Dissociative Adsorption Of Hydrogen Research Articles

Thermal Rates and High-Temperature Tunneling from Surface Reaction Dynamics and First-Principles.

Studying dynamics of the dissociative adsorption and recombinative desorption of hydrogen on copper surfaces has shaped our atomic-scale understanding of surface chemistry, yet experimentally determining the thermal rates for these processes, which dictate the outcome of catalytic reactions, has been impossible so far. In this work, we determine the thermal rate constants for dissociative adsorption and recombinative desorption of hydrogen on Cu(111) between 200 and 1000 K using data from reaction dynamics experiments. Contrary to current understanding, our findings demonstrate the predominant role of quantum tunneling, even at temperatures as high as 400 K. We also provide precise values for the reaction barrier (0.619 ± 0.020 eV) and adsorption energy (0.348 ± 0.026 eV) for H2 on Cu(111). Remarkably, the thermal rate constants are in excellent agreement with a first-principles quantum rate theory based on a new implementation of ring polymer molecular dynamics for reactions on surfaces, paving the way to discovering better catalysts using reliable and efficient computational methods.

Hydrogen transport through Mo2C/PdCu composite membranes

First-principle calculations have been adopted to systematically investigate the adsorption and dissociation of H2 on Mo2C (100) and (001) surfaces, as well as the fluxes of hydrogen dissociative adsorption on the Mo2C surfaces and the hydrogen across the Mo2C(100)/PdCu(110) and Mo2C(001)/PdCu(110) interfaces. It is observed that the adsorption and dissociation of H2 as well as the hydrogen flux on the Mo2C (100) surfaces are energetically more favorable than on the Mo2C (001) surfaces. Calculations also reveal that the hydrogen flux across the Mo2C(100)/PdCu(110) and Mo2C(001)/PdCu(110) interfaces is one and six order of magnitude greater than the hydrogen desorption flux on the permeation side of PdCu, respectively. In other words, the hydrogen diffusion in BCC PdCu bulk and hydrogen desorption are the dominant steps of hydrogen transport through Mo2C/PdCu composite membranes, rather than the hydrogen diffusion across the Mo2C/PdCu interfaces. The calculational findings are consistent with the similar experimental observations in the literature, and provide a profound understanding regarding the hydrogen transport across Mo2C-coated PdCu membrane.

Unraveling Hydrogen Adsorption on Transition Metal-Doped [Mo3S13]2- Clusters: Insights from Density Functional Theory Calculations.

Molecular and dissociative hydrogen adsorption of transition metal (TM)-doped [Mo3S13]2- atomic clusters were investigated using density functional theory calculations. The introduced TM dopants form stable bonds with S atoms, preserving the geometric structure. The S-TM-S bridging bond emerges as the most stable configuration. The preferred adsorption sites were found to be influenced by various factors, such as the relative electronegativity, coordination number, and charge of the TM atom. Notably, the presence of these TM atoms remarkably improved the hydrogen adsorption activity. The dissociation of a single hydrogen molecule on TM[Mo3S13]2- clusters (TM = Sc, Cr, Mn, Fe, Co, and Ni) is thermodynamically and kinetically favorable compared to their bare counterparts. The extent of favorability monotonically depends on the TM impurity, with a maximum activation barrier energy ranging from 0.62 to 1.58 eV, lower than that of the bare cluster (1.69 eV). Findings provide insights for experimental research on hydrogen adsorption using TM-doped molybdenum sulfide nanoclusters, with potential applications in the field of hydrogen energy.

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.

Chemical Vapor Deposition of Elemental Crystallogen Thin Films

A consolidation of the fundamentals of elemental crystallogen chemical vapor deposition (CVD) is a necessity in view of the extensive evidence accumulated over the last few decades. An in-depth understanding of deposition mechanisms via hydrides asks for a discerning understanding of molecular hydrogen dissociative adsorption, precursor thermal decomposition, and CVD growth rates. With those, a groundbreaking paradigm shift comes to light. GR activation energy E(GR) fingerprints the surface energy. SE ≈ 2 × E(GR)/(a×a), where SE is surface energy, E(GR) activation energy, a lattice parameter. Hydride precursor thermal decomposition consistency with the corresponding solid growth kinetics is demonstrated. Heterogeneous TD kinetics captures a solid deposition and not a gas phase molecular reaction. Thermodynamic equilibrium is achieved during the heterogeneous thermal decomposition of silicon precursors. The popular split between mass-transfer and kinetic regimes is not supported by evidence. Three mechanisms are apparent. The first is controlled by a Si–H bond dissociation energy. The second is controlled by an H–H bond dissociation energy. The last is controlled by a Si–Si bond dissociation energy as lattice sites are sealed off with Si–H bonds.

