sensing platform.

Characteristics of the Frustrated Lewis Pairs (FLPs) on the Surface of Albite and the Corresponding Mechanism of H2 Activation.

The characteristics of frustrated Lewis pairs (FLPs) on albite surfaces were analyzed with density functional theory, and the reaction mechanism for H2 activation by the FLPs was studied. The results show that albite is an ideal substrate material with FLPs, and its (001) and (010) surfaces have the typical characteristics of FLPs. In the case of H2 activation, the interaction between the HOMO of H2 and the SOMO of the Lewis base and the electron acceptance characteristics of the Lewis acid are the key factors. In fact, the activation energy of H2 is the required activation energy from the ground state to the excited state, and once the excited state is produced, the dissociative adsorption of H2 will occur directly. This study provides a new ideas and a reference for research on the construction of novel solid FLPs catalysts using ultramicro channel materials.

Exploring hydrogen adsorption on nanocluster systems: Insights from DFT calculations of Au[formula omitted] ([formula omitted] = Sc-Ni)

The superatomic cluster Au102+ is the smallest known replica of the golden pyramid Au20, and its complex analogues, Au9M2+ (M = Sc-Ni), have exhibited stable and intriguing electronic properties. In this study, we employ density-functional theory (DFT) to investigate the interaction between hydrogen molecules and Au9M2+ clusters. Our DFT calculations reveal that the structures of these clusters are influenced by the dissociative adsorption of H2, but not by the molecular adsorption. By conducting a comprehensive analysis, we show that the preferred adsorption configuration is a result of the competition between various factors, including the surface/encapsulated position, relative electronegativity, and coordination number of atoms. Although Au9M2+-2H (M = Sc, Ti, Mn, and Fe) are remarkably more stable than their corresponding Au9M2+-H2 configurations, different activation energies ranging from 0.15 to 0.99 eV are required during the dissociative reaction pathway. The finding results shed light on the mechanisms underlying the preferred adsorption site of hydrogen on the studied species and provide insights into adsorption kinetics that can aid further theoretical and experimental studies of hydrogenation in nanostructured materials.

Potential-Dependent Competitive Adsorption of Electrolytes Kinetically Inhibits High-Current Hydrogen Oxidation in Non-Aqueous Media

Performing electrochemical transformations in non-aqueous media can unlock new reactivities and selectivities unattainable in aqueous media. For instance, non-aqueous media (e.g. acetonitrile (MeCN), dimethylformamide (DMF), dimethylsulfoxide (DMSO)) allow the dissolution of organic compounds and enable water-incompatible organic electrosynthesis reactions. On the other hand, the lithium-mediated nitrogen reduction reaction (NRR) carried out electrochemically in tetrahydrofuran (THF) is one of the most promising routes towards electrifying the Haber-Bosch reaction and thus eliminating the need for hundreds of atmospheres of pressure and high temperatures. However, many cathodic non-aqueous electrochemical reactions either do not operate with well-defined anodic reactions (e.g. unproductive solvent and/or electrolyte oxidation) or are often coupled with the sacrificial oxidation of a dissolving metal electrode (e.g. magnesium, zinc). Hence, the development of a non-sacrificial anodic reaction is essential for engineering practical, sustainable non-aqueous electrochemical transformations. In this study, we investigate the hydrogen oxidation reaction (HOR) in a variety of non-aqueous solvents (MeCN, DMF, DMSO, THF) and buffers (AcO-/AcOH, Cl-/HCl, etc.) whose base serves as the proton acceptor for HOR. The reason why HOR can be sustainable is because it produces protons, which can be paired with a number of proton-demanding electro-reductions such as NRR, carbon dioxide reduction (CDR) to form oxalic acid or carboxylic acids, and more. Hydrogen is also stoichiometrically cheaper and less greenhouse-emitting than sacrificial metals, making HOR both cheaper and greener. First, we demonstrate a fabricated gas diffusion electrode (GDE) architecture that supports high H2 transport-limited current densities in non-aqueous solvents at ambient pressure. We find that the maximum HOR current density obtained is not H2 transport-limited but instead depends on the kinetics influenced by the pKa and steric profile of the buffer electrolytes present. By also varying the buffer electrolyte concentration and H2 partial pressure and conducting voltametric studies in the absence of H2, we demonstrate that the HOR current density reaches a maximum due to a competitive adsorption mechanism of the buffer electrolytes, limiting the surface concentration of adsorbed hydrogen atoms from H2 dissociative adsorption and thus inhibiting HOR. Based on these findings, we outline strategies for mitigating HOR inhibition via this competitive adsorption mechanism to unlock higher HOR current densities.

