Basic Hydroxyl Research Articles

Roles of Metal‐Organic Framework Supports in Base‐Free Oxidation of 5‐Hydroxymethylfurfural to Furan‐2,5‐Dicarboxylic Acid over Pt‐Based Catalysts

AbstractThe catalytic oxidation of 5‐hydroxymethylfurfural (HMF) is an important process for producing a renewable furan‐2,5‐dicarboxylic acid (FDCA). Although several Pt‐based catalysts have been reported to efficiently catalyze the oxidation of HMF, the functional roles of catalyst supports, as corroborated by spectroscopic techniques, have not been fully described. Here, we report on Pt nanoparticles supported on metal‐organic frameworks (MOFs) that catalyze the oxidation of HMF to FDCA in the absence of base. Several catalysts are prepared by immobilizing pre‐synthesized Pt nanoparticles on supports, allowing for the investigation of the role of the supports without the influence of the size of Pt nanoparticles (Pt NPs). By analyzing the catalytic performances along with the electronic properties using X‐ray photoelectron spectroscopy and surface properties of the catalysts using in situ infrared spectroscopy of adsorbed CO2, we find that the presence of Pt NPs in the metallic phase and basic hydroxyl groups on the support promote the HMF oxidation.

Optimizing the Incorporation Modes of TiO2 in TiO2-Al2O3 Composites for Enhancing Hydrodesulfurization Performance of Corresponding NiMoP-Supported Catalysts

TiO2-Al2O3 supports with different incorporation methods of titania were synthesized via three methods: impregnation (TA-I), co-precipitation (TA-CP), and co-precipitation–hydrothermal treatment (TA-HT). And the NiMoP catalysts prepared on the corresponding supports were evaluated for hydrodesulfurization (HDS) reactions. The results demonstrated that the Ti atoms in TA-I are attached to alumina through hydroxyl groups, while the Ti atoms in TA-CP and TA-HT can be dispersed in the alumina skeleton. Variations in the incorporation modes of TiO2 affect the support properties, consequently influencing the nature of the active metal on the supports. The Ti atoms dispersed in the Al2O3 skeleton allow an increase in the basic hydroxyl groups. Meanwhile, TiO2 in TA-CP and TA-HT can absorb hydrogen molecules and be partially reduced. Furthermore, metal species supported on the TA-CP and TA-HT are more easily reduced and better dispersed. For the NiMoP catalysts prepared with TA-CP and TA-HT, the Ti element promotes the sulfidation degree of Mo, besides shortening the average (Ni)MoS2 slab. The catalysts prepared with TA-CP exhibited superior activity for 4,6-DMDBT hydrodesulfurization. This can be ascribed not only to the relatively high sulfidation degree of Mo and proportion of the NiMoS active phase but also to the well-dispersed (Ni)MoS2 slabs. Moreover, the Ti4+ ions dispersed in the Al2O3 skeleton can be partially reduced to act as electron donors, enhancing the metallic character of the S layers in MoS2, which facilitates the improvement of the hydrogenation desulfurization activity.

Defect-Induced All-Solid-State Frustrated Lewis Pair on Metal-Organic Monolayer Accelerating Photocatalytic CO2 Reduction with H2O Vapor.

Understanding the structure-performance relationships of a frustrated Lewis pair (FLP) at the atomic level is key to yielding high efficiency in activating chemically "inert" molecules into value-added products. A sound strategy was developed herein through incorporating oxygen defects into a Zr-based metal-organic layer (Zr-MOL-D) and employing Lewis basic proximal surface hydroxyls for the in situ formation of solid heterogeneous FLP (Zr4-δ-VO-Zr-OH). Zr-MOL-D exhibits a superior CO2 to CO conversion rate of 49.4 μmol g-1 h-1 in water vapor without any sacrificing agent or photosensitizer, which is about 12 times higher than that of pure MOL (Zr-MOL-P), with extreme stability even after being placed for half a year. Theoretical and experimental results reveal that the introduction of FLP converts the process of the crucial intermediate COOH* from an endothermic reaction to an exothermic spontaneous reaction. This work is expected to provide new prospects for developing efficient MOL-based photocatalysts in FLP chemistry through a sound defect-engineering strategy.

