Surface OH Groups Research Articles

Influence of Temperature on the Shape Evolution of IrO2-SiO2 coating and Its Electrochemical Activity for Oxygen Evolution

The surface structure of IrO2-SiO2/Ti anode was modified by thermal decomposition of non-aqueous solution of iridium chloride (IrCl3) and tetraethoxysilane (TEOS) with different calcination temperatures. The surface morphology and microstructure of these anodes were characterized by scanning electron microscope (SEM), X-ray diffraction (XRD) and X-ray-photoelectron spectroscopy (XPS). At low calcination temperature, the surface was dominated by biphase solid solution, in which the IrO2-rich phase, spiculate IrO2-SiO2 crystal, was encircled by the SiO2-rich phase, the amorphous SiO2­-IrO2 solution. As the calcination temperature increase, more amorphous IrO2 from the SiO2-rich phase converted into rutile nanothorn, then into nanoparticle composed by fine crystallites nanosheet aggregated on the surface of coatings and grew larger constantly. The electrochemical behaviors of the oxygen evolution reaction (OER) were evaluated by cyclic voltammetry (CV) and quasi steady-state polarization. The results showed that the electrocatalytic activities and the electrochemically active sites on the surface of the oxide electrodes for OER decreased with the increase of calcination temperature. This could be explained by the decrease of amorphous IrO2 and the surface active sites, such as surface OH groups and crystal edges. The onset potentials of IrO2-SiO2/Ti anode were significantly affected by the calcinations temperature. The onset potential of amorphous film was shifted toward the negative side from that of the crystalline ones. The Tafel slopes indicated that the electron charge transfer process is the rate determining step of IrO2-SiO2/Ti electrodes for the OER.

Enhancing the photoelectrochemical water splitting activity of rutile nanorods by removal of surface hydroxyl groups

The effect of the surface hydroxyl (OH) groups on the photoelectrochemical (PEC) water splitting performance of rutile nanorod is examined. Based on the results of characterization, the surface OH groups are demonstrated to serve as the recombination centers for the photo-generated charges and to hinder the charge transfer at the interfaces. H2O2 thermal and anneal treatments are applied to reduce the OH group surface coverage and both are proved effective. With reducing the relative surface OH coverage from 47 to 8%, the photocurrent density is increased from 0.419 to 0.642mAcm−2 at 0V (vs. Ag/AgCl), achieving 53.2% improvement. Compared with the thermal treatment, H2O2 treatment is more effective and energy-efficient. This work highlights that the surface property of the material itself is an important factor in the design of the PEC electrodes for water splitting.

Selective Photocatalytic CC Coupling of Bioethanol into 2,3‐Butanediol over Pt‐Decorated Hydroxyl‐Group‐Tunable TiO2 Photocatalysts

Abstract2,3‐Butanediol (2,3‐BD) was synthesized through TiO2‐photocatalytic CC coupling of bioethanol synchronously with the liberation of an energy H2 molecule in an anaerobic atmosphere. It was found that the selectivity of 2,3‐BD is controlled by the amount of .OH. The less the .OH, the higher the 2,3‐BD selectivity. Furthermore, it was revealed that the amount of .OH increases with the increasing of the surface OH groups on TiO2 photocatalyst. The introduction of water is in favor of the CC coupling pathway. This can be attributed to the stronger interaction between water and TiO2, which is beneficial to recovering the OH groups and promoting the desorption of .CH(OH)CH3 intermediates, thus suppressing the thermodynamically favorable overoxidation of .CH(OH)CH3 into acetaldehyde and promoting the CC coupling into 2,3‐BD. Based on the findings, the 2,3‐BD selectivity was greatly enhanced from approximately 2.6 % to approximately 65 % over Degussa P25‐TiO2 photocatalyst through fluorine substitution of surface OH groups.

