Density Of Surface Hydroxyl Groups Research Articles

Upside-Down Adsorption: The Counterintuitive Influences of Surface Entropy and Surface Hydroxyl Density on Hydrogen Spillover.

Although hydrogen spillover is often invoked to explain anomalies in catalysis, spillover remains a poorly understood phenomenon. Hydrogen spillover (H*) is best described as highly mobile H atom equivalents that arise when H2 migrates from a metal nanoparticle to an oxide or carbon support. In the 60 years since its discovery, few methods have become available to quantify or characterize H*-support interactions. We recently showed in situ infrared spectroscopy and volumetric chemisorption can quantify reversible H2 adsorption on Au/TiO2 catalysts, where adsorbed hydrogen exists as H* and interacts with titania surface hydroxyl (TiOH) groups. Here, we report parallel thermogravimetric analysis and Fourier transform infrared spectroscopy methods for systematically manipulating the surface TiOH density. We examine the role of surface hydroxylation on spillover thermodynamics using van't Hoff studies to determine apparent adsorption enthalpies and entropies at constant H* coverage, which is necessary to maintain constant H* translational entropy. Although surface TiOH groups are the likely adsorption sites, the data show removing hydroxyl groups increases spillover. This surprising finding─that adsorption increases as the adsorption site density decreases─is associated with improved thermodynamics on dehydroxylated surfaces. A strong adsorption enthalpy-entropy correlation implicates the changing surface entropy of the titania support itself (i.e., an initial state effect) is deeply intertwined with the H* configurational entropy. These effects are surprising and should apply to all low-coverage adsorbates where entropy terms dominate more traditional enthalpic considerations. Moreover, this study points toward a kinetic test for invoking spillover in a reaction mechanism: namely, in situ dehydroxylation should enhance spillover processes.

Wetting Preference of Silica Surfaces in the Context of Underground Hydrogen Storage: A Molecular Dynamics Perspective.

The growing interest in large-scale underground hydrogen (H2) storage (UHS) emphasizes the need for a comprehensive understanding of the fundamental characteristics of subsurface environments. The wetting preference of subsurface rock is a crucial parameter influencing the H2 flow behavior during storage and withdrawal processes. In this study, we utilized molecular dynamics simulation to evaluate the wetting preference of the silica surface in subsurface hydrogen systems, with the aim of addressing disparities observed in experimental results. We conducted an initial comprehensive assessment of potential models, comparing the wettability of five common silica surfaces with different surface morphologies and hydroxyl densities in CO2-H2/water/silica systems against experimental data. After introducing the INTERFACE force field as the most accurate potential model for the silica surface, we evaluated the wetting behavior of the α-quartz (101) surface with a hydroxyl density of 5.9 number/nm2 under the impact of actual geological storage conditions (333-413 K and 10-30 MPa), the coexistence of cushion gases (i.e., CO2, CH4, and N2) at various mole fractions, and pH levels ranging from 2 to 11 characterized through considering the negative charges of 0 to -0.12 C/m2 via deprotonation of silanol on the silica surface. Our results indicate that neither pressure nor temperature has a significant impact on the wetness of the silica in the case of pure H2 (single component UHS operations). However, when CO2 coexists with H2, especially at higher mole fractions, an increase in pressure and a decrease in temperature lead to higher contact angles. Moreover, when the mole fraction of cushion gas ranges from 0 to 1, the contact angle increases 20, 9.5, and 4.5° for CO2, CH4, and N2, respectively, on the neutral silica substrate. Interestingly, at higher pH conditions where the silica surface carries a negative charge, the contact angle considerably reduces where surface charges of -0.03 and -0.06 C/m2 result in an average reduction of 20 and 80% in the contact angle, respectively. More importantly, at a pH of ∼11 (-0.12 C/m2), a 0° contact angle is observed for the silica surface under all temperatures, pressures, types of cushion gases, and varying mole fractions.

