The solar energy conversion efficiency of photoelectrochemical (PEC) devices is usually limited by poor interface energetics, limiting the onset potential, and light reflection losses. Here, a three-pronged approach to obtain excellent performance of an InP-based photoelectrode for water reduction is presented. First, a buried p–n⁺ junction is fabricated, which shifts the valence band edge favorably with respect to the hydrogen redox potential. Photoelectron spectroscopy substantiates that the shift of the surface photovoltage is mainly determined by the buried junction. Second, a periodic array of InP nanopillars is created at the surface of the photoelectrode to substantially reduce the optical reflection losses. This device displays an unprecedented photocathodic power-saved efficiency of 15.8% for single junction water reduction. Third, a thin TiO₂ protection layer significantly increases the stability of the InP-based photoelectrode. Careful design of the interface energetics based on surface photovoltage spectroscopy allows obtaining a PEC cell with stable record performance in water reduction.

Microkinetics simulations are presented based on DFT-determined elementary reaction steps of the Fischer–Tropsch (FT) reaction. The formation of long-chain hydrocarbons occurs on stepped Ru surfaces with CH as the inserting monomer, whereas planar Ru only produces methane because of slow CO activation. By varying the metal–carbon and metal–oxygen interaction energy, three reactivity regimes are identified with rates being controlled by CO dissociation, chain-growth termination, or water removal. Predicted surface coverages are dominated by CO, C, or O, respectively. Optimum FT performance occurs at the interphase of the regimes of limited CO dissociation and chain-growth termination. Current FT catalysts are suboptimal, as they are limited by CO activation and/or O removal.

Microkinetics simulations are presented based on DFT-determined elementary reaction steps of the Fischer–Tropsch (FT) reaction. The formation of long-chain hydrocarbons occurs on stepped Ru surfaces with CH as the inserting monomer, whereas planar Ru only produces methane because of slow CO activation. By varying the metal–carbon and metal–oxygen interaction energy, three reactivity regimes are identified with rates being controlled by CO dissociation, chain-growth termination, or water removal. Predicted surface coverages are dominated by CO, C, or O, respectively. Optimum FT performance occurs at the interphase of the regimes of limited CO dissociation and chain-growth termination. Current FT catalysts are suboptimal, as they are limited by CO activation and/or O removal.

Invited for this month′s cover is the group of Emiel Hensen at the Eindhoven University of Technology. The image on the right shows how soda lignin is depolymerized and stabilized under supercritical ethanol over a Cu-containing mixed oxide catalyst. The Full Paper itself is available at 10.1002/cssc.201402094

One-step valorization of soda lignin in supercritical ethanol using a CuMgAlO_x catalyst results in high monomer yield (23 wt %) without char formation. Aromatics are the main products. The catalyst combines excellent deoxygenation with low ring-hydrogenation activity. Almost half of the monomer fraction is free from oxygen. Elemental analysis of the THF-soluble lignin residue after 8 h reaction showed a 68 % reduction in O/C and 24 % increase in H/C atomic ratios as compared to the starting Protobind P1000 lignin. Prolonged reaction times enhanced lignin depolymerization and reduced the amount of repolymerized products. Phenolic hydroxyl groups were found to be the main actors in repolymerization and char formation. 2D HSQC NMR analysis evidenced that ethanol reacts by alkylation and esterification with lignin fragments. Alkylation was found to play an important role in suppressing repolymerization. Ethanol acts as a capping agent, stabilizing the highly reactive phenolic intermediates by O-alkylating the hydroxyl groups and by C-alkylating the aromatic rings. The use of ethanol is significantly more effective in producing monomers and avoiding char than the use of methanol. A possible reaction network of the reactions between the ethanol and lignin fragments is discussed.

The nature and location of copper in Cu/SSZ-13 zeolites synthesized by using a one-pot hydrothermal approach with Cu–tetraethylenepentamine as a template and by the ion exchange of SSZ-13 were investigated by applying H₂-temperature-programmed reduction, FTIR, EPR, and in situ Raman spectroscopic techniques. The one-pot synthesized Cu/SSZ-13 zeolite contains predominantly isolated copper ions in the large cages, whereas copper species in the ion-exchanged Cu/SSZ-13 zeolite occupy sites in the large cages of the chabazite (CHA) structure and the six-membered rings of the CHA structure. If the one-pot synthesized Cu/SSZ-13 zeolite is exchanged with the NH₄NO₃ solution in addition to the removal of a part of copper ions, the remaining copper ions in the CHA structure relocated from the large cages to the six-membered rings. Isolated copper dominated in all Cu/SSZ-13 zeolites. The in situ Raman spectra demonstrated that CuOCu dimers form at higher copper content. The bis-μ-oxo dicopper(III) complex is observed only in the ion-exchanged sample upon dehydration. The higher NO selective catalytic reduction activity of the one-pot synthesized sample in a wide temperature range appears to be due to the predominance of isolated Cu²⁺ sites in the large cages, and their higher reactivity is possibly owing to the lower stability of Cu²⁺ at these sites.

