The Role of Surface Complexes in Ketene Formation from Fatty Acids via Pyrolysis over Silica: from Platform Molecules to Waste Biomass

Fatty acids (FA) are the main constituents of lipids and oil crop waste, considered to be a promising 2G biomass that can be converted into ketenes via catalytic pyrolysis. Ketenes are appraised as promising synthons for the pharmaceutical, polymer, and chemical industries. Progress in the thermal conversion of short- and long-chain fatty acids into ketenes requires a deep understanding of their interaction mechanisms with the nanoscale oxide catalysts. In this work, the interactions of fatty acids with silica are investigated using a wide range of experimental and computational techniques (TPD MS, DFT, FTIR, in situ IR, equilibrium adsorption, and thermogravimetry). The adsorption isotherms of linear and branched fatty acids C1-C6 on the silica surface from aqueous solution have been obtained. The relative quantities of different types of surface complexes, as well as kinetic parameters of their decomposition, were calculated. The formation of surface complexes with a coordination bond between the carbonyl oxygens and silicon atoms in the surface-active center, which becomes pentacoordinate, was confirmed by DFT calculations, in good agreement with the IR feature at ∼1680 cm 1. Interestingly, ketenes release relate to these complexes' decomposition as confirmed by the thermal evolution of the absorption band (1680 cm-1) synchronously with the TPD peak of the ketene molecular ion. The established regularities of the ketenezation are also observed for the silica-induced pyrolysis of glyceryl trimyristate and real waste, rapeseed meals.

Reactivity of a series of triaryl borates, B(OArx)3, in hydroboration catalysis

In this paper, we compare the reactivity of a series of triaryl borates B(OArx)3 as catalysts for the hydroboration of alkenes and alkynes. It was observed that commercially available B(OPh)3 performed the poorest, whereas catalysts with o-F atoms appeared to perform much better.

Effects of oxygen functionalities on hydrous hydrazine decomposition over carbonaceous materials

Metal-free heterogeneous catalysis is promising in the context of H2 generation. Therefore, establishing structure-activity relationships is a crucial issue to improve the development of more efficient catalysts. Herein, to evaluate the reactivity of the oxygen functionalities in carbonaceous materials, commercial functionalized pyrolytically stripped carbon nanofibers (CNFs) were used as catalysts in the liquid-phase hydrous hydrazine decomposition process and its activity was compared to that of a pristine CNF material. Different oxygenated groups were inserted by treating CNFs with hydrogen peroxide for 1 h (O1-H2O2) and HNO3 for 1 h (O1-HNO3) and 6 h (O6-HNO3). An increase in activity was observed as a function of the oxidizing agent strength (HNO3 > H2O2) and the functionalization time (6 h > 1 h). A thorough characterization of the catalysts demonstrated that the activity could be directly correlated with the oxygen content (O6-HNO3 > O1-HNO3 > O1-H2O2 > CNFs) and pointed out the active sites for the reaction at carbon-oxygen double bond groups (CO and COOH). Systematic DFT calculations supported rationalization of the experimental kinetic trends with respect to each oxygen group (CO, C-O-C, C-OH and COOH).

Synthesis and lewis acidity of fluorinated triaryl borates

A series of fluorinated triaryl borates B(OAr^F)₃ (Ar^F = 2-FC₆H₄, 3-FC₆H₄, 4-FC₆H₄, 2,4-F₂C₆H₃, 3,5-F₂C₆H₃, 2,3,4-F₃C₆H₂, 2,4,6-F₃C₆H₂, 3,4,5-F₃C₆H₂) have been prepared and isolated from the reactions of the mono-, di-, or tri-fluorophenol with BCl₃. The Lewis acidity of these borates has been determined by NMR spectroscopic and theoretical methods and compared to their well-established borane counterpart.

Toward a new definition of surface energy for late transition metals

Surface energy is a top-importance stability descriptor of transition metal-based catalysts. Here, we combined density functional theory (DFT) calculations and a tiling scheme measuring surface areas of metal structures to develop a simple computational model predicting the average surface energy of metal structures independently of their shape. The metals considered are W, Ru, Co, Ir, Ni, Pd, Pt, Cu, Ag and Au. Lorentzian trends derived from the DFT data proved effective at predicting the surface energy of metallic surfaces but not for metal clusters. We used machine-learning protocols to build an algorithm that improves the Lorentzian trend's accuracy and is able to predict the surface energies of metal surfaces of any crystal structure, i.e., face-centred cubic, hexagonal close-packed, and body-centred cubic, but also of nanostructures and sub-nanometer clusters. The machine-learning neural network takes easy-to-compute geometric features to predict metallic moieties surface energies with a mean absolute error of 0.091 J m^-2 and an R² score of 0.97.

