Surface Hydroxide Groups Research Articles

Construction of high-performance NiCe-MOF derived structured catalyst for steam reforming of biomass tar model compound

High-performance and inexpensive catalysts play a large role in effective removal of biomass tar produced during biomass gasification. In this study, raw wood, with long, through, but distorted channels and a low tortuosity, was selected as a support. A layered NiCe-metal organic framework (NiCe-MOF) was grown in-situ on the surface of raw wood microchannels by using abundant surface hydroxide groups. Then, this catalyst was carbonized at 600 °C in a N2 atmosphere to obtain NiCe-MOF derived catalyst/wood carbon (NiCe-MDC/WC), which was selected as a structured reactor for the steam reforming of biomass tar. NiCe-MDC/WC achieved an excellent conversion rate of approximately 99% for toluene and a high catalytic stability of 48 h at low temperature of 550 °C. Moreover, NiCe-MDC/WC showed higher catalytic performance than Ni-MDC/WC (∼79%), crushed-NiCe-MDC/WC (∼94%), and Ni/WC (∼75%) in stability tests. These excellent results were assumed to be derived from the multilevel structure obtained from wood carbon microchannels and secondary layered MOF channels, appropriate metal-support interactions, and the presence of Ce, which could improve the dispersion of active sites and mass transfer efficiency and inhibit coke formation. Thus, such Ni-based MOF-derived structured reactors are promising for tar conversion and useful syngas production.

Mesoporous Prussian Blue as the highly effective and selective catalyst for CO2 conversion into cyclic carbonates under mild conditions

Developing low-cost, effective, and environmentally friendly heterogeneous catalysts for CO2 conversion into high-value-added products is a crucial task. In this study, Ferric ferrocyanide (Fe4[Fe(CN)6]3), the typical Prussian Blue (FeFe-PB) nanoparticles functionalized with hydroxide groups, is found to be the superior catalyst for CO2 and epoxide coupling to form cyclic carbonates with 100% selectivity and 99% yield under mild reaction conditions (1.0 MPa, 90 °C). A series of characterizations techniques showed that the FeFe-PB structure possesses excellent thermal and chemical stability and a high mesoporous surface area. The hydroxide group functionalized nanoparticles could preserve 97% of their initial activity after six reaction rounds. Additionally, we investigated the origin of epoxides activation with heterogeneous Lewis acid sites and surface hydroxide groups via hydrogen-bond interaction at the solid/liquid interface. These findings demonstrate a simple, ecologically safe, versatile, and efficient route for CO2 conversion into high-value chemicals by the FeFe-PB catalytic system.

Self-supported amorphous iridium oxide catalysts for highly efficient and durable oxygen evolution reaction in acidic media

Developing highly active and stable electrocatalysts for acidic oxygen evolution reaction is still a great challenge. Herein, we developed a novel self-supported amorphous IrOx catalyst (IrOx/Ti) encapsulated with a small number of crystalline nanoparticles for efficient acidic OER. The resulting IrOx-50-AC/Ti electrode exhibits a superior OER performance with an overpotential of 250 mV at 10 mA cm−2 and Tafel slope of 50.4 mV dec−1 in 0.1 M HClO4. High contents of IrIII species and surface hydroxide groups, hierarchically porous structure and high defective density contribute to boost the OER activity. Importantly, the amorphous IrOx-50-AC/Ti also demonstrates a super long-term durability for 190 h due to the strong interaction between IrOx film and Ti. The unique amorphous-crystalline structure also enhances the stability and the activity by promoting the synergy between amorphous phase and crystalline phase. This work provides some valuable insights on developing efficient Ir-based catalysts for acidic water splitting.

Carboxylate-assisted synthesis of highly-defected monoclinic zirconia nanoparticles

Zirconia (ZrO2) nanoparticles were prepared by hydrothermal route using zirconium oxychloride (ZrOCl2) as a single precursor with different sodium carboxylates as mineralizers. The structure, morphology and defects of the nanoparticles were characterized by XRD, HRTEM and ATR-FTIR spectroscopy. X-ray diffraction studies revealed that the as-synthesized zirconia was pure monoclinic ZrO2 phase. The crystallite sizes were in the range 8.9–19.5 nm. The defects and microstrains of ZrO2 nanoparticles were greatly influenced by the type of sodium carboxylate. Apparently, there was a direct correlation between the stability constant of Zr(IV)-carboxylate complex and the amount of defects in the produced ZrO2. Sodium tartrate produced semicrysalline ZrO2 nanoparticles with microstarin that were 5 times greater than that produced without any carboxylate salt. ZrO2 nanoparticles with increased defects and microstarins were found to be rich with surface hydroxide groups. Designing monoclinic ZrO2 with controlled defects and degree of hydroxylation may have a great impact in heterogonous organic catalysis.