Cerium-modified Pt/Al2O3 for NH3 synthesis by NO reduction with H2

The Haber–Bosch process is a mature and proven technology for the commercial production of NH3. However, with the increasing focus on achieving net-zero emissions, alternative technologies have gained interest. This paper proposes an effective catalytic system for synthesizing NH3 directly from NO and H2 at atmospheric pressures. Pt/CeOxAl2O3 synthesized via solvent-deficient precipitation (SDP) showed a much higher NH3 yield compared to conventional Pt/Al2O3 and other Pt/Ce-containing catalysts. Various characterization techniques demonstrated that achieving precise control of the electronic metal-support interaction (EMSI) is crucial for attaining the optimal electronic and geometric structure that balances the decisive factors, Pt oxidation state and Pt dispersion, to enable a high rate of NH3 production from NO and H2. Pt/CeOxAl2O3 possesses abundant metallic Pt nanoclusters that are well dispersed on CeOxAl2O3 due to the moderate Pt-Ce interactions. These small metallic Pt clusters in contact with CeOx turned out to play a critical role in the dissociative adsorption of NO and H2, leading to the formation of the reaction intermediates, the –NH2 species, which contribute to the improved NH3 productivity of Pt/CeOxAl2O3.

The elemental effects on the H2 dissociative adsorption on FeCrAl (110) surface

As a promising candidate for accident-tolerant fuel (ATF) cladding materials, FeCrAl alloys are susceptible to hydrogen embrittlement (HE) under high-temperature radioactive water environments. Understanding the HE mechanism of FeCrAl is crucial, and studying H2 dissociative adsorption can provide valuable insights since it is the first step in the HE processes. In this paper, we investigated the H2 dissociative adsorption on Fe (110) and FeCrAl (110) surfaces using first-principles calculations based on Density Functional Theory (DFT). Our results indicate that the decomposition of H2 molecules on FeCrAl (110) surfaces is significantly affected by surface elemental effects compared to Fe (110) surfaces. Specifically, Al atoms weaken both the H2 decomposition and the H binding strength of Al-containing sites. Cr atoms decrease the decomposition tendency of H2 in certain configurations, but Cr aggregation enhances the binding strength of H atoms on Cr-containing sites. The mechanism underlying the Al/Cr weakening effects on H2 decomposition is due to the charge deficiency through alloying. Our findings have generalized implications for studying the interactions between iron-based alloys and hydrogen in various applications such as hydrogen storage, transportation, and prevention.

Dissociative H2 adsorption on 1-butyl-3-methylimidazolium-exchanged mordenite monitored by Infrared spectroscopy

Dissociative H2 adsorption on 1-butyl-3-methylimidazolium-exchanged mordenite monitored by Infrared spectroscopy

Ab initio molecular dynamics study of dissociative adsorption of H2 on defective graphene-supported Cu19 cluster

The dissociative adsorption of H2 on Cu19 and defective graphene-supported Cu19 clusters (Cu19G) are investigated using ab initio molecular dynamics. The molecular-level trajectories show that, on Cu19, the preferred adsorption site is the bridge-hollow site, where the two H atoms are adsorbed at the bridge and hollow sites beside a Cu atom, with an adsorption energy of −0.74 eV. In contrast, on the defective graphene-supported Cu19 cluster, the favorite adsorption site is located where the two H atoms are adsorbed at hollow-hollow sites with an adsorption energy of −1.27 eV. In general, the average adsorption energy on the defective graphene-supported Cu19 cluster is −1.07 eV, which is about 84% larger than that of −0.58 eV on the Cu19 cluster. This indicates that the adsorption capacity is greatly enhanced for the dissociative adsorption of H2 on the defective graphene-supported Cu19 cluster. The d-band center shifts to the Fermi level, illustrating the enhanced adsorption capacity on the defective graphene-supported Cu19 cluster. The integrated crystal orbital Hamilton population analysis reveals that stronger bond interactions between hydrogen atoms with their bonded Cu atoms lead to much larger adsorption energies on the defective graphene-supported Cu19 cluster compared to the Cu19 cluster.