A Kinetic Model of Furfural Hydrogenation to 2-Methylfuran on Nanoparticles of Nickel Supported on Sulfuric Acid-Modified Biochar Catalyst

Lignocellulosic biomass can uptake CO2 during growth, which can then be pyrolysed into three major products, biochar (BC), syngas, and bio-oil. Due to the presence of oxygenated organic compounds, the produced bio-oil is not suitable for direct use as a fuel and requires upgrading via hydrodeoxygenation (HDO) and hydrogenation. This is typically carried out over a supported metal catalyst. Regarding circular economy and sustainability, the BC from the pyrolysis step can potentially be activated and used as a novel catalyst support, as reported here. A 15 wt% Ni/BC catalyst was developed by chemically modifying BC with sulfuric acid to improve mesoporous structure and surface area. When compared to the pristine Ni/BC catalyst, sulfuric activated Ni/BC catalyst has excellent mesopores and a high surface area, which increases the dispersion of Ni nanoparticles and hence improves the adsorptive effect and thus catalytic performance. A liquid phase hydrogenation of furfural to 2-methylfuran was performed over the developed 15 wt% Ni/BC catalyst. Langmuir–Hinshelwood–Hougen–Watson (LHHW) kinetic type models for adsorption of dissociative H2 were screened based on an R2 value greater than 99%, demonstrating that the experimental data satisfactorily fit to three plausible models: competitive (Model I), competitive at only one type of adsorption site (Model II), and non-competitive with two types of adsorption sites (Model III). With a correlation coefficient greater than 99% between the experimental rates and the predicted rate, Model III, which is a dual-site adsorption mechanism involving furfural adsorption and hydrogen dissociative adsorption and surface reaction, is the best fit. The Ni/BC catalyst demonstrated comparative performance and significant cost savings over previous catalysts; a value of 24.39 kJ mol−1 was estimated for activation energy, −11.43 kJ mol−1 for the enthalpy of adsorption for H2, and −5.86 kJ mol−1 for furfural. The developed Ni/BC catalyst demonstrated excellent stability in terms of conversion of furfural (96%) and yield of 2-methylfuran (54%) at the fourth successive experiments. Based on furfural conversion and yield of products, it appears that pores are constructed slowly during sulfuric acid activation of the biochar.

Inhibiting effect of carbon monoxide on gaseous hydrogen embrittlement of pipelines transporting hydrogen

Hydrogen embrittlement (HE) potentially occurs on pipe steels in hydrogen-blended natural gas environments, compromising integrity of the pipelines. The mechanistic aspect about the role of CO in hydrogen dissociative adsorption and steel has remained unknown. In this work, in-situ high-pressure gaseous hydrogen-charging and tensile experiments were conducted to investigate hydrogen atom entry and the resulting mechanical responses of an X52 pipe steel in the presence of CO. The density functional theory and first-principle molecular dynamics were used to reveal the effect of CO on hydrogen dissociative adsorption. A feasible solution to address the HE risk was proposed.