Fluoride-capped nanoceria as a highly efficient oxidase-mimicking nanozyme: inhibiting product adsorption and increasing oxygen vacancies.

Nanozymes aim to mimic enzyme activities using nanomaterials. Nanoceria (CeO2 nanoparticles) is an important model nanozyme for its rich redox chemistry. In particular, its oxidase-like activity allows oxidation reactions without the need of unstable and toxic H2O2. Fluoride can significantly improve its oxidase-like activity, and this work aims to understand the mechanism of fluoride-promoted catalysis. First, fluoride can adsorb on CeO2 tighter than other halides, but not as strong as phosphate as characterized by isothermal titration calorimetry (ITC). FT-IR spectroscopy indicates adsorption of fluoride likely via exchange with surface hydroxide groups. Fluoride capping inverses the surface charge of CeO2, facilitating desorption of the ABTS oxidation product, significantly increasing the turnover number. The Raman, EPR and XPS spectroscopy results demonstrate that the concentration of Ce3+ and the accompanying oxygen vacancy significantly increased upon adding F-, which can explain the enhanced catalytic activity. Finally, the electron transfer properties of fluoride-capped CeO2 were more efficient than that of the bare CeO2 as determined by a direct electrochemical measurement on a glass carbon electrode. This study has provided new insight into nanoceria, and can also further confirm the role of nanoceria as a model for engineering the surface of nanozymes.

Probing the Reactivity of Pt/Ceria Nanocatalysts toward Methanol Oxidation: From Ionic Single-Atom Sites to Metallic Nanoparticles

Single-atom catalysts represent the ultimate extreme in heterogeneous catalysis for the maximum dispersion of mononuclear catalytic metal particles on supporting surfaces. Ultralow Pt loading has been achieved on nanostructured ceria surfaces that allow for stabilizing metallic and ionic Pt sites that are anchored at surface defects. Here, we assess the chemical reactivity of these different Pt species, which are experimentally known to coexist on Pt–ceria nanocatalysts, by taking methanol oxidation as a chemical probe. Our density functional theory calculations demonstrate that Pt2+ and Pt4+ single-ion species do not promote methanol oxidation by themselves. Instead, metallic sites of supported sub-nanometer Pt particles are always required to promote the oxidation reaction. Our finding generalizes the conclusions of recent photoemission experiments in the context of H2 oxidation by ceria/Pt nanocatalysts. Moreover, the simulations predict that surface hydroxide groups may act as cocatalyst for the direc...

Hydrogen-Bond Relations for Surface OH Species

This paper concerns thin water films and their hydrogen-bond patterns on ionic surfaces. As far as we are aware, this is the first time H-bond correlations for surface water and hydroxide species are presented in the literature while hydrogen-bond relations in the solid state have been scrutinized for at least five decades. Our data set, which was derived using density functional theory, consists of 116 unique surface OH groups–intact water molecules as well as hydroxides–on MgO(001), CaO(001) and NaCl(001), covering the whole range from strong to weak to no H-bonds. The intact surface water molecules are found to always be redshifted with respect to the gas-phase water OH vibrational frequency, whereas the surface hydroxide groups are either redshifted (OsH) or blueshifted (OHf) compared to the gas-phase OH– frequency. The surface H-bond relations are compared with the traditional relations for bulk crystals. We find that the “ν(OH) vs R(H···O)” correlation curve for surface water does not coincide with ...

Behaviour of the surface hydroxide groups of exfoliated kaolinite in the gas phase and during water adsorption.

