Stavros Caratzoulas

Nickel supported on nitrogen-doped carbon nanotubes as hydrogen oxidation reaction catalyst in alkaline electrolyte

The development of a low-cost, high-performance platinum-group-metal-free hydroxide exchange membrane fuel cell is hindered by the lack of a hydrogen oxidation reaction catalyst at the anode. Here we report that a composite catalyst, nickel nanoparticles supported on nitrogen-doped carbon nanotubes, has hydrogen oxidation activity similar to platinum-group metals in alkaline electrolyte. Although nitrogen-doped carbon nanotubes are a very poor hydrogen oxidation catalyst, as a support, it increases the catalytic performance of nickel nanoparticles by a factor of 33 (mass activity) or 21 (exchange current density) relative to unsupported nickel nanoparticles. Density functional theory calculations indicate that the nitrogen-doped support stabilizes the nanoparticle against reconstruction, while nitrogen located at the edge of the nanoparticle tunes local adsorption sites by affecting the d-orbitals of nickel. Owing to its high activity and low cost, our catalyst shows significant potential for use in low-cost, high-performance fuel cells.

Converting fructose to 5-hydroxymethylfurfural: a quantum mechanics/molecular mechanics study of the mechanism and energetics

We studied the energetics of the closed-ring mechanism of the acid-catalysed dehydration of d-fructose to 5-hydroxymethylfurfural (HMF) by carrying out canonical ensemble free-energy calculations using bias-sampling, hybrid Quantum Mechanics/Molecular Mechanics Molecular Dynamics simulations with explicit water solvent at 363 K. The quantum mechanical calculations are performed at the PM3 theory level. We find that the reaction proceeds via intramolecular proton and hydride transfers. Solvent dynamics effects are analysed, and we show that the activation energy for the hydride transfers is due to re-organization of the polar solvent environment. We also find that in some instances intramolecular proton transfer is facilitated by mediating water, whereas in others the presence of quantum mechanical water has no effect. From a micro-kinetic point of view, we find that the rate-determining step of the reaction involves a hydride transfer prior to the third dehydration step, requiring an activation free energy of 31.8 kcal/mol, and the respective rate is found in good agreement with reported experimental values in zeolites. Thermodynamically, the reaction is exothermic by ΔF=20.5 kcal/mol.

Origin of 5‐Hydroxymethylfurfural Stability in Water/Dimethyl Sulfoxide Mixtures

In the present work, we combined vibrational spectroscopy with electronic structure calculations to understand the solvation of HMF in DMSO, water, and DMSO/water mixtures and to provide insights into the observed hindrance of HMF rehydration and aldol condensation reactions if it is dissolved in DMSO/water mixtures. To achieve this goal, the attenuated total reflection FTIR spectra of a wide composition range of binary and ternary mixtures were measured, analyzed, and compared to the findings of ab initio DFT calculations. The effect of solvent on the HMF C-O and O-H vibrational modes reveals significant differences that are ascribed to different intermolecular interactions between HMF and DMSO or water. We also found that DMSO binds to HMF more strongly than water, and interactions with the HMF hydroxyl group are stronger than those with the HMF carbonyl group. We also showed the preferential solvation of HMF C-O groups by DMSO if HMF is dissolved in DMSO/water mixed solvent. Frontier molecular orbital theory was used to examine the influence of the solvent on side reactions. The results show that HMF solvation by DMSO increases its LUMO energy, which reduces its susceptibility to nucleophilic attack and minimizes undesirable hydration and humin-formation reactions. This result, together with the preferential solvation of HMF by DMSO, provide an explanation for the enhanced HMF stability in DMSO/water mixtures observed experimentally.

Insights into the isomerization of xylose to xylulose and lyxose by a Lewis acid catalyst

We present electronic structure calculations on the isomerization and epimerization of xylose to xylulose and lyxose, respectively, by a zeolite Sn-BEA model at the MP2 and B3LYP theory levels. Benchmarking calculations at the CCSD(T) theory level are also presented. We show that lyxose is formed from a stable intermediate in the xylose-xylulose isomerization pathway. In agreement with experimental observations, we predict that xylulose is thermodynamically and kinetically favoured over lyxose. We find that the slowest step for both reactions involves hydrogen transfer from the C2 to the C1 carbon of the carbohydrate molecule and we characterize it using natural population analysis. We conclude that the hydrogen transfer does not take place as a hydride ion but rather as concerted neutral hydrogen-electron transfer that involves different centres for the hydrogen and electron transfer.

A perspective on the modeling of biomass processing

In this perspective, the state of the art in modeling of biomass processing to produce fuels and chemicals is reviewed, and the potential impact of modeling, along with the computational challenges, is presented. First, we discuss metal catalyzed chemistry, such as hydrogenation, hydrodeoxygenation, and reforming in vapor/solid reactions. Density functional theory (DFT) and microkinetic modeling of ethylene glycol and glycerol are reviewed and recent progress and challenges in modeling of unsaturated aldehydes and furans are discussed. A computational engine is presented that enables computer-based high throughput screening to derive performance maps and identify optimal catalysts. Condensed-phase processes are then covered with emphasis on solvent effects (e.g., solvation of biomass molecules, participation of solvent in the chemistry), thermodynamic properties, and separations. The conversion of fructose to 5-hydroxymethylfurfural (HMF) is taken as a prototype case of this class of processes. Finally, an outlook is provided.