Enhanced stability of sub-nanometric iridium decorated graphitic carbon nitride for H2 production upon hydrous hydrazine decomposition

Stabilizing metal nanoparticles is vital for large scale implementations of supported metal catalysts, particularly for a sustainable transition to clean energy, e.g., H₂ production. In this work, iridium sub-nanometric particles were deposited on commercial graphite and on graphitic carbon nitride by a wet impregnation method to investigate the metal-support interaction during the hydrous hydrazine decomposition reaction. To establish a structure-activity relationship, samples were characterized by transmission electron microscopy and X-ray photoelectron spectroscopy. The catalytic performance of the synthesized materials was evaluated under mild reaction conditions, i.e. 323 K and ambient pressure. The results showed that graphitic carbon nitride (GCN) enhances the stability of Ir nanoparticles compared to graphite, while maintaining remarkable activity and selectivity. Simulation techniques including Genetic Algorithm geometry screening and electronic structure analyses were employed to provide a valuable atomic level understanding of the metal-support interactions. N anchoring sites of GCN were found to minimise the thermodynamic driving force of coalescence, thus improving the catalyst stability, as well as to lead charge redistributions in the cluster improving the resistance to poisoning by decomposition intermediates.

Selective decomposition of hydrazine over metal free carbonaceous materials

Herein we report a combined experimental and computational investigation unravelling the hydrazine hydrate decomposition reaction on metal-free catalysts. The study focuses on commercial graphite and two different carbon nanofibers, pyrolytically stripped (CNF-PS) and high heat-treated (CNF-HHT), respectively, treated at 700 and 3000 °C to increase their intrinsic defects. Raman spectroscopy demonstrated a correlation between the initial catalytic activity and the intrinsic defectiveness of carbonaceous materials. CNF-PS with higher defectivity (I_D/I_G = 1.54) was found to be the best performing metal-free catalyst, showing a hydrazine conversion of 94% after 6 hours of reaction and a selectivity to H₂ of 89%. In addition, to unveil the role of NaOH, CNF-PS was also tested in the absence of alkaline solution, showing a decrease in the reaction rate and selectivity to H₂. Density functional theory (DFT) demonstrated that the single vacancies (SV) present on the graphitic layer are the only active sites promoting hydrazine decomposition, whereas other defects such as double vacancy (DV) and Stone-Wales (SW) defects are unable to adsorb hydrazine fragments. Two symmetrical and one asymmetrical dehydrogenation pathways were found, in addition to an incomplete decomposition pathway forming N₂ and NH₃. On the most stable hydrogen production pathway, the effect of the alkaline medium was elucidated through calculations concerning the diffusion and recombination of atomic hydrogen. Indeed, the presence of NaOH helps the extraction of H species without additional energetic barriers, as opposed to the calculations performed in a polarizable continuum medium. Considering the initial hydrazine dissociative adsorption, the first step of the dehydrogenation pathway is more favourable than the scission of the N-N bond, which leads to NH₃ as the product. This first reaction step is crucial to define the reaction mechanisms and the computational results are in agreement with the experimental ones. Moreover, comparing two different hydrogen production pathways (with and without diffusion and recombination), we confirmed that the presence of sodium hydroxide in the experimental reaction environment can modify the energy gap between the two pathways, leading to an increased reaction rate and selectivity to H₂.

Hydrodeoxygenation of guaiacol over orthorhombic molybdenum carbide: a DFT and microkinetic study

The hydrodeoxygenation of guaiacol is modelled over a (100) β-Mo2C surface using density functional theory and microkinetic simulations.

The Effect of Pristine and Hydroxylated Oxide Surfaces on the Guaiacol HDO Process: A DFT Study

The acid‐base character of oxide supports is crucial for catalytic reactions. In this work, the acid‐base properties of five oxide surfaces common in heterogeneous catalysis were investigated and related to their interaction with monolignol compounds derived from lignin. We have used density functional theory simulations also to understand the role of the surfaces’ hydroxylation state. The results show that moderate hydroxyl coverage on the amphoteric γ‐Al₂O₃ (110) slightly strengthens the oxy‐compounds’ adsorption due to an increase in Lewis acidity. Similarly, low hydroxyl coverage on the reducible TiO₂ (101) enlarges its adsorption capacity by up to 42 % compared with its clean surface. The higher affinity is attributed to the more favourable interaction between the surface‐OH groups and the aromatic rings. Overall, the results indicate that hydroxyl coverage enhances the amphoteric and reducible adsorption capacity towards aromatic species.