The chemical and physical properties, and thus the reactivity of phylloaluminosilicates can be tailored by intercalation, delamination, and exfoliation processes. In going from the periodic crystalline to the molecular exfoliated phase, surface defects and modifications gain importance as each face of the phylloaluminosilicate comes in direct contact with the external chemical environment. In this work, we extend our earlier studies on the molecular cluster modelling of exfoliated kaolinite sheets by evaluating the positions and orientations of surface hydroxide groups and bridging oxide anions, as the sites of reactivity. The previous focus on the inner chemical environment of a single kaolinite layer is shifted to the surface exposed octahedral aluminium-hydroxide and tetrahedral silicon-oxide sheets. The combination of semi-empirical, ab initio wave function, and density functional calculations unanimously support the amphoteric nature of the surface hydroxide groups with respect to H-bonding donor/acceptor capabilities. To a lesser extent, we observe the same for the bridging oxide anions. This is in contrast to the crystalline phase, which manifests only donor orientation for maintaining an inter-layer H-bond network. These results suggest that both electrophilic and nucleophilic characteristics of the octahedral and tetrahedral sheets need to be considered during intercalation and concomitant exfoliation of the kaolinite sheets.

Illuminating CO2 reduction on frustrated Lewis pair surfaces: investigating the role of surface hydroxides and oxygen vacancies on nanocrystalline In2O(3-x)(OH)y.

Designing catalytic nanostructures that can thermochemically or photochemically convert gaseous carbon dioxide into carbon based fuels is a significant challenge which requires a keen understanding of the chemistry of reactants, intermediates and products on surfaces. In this context, it has recently been reported that the reverse water gas shift reaction (RWGS), whereby carbon dioxide is reduced to carbon monoxide and water, CO2 + H2 → CO + H2O, can be catalysed by hydroxylated indium oxide nanocrystals, denoted In2O(3-x)(OH)y, more readily in the light than in the dark. The surface hydroxide groups and oxygen vacancies on In2O(3-x)(OH)y were both shown to assist this reaction. While this advance provides a first step toward the rational design and optimization of a single-component gas-phase CO2 reduction catalyst for solar fuels generation, the precise role of the hydroxide groups and oxygen vacancies in facilitating the reaction on In2O(3-x)(OH)y nanocrystals has not been resolved. In the work reported herein, for the first time we present in situ spectroscopic and kinetic observations, complemented by density functional theory analysis, that together provide mechanistic information into the surface reaction chemistry responsible for the thermochemical and photochemical RWGS reaction. Specifically, we demonstrate photochemical CO2 reduction at a rate of 150 μmol gcat(-1) hour(-1), which is four times better than the reduction rate in the dark, and propose a reaction mechanism whereby a surface active site of In2O(3-x)(OH)y, composed of a Lewis base hydroxide adjacent to a Lewis acid indium, together with an oxygen vacancy, assists the adsorption and heterolytic dissociation of H2 that enables the adsorption and reaction of CO2 to form CO and H2O as products. This mechanism, which has its analogue in molecular frustrated Lewis pair (FLP) chemistry and catalysis of CO2 and H2, is supported by preliminary kinetic investigations. The results of this study emphasize the importance of engineering the surfaces of nanostructures to facilitate gas-phase thermochemical and photochemical carbon dioxide reduction reactions to energy rich fuels at technologically significant rates.

Low capping group surface density on zinc oxide nanocrystals.

The ligand shell of colloidal nanocrystals can dramatically affect their stability and reaction chemistry. We present a methodology to quantify the dodecylamine (DDA) capping shell of colloidal zinc oxide nanocrystals in a nonpolar solvent. Using NMR spectroscopy, three different binding regimes are observed: strongly bound, weakly associated, and free in solution. The surface density of bound DDA is constant over a range of nanocrystal sizes, and is low compared to both predictions of the number of surface cations and maximum coverages of self-assembled monolayers. The density of strongly bound DDA ligands on the as-prepared ZnO NCs is 25% of the most conservative estimate of the maximum surface DDA density. Thus, these NCs do not resemble the common picture of a densely capped surface ligand layer. Annealing the ZnO NCs in molten DDA for 12 h at 160 °C, which is thought to remove surface hydroxide groups, resulted in a decrease of the weakly associated DDA and an increase in the density of strongly bound DDA, to ca. 80% of the estimated density of a self-assembled monolayer on a flat ZnO surface. These findings suggest that as-prepared nanocrystal surfaces contain hydroxide groups (protons on the ZnO surfaces) that inhibit strong binding of DDA.