Exploring Furfural Catalytic Conversion on Cu(111) from Computation

The full potential energy surface of the catalytic conversion of furfural to 2-methylfuran on the Cu(111) surface has been systematically computed on the basis of density functional theory, including dispersion and zero-point energy corrections. For furfuryl alcohol formation, the more favorable step is the first H addition to the carbon atom of the C═O group, forming an alkoxyl intermediate (F-CHO +H → F-CH2O); the second H atom addition, leading to furfuryl alcohol formation (F-CH2O + H → F-CH2OH), is the rate-determining step. For 2-methylfuran formation from furfuryl alcohol dissociation into surface alkyl (F-CH2) and OH groups, H2O formation is the rate-determining step (OH + H → H2O). Our results explain perfectly the experimentally observed selective formation of furfuryl alcohol and the equilibrium of furfural/furfuryl alcohol conversion under hydrogen-rich conditions as well as the effect of H2O suppressing furfural conversion. In addition, it is found that dispersion correction (PBE-D3) overesti...

Hydrothermally Synthesized CuO Powders for Photocatalytic Inactivation of Bacteria

Various morphologies of monoclinic CuO powders were synthesized by hydrothermal treatment of copper nitrate, copper acetate or copper sulfate. The synthesized samples were characterized by scanning electron microscopy, X-ray di ractometry, the Fourier transform infrared spectroscopy, and di use re ectance spectrophotometry. Antibacterial activity of the samples was studied against Escherichia coli bacteria in dark and under visible light irradiation. Although the di erent precursors yielded the same band gap energies (≈1.6 eV) for the synthesized CuO samples, they resulted in various morphologies (hierarchy of stabilized micro/nanostructures), speci c surface areas, concentrations of OH-surface groups, and visible light photocatalytic performances. The CuO nanorods synthesized from nitrate hydrothermal bath not only exhibited a considerable e ective surface area, but also showed the highest concentration of absorbed OH-groups and subsequently, the strongest (photo)catalytic antibacterial properties (≈37 and 94% inactivation of the bacteria in dark and under visible light irradiation, respectively).

Synthesis and characterization of robust zero valent iron/mesoporous carbon composites and their applications in arsenic removal

Nanoscale zero valent iron particles (nZVI) have been developed by in situ reduction of Fe3+ ions onto a mesoporous type of carbon matrix – Starch-Derived Mesoporous Carbonaceous Material previously reported and marketed commercially as “Starbon”. The obtained nZVI/Starbon hybrid material exhibits homogeneous distribution of nZVI (10–20nm) within the carbon matrix, surface area of 141m2/g and a total iron loading of 1mmol per gram of the composite, in accordance with transmission electron microscopy (TEM), X-ray diffraction (XRD), N2 adsorption–desorption measurements, Infrared (IR)/Raman spectroscopy and Thermal gravimetric (TG)/Differential Thermal analysis (DTA). Electron Paramagnetic Resonance (EPR) and proton binding measurements show that the nanoparticles have a core–shell structure with iron(III) oxide/hydroxide shell due to partial air-oxidation of nZVI and the composite exhibits four different types of proton binding groups. Most importantly, the nZVI/Starbon hybrid has been tested as absorbent for As(III) removal showing a total removal of 358μmol (26.8mg) of As(III) per gram of the composite at pH=7. We also discuss the principal role of surface OH groups of iron oxide in arsenic uptake and the crucial effect of pH on removal efficiency.

New insight into electrochemical-induced synthesis of NiAl2O4/Al2O3: Synergistic effect of surface hydroxyl groups and magnetism for enhanced adsorptivity of Pd(II)

A new promising adsorbent, Ni supported on γ-Al2O3 was prepared in a simple electrolysis system (Ni/Al2O3-E) in minutes and was compared with the sample prepared by a physical mixing method (Ni/Al2O3-PM). The adsorbents were characterized by XRD, TEM, FTIR, 27Al MAS NMR, XPS, and VSM. The results showed that besides NiO nanoparticles, a NiAl2O4 spinel was also formed in Ni/Al2O3-E during the electrolysis via the dealumination and isomorphous substitution of Ni2+ ions. In contrast, only agglomerated NiO was found in the Ni/Al2O3-PM. Adsorption test on removal of Pd2+ ions from aqueous solution showed that the Pd2+ ions were exchanged with the hydrogen atoms of the surface–OH groups of both adsorbents. Significantly, the Ni/Al2O3-E demonstrated a higher adsorption towards Pd2+ ions than Ni/Al2O3-PM due to its remarkably higher degree of magnetism, which came from the NiAl2O4. The use of 0.1gL−1 Ni/Al2O3-E gave the maximum monolayer adsorption capacity (qm) of 40.3mgg−1 at 303K and pH 5. The Ni/Al2O3-E showed high potential for simultaneous removal of various noble and transition metal ions and could be also used repetitively without affecting the high adsorptivity for Pd2+ ions. This work may provide promising adsorbents for recovery of various metals as well as other materials for such related applications.