Low-Valence Znδ+ (0<δ<2) Single-Atom Material as Highly Efficient Electrocatalyst for CO2 Reduction

Electrochemical CO2 reduction represents a promising approach to sustainably produce carbon-based chemicals and fuels but has been experiencing challenges in developing low-cost and efficient electrocatalysts. Herein, a nitrogen-stabilized single-atom catalyst containing low-valence zinc atoms (Znδ+-NC) is reported. It is revealed that Znδ+-NC contains a mixture of saturated four-coordinate (Zn-N4) and unsaturated three-coordinate (Zn-N3) sites. The latter makes Zn a low-valence state, as deduced from X-ray photoelectron spectroscopy, X-ray absorption fine structure spectroscopy, electron paramagnetic resonance, and density functional theory (DFT) simulation. As a result, Znδ+-NC catalyzes electrochemical reduction of CO2 to CO with near-unity selectivity in water at an overpotential as low as 310 mV. Importantly, a record-high current density up to 1 A cm-2 can be achieved together with high CO selectivity of >95% using Znδ+-NC in a flow cell reactor. DFT calculations suggest that the unsaturated Zn-N3 site could dramatically reduce the energy barrier by stabilizing the COOH* (* represents active sites) intermediate due to the electron-rich environment of Zn. This work not only sheds light on the relationship among coordination number, valence state, and catalytic performance of Zn single-atom sites, but also succeeds in achieving high current densities relevant for industrial applications.

Kinetic and mechanistic analysis of NH3 decomposition on Ru(0001), Ru(111) and Ir(111) surfaces

We investigated the catalytic NH₃ decomposition on Ru and Ir metal surfaces using density functional theory. The reaction mechanisms were unraveled on both metals, considering that, on the nano-scale, Ru particles may also present an fcc structure, hence, leading to three energy profiles. We implemented thermodynamic and kinetic parameters obtained from DFT into microkinetic simulations. Batch reactor simulations suggest that hydrogen generation starts at 400 K, 425 K and 600 K on Ru(111), Ru(0001) and Ir(111) surfaces, respectively, in excellent agreement with experiments. During the reaction, the main surface species on Ru are NH, N and H, whereas on Ir(111), it is mainly NH. The rate-determining step for all surfaces is the formation of molecular nitrogen. We also performed temperature-programmed reaction simulations and inspected the desorption spectra of N₂ and H₂ as a function of temperature, which highlighted the importance of N coverage on the desorption rate.

Stable and Antisintering Tungsten Carbides with Controllable Active Phase for Selective Cleavage of Aryl Ether C-O Bonds

Transition-metal carbides are important materials in heterogeneous catalysis. It remains challenging yet attractive in nanoscience to construct the active phase of carbide catalysts in a controllable manner and keep a sintering-resistant property in redox reactions, especially hydroprocessing. In this work, an integrated strategy was presented to synthesize stable and well-defined tungsten carbide nanoparticles (NPs) by assembling the metal precursor onto carbon nanotubes (CNTs), wrapping a thin polymeric layer, and following a controlled carburization. The polymer served as a soft carbon source to modulate the metal/carbon ratio in the carbides and introduced amorphous carbons around the carbides to prevent the NPs from sintering. The as-built p-W_xC/CNT displayed high stability in the hydrogenolysis of aryl ether C-O bond in guaiacol for more than 150 h. Its activity was more than two and six times higher than those prepared via typical temperature-programmed reduction with gaseous carbon (W_xC/CNT-TPR) and carbothermal reduction with intrinsic carbon support (W_xC/CNT-CTR), respectively. Our p-W_xC/CNT catalyst also achieved high efficiency for selective cleavage of the aryl ether C-O bonds in lignin-derived aromatic ethers, including anisole, dimethoxylphenol, and diphenyl ether, with a robust lifespan.

CO2 reduction to acetic acid on the greigite Fe3S4{111} surface

Acetic acid (CH3-COOH) is an important commodity chemical widely used in a myriad of industrial processes, whose production still largely depends on homogeneous catalysts based on expensive rare metals. Here, we report a computational study on the formation of CH3-COOH from carbon dioxide (CO2) as an alternative chemical feedstock on the {111} surface of the low-cost greigite Fe3S4 catalyst. We have used density functional theory calculations with a Hubbard Hamiltonian approach and long-range dispersion corrections (DFT+U-D2) to simulate the various stages of the direct combination of C1 species of different composition to produce glyoxylic acid (CHO-COOH) as a key intermediate in the formation of CH3-COOH. Three reaction mechanisms are considered: (i) the main pathway where the direct formation of the C-C bond takes place spontaneously, followed by a step-wise reduction of CHO-CHOO to CH3-COOH; and the competitive pathways for the non-promoted and H-promoted elimination of hydroxy groups (OH) and water (H2O), respectively from (ii) the carboxyl; and (iii) the carbonyl end of the glyoxylate intermediates. The thermodynamic and kinetic profiles show that the energies for the intermediates on the main pathway are very similar for the two catalytic sites considered, although the activation energies are somewhat larger for the exposed tetrahedral iron (FeA) ion. In most cases, the intermediates for the deoxygenation of the carboxylic acid are less stable than the intermediates on the main pathway, which suggests that the molecule prefers to lose the carbonylic oxygen. The suitable surface properties of the Fe3S4{111} surface show that this material could be a promising sustainable catalyst in future technologies for the conversion of CO2 into organic acid molecules of commercial interest.