Optimization of surface silanol groups in mesoporous SBA-15 and KIT-6 materials: effects on APTES functionalization and CO2 adsorption

Hybrid mesoporous silicas are potentially reusable promising adsorbents for CO2 capture. Here SBA-15 and KIT-6 mesoporous silicas were synthetized by the sol-gel technique using a lower calcination temperature than the conventional one: 300 ºC vs. 550 ºC. This strategy aimed to maximize the surface hydroxyl group density and consequently, to improve functionalizing agent-loading performed by the post-synthesis grafting method. The as-obtained silica-based samples were then characterized and applied to CO2 capture. A correlation between chemical-textural properties and adsorption capacities of pure and modified materials was discussed. The calcined at 300 ºC silica supports, SBA-15-300-1 and KIT-6-300-1, with high silanol group densities showed better adsorption capacities overall pressure range. It is worth highlighting that the KIT-6-300-1: APTES adsorbent had a good stability performance, showing a drop of around 8 % over the aging time of a year. While, the SBA-15-300-1: APTES solid presented an excellent recycled performance after 10 adsorption-desorption cycles, maintaining about 95 % of its initial capacity.

Activation of peroxymonosulfate by β-FeOOH@Cia-MoS2 for enhancing degradation of tetracycline: Significant roles of surface functional groups and Fe/Mo redox reactions

Vertically oriented interstitial atom carbon-anchored molybdenum disulfide (Cia-MoS2) nanospheres loaded with iron oxyhydroxide (β-FeOOH) were proposed for modulating the surface catalytic activity and stability of the unsaturated catalytic system. The β-FeOOH@Cia-MoS2 efficiently activated peroxymonosulfate (PMS) to degrade 95.4% of tetracycline (TC) within 30 min, owing to the more sulfur vacancies, higher surface hydroxyl density, redox ability and electronic transmission rate of β-FeOOH@Cia-MoS2. According to the characterization and analysis data, the multiple active sites (Fe, Mo and S sites) and oxygen-containing functional groups (CO, –OH) of β-FeOOH@Cia-MoS2 could promote the activation of PMS to form reactive oxygen species (ROS). The oxidation cycle of Fe(II)/Fe(III) and Mo(IV)/Mo(VI), the electron transfer mediator of rich sulfur vacancies, as well as oxygen-containing functional groups on the surface of β-FeOOH@Cia-MoS2 synergistically promoted the formation of ROS (1O2, FeIVO, SO4•- and •OH), among which 1O2 was the main active oxidant. In particular, the β-FeOOH@Cia-MoS2/PMS system could still degrade pollutants efficiently and stably after five recycling cycles. Furthermore, this system had a strong anti-interference ability in the actual water body. This study provided a promising strategy for the removal of difficult-to-degrade organic pollutants.

Effect of surface hydroxyl group density of support membrane on water permeance of polyamide thin-film composite membrane

Polyamide-based thin-film composite (TFC) membranes are widely used in water treatment processes. Recently, the influence of support membranes on the performance of polyamide layers has garnered considerable attention. Regarding the chemical properties of the support membranes, the surface wettability and the interaction between the amine monomer used for interfacial polymerization and the membrane surface are considered to play crucial roles. In this study, we investigated the effect of the wettability of the support membrane surface and its interaction with the amine monomer on the water permeance of TFC membranes. We prepared TFC membranes on polyketone support membranes with varying surface hydroxyl group densities and aqueous solution wettabilities. While no clear correlation was observed between the support membrane's wettability and the water permeance of the TFC membrane, an increase in hydroxyl group density did result in higher water permeance. The hydroxyl groups interact with PIP, inhibiting PIP diffusion and potentially leading to the formation of a thinner polyamide layer, which enhances water permeance. Therefore, the interaction between the amine monomer and support membrane surface strongly affects the formation of the polyamide layer.

Regulating pore structure of diatomite with alkali dissolution and its influence on humidity control performance

Alkali dissolution is an effective method to regulate the pore structure of porous mineral material. The main chemical composition of diatomite is amorphous SiO2, which can be dissolved in alkali dissolution. The diatomite samples with alkali dissolution treatment were systematacially characterized by the particle size analysis, low temperature nitrogen adsorption, MIP, fractal theory, SEM, TEM, XRD, FTIR and surface hydroxyl density to analyze the pore structure and surface properties. And the humidity control performance of diatomite was tested under different temperatures and relative humidities. The relationship among pore structure, surface properties and humidity control performance were analyzed. The results show that alkali dissolution can regulate the mesoporous and macroporous of diatomite, and some new microporous can generate at high alkali dosage. The specific surface area, mesoporous pore volume, proportion of macroporous volume, surface roughness, heterogeneity of pore structure and number of hydroxyl groups on the surface of diatomite are important factors which determining the humidity control performance. The humidity control performance of diatomite is positively correlated with the specific surface area, mesoporous volume, surface roughness, heterogeneity of pore structure and number of hydroxyl groups on the surface, while is negatively correlated with the proportion of macroporous volume.