Dissociative adsorption of hydrogen and methane molecules at high-angle grain boundaries of pipeline steel studied by density functional theory modeling

Pipelines provide an economic and efficient means for hydrogen transport, contributing to accelerated realization of a full-scale hydrogen economy. Dissociative adsorption of hydrogen molecules (H2) occurring on pipe steels generates hydrogen atoms (H), potentially resulting in hydrogen embrittlement of the pipelines. This is particularly important for existing pipelines transporting hydrogen in blended form with methane (CH4). In this work, a density functional theory model was developed to investigate the dissociative adsorption of H2 and CH4 at high-angle grain boundaries (HAGB), a typical type of hydrogen traps contained in steels, and the stable adsorption configurations. Results demonstrate that the dissociative adsorption of both H2 and CH4 at the HAGB is thermodynamically feasible under pipeline operating conditions. Compared with crystalline lattice sites, the HAGB possesses the most negative free energy change, a lower energy barrier and the lowest H-adsorption energy, making the HAGB, especially the quasi three-fold site, become the most stable site for hydrogen adsorption. The saturation coverage of hydrogen at HAGB is calculated to be 1.33. The iron-H bonds are formed at the HAGB by charge consumption at Fe atoms and electron accumulation at H atoms, following a so-called electron hybridization mechanism. The CH4 adsorption at HAGB affects the H2 adsorption. Without pre-adsorption of CH4, the hydrogen adsorption at the HAGB is more stable. Although an elevated CH4 partial pressure decreases the thermodynamic tendency for H2 adsorption, it cannot hinder occurrence of the H2 dissociative adsorption.

C–O Hydrogenolysis of C3–C4 Polyols Selectively to Terminal Diols over Pt/W/SBA-15 Catalysts

Pt/W/SBA-15 catalysts (with Pt-loading = 0.5–4 wt% and W-loading = 1 wt%) prepared by the sequential impregnation method were evaluated for selective C–O cleavage of erythritol and glycerol in an aqueous medium. The Pt and W particles dispersed on SBA-15 approached close proximity at higher Pt loadings and afforded synergistic enhancement in C–O hydrogenolysis activity/selectivity. 1,4-Butanediol yields of 30.9% (at 190 °C, 50 bar H2 and 24 h) and 1,3-propanediol yields of 34.4% (at 190 °C, 50 bar H2 and 12 h of reaction) were obtained over these catalysts. Pt nanoparticles (facilitating dissociative H2 adsorption and spillover) and W (present as acidic oligomeric WOx species; activating and coordinating the polyol via 1°-OH group) worked in tandem for the selective hydrogenolysis of polyols yielding terminal diols of industrial demand.

Origin of the High Selectivity of the Pt-Rh Thin-Film H2 Gas Sensor Studied by Operando Ambient-Pressure X-ray Photoelectron Spectroscopy at Working Conditions.

The Pt-Rh thin-film sensors exhibit excellent sensitivity and selectivity for H2 gas detection. Here, we studied the mechanism of highly selective detection of H2 by the Pt-Rh thin-film sensors with ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) measurements at working conditions, which were paralleled with electric resistivity measurements. The elemental composition and chemical state of surface Pt and Rh drastically change depending on the background gas environments, which directly link to the sensor response. It is revealed that surface segregated Pt atoms accelerate dissociative adsorption of H2, resulting in a reduction of the sensor surface and then a decrease of electric resistivity of the film, whereas a thin oxidized Rh layer blocks dissociation of the other reducing agent, that is, NH3. This is supported from the adsorption energetics obtained by the density functional theory (DFT) calculations.

Adsorption and dissociation of high-pressure hydrogen on Fe (100) and Fe2O3 (001) surfaces: Combining DFT calculation and statistical thermodynamics

Hydrogen (H2) pipeline systems are fundamentally the same as natural gas pipeline networks, but face a more serious safety challenge due to potential hydrogen embrittlement (HE) risk. At present, H2 transportation and storage are intentionally operated under high-pressure (HP) conditions (i.e., 5–20 MPa for transportation, 35–100 MPa for storage), which make H2 under supercritical state (i.e., supercritical H2, s-H2). In this study, the thermodynamics of H2 at a wide combination of temperatures (300–900 K) and pressures (0.1–100 MPa) has firstly been established based on a lattice-molecule model for predicting the adsorption and dissociation of gaseous and supercritical H2 on the Fe-based steel surface. The configurations of H2 adsorption and dissociation on Fe (100) and Fe2O3 (001) surfaces were investigated through the density functional theory (DFT) calculation, and the corresponding mechanism was elucidated using hybrid orbital theory. By applying the combination of DFT calculation and statistical thermodynamics, the dissociative adsorption of HP H2 on Fe (100) and Fe2O3 (001) surfaces upon varying temperature and pressure was predicted and the results well aligned with previously published experimental studies. Compared to the gaseous H2, s-H2 was likely to be more active on the iron (Fe) and its oxide (Fe2O3) surface in terms of dissociating into H atoms and could cause steels more susceptible to HE. The results also confirmed that the presence of the Fe2O3 scale could protect pipeline steels from environmental hydrogen permeation under the investigated HP conditions.