DFT+U Study of the Electronic, Magnetic and Mechanical Properties of Co, CoO, and Co3O4

ABSTRACT Cobalt nanoparticles play an important role as a catalyst in the Fischer-Tropsch synthesis. During the reaction process, cobalt nanoparticles can become oxidized leading to the formation of two phases: CoO rock-salt and Co3O4 cubic spinel. Experimentally, it is possible to evaluate the phase change and follow the catalyst degradation by measuring the magnetic moment, as each material presents a different magnetic structure. It is therefore important to develop a fundamental description, at the atomic scale, of cobalt and its oxide phases which we have done here using density functional theory with the Dudarev approach to account for the on-site Coulomb interactions (DFT+U). We have explored different Ueff values, ranging from 0 to 5 eV, and found that Ueff = 3.0 eV describes most appropriately the mechanical properties, as well as the electronic and magnetic structures of Co, CoO and Co3O4. We have considered a ferromagnetic ordering for the metallic phase and the antiferromagnetic structure for the oxide phases. Our results support the interpretation of the catalytic performance of metallic cobalt as it transforms into its oxidized phases under experimental conditions. Keywords: Materials chemistry, cobalt oxides, cobalt metal, spinel, density functional theory, on-site Coulomb correction.

Role of defects in carbon materials during metal-free formic acid dehydrogenation

Commercial graphite (GP), graphite oxide (GO), and two carbon nanofibers (CNF-PR24-PS and CNF-PR24-LHT) were used as catalysts for the metal-free dehydrogenation reaction of formic acid (FA) in the liquid phase. Raman and XPS spectroscopy demonstrated that the activity is directly correlated with the defectiveness of the carbon material (GO > CNF-PR24-PS > CNF-PR24-LHT > GP). Strong deactivation phenomena were observed for all the catalysts after 5 minutes of reaction. Density functional theory (DFT) calculations demonstrated that the single vacancies present on the graphitic layers are the only active sites for FA dehydrogenation, while other defects, such as double vacancies and Stone-Wales (SW) defects, rarely adsorb FA molecules. Two different reaction pathways were found, one passing through a carboxyl species and the other through a hydroxymethylene intermediate. In both mechanisms, the active sites were poisoned by an intermediate species such as CO and atomic hydrogen, explaining the catalyst deactivation observed in the experimental results.

Biomass hydrodeoxygenation catalysts innovation from atomistic activity predictors

Circular economy emphasizes the idea of transforming products involving economic growth and improving the ecological system to reduce the negative consequences caused by the excessive use of raw materials. This can be achieved with the use of second-generation biomass that converts industrial and agricultural wastes into bulk chemicals. The use of catalytic processes is essential to achieve a viable upgrade of biofuels from the lignocellulosic biomass. We carried out density functional theory calculations to explore the relationship between 13 transition metals (TMs) properties, as catalysts, and their affinity for hydrogen and oxygen, as key species in the valourization of biomass. The relation of these parameters will define the trends of the hydrodeoxygenation (HDO) process on biomass-derived compounds. We found the hydrogen and oxygen adsorption energies in the most stable site have a linear relation with electronic properties of these metals that will rationalize the surface's ability to bind the biomass-derived compounds and break the C-O bonds. This will accelerate the catalyst innovation for low temperature and efficient HDO processes on biomass derivates, e.g. guaiacol and anisole, among others. Among the monometallic catalysts explored, the scaling relationship pointed out that Ni has a promising balance between hydrogen and oxygen affinities according to the d-band centre and d-band width models. The comparison of the calculated descriptors to the adsorption strength of guaiacol on the investigated surfaces indicates that the d-band properties alone are not best suited to describe the trend. Instead, we found that a linear combination of work function and d-band properties gives significantly better correlation. This article is part of a discussion meeting issue 'Science to enable the circular economy'.

Role of the Support in Gold-Containing Nanoparticles as Heterogeneous Catalysts

In this review, we discuss selected examples from recent literature on the role of the support on directing the nanostructures of Au-based monometallic and bimetallic nanoparticles. The role of support is then discussed in relation to the catalytic properties of Au-based monometallic and bimetallic nanoparticles using different gas phase and liquid phase reactions. The reactions discussed include CO oxidation, aerobic oxidation of monohydric and polyhydric alcohols, selective hydrogenation of alkynes, hydrogenation of nitroaromatics, CO₂ hydrogenation, C–C coupling, and methane oxidation. Only studies where the role of support has been explicitly studied in detail have been selected for discussion. However, the role of support is also examined using examples of reactions involving unsupported metal nanoparticles (i.e., colloidal nanoparticles). It is clear that the support functionality can play a crucial role in tuning the catalytic activity that is observed and that advanced theory and characterization add greatly to our understanding of these fascinating catalysts.