How do defects affect hydrogen spillover on graphene-supported Pt? A DFT study

Hydrogen dissociation, adsorption, and spillover on Pt4 supported on defective graphene that contains vacancy defects, such as single and di-vacancy graphenes (SVG and DVG), have been studied using density functional theory (DFT) with dispersion correction (Grimme-D3). Vacancy defects lead to n-type doping of graphene from the Pt4 cluster, which is contrary to the p-type doping on pristine graphene. Also, the metal–support interactions increase with the presence of a vacancy defect as indicated by more negative Pt4 binding energies on the SVG, and 5-8-5, 5555-6-7777, and 555–777 DVGs of −8.60, −4.51, −4.15 and −3.63 eV, respectively, compared to that on pristine graphene (−2.16 eV). Atomic hydrogen adsorption is more stable on all defective graphenes with adsorption energies of −2.24, −0.16, 0.05, and −0.27 eV for SVG, 5-8-5, 5555-6-7777, and 555–777 DVGs, respectively, as compared to that on pristine graphene (+1.38 eV). Spontaneous hydrogen dissociative adsorption on the two-fold coordinated C site of bare SVG occurs with an adsorption energy per hydrogen atom of −1.77 eV, while those on the three types of DVGs are in the range of ∼0.03–0.29 eV; these are all more stable than those on pristine graphene that fall in the range of 0.89–1.53 eV. The energy barriers for hydrogen spillover from Pt4 to the defective graphene are 0.91, 1.09, 0.90, and 0.71 eV for SVG, 5-8-5, 5555-6-7777, and 555–777 DVGs, respectively. Ab initio molecular dynamics (AIMD) simulations show that the adsorption of up to six hydrogen molecules, with and without spillover, on the four Pt4/defective graphene systems, is stable at room temperature. This study demonstrates that the presence of vacancy defects on graphene could improve hydrogen adsorption and spillover. A trend is discovered—when the magnitude of binding energies of metal–support interaction is stronger than −3.5 eV and the adsorption energies of atomic hydrogen on the support are negative or close to zero, there is potential to facilitate the occurrence of hydrogen spillover, correlating with the hydrogen storage capacity of the materials. Thus, our findings could provide useful criteria for designing hydrogen storage materials and a fundamental understanding of the correlation between metal–support interactions and hydrogen spillover mechanisms.

Unveiling the promotion effect of ethylenediamine on preparation of Ni/CeO2 catalyst for low-temperature CO2 methanation

Cost-effective CO2 conversion to value-added products remains a great challenge until present. Here, we have discovered that the ethylenediamine (EDA) assisted preparation of Ni/CeO2 catalyst remarkably promotes the CO2 methanation performance at low temperatures. The Ni/CeO2-EDA catalyst exhibits a CO2 conversion efficiency of 84% and a CH4 selectivity of 98% at 225 °C, and no visible decline of CO2 conversion is observed even after 70h of continuous reaction. The involvement of EDA is shown to effectively result in a more uniform distribution of Ni species, coupled with a reduction in particle size. In addition, the smaller particle size of Ni results in a rapid dissociative adsorption of hydrogen and H2 spillover. Accordingly, oxygen vacancies on the CeO2 surface are created and adequate H atoms are engendered for CO2 hydrogenation. Furthermore, both the formate and CO pathways occur simultaneously on the Ni/CeO2-EDA catalyst, enhancing the low-temperature methanation activity. Herein, the EDA-assisted strategy provides an effective methodology in terms of well-dispersion and highly active catalysts for low-temperature CO2 methanation.

Dissociative adsorption of hydrogen at edge dislocation emergence on α-Fe studied by density functional theory

Pipelines provide an efficient and economical method for hydrogen transportation with a high capacity over wide ranges and long distances, contributing to accelerated realization of a full-scale hydrogen economy. However, steel pipelines are prone to hydrogen embrittlement in high-pressure gaseous environments once the generated hydrogen (H) atoms enter the steels. Dislocations are one of the most common defects contained in steels. The H atom generation and adsorption at dislocation emergences on pipeline steels have not been fully understood. This work investigated and determined the stable hydrogen adsorption configurations at the emergence of an edge dislocation on iron (Fe) (100) crystalline plane by density functional theory. The partition function was used to study the thermodynamics of dissociative adsorption of hydrogen, as well as the effects of dissociated impurity gases (i.e., oxygen, methane and water vapor) existing in the carried fluid, under pipeline operating conditions. Results demonstrate that the dislocation core is the stable site for hydrogen adsorption, followed by the sites with a tensile strain at the dislocation, although the dissociative adsorption of hydrogen can also occur at the compressive strain sites. An elevated hydrogen gas pressure and low temperature favor the hydrogen adsorption. The three impurity gases can have dissociative adsorption occur at the edge dislocation. Moreover, their adsorptions are more thermodynamically favorable than hydrogen. Oxygen has the most stable adsorption configuration, followed by water vapor and then methane. Pre-adsorbed O-containing gases (i.e., oxygen and water vapor) compete with the hydrogen adsorption and decrease the stability of the hydrogen adsorption configuration. The competing effect of the impurity gases with hydrogen adsorption occurs only when they adsorb at the compressive sites of the dislocation. When the impurity gases pre-adsorb at the tensile strain sites of the dislocation, they have little influence on hydrogen adsorption.