Mechanism of Brønsted Acid‐Catalyzed Glucose Dehydration

We present the first DFT-based microkinetic model for the Brønsted acid-catalyzed conversion of glucose to 5-hydroxylmethylfurfural (HMF), levulinic acid (LA), and formic acid (FA) and perform kinetic and isotopic tracing NMR spectroscopy mainly at low conversions. We reveal that glucose dehydrates through a cyclic path. Our modeling results are in excellent agreement with kinetic data and indicate that the rate-limiting step is the first dehydration of protonated glucose and that the majority of glucose is consumed through the HMF intermediate. We introduce a combination of 1) automatic mechanism generation with isotopic tracing experiments and 2) elementary reaction flux analysis of important paths with NMR spectroscopy and kinetic experiments to assess mechanisms. We find that the excess formic acid, which appears at high temperatures and glucose conversions, originates from retro-aldol chemistry that involves the C6 carbon atom of glucose.

A First Principles-Based Microkinetic Model for the Conversion of Fructose to 5-Hydroxymethylfurfural

We have tested and discussed the accuracy of hybrid quantum mechanics/molecular mechanics molecular dynamics free energy calculations for the investigation of the mechanism of dehydration of biomass‐derived carbohydrates in solution. In this respect and taking into account earlier calculations of this type, we have developed a microkinetic model for the dehydration of fructose to 5‐hydroxymethylfurfural (HMF) in acidic water and embedded it in a reaction network that includes fructose and HMF degradation reactions. Sensitivity analysis of the kinetic model has shown the rate‐limiting step of the reaction network under consideration to be an intramolecular hydride transfer that takes place right after the first water removal from fructose. We predict the formation of two stable intermediates, one of which is structurally similar to the (4R,5R)‐4‐hydroxy‐5‐hydroxymethyl‐4,5‐dihydrofuran‐2‐carbaldehyde intermediate identified by NMR studies in pure DMSO solution. We find remarkable agreement between calculated and experimental concentration profiles over a wide range of temperatures and over the entire range of timescales considered in the kinetic study of Asghari and Yoshida. We demonstrate that the microkinetic model cannot capture the correct temperature dependence of the rates unless one uses Marcus theory rate constants for those elementary steps of the mechanism that involve hydride transfer. The computed apparent activation energy and Arrhenius frequency factor for fructose conversion to HMF are also found to be in excellent agreement with those obtained from experiments.

On the Brønsted Acid-Catalyzed Homogeneous Hydrolysis of Furans

Furan affairs: Electronic structure calculations of the homogeneous Brønsted acid-catalyzed hydrolysis of 2,5-dimethylfuran show that proton transfer to the β-position is rate-limiting and provides support that the hydrolysis follows general acid catalysis. By means of projected Fukui indices, we show this to be the case for unsubstituted, 2-, and 2,5-substituted furans with electron-donating groups.

Understanding mixing of Ni and Pt in the Ni/Pt(111) bimetallic catalyst via molecular simulation and experiments

Molecular dynamics (MD) simulations employing embedded atom method potentials and ultrahigh vacuum (UHV) experiments were carried out to study the mixing process between the Ni and Pt atoms in the Ni/Pt(111) bimetallic system. The barrier for a Ni atom to diffuse from the top surface to the subsurface layer is rather high (around 1.7 eV) as calculated using the nudged elastic band (NEB) method. Analysis of the relaxation dynamics of the Ni atoms showed that they undergo diffusive motion through a mechanism of correlated hops. At 600 K, all Ni atoms remain trapped on the top surface due to large diffusion barriers. At 900 K, the majority of Ni atoms diffuse to the second layer and at 1200 K diffusion to the bulk is observed. We also find that smaller Ni coverages and the presence of Pt steps facilitate the Ni-Pt mixing. By simulated annealing simulations, we found that in the mixed state, the Ni fraction oscillates between layers, with the second layer being Ni-richer at equilibrium. The simulation results at multiple time scales are consistent with the experimental data.

Adsorption in zeolites using mechanically embedded ONIOM clusters

We have explored mechanically embedded three-layer QM/QM/MM ONIOM models for computational studies of binding in Al-substituted zeolites. In all the models considered, the high-level-theory layer consists of the adsorbate molecule and of the framework atoms within the first two coordination spheres of the Al atom and is treated at the M06-2X/6-311G(2df,p) level. For simplicity, flexibility and routine applicability, the outer, low-level-theory layer is treated with the UFF. We have modelled the intermediate-level layer quantum mechanically and investigated the performance of HF theory and of three DFT functionals, B3LYP, M06-2X and ωB97x-D, for different layer sizes and various basis sets, with and without BSSE corrections. We have studied the binding of sixteen probe molecules in H-MFI and compared the computed adsorption enthalpies with published experimental data. We have demonstrated that HF and B3LYP are inadequate for the description of the interactions between the probe molecules and the framework surrounding the metal site of the zeolite on account of their inability to capture dispersion forces. Both M06-2X and ωB97x-D on average converge within ca. 10% of the experimental values. We have further demonstrated transferability of the approach by computing the binding enthalpies of n-alkanes (C1-C8) in H-MFI, H-BEA and H-FAU, with very satisfactory agreement with experiment. The computed entropies of adsorption of n-alkanes in H-MFI are also found to be in good agreement with experimental data. Finally, we compare with published adsorption energies calculated by periodic-DFT for n-C3 to n-C6 alkanes, water and methanol in H-ZSM-5 and find very good agreement.