DFT-Assisted Spectroscopic Studies on the Coordination of Small Ligands to Palladium: From Isolated Ions to Nanoparticles

A combination of experimental spectroscopies (UV-vis and Fourier-transform infrared) and computational modeling was used to investigate the coordination of small ligands (aminopropanol and propanediol) to Pd species during the metal nanoparticle formation process. Differences emerged between O- (propanediol) and N-containing (aminopropanol) ligands. In particular, a strong interaction between the NH amino group and Pd²⁺ ions could be inferred on the basis of spectroscopic evidences, which was corroborated by theoretical simulations, which confirmed the preferential coordination of aminopropanol through the NH group. This interaction seems to potentially cause the aminopropanol ligand to control the particle shape through a selective blocking of Pd(100) facets, which promote the growth on the Pd(111) facets.

Mechanistic study of hydrazine decomposition on Ir(111)

Hydrogen transport and storage technology remain one of the critical challenges of the hydrogen economy. Hydrazine (N2H4) is a carbon-free hydrogen carrier which has been widely used as fuel in the field of space exploration. We have combined experiments and computer simulations in order to gain a better understanding of the N2H4 decomposition on Ir catalyst, the most efficient catalyst for hydrazine decomposition up to date. We have identified metallic Ir rather than IrO2 as the active phase for hydrazine decomposition and carried out density functional theory (DFT) calculations to systematically investigate the changes in the electronic structure along with the catalytic decomposition mechanisms. Three catalytic mechanisms to hydrazine decomposition over Ir(111) have been found: (i) intramolecular reaction between hydrazine molecules, (ii) intramolecular reaction between co-adsorbed amino groups, and (iii) hydrazine dehydrogenation assisted by co-adsorbed amino groups. These mechanisms follow five different pathways for which transition states and intermediates have been identified. The results show that hydrazine decomposition on Ir(111) starts preferentially with an initial N-N bond scission followed by hydrazine dehydrogenation assisted by the amino group produced, eventually leading to ammonia and nitrogen production. The preference for N-N scission mechanisms was rationalized by analyzing the electronic structure. This analysis showed that upon hydrazine adsorption, the π bond between nitrogen atoms becomes weaker.

Benzyl alcohol oxidation with Pd-Zn/TiO2: computational and experimental studies

Pd-Zn/TiO2 catalysts containing 1 wt% total metal loading, but with different Pd to Zn ratios, were prepared using a modified impregnation method and tested in the solvent-free aerobic oxidation of benzyl alcohol. The catalyst with the higher Pd content exhibited an enhanced activity for benzyl alcohol oxidation. However, the selectivity to benzaldehyde was significantly improved with increasing presence of Zn. The effect of reduction temperature on catalyst activity was investigated for the catalyst having a Pd to Zn metal molar ratio of 9:1. It was found that lower reduction temperature leads to the formation of PdZn nanoparticles with a wide particle size distribution. In contrast, smaller PdZn particles were formed upon catalyst reduction at higher temperatures. Computational studies were performed to compare the adsorption energies of benzyl alcohol and the reaction products (benzaldehyde and toluene) on PdZn surfaces to understand the oxidation mechanism and further explain the correlation between the catalyst composition and its activity.

Hydrogen adsorption on transition metal carbides: a DFT study

Transition metal carbides are a class of materials widely known for both their interesting physical properties and catalytic activity. In this work, we have used plane-wave DFT methods to study the interaction with increasing amounts of molecular hydrogen on the low-index surfaces of four major carbides - TiC, VC, ZrC and NbC. Adsorption is found to be generally exothermic and occurs predominantly on the surface carbon atoms. We identify trends over the carbides and their surfaces for the energetics of the adsorption as a function of their electronic and geometrical characteristics. An ab initio thermodynamics formalism is used to study the properties of the slabs as the hydrogen coverage is increased.

A density functional theory study of the hydrogenation and reduction of the thio-spinel Fe3S4{111} surface

The mineral greigite, Fe3S4, shows promising electro-reduction activity, especially towards carbon dioxide conversion to small organic molecules. We have employed density functional theory calculations with correction for the long-range dispersion forces to investigate the behavior of hydrogen on the greigite{111} surface. We have studied the adsorption, diffusion, surface reduction and associative (i.e. Volmer-Tafel mechanism) and molecular desorption of hydrogen as a function of its coverage. We found that (i) the H ad-atoms adsorb on S sites far from metallic centres in the topmost surface layer; (ii) the reduction of greigite by hydrogen is energetically unfavorable at any surface coverage; and (iii) molecular hydrogen evolution has a transition state at ∼0.5 eV above the energy of the reactants on Fe3S4{111}, which is very similar to the barrier found experimentally on Pt{111}. We have also determined the electrode potential under room conditions at which the H2 evolution reaction becomes energetically barrierless.

The influence of support materials on the structural and electronic properties of gold nanoparticles – a DFT study

This study investigates the effect of commonly used support materials (MgO, C, CeO₂) on small gold particles using dispersion corrected density functional theory (DFT-D).