Enhanced catalytic performance for gas phase epoxidation of propylene with H2 and O2 by multiple pretreated Au/TS-1 catalysts

Direct epoxidation of propylene (C3H6) with hydrogen (H2) and oxygen (O2) over Au/TS-1 catalyst was promising way for the synthesis of propylene oxide (PO). However, high selectivity of PO (>95 %) and conversion of C3H6 (>5 %) cannot be achieved at the same time. The industrialization process was severely restricted. In this paper, Au/TS-1 catalysts were synthesized by deposition precipitation method with alkali metal promoter. And alkaline gases and silane coupling post-treatment were performed to promote the catalytic activity. The prepared samples exhibited a much better C3H6 conversion (6.5 %) and PO selectivity (95.1 %) compared with general catalyst using normal synthetic technology. Cs2CO3 was beneficial to increasing Au nanoparticles uptake efficiency. Both NH3 and triethoxyvinylsilane post-treatment were good for smaller and more uniform loading of Au nanoparticles. The decreasing in surface hydroxyl density effectively alleviated the occurrence of PO ring opening isomerization. The increased hydrophobicity of TS-1 surface was conducive to the rapid desorption of generated PO and significantly increased PO selectivity. Multiple pretreatment was also favorable for extending the lifetime of Au/TS-1 catalyst.

Understanding the relationship between pore size, surface charge density, and Cu2+ adsorption in mesoporous silica

This research delved into the influence of mesoporous silica’s surface charge density on the adsorption of Cu2+. The synthesis of mesoporous silica employed the hydrothermal method, with pore size controlled by varying the length of trimethylammonium bromide (CnTAB, n = 12, 14, 16) chains. Gas adsorption techniques and transmission electron microscopy characterized the mesoporous silica structure. Surface charge densities of the mesoporous silica were determined through potentiometric titration, while surface hydroxyl densities were assessed using the thermogravimetric method. Subsequently, batch adsorption experiments were conducted to study the adsorption of Cu2+ in mesoporous silica, and the process was comprehensively analyzed using Atomic absorption spectrometry (AAS), Fourier transform infrared (FTIR), and L3 edge X-ray absorption near edge structure (XANES). The research findings suggest a positive correlation between the pore size of mesoporous silica, its surface charge density, and the adsorption capacity for Cu2+. More specifically, as the pore size increases within the 3–4.1 nm range, the surface charge density and the adsorption capacity for Cu2+ also increase. Our findings provide valuable insights into the relationship between the physicochemical properties of mesoporous silica and the adsorption behavior of Cu2+, offering potential applications in areas such as environmental remediation and catalysis.

Effects of synthesis conditions on particle size and pore size of spherical mesoporous silica

The particle size and pore size of spherical mesoporous silica materials play significant roles in their application. However, relatively limited systematic research has been conducted on how preparation conditions influence these properties. In particular, the effects of some important factors have not been adequately studied, including reaction time, reaction temperature, and organic solvent type. In this work, octane and water were used as solvents, and tetraethyl orthosilicate was used as the silicon source to systematically study the effects of reaction time, reaction temperature, different organic solvents, octane/water mass ratio, styrene template concentration, and surfactant (cetyltrimethylammonium bromide, CTAB)/H2O mass ratio on the particle morphology, particle size, and pore size of silica. The results suggest that the above-mentioned neglected factors exert a substantial influence on both particle size and pore size. In the experimental temperature range, the pore diameter decreases and the particle size increases with increasing temperature. The maximum particle size and pore size are achieved after a reaction time of 3 h, and a further increase in reaction time leads to a smaller particle size and pore size. As the number of carbon atoms in the organic solvent decreases, the pore size also gradually increases. Styrene and organic solvents that dissolve in CTAB micelles are crucial factors in pore formation, while the aggregation of the swollen CTAB micelles influences the particle size. The changes in the pore structure stability and hydroxyl density of the synthesized samples in water were also studied. After undergoing water treatment at temperatures ranging from 20 to 60 °C for 72 h, both the pore structure and morphology remain relatively unchanged. When the temperature increases, the surface hydroxyl density exhibits a more pronounced increase in the presence of water. After water treatment for 5 h, the surface hydroxyl density reaches saturation.