Influence of shear deformation on the formation of dynamic structure in protic ionic liquids. Representation of the temperature dependence of viscosity within the framework of Angell’s fragility concept

The rheological properties of a liquid containing two types of basic centers and hydroxyl groups (BGE-Im), as well as protic ionic liquids (ILs) derived from it, were studied in a wide range of shear rates (γ̇) (10−6–103 s−1) at various temperatures from 10 to 50 °C. Viscosity (η) of the BGE-Im does not depend on the scanning direction of shear stress (σ), i.e. this compound behaves like a Newtonian fluid over the entire range of σ. The dependence log η (1/T) of BGE-Im in Arrhenius coordinates is linear. Protonation of basic centers of BGE-Im leads to a strong increase in the viscosity of ILs obtained on its basis, and the temperature dependence of the viscosity of these ILs is described by the Vogel–Fulcher–Tammann (VFT) equation. It has been established that in the region of low shear rates, the viscosity of the ILs increases with decreasing shear rate, which leads to the appearance of a yield stress (σY). This indicates the formation of a fluctuation rheopectic structure in the ILs. In this sense, ILs are heterogeneous systems and their behavior is similar to that of highly concentrated suspensions and emulsions. For the first time, a quantitative comparison between the parameters of the VFT equation, the fragility (according to Angell’s model) of ILs, the relative coordination number of ILs molecules and the rheological yield stress characterizing the shear-induced fluctuation rheopectic structure of ILs has been made and the relationship between these characteristics has been shown.

Activated-carbon-induced morphological and crystal transformation of α-FeOOH for efficient catalytic removal of hydrogen sulfide from blast furnace gas

The performance of FeOOH for the removal of hydrogen sulfide (H2S) has been widely investigated. However, the harsh conditions of use and the difficulty of regeneration limit its practical application. In this study, FeOOH was loaded on activated carbon (AC) by a hydrothermal method to obtain a series of XF-AC catalysts for the removal of H2S from blast furnace gas (BFG). Among these catalysts, 3F-AC exhibited a high penetrating sulfur capacity of 482.3 mg/g and good regeneration performance. It is noteworthy that the use of AC as the support induced the transformation of α-FeOOH crystals into amorphous FeOOH and affected their morphology, and uniformly dispersed nanoparticles rather than agglomerated acicular crystals appeared on the surface of AC. The presence of amorphous FeOOH modulated the basic sites and surface hydroxyl groups of the catalysts while generating hydroxyl radicals during the reaction, thus improving the adsorption and oxidation of H2S. More importantly, the structural properties (surface area and pore volume) of 3F-AC enhanced the storage capacity of elemental sulfur and reduced the formation of by-products. The cyclic catalytic role of Fe3+/Fe2+ played an important role in the oxidation process. This work provides a simple and effective strategy for the design of BFG desulfurization catalysts.

Preparation, characterization, and application of stable naphthalenediimide‐based metal–organic framework for cooperative capture low concentration uranium from wastewater

AbstractExtracting uranium from real water samples remains a great challenge due to low uranium concentration, concentrated competing ions and volumes of water. The design and preparation of uranium adsorbents with high efficiency and affinity are still difficult. Herein, we presented a facile one‐pot strategy to obtain a novel metal organic framework (denoted as Mn‐NDISA) for stable and efficient trapping of low concentration uranium. Mn‐NDISA with a built‐in hydrophobic cavity can boost the absorption affinity to 1.99 × 106 mL g−1 through the cooperative capture composed of electrostatic interaction, coordination force and hydrogen binding. Owing to the coordination‐available oxygen sites in flexible framework, a rapid kinetic equilibrium was achieved in just 25 min. Moreover, these exceptional adsorption features enabled Mn‐NDISA to successfully capture the naturally occurring uranium traces (~ppb) in wastewater samples, making it one of the most influential absorbents toward UO22+ ever reported. The experimental and theoretical studies revealed that the electrostatic attraction came from the surface negatively charged Mn‐NDISA and the positively charged UO22+. The coordination originated from Lewis basic hydroxyl, carbonyl groups, and Lewis acid UO22+, while hydrogen bonds further reinforced the as‐formed uranium binding complex. This research offered a promising cooperative capture strategy to improve the uranium affinity of the pristine MOF for trace contaminants removal in environmental remediation fields.