An apparatus for investigating the kinetics of plasmonic catalysis

Plasmonic catalysis, which is driven by the localized surface plasmon resonance of metal nanoparticles, has become an emerging field in heterogeneous catalysis. The microscopic mechanism of this kind of reaction, however, remains controversial partly because of the inaccuracy of temperature measurement and the ambiguity of reagent adsorption state. In order to investigate the kinetics of plasmonic catalysis, an online mass spectrometer-based apparatus has been built in our laboratory, with emphases on dealing with temperature measurement and adsorption state identification issues. Given the temperature inhomogeneity in the catalyst bed, three thermocouples are installed compared with the conventional design with only one. Such a multiple-point temperature measuring technique enables the quantitative calculation of equivalent temperature and thermal reaction contribution of the catalysts. Temperature-programmed desorption is incorporated into the apparatus, which helps to identify the adsorption state of reagents. The capabilities of the improved apparatus have been demonstrated by studying the kinetics of a model plasmon-induced catalytic reaction, i.e., H2+D2→HD over Au/TiO2. Dissociative adsorption of molecular hydrogen at Au/TiO2 interface and non-thermal contribution to HD production have been confirmed.

Rational Development of IT-SOFC Electrodes Based on the Nanofunctionalization of La0.6Sr0.4Ga0.3Fe0.7O3 with Oxides. Part 2: Anodes by Means of Manganite Oxide.

To promote the diffusion on the market of solid oxide fuel cell (SOFC) devices, the use of fuels other than the most appealing hydrogen and also decreasing the working temperature could show the way forward. In the first part, we concentrated our efforts on cathodes; hereby, we focused on anodes and concentrated our efforts to develop a sustainable multifuel anode. We decided to develop LSGF (La0.6Sr0.4Ga0.3Fe0.7O3)-based nanocomposites by depositing manganite oxide to enhance the performance toward propane. MnOx has been deposited by a wet impregnation method, and the powders have been largely characterized by X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray analysis, X-ray photoelectron spectroscopy, hydrogen temperature-programmed reduction, oxygen temperature-programmed desorption, and N2 adsorption. Cell performances were first collected in hydrogen as a function of both the temperature and hydrogen content. EIS measurements were studied using Nyquist and Bode plots, and they show two processes at high frequency, assigned to charge transfer at the electrode/electrolyte interface, and at low frequency due to the dissociative adsorption of hydrogen. The Arrhenius plot of area specific resistance suggests two different trends, and the activation energy decreases from 117 kJ/mol at 750 °C to 46 kJ/mol above that temperature. This behavior is often connected to chemical modification of the catalyst or changes in the limiting step processes. Power densities in hydrogen and propane were determined at 744 °C after 1 h of operation, achieving 70 mW/cm2 in H2 and 67 mW/cm2 in C3H8. The open-circuit voltage increases from 1.10 V in hydrogen to 1.13 V in propane.

Enhancing Tungsten Oxide Gasochromism with Noble Metal Nanoparticles: The Importance of the Interface.

Crystalline tungsten trioxide (WO3 ) thin films covered by noble metal (gold and platinum) nanoparticles are synthesized via wet chemistry and used as optical sensors for gaseous hydrogen. Sensing performances are strongly influenced by the catalyst used, with platinum (Pt) resulting as best. Surprisingly, it is found that gold (Au) can provide remarkable sensing activity that tuned out to be strongly dependent on the nanoparticle size: devices sensitized with smaller nanoparticles display better H2 sensing performance. Computational insight based on density functional theory calculations suggested that this can be related to processes occurring specifically at the Au nanoparticle-WO3 interface (whose extent is in fact dependent on the nanoparticle size), where the hydrogen dissociative adsorption turns out to be possible. While both experiments and calculations single out Pt as better than Au for sensing, the present work reveals how an exquisitely nanoscopic effect can yield unexpected sensing performance for Au on WO3 , and how these performances can be tuned by controlling the nanoscale features of the system.