Carbon dioxide and water co-adsorption on the low-index surfaces of TiC, VC, ZrC and NbC: a DFT study

We present a theoretical DFT study into the activation of CO₂ and H₂O by four low-index surfaces of TiC, VC, ZrC and NbC. Two distinct chemisorption pathways are found for CO₂ activation, whilst multiple surface mediated interactions between H₂O and CO₂ are reported.

Selective hydrogenation of CO on Fe3S4{111}: a computational study

Fischer-Tropsch (FT) synthesis has been a recursive method to form valuable molecules from syngas. Metal surfaces have been extensively studied as FT catalysts; among them, iron presents several phases under reaction conditions, oxides and carbides, as active sites for the FT and reverse water gas shift reaction. We present CO reduction on an iron sulfide phase with spinel structure, Fe₃S₄, also considering the pathways where C-O dissociates leaving CH_x species on the surface, which may feed longer aliphatic chains via the FT process. We analysed the thermodynamic and kinetic availability of each step leading to O and OH species co-adsorbed on the surface as well as the formation of H₂O from the hydrogenation of the alcohol group in the molecule. This detailed analysis led to energy profiles on both active sites of the surface, and we conclude that this Fe₃S₄ surface is highly selective towards the formation of methanol, in full agreement with experimental results. These findings point out that the C-C bond formation on greigite takes place through a hydroxycarbene FT mechanism.

Insight into the Nature of Iron Sulfide Surfaces During the Electrochemical Hydrogen Evolution and CO2 Reduction Reactions

Greigite and other iron sulfides are potential, cheap, earth-abundant electrocatalysts for the hydrogen evolution reaction (HER), yet little is known about the underlying surface chemistry. Structural and chemical changes to a greigite (Fe₃S₄)-modified electrode were determined at -0.6 V versus standard hydrogen electrode (SHE) at pH 7, under conditions of the HER. In situ X-ray absorption spectroscopy was employed at the Fe K-edge to show that iron-sulfur linkages were replaced by iron-oxygen units under these conditions. The resulting material was determined as 60% greigite and 40% iron hydroxide (goethite) with a proposed core-shell structure. A large increase in pH at the electrode surface (to pH 12) is caused by the generation of OH^- as a product of the HER. Under these conditions, iron sulfide materials are thermodynamically unstable with respect to the hydroxide. In situ infrared spectroscopy of the solution near the electrode interface confirmed changes in the phosphate ion speciation consistent with a change in pH from 7 to 12 when -0.6 V versus SHE is applied. Saturation of the solution with CO₂ resulted in the inhibition of the hydroxide formation, potentially due to surface adsorption of HCO₃^-. This study shows that the true nature of the greigite electrode under conditions of the HER is a core-shell greigite-hydroxide material and emphasizes the importance of in situ investigation of the catalyst under operation to develop true and accurate mechanistic models.

Platinum Nanoparticle Inclusion into a Carbonized Polymer of Intrinsic Microporosity: Electrochemical Characteristics of a Catalyst for Electroless Hydrogen Peroxide Production

The one-step vacuum carbonization synthesis of a platinum nano-catalyst embedded in a microporous heterocarbon (Pt@cPIM) is demonstrated. A nitrogen-rich polymer of an intrinsic microporosity (PIM) precursor is impregnated with PtCl₆2- to give (after vacuum carbonization at 700 °C) a nitrogen-containing heterocarbon with embedded Pt nanoparticles of typically 1⁻4 nm diameter (with some particles up to 20 nm diameter). The Brunauer-Emmett-Teller (BET) surface area of this hybrid material is 518 m² g-1 (with a cumulative pore volume of 1.1 cm³ g-1) consistent with the surface area of the corresponding platinum-free heterocarbon. In electrochemical experiments, the heterocarbon-embedded nano-platinum is observed as reactive towards hydrogen oxidation, but essentially non-reactive towards bigger molecules during methanol oxidation or during oxygen reduction. Therefore, oxygen reduction under electrochemical conditions is suggested to occur mainly via a 2-electron pathway on the outer carbon shell to give H₂O₂. Kinetic selectivity is confirmed in exploratory catalysis experiments in the presence of H₂ gas (which is oxidized on Pt) and O₂ gas (which is reduced on the heterocarbon surface) to result in the direct formation of H₂O₂.