Exploration of surface hydroxyl on goethite during heterogeneous catalytic ozonation process: Generation, category, charged property, and contribution

The surface hydroxyl of a catalyst is believed to play a crucial role in O3 decomposition during heterogeneous catalytic ozonation. However, studies related to the generation, category, charged properties and the respective contributions of surface hydroxyl are inadequate. In this study, a hydroxyl-rich goethite (α-FeOOH) was selected as the catalyst for the ozonation of ibuprofen (IBP). By heating the catalyst and changing the reaction pH, the state of the hydroxyl groups on the catalyst surface was successfully adjusted, and the main functional hydroxyl group for ozone catalysis was identified. The results indicated that the addition of α-FeOOH during ozonation effectively degraded and mineralized IBP, and the main action was to promote the decomposition of O3 into •OH. Thermogravimetry and X-ray photoelectron spectroscopy (XPS) analyses showed that four types of hydroxyl groups coexisted on the surface of the catalyst. While adsorbed hydroxyl presented the least, the vacancy, metal, and basic site hydroxyls were dominant. Fourier transform infrared spectroscopy (FTIR) and XPS analyses revealed that heating the catalyst to 350 °C could eliminate the vacancy hydroxyl, whereas the metal and basic site hydroxyls recovered to their before-heating levels, which is related to the crystal transformation of the catalyst from goethite to hematite. Changing the solution pH from 3.0 to 9.0 adjusted the surface hydroxyl charge from positive to negative, and the neutral vacancy hydroxyl was judged as the main functional hydroxyl for ozone catalysis. Finally, a route involving the decomposition of O3 into •OH on the surface hydroxyl is proposed.

Plasma Surface Modification of 3Y-TZP at Low and Atmospheric Pressures with Different Treatment Times.

The efficiency of plasma surface modifications depends on the operating conditions. This study investigated the effect of chamber pressure and plasma exposure time on the surface properties of 3Y-TZP with N2/Ar gas. Plate-shaped zirconia specimens were randomly divided into two categories: vacuum plasma and atmospheric plasma. Each group was subdivided into five subgroups according to the treatment time: 1, 5, 10, 15, and 20 min. Following the plasma treatments, we characterized the surface properties, including wettability, chemical composition, crystal structure, surface morphology, and zeta potential. These were analyzed through various techniques, such as contact angle measurement, XPS, XRD, SEM, FIB, CLSM, and electrokinetic measurements. The atmospheric plasma treatments increased zirconia's electron donation (γ-) capacity, while the vacuum plasma treatments decreased γ- parameter with increasing times. The highest concentration of the basic hydroxyl OH(b) groups was identified after a 5 min exposure to atmospheric plasmas. With longer exposure times, the vacuum plasmas induce electrical damage. Both plasma systems increased the zeta potential of 3Y-TZP, showing positive values in a vacuum. In the atmosphere, the zeta potential rapidly increased after 1 min. Atmospheric plasma treatments would be beneficial for the adsorption of oxygen and nitrogen from ambient air and the generation of various active species on the zirconia surface.

Differentiating between Acidic and Basic Surface Hydroxyls on Metal Oxides by Fluoride Substitution: A Case Study on Blue TiO2 from Laser Defect Engineering.

Both oxygen vacancies and surface hydroxyls play a crucial role in catalysis. Yet, their relationship is not often explored. Herein, we prepare two series of TiO2 (rutile and P25) with increasing oxygen deficiency and Ti3+ concentration by pulsed laser defect engineering in liquid (PUDEL), and selectively quantify the acidic and basic surface OH by fluoride substitution. As indicated by EPR spectroscopy, the laser-generated Ti3+ exist near the surface of rutile, but appear to be deeper in the bulk for P25. Fluoride substitution shows that extra acidic bridging OH are selectively created on rutile, while the surface OH density remains constant for P25. These observations suggest near-surface Ti3+ are highly related to surface bridging OH, presumably the former increasing the electron density of the bridging oxygen to form more of the latter. We anticipate that fluoride substitution will enable better characterization of surface OH and its correlation with defects in metal oxides.