Effect of interaction between Ni and YSZ on coke deposition during steam reforming of methane on Ni/YSZ anode catalysts for an IR-SOFC

Ni cermet anodes were prepared from nanoparticle 8 mol% yttria-stabilized zirconia (8YSZ) with various crystallite sizes. Results of temperature-programmed reduction of the anode suggested strong interaction between Ni and 8YSZ with a small crystallite size. The performance of an internal reforming solid oxide fuel cell (IR-SOFC) with anodes prepared from 8YSZ was almost the same regardless of interaction between Ni and 8YSZ. However, the rate of coke deposition decreased with an increase in interaction between Ni and 8YSZ. This strong interaction was caused by the presence of surface hydroxide groups indicated by in situ FT-IR. Carbon deposition resistance was enhanced by controlling the interaction between Ni and 8YSZ.

XANES and XPS investigations of (TiO2)x(SiO2)1−x: the contribution of final-state relaxation to shifts in absorption and binding energies

The (TiO2)x(SiO2)1−x system (0 ≤ x ≤ 0.33) was synthesized by the sol–gel method and investigated by X-ray absorption near-edge spectroscopy (XANES) and X-ray photoelectron spectroscopy (XPS). The use of both hard (Ti K-edge) and soft (Ti L-edge) X-rays provides a useful way to monitor changes in the bulk and surface, respectively, of these amorphous materials. The average CN of both bulk-Ti and surface-Ti increases with greater x in the chemical formula, due to the larger ionic radius of Ti. Comparison of Ti K- and L-edge spectra of annealed samples revealed that Ti atoms at the surface have a higher average CN than in the bulk, likely due to the presence of surface hydroxide and water groups that can coordinate to the Ti centres. The O K-edge, Ti L-edge, and Si L-edge XANES absorption energies showed little to no change with Ti content, while the O 1s, Ti 2p, and Si 2p XPS BEs were found to decrease with increasing Ti content due to nearest-neighbour and next-nearest-neighbour effects, which lead to increased final-state relaxation. The degree of final-state relaxation is more significant than previously believed for these amorphous powders.

High temperature proton exchange membranes based on triazoles attached onto SBA-15 type mesoporous silica

Triazole shows rather high proton conductivity under anhydrous condition and high temperature, where the proton conduction arises from the transfer of protons between neighboring protonated and unprotonated guest molecules, i.e., intermolecular proton transfer between adjacent triazole groups. When triazole is incorporated into a polymeric matrix for proton exchange membranes, it may leach out gradually because of no bonds formed between the triazole and the polymeric matrix, leading to the decrease in the proton conductivity. Therefore, a proton conductive pathway is constructed through mesoporous silica SBA-15 by reacting benzotriazole-5-carboxylic acid with the surface hydroxide groups. SBA-15 has hydroxyl groups inside pores. Thus, triazole groups were attached to the inside of the nanopores of SBA-15, which was subsequently dispersed in Nafion membrane. The proton exchange membrane showed the proton conductivity of 8.52 × 10 −4 S/cm at low humidity of ∼10% and 140 °C. It also exhibited a thermal stability up to 200 °C. Therefore it was concluded that the new proton exchange membrane showed no leaching problem, by rather low proton conductivity, which should be increased further for practical applications for high temperature fuel cells.

Atomic Layer Deposition of NiO by the Ni(thd)2/H2O Precursor Combination

AbstractPolycrystalline nickel oxide is deposited on SiO2 substrates by alternating pulses of bis(2,2,6,6‐tetramethylheptane‐3,5‐dionato)nickel(II) (Ni(thd)2) and H2O. The deposition process shows atomic layer deposition (ALD) characteristics with respect to the saturation behavior of the two precursors at deposition temperatures up to 275 °C. The growth of nickel oxide is shown to be highly dependent on surface hydroxide groups, and a large excess of H2O is required to achieve saturation. Throughout the deposition temperature range the amount of carbon in the film, originating from the metal precursor ligand, is in the range 1–2%. Above 275 °C ALD growth behavior is lost in favor of thermal decomposition of the metal precursor. The initial nucleation process is studied by atomic force microscopy (AFM) and reveals nucleation of well‐separated grains which coalesce to a continuous film after about 250 ALD cycles.

Laser-induced luminescence associated with surface hydroxide groups in Al2O3

Laser-induced luminescence of OHs groups for undoped Al2O3 oxides of various phase compositions was excited by pulsed nitrogen laser radiation at 337.1 nm. The luminescence band at 500–650 nm assigned to hydroxide groups of Al2O3, actually, consists of several lines at 500–515, 553, 567, 577, 607, and 633 nm; these constituent bands can be assigned to various types of OHs surface groups. In the low-temperature phases of the γ→δ→θ-Al2O3 series, excitation at a wavelength of 337.1 nm gave rise to a characteristic luminescence band associated with surface hydroxide groups of Al2O3 that appeared at 770 nm.