Electrocatalytic Oxidation of Ethanol and Formic Acid on Bimetallic Nanoalloys and Core-Shell Nanoparticles

Mechanism of ethanol electrooxidation on numerous materials has been widely investigated in the past. It is well known that in acidic conditions the electrooxidation of ethanol on platinum surface leads to strongly adsorbed surface groups (i.e. CO), poisoning the surface of catalyst and inhibiting further reactions. Presence of metals other than platinum will make it possible to oxidize adsorbed CO by surface OH groups, present on the other metal (so called “bi-functional mechanism”). Such bi-metallic surface is more resistant to poisoning and as a result more catalytically active. Hovewer it is still not well understood, how the presence of the other metal influences the surface’s electronic properties, which in turn alter adsorption properties, and reaction mechanism. Electronic properties of the catalyst can be modified by the following methods: they can be related directly to charge transfer from other component of the system, such as another component of the alloy, from substrate, or they can be induced by geometric factors, such as change in lattice parameter. Change in lattice parameter influences the degree of overlapping of valence orbitals forming the conduction band. When lattice is contracted, the conduction band becomes broader. When the conduction band is filled in more than half the broadening of the conduction band leads to shift of the d-band center towards Fermi edge. The lattice parameter can be (to some degree) modified, and as a result, the degree of orbital overlapping can be changed, for instance by alloying. It is known that in alloys the atom-atom distance changes linearly as a function of composition, in the range determined by inter-atomic distances of the pure elements (Vegard’s law). Similarly, when thin layers are formed, the layer forming metal will exhibit the lattice parameter of the substrate (so called “pseudomorphic monolayer”). Consequently experimental investigation of materials, where lattice parameter has been intentionally modified, allows for experimental determination of the relation between reaction mechanism and electronic properties of the material used.

Tuning of product selectivity in the conversion of ethanol to hydrocarbons over H-ZSM-5 based zeolite catalysts

Conversion of ethanol to hydrocarbons was investigated over H-ZSM-5 zeolite and its modification by addition of phosphorus, iron and nickel. Reaction is realized in a tubular flow reactor at atmospheric pressure. Catalyst characterization was performed by UV–vis, TG–DTA, XRD, NH3-TPD, and IR spectroscopy of the surface OH groups and of adsorbed pyridine. High P content enhances the selectivity to lower olefins, in particular ethylene, as also occurs in the case of Fe addition. Over Ni-modified zeolites, C4 and aromatic production is promoted. The role of acidity on the conversion path is briefly discussed.

Role of chemical hardness in the adsorption of hexavalent chromium species onto metal oxide nanoparticles

We employed chemical hardness to explain the adsorption behaviour of Cr(VI) anions on metal oxide sorbents. In the absence of electrostatic attraction at pH values around the point of zero charge, silica and titania nanoparticles adsorbed a very small amount of Cr(VI) anions. This lack of adsorption is because the magnitude of the chemical hardness is too high to pair with the less hard Cr(VI) anions. In contrast, maghemite nanoparticles with relatively weak values of hardness adsorbed 0.021mol of Cr(VI) per one mole of OH groups on the particle surface, 2.7 times that of titania. The magnitude of the chemical hardness decreases in the order of SiO2≫TiO2>γ-Fe2O3, which is determined by the hardness of constituting metal cations. This result indicated that the adsorption onto an oxide surface is governed by metal cations in that their chemical hardness determines the basicity of surface OH groups, which regulates adsorption as well as protonation/deprotonation. The hardness match between a sorbent and a sorbate may be interpreted as their chemical affinity. We demonstrated that the chemical affinity is the principal driving force for adsorption of Cr(VI) anions onto the oxides.