Unterscheidung zwischen sauren und basischen Oberflächenhydroxylgruppen auf Metalloxiden durch Fluoridsubstitution: Eine Fallstudie am Beispiel von defektreichem, blauem TiO2 aus der laserbasierten Defekterzeugung

AbstractSowohl oberflächennahe Sauerstofffehlstellen als auch Hydroxylgruppen an der Oberfläche spielen eine entscheidende Rolle in der Katalyse. Dennoch bleiben die genauen Zusammenhänge durch die limitierte analytische Zugänglichkeit dieser Struktureigenschaften oft wenig verstanden. Zur selektiven Quantifizierung der sauren und basischen Oberflächenhydroxide wurde in dieser Studie deshalb eine Fluoridsubstitution am Beispiel zweier Serien von TiO2 (Rutil und P25) untersucht, welche eine zunehmende Dichte an Ti3+‐Sauerstofffehlstellen aufwiesen. Die Materialien sind durch gepulster Laserdefekt‐Engineering in Flüssigkeit (PUDEL) hergestellt worden. Aus EPR‐ und EEL‐Spektroskopie geht hervor, dass sich die lasergenerierten Ti3+ beim Rutil potentiell in der Nähe der Partikeloberfläche befinden, während diese beim P25 tiefer in den Partikeln vorzuliegen scheinen. Anhand der pH‐abhängigen Fluoridsubstitution zeigt sich, dass sich auf dem laserbehandelten Rutil zunehmend saure (verbrückende) Hydroxylgruppen gebildet haben, welche potentiell auf die Hydrierung der lasergenerierten, oberflächennahen Ti3+‐Zentren zurückgeht. Beim P25 blieb die Dichte an Hydroxylgruppen trotz nachweislicher Bildung von Ti3+‐Zentren bei der Laserbehandlung unverändert. Wir gehen davon aus, dass die hier eingesetzte pH‐abhängige Substitutionsmethode von Hydroxiden durch Fluoride zukünftig eine einfache und robuste Charakterisierung von Oberflächen‐OH und deren Korrelation mit Defekten in Metalloxiden zulassen wird.

Surface Hydroxyl Chemistry of Titania- and Alumina-Based Supports: Quantitative Titration and Temperature Dependence of Surface Brønsted Acid-Base Parameters.

Surface hydroxyl groups on metal oxides play significant roles in catalyst synthesis and catalytic reactions. Despite the importance of surface hydroxyls in broader material applications, quantitative measurements of surface acid-base properties are not regularly reported. Here, we describe direct methods to quantify fundamental properties of surface hydroxyls on several titania- and alumina-based supports. Comparing commercially available anatase, rutile, P25, and P90 titania, thermogravimetric analysis (TGA) indicated that the total surface hydroxyl density varied by a factor of 2, and each surface hydroxyl is associated with approximately one weakly adsorbed water molecule. Proton-exchange site densities, determined at 25 °C with slurry acid-base titrations, led to several conclusions: (i) the intrinsic acidity/basicity of surface hydroxyls were similar regardless of the titania source; (ii) differences in the surface isoelectric point (IEP) were primarily attributable to differences in the surface concentration of acid and base sites; (iii) rutile has a higher surface concentration of basic hydroxyls, leading to a higher IEP; and (iv) P25 and P90 titania have slightly higher surface concentrationsof acidic hydroxyls relative to anatase or rutile. Temperature effects on surface acid-base properties are rarely reported yet are significant: from 5 to 65 °C, IEP values change by roughly one pH unit. The IEP changes were associated with large changes to the intrinsic acid-base equilibrium constants over this temperature range, rather than changes in the composition or concentration of the surface sites.

Hydrogeochemical assessment of carcinogenic and non-carcinogenic health risks of potentially toxic elements in aquifers of the Hindukush ranges, Pakistan: insights from groundwater pollution indexing, GIS-based, and multivariate statistical approaches.