Gold supported on mixed oxides for the oxidation of carbon monoxide

Abstract Gold catalysts (∼1%) supported on CeO 2 , on CeO 2 modified by inclusion of lanthanum (10%) or iron (5%), and on CeZrO 4 , have been prepared, characterised (N 2 physisorption, XRD) and tested for CO oxidation (200–300 K); stability tests for up to 75 h (Salford) and 24 h (Delft) have also been conducted. All three ‘modified’ catalysts show 10–20% conversion at ∼200–243 K, but lanthanum and zirconium modifiers lead to slightly decreased activity at 243–293 K. Au/La-CeO 2 and Au/Fe-CeO 2 in particular, however, show exceptional stability in life-tests at ambient temperature. DRIFTS results on fresh and used catalysts suggest that this may be due to both better retention of surface hydroxide groups and lesser formation of carbonate/bicarbonate ions near gold particles during reaction. Mechanisms to account for this behaviour are considered and hydroxyl groups acting catalytically may be responsible for stable activity.

Protection of Lithium Metal Surfaces Using Chlorosilanes

In this paper, we present a new approach for protecting metallic lithium surfaces based on a reaction between the thin native layer of lithium hydroxide present on the surface and various chlorosilane derivatives. The chemical composition of the resulting layer and the chemistry involved in layer formation were analyzed by polarization modulated infrared reflection absorption spectroscopy (PM-IRRAS), X-ray photoelectron spectroscopy (XPS), and energy dispersive X-ray analysis (EDX). Spectroscopy shows the disappearance of surface hydroxide groups and the appearance of silicon and chloride on the lithium surface. Differential scanning calorimetry (DSC) and electrochemical impedance spectroscopy (EIS) show that this surface treatment protects the lithium from certain gas-phase reactions and is ionically conductive.

Interaction of neptunyl with goethite (α-FeOOH), maghemite (γ-Fe2O3), and hematite (α-Fe2O3) in water as probed by X-ray photoelectron spectroscopy

The sorption behavior is studied and the physicochemical neptunium species existing on the surface of goethite (α-FeOOH), maghemite (γ-Fe2O3), and hematite (α-Fe2O3) are determined. Solvent extraction and X-ray photoelectron spectroscopy (XPS) are used to determine the neptunium surface species. The ion and elemental composition of the surface of the minerals and surface neptunyl NpO 2 + complexes is determined using these data. Compounds containing neptunium(IV) or neptunium(VI) ions do not appear; rather, neptunyl (Np(V)O 2 + group is complexed with surface hydroxide groups of α-FeOOH, γ-Fe2O3, and α-Fe2O3. Presumably, the oxygen atoms of iron oxides and water and/or carbonate (CO 3 2- ) or nitrate (NO 3 - ) group lie in the equatorial plane of the neptunyl (NpO 2 + ) group.

Structures of UO 2 and PuO 2 surfaces with hydroxide coverage

Atomic scale computer simulation is used to predict the energies and structures associated with surface hydroxide groups on the (1 0 0), (1 1 0) and (1 1 1) surfaces of UO 2 and PuO 2. The (1 0 0) surface is of particular interest since it is dipolar and therefore must be stabilised by a series of surface defects. Two different (1 0 0) defect terminations are considered, one a conventional series of surface layer defects, the other a partial reconstruction with (1 1 1) like facets. The latter is the most stable termination in the absence of hydroxide. In the presence of hydroxide this preference is reversed.

Effect of Surface Acidity of Al2O3 on Its Interactions with Lubricant Additives

The effect of the surface acidities of various aluminum oxides on the energy of adsorption of selective organic species has been studied by means of flow adsorption microcalorimetry. The strength of interaction of dioctyl sebacate (one typical lubricant additive) with the surface of Al2O3 depends on the amount of surface water and on the charge of surface hydroxide groups, with the positive and neutral charges being favorable. Temperature affects dioctyl sebacate adsorption by partially removing water molecules from the surface and exposing active sites, which then become available for adsorptive interactions. Al2O3 surfaces of various acidities have also been characterized using flow microcalorimetry and 1-butanol as a probe molecule. The adsorption capacity of 1-butanol on acidic Al2O3 is twice as much as that on the neutral and basic oxides.