Photocatalytic activity of nanostructured ZnO–ZrO2 binary oxide using fluorometric method

Evaluation of the photocatalytic activity of ZnO–ZrO2 nanomaterials using fluorescence based technique has rarely been reported. In the present work, ZnO–ZrO2 mixed oxides coupled with various ZnO dosages (0, 10, 30, 50, 70wt%) were prepared by impregnation method. These nanomaterials were characterized by studying their structural, surface and optical properties. The photocatalytic activity in term of quantitative determination of the active oxidative species (OH) produced on the surface of binary oxide was evaluated using fluorescent probe method. The interaction between ZnO and ZrO2 was affected on the photocatalytic efficiency of mixture. The results show that, the addition of ZnO to ZrO2 decreased the electron–hole recombination and increased the rate of OH radicals formation. 50wt% ZnO–ZrO2 photocatalyst exhibited much higher photocatalytic activity. The profound effect of binary oxide catalyst was generally considered due to the high surface area, small particle size, high monoclinic phase of ZrO2 content, low band gap and the presence of surface OH groups.

Solar Hydrogen Production by Plasmonic Au–TiO2 Catalysts: Impact of Synthesis Protocol and TiO2 Phase on Charge Transfer Efficiency and H2 Evolution Rates

The activity of plasmonic Au–TiO2 catalysts for solar hydrogen production from H2O/MeOH mixtures was found to depend strongly on the support phase (anatase, rutile, brookite, or composites thereof) as well as on specific structural properties caused by the method of Au deposition (sol-immobilization, photodeposition, or deposition–precipitation). Structural and electronic rationale have been identified for this behavior. Using a combination of spectroscopic in situ techniques (EPR, XANES, and UV–vis spectroscopy), the formation of plasmonic Au particles from precursor species was monitored, and the charge-carrier separation and stabilization under photocatalytic conditions was explored in relation to H2 evolution rates. By in situ EPR spectroscopy, it was directly shown that abundant surface vacancies and surface OH groups enhance the stabilization of separated electrons and holes, whereas the enrichment of Ti3+ in the support lattice hampers an efficient electron transport. Under the given experimental c...

Synthesis of mesoporous cerium–zirconium binary oxide nanoadsorbents by a solvothermal process and their effective adsorption of phosphate from water

Cerium–zirconium binary oxide nanoparticles were synthesized by a simple and low-cost solvothermal process without templates, and their phosphate removal performance was investigated in batch studies. Through careful modulation of the Ce/Zr ratio, Ce/Zr binary oxide nanoparticles were created with different structure, crystal size, surface properties, and phosphate adsorption performance. It was found that Ce0.8Zr0.2O2 nanoparticles had the best phosphate adsorption performance among these Ce/Zr binary oxide nanoparticles, which could be attributed to their desirable structure and surface properties. The phosphate adsorption capacity of Ce0.8Zr0.2O2 nanoparticles was determined at ∼112.23mg/g, the highest among various adsorbents reported in literature. The adsorption of phosphate onto Ce0.8Zr0.2O2 nanoparticles followed the inner-sphere complexing mechanism, and the surface OH groups played the major role. Commonly coexisting anions showed no or moderate effect on the phosphate adsorption onto Ce0.8Zr0.2O2 nanoparticles, and little pH dependence was observed for their phosphate adsorption in the range from pH 2 to 6. After adsorption, phosphate adsorbed on these Ce0.8Zr0.2O2 nanoparticles could be easily desorbed by NaOH solution washing.