Globally, potentially toxic elements (PTEs) and bacterial contamination pose health hazards, persistency, and genotoxicity in the groundwater aquifer. This study evaluates PTE concentration, carcinogenic and noncarcinogenic health hazards, groundwater quality indexing (GWQI-model), source provenance, and fate distribution in the groundwater of Hindukush ranges, Pakistan. The new estimates of USEPA equations record new research dimensions for carcinogenic and noncarcinogenic hazards. The principal component analysis (PCA), mineral phases, and spatial distribution determine groundwater contamination and its impacts. The average concentrations of PTEs, viz., Cd, Cu, Co, Fe, Pb, and Zn, were 0.06, 0.27, 0.07, 0.55, 0.05, and 0.19mg/L, and E. coli, F. coli, and P. coli were 27.5, 24.0, and 19.0CFU/100ml. Moreover, the average values of basic minerals, viz., anhydrite, aragonite, calcite, dolomite, gypsum, halite, and hydroxyl apatite, were 0.4, 2.4, 2.6, 5.1, 0.6, and - 4.0, 11.2, and PTE minerals like monteponite, tenorite, cuprite, cuprous ferrite, cupric ferrite, ferrihydrite, goethite, hematite, lepidocrocite, maghemite, magnetite, massicot, minium, litharge, plattnerite, and zincite were - 5.5, 2.23, 4.65, 18.56, 20.0, 4.84, 7.54, 17.46, 6.66, 9.67, 22.72, - 3.36, 22.9, 3.16, - 18.0, and 1.46. The groundwater showed carcinogenic and non-carcinogenic health hazards for children and adults. The GWQI-model showed that 58.3% of samples revealed worse water quality. PCA revealed rock weathering, mineral dissolution, water-rock interaction, and industrial effluents as the dominant factors influencing groundwater chemistry. Carbonate weathering and ion exchange play vital roles in altering CaHCO3 type to NaHCO3 water. In this study, E. coli, F. coli, P. coli, EC, turbidity, TSS, PO43─, Na+, Mg+2, Ca+2, Cd, Co, Fe, and Pb have exceeded the World Health Organization (WHO) guidelines. The carcinogenic and non-carcinogenic impacts of PTEs and bacterial contamination declared that the groundwater is unfit for drinking and domestic purposes.

Dynamic simulation on surface hydration and dehydration of monoclinic zirconia

The commonly used oxide-supported metal catalysts are usually prepared in aqueous phase, which then often need to undergo calcination before usage. Therefore, the surface hydration and dehydration of oxide supports are critical for the realistic modeling of supported metal catalysts. In this work, by ab initio molecular dynamics (AIMD) simulations, the initial anhydrous monoclinic ZrO2(1¯11) surfaces are evaluated within explicit solvents in aqueous phase at mild temperatures. During the simulations, all the two-fold-coordinated O sites will soon be protonated to form the acidic hydroxyls (HOL), remaining the basic hydroxyls (HO*) on Zr. The basic hydroxyls (HO*) can easily diffuse on surfaces via the active proton exchange with the undissociated adsorption water (H2O*). Within the temperatures ranging from 273 K to 373 K, in aqueous phase a certain representative equilibrium hydrated m-ZrO2(1¯11) surface is obtained with the coverage (θ) of 0.75 on surface Zr atoms. Later, free energies on the stepwise surface water desorption are calculated by density functional theory to mimic the surface dehydration under the mild calcination temperatures lower than 800 K. By obtaining the phase diagrams of surface dehydration, the representative partially hydrated m-ZrO2(1¯11) surfaces (0.25≤θ<0.75) at various calcination temperatures are illustrated. These hydrated m-ZrO2(1¯11) surfaces can be crucial and readily applied for more realistic modeling of ZrO2 catalysts and ZrO2-supported metal catalysts.