A kinetic model of water adsorption, clustering and dissociation on the Fe3S4{001} surface

The interaction of water with catalyst surfaces is a common process which requires investigation. Here, we have employed density functional theory calculations to investigate the adsorption of up to ten water molecules on the {001} surface of greigite (Fe₃S₄), which owing to its redox properties, is of increasing interest as a catalyst, e.g. in electro-catalysis. We have systematically analyzed and characterized the modes of water adsorption on the surface, where we have considered both molecular and dissociative adsorption processes. The calculations show that molecular adsorption is the predominant state on these surfaces, from both a thermodynamic and kinetic point of view. We have explored the molecular dispersion on the surface under different coverages and found that the orientation of the molecule, and therefore the surface dipole, depends on the number of adsorbed molecules. The interactions between the water molecules become stronger with an increasing number of water molecules, following an exponential decay which tends to the interaction energy found in bulk water. We have also shown the evolution of the infra-red signals as a function of water coverage relating to the H-bond networks formed on the surface. Next we have included these results in a classical micro-kinetic model, which introduced the effects of temperature in the simulations, thus helping us to derive the water cluster size on the greigite surface as a function of the initial conditions of pressure, temperature and external potential. The kinetic model concluded that water molecules agglomerate in clusters instead of wetting the surface, which agrees with the low hydrophilicity of Fe₃S₄. Clusters consisting of four water molecules was shown to be the most stable cluster under a wide range of temperatures and external potential.

Hydrogen Generation from Additive-Free Formic Acid Decomposition Under Mild Conditions by Pd/C: Experimental and DFT Studies

Safe and efficient hydrogen generation and storage has received much attention in recent years. Herein, a commercial 5 wt% Pd/C catalyst has been investigated for the catalytic, additive-free decomposition of formic acid at mild conditions, and the experimental parameters affecting the process systematically have been investigated and optimised. The 5 wt% Pd/C catalyst exhibited a remarkable 99.9% H2 selectivity and a high catalytic activity (TOF = 1136 h-1) at 30 °C toward the selective dehydrogenation of formic acid to H2 and CO2. The present commercial catalyst demonstrates to be a promising candidate for the efficient in-situ hydrogen generation at mild conditions possibiliting practical applications of formic acid systems on fuel cells. Finally DFT studies have been carried out to provide insights into the reactivity and decomposition of formic acid along with the two-reaction pathways on the Pd (111) surface.

Reactivity of CO2 on the surfaces of magnetite (Fe3O4), greigite (Fe3S4) and mackinawite (FeS)

The growing environmental, industrial and commercial interests in understanding the processes of carbon dioxide (CO2) capture and conversion have led us to simulate, by means of density functional theory calculations, the application of different iron oxide and sulfide minerals to capture, activate and catalytically dissociate this molecule. We have chosen the {001} and {111} surfaces of the spinel-structured magnetite (Fe3O4) and its isostructural sulfide counterpart greigite (Fe3S4), which are both materials with the Fe cations in the 2+/3+ mixed valence state, as well as mackinawite (tetragonal FeS), in which all iron ions are in the ferrous oxidation state. This selection of iron-bearing compounds provides us with understanding of the effect of the composition, stoichiometry, structure and oxidation state on the catalytic activation of CO2 The largest adsorption energies are released for the interaction with the Fe3O4 surfaces, which also corresponds to the biggest conformational changes of the CO2 molecule. Our results suggest that the Fe3S4 surfaces are unable to activate the CO2 molecule, while a major charge transfer takes place on FeS{111}, effectively activating the CO2 molecule. The thermodynamic and kinetic profiles for the catalytic dissociation of CO2 into CO and O show that this process is feasible only on the FeS{111} surface. The findings reported here show that these minerals show promise for future CO2 capture and conversion technologies, ensuring a sustainable future for society.This article is part of a discussion meeting issue 'Providing sustainable catalytic solutions for a rapidly changing world'.

Bulk and surface properties of metal carbides: implications for catalysis

We present a comprehensive study of the bulk and surface properties of transition metal carbides with rock salt structures and discuss their formation energies, electronic structure and potential catalytic activity.

Stability and mobility of supported Nin (n = 1–10) clusters on ZrO2(111) and YSZ(111) surfaces: a density functional theory study

We have used spin polarized density functional theory (DFT) to evaluate the geometrical resilience of Ni clusters on ZrO₂(111) and YSZ(111).

CO2 interaction with violarite (FeNi2S4) surfaces: a dispersion-corrected DFT study

The interaction between the CO₂ molecule and the violarite FeNi₂S₄{001} and {111} surfaces is studied using different exchange–correlation functionals and long-range dispersion correction approximations.

Periodic modeling of zeolite Ti-LTA

We have proposed a combination of density functional theory calculations and interatomic potential-based simulations to study the structural, electronic, and mechanical properties of pure-silica zeolite Linde Type A (LTA), as well as two titanium-doped compositions. The energetics of the titanium distribution within the zeolite framework suggest that the inclusion of a second titanium atom with configurations Ti-(Si)₀-Ti, Ti-(Si)₁-Ti, and Ti-(Si)₂-Ti is more energetically favorable than the mono-substitution. Infra-red spectra have been simulated for the pure-silica LTA, the single titanium substitution, and the configurations Ti-(Si)₀-Ti and Ti-(Si)₂-Ti, comparing against experimental benchmarks where available. The energetics of the direct dissociation of water on these Lewis acid sites indicate that this process is only favored when two titanium atoms form a two-membered ring (2MR) sharing two hydroxy groups, Ti-(OH)₂-Ti, which suggests that the presence of water may tune the distribution of titanium atoms within the framework of zeolite LTA. The electronic analysis indicates charge transfer from H₂O to the Lewis acid site and hybridization of their electronic states.