Revealing the interfacial phenomenon of metal-hydride formation and spillover effect on C48H16 sheet by platinum metal catalyst (Pt (n=1-4)) for hydrogen storage enhancement

Hydrogen is a worldwide green energy carrier, however due its low storage capacity, it has yet to be widely used as an energy carrier. Therefore, the quantum chemical method is being employed in this investigation for better understand the hydrogen storage behaviour on Pt (n = 1-4) cluster decorated C48H16 sheet. The Pt(n = 1-4) clusters are strongly bonded on the surface of C48H16 sheet with binding energies of −3.06, −4.56, −3.37, and −4.03 eV respectively, while the charge transfer from Pt(n = 1-4) to C48H16 leaves an empty orbital in Pt atom, which will be crucial for H2 adsorption. Initially, the molecular hydrogen is adsorbed on Pt(n = 1-4) decorated C48H16 sheet through the Kubas interaction with adsorption energies of −0.85, −0.66, −0.72, and −0.57 eV respectively, while H–H bond is elongated due to the transfer of electron from σ (HH) orbital to unfilled d orbital of the Pt atom, resulting in a Kubas metal-dihydrogen complexes. Furthermore, the dissociative hydrogen atoms adsorbed on Pt(n = 1-4) decorated C48H16 sheet have adsorption energies of −1.14 eV, −1.02 eV, −0.95 eV, and −1.08 eV, which are greater than the molecular hydrogen adsorption on Pt(n = 1-4) cluster supported C48H16 sheet with lower activation energy of 0.007, 0.109, 0.046, and 0.081 eV respectively. To enhance the dissociative hydrogen adsorption energy, positive and negative external electric fields are applied in the charge transfer direction. Increasing the positive electric field makes H–H bond elongation and good adsorption, whereas increasing the negative electric field results H–H bond contraction and poor adsorption. Thus, by applying a sufficient electric field, the H2 adsorption and desorption processes are can be easily tailored.

Potential of nickel nanoclusters supported on [formula omitted]-Al[formula omitted]O[formula omitted](0001) surface for CO[formula omitted] capture, energy production, and dry reforming of methane

The structural, electronic and magnetic properties of Nin@α-Al2O3(0001) (with n = 1 to 7) systems have been studied using density functional theory. Grafting energies, Bader charges and DOS analysis showed that the nickel clusters strongly interact with the α-Al2O3(0001) surface. In a second step, the adsorption performances of these materials towards CH4, CO2, CO, H2 gases have been evaluated. Pure alumina surface and supported Ni systems weakly interact with methane, which are therefore promising for an efficient purification of methane. Additionally, it was found that the pure alumina surface and Ni5/α-Al2O3(0001) selectively adsorb the CO2, facilitating its storage and utilization. A selective adsorption was also observed for CO on Ni3/α-Al2O3(0001). Looking now at the dry reforming of methane, a strong adsorption of CO was shown in all supported systems. The dissociative adsorption of hydrogen on all supported systems (except for n = 4) was observed. A high resistance against carbon deposition was observed on our modeled systems compared to those reported in the literature, which is a promising result since the rapid deactivation by coke deposition is the main problem for the catalysts used in the dry reforming of methane process.

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.