Basic silica catalysts for the efficient dehydration of biomass-derived compounds – Elucidating structure-activity relationships for Na2O/SiO2-type materials

The gas-phase dehydration of biomass-derived N-(2-hydroxyethyl)-2-pyrrolidone by Na2O/SiO2-type catalysts propounds a promising route to polyvinylpyrrolidone, a high-value polymer. Thus, an optimization of said catalysts via variations of the silica parent, sodium precursor and sodium loading was performed. The combination of resulting performance data with in-depth characterization via FTIR spectroscopy, CO2-TPD and physisorption techniques leads to a clear structure-activity correlation. Most importantly, catalyst activity is promoted by a balance between weak basic sites (Si-ONa) and surface hydroxyls (Si-OH), which comes about through ion exchanging a fraction of the initial silanol in mild conditions. The optimal synthesis procedure and recipe are thus a function of the type of parent silica. With this renewed molecular understanding of the reaction, the scope of application of the Na2O/SiO2 materials was expanded to the dehydration of other biomass-derived chemicals of interest in the polymer industry.

Towards highly efficient hydrotalcite/hydroxyapatite composites as novel catalysts involved in eco-synthesis of chromene derivatives

A series of hydrotalcite/hydroxyapatite composites were investigated as novel catalysts in the green synthesis of chromenes in high conversions (up to 78% after 2 h), under solvent-free and mild conditions, from salycilaldehydes and cyano compounds. Materials were prepared by three different methods: impregnation of hydroxyapatite over hydrotalcite already crystallized; impregnation of hydrotalcite over hydroxyapatite and crystallization of both the components simultaneously. The composites resulted in the materials with high activity and selectivity for the 2-amino-4H-chromene synthesis. The catalytic performance was depending on the catalyst synthesis method and governed by the basicity of the samples and textural properties also varying as a function of the used cyano compound. Observed reactivity was attributed to the high dispersion of the hydrotalcite particles on the hydroxyapatite component, resulting in a considerable increase of the surface areas from 3 to 154 m2/g, and thus, given rising to a high concentration of superficial hydroxyls reaching 7.1 × 1014 sites/g catalyst. Broad pore size distributions in composites enabled a rapid diffusion of the reactants to reach the basic hydroxyl sites.

Hydrodesulfurization Properties of Nickel Phosphide on Boron‐treated Alumina Supports

AbstractThe deep hydrodesulfurization (HDS) properties of nickel phosphide (Ni2P) on boron‐modified alumina (xB−Al2O3) supports having different B contents have been investigated. Ni2P precursors were prepared on the xB−Al2O3 supports using hypophosphite (‐hypo) or phosphate (‐phos) as the P source and were subsequently reduced in flowing hydrogen. The 4,6‐dimethyldibenzothiophene HDS activities and turnover frequencies of the Ni2P/xB−Al2O3 catalysts showed a strong dependence on B loading, with a maximum observed for the hypophosphite‐ and phosphate‐based Ni2P/B−Al2O3 catalysts corresponding to 0.8 wt % and 1.2 wt % B loadings, respectively. Based on XPS and IR spectral measurements, the optimal B loadings corresponded to ∼20 % B2O3 coverage of the γ‐Al2O3 support and coincided with removal of the most basic hydroxyl groups on the alumina surface. At 573 K, a Ni2P/0.8B−Al2O3‐hypo catalyst was 2.5 times more active than a B‐free Ni2P/Al2O3‐hypo catalyst, while a Ni2P/1.2B−Al2O3‐phos catalyst was 8.6 times more active than a B‐free Ni2P/Al2O3‐phos catalyst. Overall, the hypophosphite‐based Ni2P/B−Al2O3 catalysts exhibited higher HDS activities than the phosphate‐based Ni2P/B−Al2O3 catalysts, in part due to smaller Ni2P particle sizes. The optimized catalysts, Ni2P/0.8B−Al2O3‐hypo and Ni2P/1.2B−Al2O3‐phos, were more active than a sulfided Ni−Mo/Al2O3 catalyst.