A DFT+U Study of the Electrochemical Oxidation of H2 and CO on SrLaFeO4-Δ

INTRODUCTION Solid oxide fuel cell (SOFC) anodes are commonly composed of cermet materials, typically Ni-YSZ. Despite success in electrochemical performance, cermet materials suffer from sulfur poisoning, coke formation under low operating temperatures, and metal coarsening. Due to success of material design focused on the cathode such as LSCF, single-phase mixed ionic-electronic conducting (MIEC) materials have displayed promising oxidation activity [1]. The Sr-based Ruddlesden-Popper (RP) family has found recent use as a base stoichiometry in anodes where experimentalists have used both A and B-site doping and nanoparticle exsolvation strategies in order to tune activity [2,3]. In order to understand the fundamental electrochemical activity, we want to elaborate on the mechanism of H2 and CO oxidation on the promising material SrLaFeO4- \U0001d6ff (SLF). In the present study, we used ab initio methods to model H2 and CO oxidation on two FeO2-based models of SLF with and without B-site doping with Co and Ni. COMPUTATIONAL DETAILS All calculations presented in this work were performed using the spin-polarized DFT+U method with periodic boundary conditions as implemented in the VASP 5.4.4 code. We choose the PBE functional to describe exchange and correlation effects. Dudarev’s approach for DFT+U calculations is used with a U-J value of 4.0, 3.32, and 6.2 eV, respectively. We used the projector-augmented wave (PAW) method to represent the inner core potentials. The kinetic energy cutoff was set for all calculations to 700 eV, and integration over the Brillouin zone was performed with 3 × 3 × 1 Monkhorst-Point k-point mesh and the Gaussian smearing method with a sigma value of 0.05 eV. Slab models utilized a 15 Å vacuum layer and the bottom two layers were fixed to bulk position values. The DFT-derived parameters were employed in a microkinetic model to calculate the surface coverage of all adsorbed species for a given reaction condition. RESULTS AND DISCUSSION To begin our study, we tested all non-symmetrical conformers of SLF for the lowest relative energy on a model 2 × 2 × 1 supercell, as displayed in Figure 1a. A total of 3 structures were tested – P4/mmm, P4/nmm, and I4mm. We confirm that the lowest energy structure (I4mm) is equivalent to the experimentally reported structure. Next, we develop two 1.5 × 1.5 × 1 non-identical (001) FeO2-based slab models – so-called FeO2-LaO and FeO2-SrO (Figure 2b). The oxidation mechanism of H2 suggests oxidation begins with a dissociative adsorption step. Subsequent steps include formation of surface H2O, a surface vacancy, and a subsurface vacancy. To complete the catalytic cycle, we computed the bulk vacancy migration of SLF and assumed fast cathode kinetics for the overall cell model. At an operating voltage of 0.7 V, we predict that the highest rate occurs on the FeO2-SrO surface. We predict that H2 dissociative adsorption step is the rate-determining process based on Campbell’s degree of rate control. With the application of a single B-site dopant (Co and Ni), Co and Ni both decrease the barrier of the H2 dissociative adsorption step. Ni causes a slight decrease in energy for the formation energy of H2O. The oxidation mechanism of CO suggests oxidation begins with adsorption of the CO molecule on a reduced Fe site. The CO migrates to a neighboring Fe-O complex in order to form surface CO2. Based on degree of rate control, CO2 formation is likely the rate-determining step for both surface models.In conclusion, this is one of the first mechanism investigations on the anodic oxidation reactions of H2 and CO on RP-based oxides. We find that H2 has promising catalytic performance similar to the experimental evidence. The inclusion of B-site dopants displays that catalytic performance can be tuned with the introduction of different transition metals. Future work will examine the role of exsolved particles for RP-based oxides. ACKNOWLEDGMENTS This work has been funded by the Division of Materials Research, a National Science Foundation organization under Award Number 1832809. Computing resources provided by the San Diego Supercomputer Center (SDSC) and Texas Advanced Computing Center (TACC) under XSEDE grant number TG-CTS090100 and USC High-Performance Computing clusters are gratefully acknowledged. REFERENCES L. Xu, Y. Yin, N. Wang, Z. Ma, Electrochem. Commun., 76, 51–54 (2017)N. Wu, W. Wang, Y. Zhong, G. Yang, J. Qu, Z. Shao, ChemElectroChem, 4, 2378–2384 (2017)H. Chong, H. Chen, Z. Shao, J. Shi, J. Bai, S. Li, J. Mater. Chem. A, 4, 13997–14007 (2016) FIGURE 1a: 2×2×1 supercell model of I4mm SrLaFeO4 FIGURE 1b: Slab models of (001) FeO2-LaO and FeO2-SrO Figure 1