On the mechanism of the preferential oxidation of carbon monoxide over Cu n Pd (n = 3–12) catalysts

Preferential oxidation of CO (CO-PROX) is an important practical process for the purification of H2 for use in proton exchange membrane fuel cells. In order to clarify the mechanism of CO-PROX, the reaction promoted by CunPd (n = 3–12) clusters has been studied by density functional theory calculations. The CO-PROX reaction via carboxylic and hydroxyl intermediates is found to be the most likely mechanism. The theoretical results show that CO oxidation is promoted by the attack of surface OH groups, which is different from the usual oxidation of CO with O2. The Cu6Pd cluster is predicted to have a lower activation barrier compared with other CunPd clusters and is therefore proposed as the most effective nanocatalyst. To gain insights into the high catalytic activity of the CunPd nanoparticles, the nature of the interaction between adsorbate and substrate has been analyzed by the detailed electronic local density of states. The results should be helpful in developing efficient catalysts for the CO-PROX reaction.

Dehydration of ethanol over zeolites, silica alumina and alumina: Lewis acidity, Brønsted acidity and confinement effects

Ethanol dehydration was investigated over commercial H-FER, H-MFI, H-MOR, H-BEA, H-Y and H-USY zeolite samples, and alumina and silica alumina for comparison. The catalysts were characterized using FT-IR spectroscopy of the surface OH groups and of adsorbed CO and pyridine. UV–vis, Raman and TG-DTA were applied to characterize coke, formed more on H-MOR and H-BEA. H-zeolites are definitely more active than silica alumina and alumina on catalyst weight base. The H-MOR sample is the most active but the H-MFI samples with Si/Al2 ratios 280 and 50 show higher reaction rates per Al ion, H-FER and faujasites show highest ethylene yield (99.9% at 573K). At lower temperature and higher space velocities, diethyl ether is formed with high yield (>70% at 453–473K on H-BEA and H-MFI (50)). Overconversion of ethylene mainly to aromatics is observed on H-MFI (50). The different behaviour of protonic zeolites can predominantly be explained by confinement effects on the different zeolite cavities.

Nanoniobia modification of CdS photoanode for an efficient and stable photoelectrochemical cell.

Herein we report the surface modification of a CdS film by niobia nanoparticles via thioglycerol as an organic linker and thus fabricate an efficient and a stable photoanode for a photoelectrochemical (PEC) cell. We have synthesized three differenly sized (∼3, ∼6 ,and ∼9 nm) niobia nanoparticles by a hydrothermal synthesis approach and have further investigated the particle-size-dependent PEC performance of the nanoparticle-modified CdS photoanode. Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) confirm the formation of Nb2O5 nanoparticles that are prepared via decomposition of the niobium peroxo complex during the hydrothermal reaction and reveal the presence of surface OH(-) groups over niobia nanoparticles that impart a high catalytic property to a material. The nano-Nb2O5-modified photoanode displayed a 23-fold higher power conversion efficiency compared to that of CdS. This modified structure increases the open circuit voltage (OCV) from 0.65 to 0.77 V, which is attributed to the nano-Nb2O5-induced surface passivation effect over bare CdS. Linking of nanoparticles on the CdS surface improves the photocorrosion stability of the CdS photoanode for even longer than 4 h in contrast to the tens of minutes for the base CdS surface. The uniform coverage of the CdS photoanode surface by niobia nanoparticles is thus found to be the controlling parameter for achieving a higher PEC performance and stability of the photoanode. This finding directed us to design an improved CdS photoanode for efficient and prolonged PEC hydrogen generation from a PEC cell.

Molecular catalysis for the steam reforming of ethanol

In this paper, the application of molecular catalysis for steam reforming of ethanol (SRE) is reviewed. Eight metals (Ni, Co, Cu, Pt, Rh, Pd, Ir and Ru) have shown high catalytic activity for SRE. Among them Ni and Rh are very promising because of high d character in the metal bond and low metal-oxygen bonding (vs. metal-carbon). They can effectively promote C-C bond cleavage in the rate-determining process during SRE. However, Rh is weak in water-gas-shift so that CH4 and CO become the main by-products at low reaction temperatures, while Ni catalysts suffer from rapid deactivation due to coking and sintering. Two low-temperature CO-free catalysts have been developed in our lab, namely Rh-Fe/Ca-Al2O3 and carbonyl-derived Rh-Co/CeO2, in which the presence of iron oxide or Co can promote water-gas-shift reaction and significantly improve the SRE performance. On the other hand, adding 3 wt% CaO to Ni/Al2O3 can greatly improve the catalyst stability because the Ca modification not only increases Ni concentration on the Ni/Ca-Al2O3 surface and 3d valence electron density, but also facilitates the water adsorption and coke gasification via water-gas-shift. The availability of abundant surface OH groups helps the formation and conversion of adsorbed formate intermediate. Hence, ethanol reaction on Ca-Al2O3-supported Ni, Pt, Pd and Rh catalysts are found to follow the formate-intermediated pathway, a new reaction pathway alternative to the traditional acetate-intermediated pathway.