Impact of greenhouse gas CO2 on the heterogeneous reaction of SO2 on alpha-Al2O3

The heterogeneous reaction of SO2 on mineral dust surfaces is generally considered as an important chemical pathway for secondary sulfate formation in the troposphere. To this day, there are no reported studies that assess the impact of atmospheric CO2 in sulfate production on mineral dust surfaces. In this work, we investigate the impact of CO2 on SO2 uptake on dust proxy aluminum oxide particles using a diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). CO2 is demonstrated to suppress the heterogeneous oxidation of SO2 on alpha-Al2O3. Compared to that measured in the CO2-free case, the uptake coefficient is decreased by nearly 57% when Al2O3 particles are exposed to the gas flow with atmospheric CO2 at a relative humidity (RH) of 25%. It is also found that there is a balance between the yield of active moiety −OH provided by Al(OH)3(CO)(OH)2 clusters and the loss of basic hydroxyl group on aluminum oxide surfaces blocked by CO2-derived (bi)carbonate species. This work, for the first time, reveals a negative effect of atmospheric CO2 on the sulfate formation, which potentially decreases solar-radiation scattering and further exacerbates global warming.

Water Poisons H2 Activation at the Au-TiO2 Interface by Slowing Proton and Electron Transfer between Au and Titania.

Understanding the dynamic changes at the active site during catalysis is a fundamental challenge that promises to improve catalytic properties. While performing Arrhenius studies during H2 oxidation over Au/TiO2 catalysts, we found different apparent activation energies (Eapp) depending on the feedwater pressure. This is partially attributed to changing numbers of metal-support interface (MSI) sites as water coverage changes with temperature. Constant water coverage studies showed two kinetic regimes: fast heterolytic H2 activation directly at the MSI (Eapp ∼ 25 kJ/mol) and significantly slower heterolytic H2 activation mediated by water (Eapp ∼ 45 kJ/mol). The two regimes had significantly different kinetics, suggesting a complicated mechanism of water poisoning. Density functional theory (DFT) showed water has minor effects on the reaction thermodynamics, primarily attributable to intrinsic differences in surface reactivity of different Au sites in the DFT model. The DFT model suggested significant surface restructuring of the TiO2 support during heterolytic H2 adsorption; evidence for this phenomenon was observed during in situ infrared spectroscopy experiments. A monolayer of water on the hydroxylated TiO2 surface increased the H2 dissociation activation barrier by ∼0.2 eV, in good agreement the difference in experimentally measured values. DFT calculations suggested H2 activation goes through a proton-coupled electron-transfer-like mechanism. During proton transfer to a basic support hydroxyl group, electron density is distributed through the gold nanorod and partially localized on the protonated support hydroxyl group. Water slows H2 activation by slowing this H+ transfer, forcing negative charge buildup on the Au and increasing the transition state energy.

Observation of Photoinduced Proton Transfer between the Titania Surface and Dye Molecule

The photocatalyst titania film surface acquires a high hydrophilicity after it is exposed to UV light, which is induced by changes in the densities of the surface hydroxyl groups and charge. A xanthene dye, fluorescein, was deposited from a solution onto a titania film after UV irradiation in order to probe the titania surface change. The change in the surface acidity was confirmed by the ratio of the dianion to monoanion of fluorescein by Raman spectroscopy. The ratio increased by the UV irradiation, indicating that the surface became more basic. Transient absorption spectroscopy revealed the transformation from the monoanion to the dianion via the excited states, i.e., photoinduced proton transfer from the fluorescein to the titania surface. The UV irradiation increases the basic hydroxyl groups on the titania surface, which accepts protons from the water molecules or proton donors on the surface and has a positive charge.

In Situ Probing of Photoinduced Hydrophilicity on Titania Surface Using Dye Molecules.

The titania film surface exhibits superhydrophilicity after UV irradiation due to its photocatalytic function. This is caused by a change in the density of the surface hydroxyl groups, which affects the surface charge density. Titania films were prepared to observe the change in the surface charge during UV irradiation. The amounts of some xanthene dyes adsorbed and deposited on the titania films after UV irradiation were evaluated as a function of the irradiation time. The increase in the adsorption and deposition amounts of the more negative dye versus the UV irradiation time was greater. These results indicated that the titania surface charge became more positive by the UV irradiation. It was reported that basic hydroxyl groups formed on the titania particle surface during the UV irradiation. The surface dissociates hydroxyl ions into the water phase and has a positive charge, which was supported by the present study.