A significant support effect was observed for the aqueous-phase reforming (APR) of glycerol over a series of Pt- and PtRe-loaded ceria-, ceria–zirconia-, zirconia-, and titania-supported catalysts. Glycerol conversion rates decreased in the order Pt/TiO₂>Pt/ZrO₂>Pt/CeZrO₂>Pt/CeO₂. Upon addition of Re, APR activities of all the Pt catalysts increased. Re promotion for glycerol APR was strongest for PtRe/CeO₂. Pt/CeO₂ was least active because of the incomplete reduction of Pt. Pt was more easily reduced in PtRe/CeO₂ because of the strong Pt–Re interaction. On comparison with previous data on Pt/C and PtRe/C, the oxide-supported catalysts were more active in the WGS reaction, with the titania-supported catalyst as the most active, which was a result of the activation of water on the titania support. The acidity of these groups also resulted in increased CO cleavage rates of Pt/TiO₂ relative to the rates of Pt/C in APR. A more detailed comparison of Pt(Re)/C and Pt(Re)/TiO₂ is made. Monitoring acetaldehyde decomposition to determine the rate of CC bond cleavage evidenced a strong promoting effect of Re on the supported Pt catalysts. All catalysts produced carbon monoxide, methane, and ethanol during acetaldehyde decomposition; notably, Pt(Re)/TiO₂ also formed products such as ethylene, ethane, propylene, and propane, indicative of a mechanism also involving ethanol dehydration and olefin coupling to CH_x surface intermediates derived from acetaldehyde. The activity in overall glycerol conversion in APR decreased in the order PtRe/C>PtRe/TiO₂>Pt/TiO₂>Pt/C.

A series of faujasite zeolites was modified by extraframework Al (Al_EF) with the goal to investigate the influence of such species on the intrinsic Brønsted acidity and catalytic activity towards paraffin cracking. The chemical state of Al_EF and zeolite acidity were investigated by ²⁷Al MAS NMR and CO_ads IR spectroscopy, H/D exchange reaction, and propane cracking. Strongly acidic defect-free Y zeolites were obtained by substitution of framework Al by Si with (NH₄)₂SiF₆. In accordance with the next-nearest-neighbor model, the intrinsic acidity of the protons increased with decreasing framework Al density. This increased acidity was evidenced by an increased shift of the OH stretching vibration upon CO adsorption in CO_ads IR spectroscopy and by an increased H/D exchange rate in H/D exchange reactions with perdeuterobenzene. All of the acid sites in these zeolites were of equal strength beyond a certain Si/Al ratio. The increased acidity resulted in an enhanced propane cracking activity. Modification of a model dealuminated Y zeolite by Al_EF only resulted in a small fraction of cationic Al_EF species, because it was difficult to control the ion exchange process. In comparison, commercial ultrastabilized Y zeolites contained less Al_EF and these species were predominantly present in cationic form. The rate of propane cracking strongly correlated to the concentration of Brønsted acid sites perturbed by cationic Al_EF species. The results of MQMAS ²⁷Al NMR spectroscopy confirmed the presence of sites perturbed by Al_EF and unaffected framework Al sites. Zeolites with higher intrinsic cracking activities contained a higher proportion of perturbed sites. Although CO_ads IR and H/D exchange methods proved to be suitable methods to probe the acidity of Y zeolites free from Al_EF, they were less suitable to predict the reactivity if the Brønsted acid sites were affected by cationic Al_EF species.

The influence of organic capping agents on the performance of Ru nanoparticles in aqueous-phase Fischer–Tropsch (FT) synthesis was investigated. Three organic capping agents were used: trimethyl(tetradecyl)ammonium bromide (TTAB), polyvinylpyrrolidone (PVP), and sodium 3-mercapto-1-propanesulfonate (SMPS). To exclude the effects of particle size, the capping agents were placed onto carbon-nanofiber-supported Ru nanoparticles of size 3.4 nm. The activity in the FT reaction increased in the order: Ru-SMPS≪Ru-PVP<Ru-TTAB<Ru. Kinetic data suggest that the FT mechanism was largely unaffected by the presence of capping agents; thus, their binding to active centers explains activity trends. Replacing water with n-hexadecane as the solvent results in an increase in the rate of formation and a decrease in the chain-growth probability for hydrocarbons, whereas the production of oxygenates is unaffected. This trend is consistent with the proposal that hydrocarbons are formed on reaction centers that involve facile CO dissociation and that termination is the rate-determining step. For oxygenates, CO dissociation is proposed to be the rate-determining step.