(High-Temperature Energy, Materials, & Processes Division Outstanding Achievement Award Address) Mechanisms of Oxide Exsolution and Electrode Applications in Solid Oxide Cells

Solid oxide cell fuel electrodes based on perovskite oxides are desirable to avoid problems with Ni-based anodes, including coking in hydrocarbon fuels and degradation due to fuel impurities or redox cycling. However, oxide anode electrochemical performance is often lacking, limited by surface processes such as the dissociative adsorption of hydrogen. One way to improve such anodes is by the addition of a reducible cation in the oxide formulation, resulting in cation exsolution and nucleation of metallic nanoparticles on oxide surfaces during cell startup and operation. For example, substitution of small amounts of Ni or Ru on the B-site of Sr(Ti,Fe)O3 results in the formation of Ni-Fe or Ru-Fe nanoparticles, respectively, during exposure of the electrode to fuel during cell operation. This talk will examine the microstructural evolution of these exsolution anodes, making use of in situ x-ray diffraction measurements to detect formation of exsolved metal alloys and oxide phase changes. Exsolved metal nanoparticles enhance electrochemical performance, usually by promoting hydrogen dissociation on electrode surfaces. Changes in perovskite stoichiometry resulting from B-site exsolution can also impact the phase stability of the oxide, in some cases resulting in the formation of Ruddlesden-Popper phases that deleteriously affect electrochemical performance. The application of exsolution in fuel electrodes in novel thin-electrolyte oxygen-electrode-supported cells is discussed; the results indicate that performance in H2/H2O fuel is superior to Ni-YSZ in both electrolysis and fuel cell modes. Electrochemical characteristics of the exsolution electrodes are especially superior to Ni-YSZ when operated in CO/CO2 fuel.

High-temperature Fischer–Tropsch synthesis over the Li-promoted FeMnMgOx catalysts

Fischer–Tropsch-to-Olefins (FTO) over iron-based catalysts is a promising process for the production of light olefins from a non-petroleum source. However, the catalytic mechanism is unclear because of the variation in the iron phase during the reduction and activation processes and the complex iron phase composition under the reaction conditions. A series of Li-promoted FeMnMgOx catalysts is synthesized using co-precipitation and impregnation methods, after which the effect of promoters on the evolution of the iron phase, chemisorption, and FTO performance are analyzed. The results demonstrate that Mg enhances dissociative CO adsorption and hydrogen adsorption, improving the FTO activity. Mg facilitates the dispersion of α-Fe2O3 and suppresses the carbonization, which weakens the chain propagation, encourages the formation of short-chain hydrocarbons, and increases the selectivity of light olefins. Li also enhances dissociative CO adsorption and increases CO conversion. Moreover, Li facilitates the formation of the ε'-Fe2.2C phase, which reveals a higher FTS activity and better C–C coupling ability than the χ-Fe5C2 phase. This results in a decrease in CO2 and increase in light olefins. At the optimal Li loading, the FeMnMg-0.75Li sample displays the highest space time yield of light olefins at 355 g/(kgcat·h).

Molecular and Dissociative Hydrogen Adsorption on Bimetallic PdAg/Pd(111) Surface Alloys: A Combined Experimental and Theoretical Study

Molecular and Dissociative Hydrogen Adsorption on Bimetallic PdAg/Pd(111) Surface Alloys: A Combined Experimental and Theoretical Study

Elucidation of the atomic-scale processes of dissociative adsorption and spillover of hydrogen on the single atom alloy catalyst Pd/Cu(111).

Hydrogen spillover is a crucial process in the selective hydrogenation reactions on Pd/Cu single atom alloy catalysts. In this study, we report the atomic-scale perspective of these processes on the single atom alloy catalyst Pd/Cu(111) based on the experimental and theoretical results, including infrared reflection absorption spectroscopy (IRAS), temperature programmed desorption (TPD), high-resolution X-ray photoelectron spectroscopy (HR-XPS), and density functional theory (DFT) calculations for core-level excitation. The hydrogen spillover onto Cu(111) was successfully observed in real time using time-resolved IRAS measurements at 80 K. The chemical shifts of Pd 3d5/2 indicate that H2 is dissociated and adsorbed at the Pd site. In addition, a "two-step" chemical shift of the Pd 3d5/2 binding energy was observed, indicating two types of hydrogen adsorption states at the Pd site. The proposed mechanism of the hydrogen dissociation and spillover processes is as follows: (i) a hydrogen molecule is dissociated at a Pd site, and the hydrogen atoms are adsorbed on the Pd site; (ii) the number of hydrogen atoms on the Pd site increases up to three; and (iii) the hydrogen atoms will spill over onto the Cu surface.