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(OArF)3 (ArF = 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.

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.

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.

Dehydrogenation and dehydration of formic acid over orthorhombic molybdenum carbide.

The dehydrogenation and dehydration of formic acid is investigated on the ß-Mo2C (100) catalyst surface using time independent density functional theory. The energetics of the two mechanisms are calculated, and the thermochemistry and kinetics are discussed using the transition state theory. Subsequently, microkinetic modelling of the system is conducted, considering the batch reactor model. The potential energy landscape of the reaction shows a thermodynamically favourable cleavage of H-COOH to form CO; however, the kinetics show that the dehydrogenation mechanism is faster and CO2 is continuously formed. The effect of HCOOH adsorption on the surface is also analysed, in a temperature-programmed desorption, with the conversion proceeding at under 350 K and desorption of CO2 is observed with a selectivity of about 100 %, in line with the experimental reports.

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 (ID/IG = 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.

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-Al2 O3 (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.

A nitrogen-stabilized single-atom catalyst containing low-valence zinc atoms (Znδ+ -NC) is reported. It contains 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 spectroscopy, electron paramagnetic resonance, and density functional theory. Znδ+ -NC catalyzes electrochemical reduction of CO2 to CO with near-unity selectivity in water at an overpotential as low as 310 mV. A 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. Calculations suggest that the unsaturated Zn-N3 could dramatically reduce the energy barrier by stabilizing the COOH* intermediate owing to the electron-rich environment of Zn. This work sheds light on the relationship among coordination number, valence state, and catalytic performance and achieves high current densities relevant for industrial applications.

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.

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-WxC/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 (WxC/CNT-TPR) and carbothermal reduction with intrinsic carbon support (WxC/CNT-CTR), respectively. Our p-WxC/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.

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.

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.

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.

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.

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

Supported gold nanoparticles are used for a wide range of catalytic processes. In this work, we use dispersion corrected density functional theory (DFT-D) to study the effect of commonly used support materials (MgO, C, CeO2) on small gold particles with up to 19 atoms. Our results show that the preferred cluster shape and morphology is highly dependent on the support material due to different adsorption strength and structural mismatch between the cluster and the surface material. We developed an algorithm to measure the mismatch between the cluster interface and the support surface. Moreover, depending on the support material, the gold clusters exhibit a positive or negative polarisation, which ultimately has strong implications on the catalytic activity of such particles. This behaviour is rationalised by an analysis of the electronic structure of the metal particles and support materials.

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 by TiC, VC, ZrC and NbC. Particular focus is given to the study of CO2/H2O co-adsorption and interaction on four carbide low-index surfaces: {001}, {011}, carbon-terminated {111} and metal-terminated {111}. The adsorption and activation of CO2 is shown to be most exothermic and indeed barrierless on the metal-terminated {111} surfaces, whilst adsorption on the {001} and {011} planes occurs via a small activation energy barrier. In contrast, the carbon-terminated {111} surface proves to be unstable in the presence of the adsorbates. Both water and carbon dioxide adsorb most strongly on TiC and most weakly on NbC, with the strongest co-adsorption interactions being seen in conformations that maximise hydrogen-bonding.