Efficient basic hydrotalcite (HT)-supported gold nanoparticle (AuNP) catalysts have been developed for the aerobic oxidative tandem synthesis of methyl esters and imines from primary alcohols catalyzed under mild and soluble-base-free conditions. The catalytic performance can be fine-tuned for these cascade reactions by simple adjustment of the Mg/Al atomic ratio of the HT support. The one-pot synthesis of methyl esters benefits from high basicity (Mg/Al=5), whereas moderate basicity greatly improves imine selectivity (Mg/Al=2). These catalysts outperform previously reported AuNP catalysts by far. Kinetic studies show a cooperative enhancement between AuNP and the surface basic sites, which not only benefits the oxidation of the starting alcohol but also the subsequent steps of the tandem reactions. To the best of our knowledge, this is the first time that straightforward control of the composition of the support has been shown to yield optimum AuNP catalysts for different tandem reactions.

A complementary computational and experimental study of the reactivity of Lewis acidic CrCl₂, CuCl₂ and FeCl₂ catalysts towards glucose activation in dialkylimidazolium chloride ionic liquids is performed. The selective dehydration of glucose to 5-hydroxymethylfurfural (HMF) proceeds through the intermediate formation of fructose. Although chromium(II) and copper(II) chlorides are able to dehydrate fructose with high HMF selectivity, reasonable HMF yields from glucose are only obtained with CrCl₂ as the catalyst. Glucose conversion by CuCl₂ is not selective, while FeCl₂ catalyst does not activate sugar molecules. These differences in reactivity are rationalized on the basis of in situ X-ray absorption spectroscopy measurements and the results of density functional theory calculations. The reactivity in glucose dehydration and HMF selectivity are determined by the behavior of the ionic liquid-mediated Lewis acid catalysts towards the initial activation of the sugar molecules. The formation of a coordination complex between the Lewis acidic Cr²⁺ center and glucose directs glucose transformation into fructose. For Cu²⁺ the direct coordination of sugar to the copper(II) chloride complex is unfavorable. Glucose deprotonation by a mobile Cl⁻ ligand in the CuCl₄²⁻ complex initiates the nonselective conversion. In the course of the reaction the Cu²⁺ ions are reduced to Cu⁺. Both paths are prohibited for the FeCl₂ catalyst.

A combined experimental and computational study of the ionic-liquid-mediated dehydration of glucose and fructose by Cr^II and Cr^III chlorides has been performed. The ability of chromium to selectively dehydrate glucose to 5-hydroxymethylfurfural (HMF) in the ionic liquid 1-ethyl-3-methyl imidazolium chloride does not depend on the oxidation state of chromium. Nevertheless, Cr^III exhibits higher activity and selectivity to HMF than Cr^II. Anhydrous CrCl₂ and CrCl₃⋅6 H₂O readily catalyze glucose dehydration with HMF yields of 60 and 72 %, respectively, after 3 h. Anhydrous CrCl₃ has a lower activity, because it only slowly dissolves in the reaction mixture. The transformation of glucose to HMF involves the formation of fructose as an intermediate. The exceptional catalytic performance of the chromium catalysts is explained by their unique ability to catalyze glucose to fructose isomerization and fructose to HMF dehydration with high selectivity. Side reactions leading to humins by means of condensation reactions take predominantly place during fructose dehydration. The higher HMF selectivity for Cr^III is tentatively explained by the higher activity in fructose dehydration compared to Cr^II. This limits the concentration of intermediates that are involved in bimolecular condensation reactions. Model DFT calculations indicate a substantially lower activation barrier for glucose isomerization by Cr^III compared to Cr^II. Qualitatively, glucose isomerization follows a similar mechanism for Cr^II and Cr^III. The mechanism involves ring opening of D-glucopyranose coordinated to a single Cr ion, followed by a transient self-organization of catalytic chromium complexes that promotes the rate-determining hydrogen-shift step.

MIL-101, a chromium-based metal–organic framework, is known for its very large pore size, large surface area and good stability. However, applications of this material in catalysis are still limited. 5-Hydroxymethylfurfural (HMF) has been considered a renewable chemical platform for the production of liquid fuels and fine chemicals. Phosphotungstic acid, H₃PW₁₂O₄₀ (PTA), encapsulated in MIL-101 is evaluated as a potential catalyst for the selective dehydration of fructose and glucose to 5-hydroxymethylfurfural. The results demonstrate that PTA/MIL-101 is effective for HMF production from fructose in DMSO and can be reused. This is the first example of the application of a metal–organic framework in carbohydrate dehydration.