Synthesis of surfactant-assisted FeVO4 nanostructure: Characterization and photocatalytic degradation of phenol

Pure triclinic FeVO4 catalysts were synthesized by hydrothermal method and modified with Hexadecyltrimethylammonium Bromide (HTAB), Sodium Dodecyl Sulfate, Polyethylene Glycol (PEG) as surfactant. The materials were characterized by XRD, BET, FT-IR, DRS and SEM techniques. These catalysts were used for the photocatalytic degradation of phenol (Ph) under natural solar light irradiation. The effect of preparation parameters on the photocatalytic activity was also examined. It is found that the surfactant and pH value a significant influence on the particle morphology and even the crystalline structure of the product. Surfactant added products resulted an improve efficiency of the triclinic FeVO4. With HTAB as surfactant at an hydrothermal temperature of 75°C and the pH 3 value of the precursor solution the highest percentage of phenol degradation (100%) and highest reaction rate (1.24mgl−1min−1) were obtained in 45min. It is concluded that the excellent photocatlytic activity of the HTAB-assisted FeVO4 catalyst, with rod-like shaped morphology, was associated its small particle size, large surface area, the presence of more surface OH groups and lower band gap energy than pure FeVO4.

A spectroscopic investigation for the adsorption of 4-nitrophenol onto synthetic zeolites

In this study, the characterization and environmental applications of faujasite type zeolites (NaX, NaY) are investigated using different instruments. Vibrational spectroscopy is used to a large extent in this research to characterize the zeolite materials and after the adsorption process. 4-nitrophenol was adsorbed on NaX and NaY zeolite and adsorption process was investigated by vibrational spectroscopy and scanning electron microscopy. Vibrational spectroscopic results showed that source of the adsorption is weak hydrogen bonding between zeolite surface silanol groups and OH group of 4-nitrophenol. This study was made to remove phenolic organics as possible substitutes for synthetic zeolites. SEM studies revealed the morphological observations of the unloaded zeolites and thereby changes in the adsorbed zeolites.

How Surface Hydroxyls Enhance MgO Reactivity in Basic Catalysis: The Case of Methylbutynol Conversion

This combined experimental and theoretical study aims at understanding why surface hydroxyl groups may enhance catalytic reactivity of MgO surfaces in basic catalysis, whereas hydroxyls are weakly deprotonating groups. We investigated that reactivity enhancement in the catalytic conversion of 2-methyl-but-3-yn-2-ol (MBOH). Reaction kinetics was experimentally determined on partially hydroxylated MgO: active sites were saturated with the reactant and catalyzed its conversion with an activation energy of 85 kJ·mol–1. Reaction pathways were calculated over fully hydroxylated, partially hydroxylated, and dehydroxylated MgO surfaces by means of first-principles simulations. To highlight the effect of reactant coverage, we also calculated the reaction pathway on MgO precovered with MBOH molecules. The results show that the surface OH groups generated by dissociative adsorption of water induce a lowering in the activation energy barriers when they keep a bare Mg2+-O2– pair available in the vicinity for MBOH to adsorb and react. Interestingly, OH groups do not directly interact with MBOH converting on the surface, but they modify the basic properties of the vicinal bare Mg2+-O2– pair on which MBOH adsorbs and converts. A similar effect is predicted when MBOH converts on a bare Mg2+-O2– pair in the vicinity of a second adsorbed MBOH molecule. The beneficial effect of coadsorbates on the reactivity of active sites is analyzed by means of a thermodynamic model and an electronic structure analysis.