Opioid analgesic use among patients presenting with acute abdominal pain and factors associated with surgical diagnoses

BACKGROUND

The prevalence of chronic opioid use among non-cancer patients presenting with acute abdominal pain (AAP) is unknown. The aim was to characterize opioid use, constipation, diagnoses, and risk factors for surgical diagnoses among non-cancer patients presenting with AAP to an emergency department (ED).

METHODS

We performed a retrospective, observational cohort study of all (n=16,121) adult patients (88% from MN, IA and WI) presenting during 2014 with AAP. We used electronic medical records, and focused on 2352 adults with AAP who underwent abdominal CT scan within 24 hours of presentation. We determined odds ratios of association with constipation and features predicting conditions that may require surgery (surgical diagnosis).

KEY RESULTS

There were 2352 eligible patients; 18.8% were opioid users. Constipation was more frequent in opioid (35.1%) compared to non-opioid users [OR 2.88 (95% CI 2.28, 3.62)]. Prevalence of surgical diagnosis in the opioid and non-opioid users was 35.3% and 41.7% respectively (P=.019). By univariate analysis, age and neutrophil count independently predicted increased risk, and chronic opioid use decreased risk of surgical diagnosis. Internal validation of logistic models using a randomly selected validation subset (25% of entire cohort, 587/2352) showed receiver operating characteristic (ROC) curves for the validation and full cohorts were similar.

CONCLUSIONS AND INFERENCES

Approximately 19% of adults presenting with AAP were opioid users; constipation is almost three times as likely in opioid users compared to non-opioid users presenting with AAP. Factors significantly associated with altered risk of surgical diagnoses were age, opioid use, and neutrophil count.

Structures and Properties of As(OH)3 Adsorption Complexes on Hydrated Mackinawite (FeS) Surfaces: A DFT-D2 Study

Reactive mineral-water interfaces exert control on the bioavailability of contaminant arsenic species in natural aqueous systems. However, the ability to accurately predict As surface complexation is limited by the lack of molecular-level understanding of As-water-mineral interactions. In the present study, we report the structures and properties of the adsorption complexes of arsenous acid (As(OH)3) on hydrated mackinawite (FeS) surfaces, obtained from density functional theory (DFT) calculations. The fundamental aspects of the adsorption, including the registries of the adsorption complexes, adsorption energies, and structural parameters are presented. The FeS surfaces are shown to be stabilized by hydration, as is perhaps to be expected because the adsorbed water molecules stabilize the low-coordinated surface atoms. As(OH)3 adsorbs weakly at the water-FeS(001) interface through a network of hydrogen-bonded interactions with water molecules on the surface, with the lowest-energy structure calculated to be an As-up outer-sphere complex. Compared to the water-FeS(001) interface, stronger adsorption was calculated for As(OH)3 on the water-FeS(011) and water-FeS(111) interfaces, characterized by strong hybridization between the S-p and O-p states of As(OH)3 and the surface Fe-d states. The As(OH)3 molecule displayed a variety of chemisorption geometries on the water-FeS(011) and water-FeS(111) interfaces, where the most stable configuration at the water-FeS(011) interface is a bidentate Fe-AsO-Fe complex, but on the water-FeS(111) interface, a monodentate Fe-O-Fe complex was found. Detailed information regarding the adsorption mechanisms has been obtained via projected density of states (PDOS) and electron density difference iso-surface analyses and vibrational frequency assignments of the adsorbed As(OH)3 molecule.

DFT-D2 simulations of water adsorption and dissociation on the low-index surfaces of mackinawite (FeS)

The adsorption and dissociation of water on mackinawite (layered FeS) surfaces were studied using dispersion-corrected density functional theory (DFT-D2) calculations. The catalytically active sites for H2O and its dissociated products on the FeS {001}, {011}, {100}, and {111} surfaces were determined, and the reaction energetics and kinetics of water dissociation were calculated using the climbing image nudged elastic band technique. Water and its dissociation products are shown to adsorb more strongly onto the least stable FeS{111} surface, which presents low-coordinated cations in the surface, and weakest onto the most stable FeS{001} surface. The adsorption energies decrease in the order FeS{111} > FeS{100} > FeS{011} > FeS{001}. Consistent with the superior reactivity of the FeS{111} surface towards water and its dissociation products, our calculated thermochemical energies and activation barriers suggest that the water dissociation reaction will take place preferentially on the FeS nanoparticle surface with the {111} orientation. These findings improve our understanding of how the different FeS surface structures and the relative stabilities dictate their reactivity towards water adsorption and dissociation.