Surface modeling of wettability transition on α-quartz: Insights from experiments and molecular dynamics simulations

The wettability of quartz surfaces is a critical property in numerous energy-related industrial processes. However, accurately quantifying wettability is challenging due to the multitude of factors that can influence wetting behavior. Herein, we proposed a generalized surface modeling procedure addressing microscopic insights to calculate the contact angles of liquids on quartz under different scenarios. An optical goniometer experimentally measured the contact angles and validated the proposed surface modeling procedure. The effects of surface density of hydroxyl groups, temperature, surfactants, and crude oil composition on the wettability of quartz surfaces were studied. The results revealed that an increased hydroxyl group density significantly improved water wettability on a quartz surface. An increased temperature reduced the water contact angle on quartz, and such decreasing rates became steeper at high temperatures. The surfactants’ adsorption morphology determined the wettability of a quartz surface. Moreover, the polar components in crude oil significantly increased the hydrophobicity of a quartz surface, making it easier to adsorb more oil molecules to alter the quartz wettability further. The valuable outcomes of this paper contribute to understanding the complex quartz wettability and pave the way for further studies involving quartz surface modeling.

Chemisorption of tetrakis(dimethylamino)zirconium on zirconium oxide: Density functional theory study

We investigated the chemisorption mechanism of tetrakis(dimethylamino)zirconium, Zr(NMe2)4, on zirconium oxide (ZrO2) to understand the first half-reaction of the atomic layer deposition (ALD) process of ZrO2 films. A hydroxylated monoclinic ZrO2 (-111) 2×2 slab model was constructed, and the surface hydroxyl group density was optimized. The obtained density through the optimization procedure was 4.5 nm−2, which closely matches the value reported in the literature. Then, the sequential ligand exchange reactions involving a Zr(NMe2)4 molecule and the –OH terminated ZrO2 surface were simulated. The first two reactions had low activation energies of 0.19 and 0.24 eV and were exothermic. Conversely, the third reaction was endothermic. Therefore, the resulting surface species after chemisorption was expected to be O2Zr(NMe2)2*, which is consistent with in situ analyses in the literature.

Water thin films on kaolinite gibbsite and edge surfaces and their effects on surface wettability in relation to geological carbon sequestration

Geological carbon sequestration (GCS) is a promising method to alleviate CO2 emission, while CO2 structural trapping is one of main GCS mechanisms, in which the storage capacity is strongly dependent on CO2-water–rock contact angle. Although CO2-water-kaolite contact angle on basal surfaces has been studied, the knowledge about kaolinite edge surface wettability remains unknown. In this work, we use molecular dynamics (MD) simulations to study CO2-water-kaolinite contact angle on kaolinite gibbsite and edge surfaces under a typical GCS condition (330 K and 200 bar). The common belief is that surface wettability in presence of water is determined by surface hydroxyl (–OH) group density. While the surface –OH group density of kaolinite edge surface is much lower than that of gibbsite surface, both gibbsite and edge surfaces are strongly water-wet. Edge surface is better hydrated than gibbsite surface as both silanol and aluminol groups can form hydrogen bonding with water molecules thanks to large effective water accessible volume around them, while pocket water can further anchor water film. Therefore, the atomic-level surface characteristics dictate interfacial water structures which determine CO2-water-kaolinite contact angle. Our study provides important insights into the effect of surface heterogeneity on interfacial water structures, and kaolinite gibbsite and edge surface wettability which is crucial to the optimization of GCS processes.

Highly hydrophilic H2TiO3 ion sieve with neodymium (Nd) doping enables fast and high-efficiency Li extraction

The metatitanic acid H2TiO3 (HTO) with high selectivity shows potential in lithium extraction from salt lake brine. Nevertheless, the sluggish adsorption kinetics and unsatisfying actual adsorption capacity hinder H2TiO3 industrial application. Herein, a rare earth element Nd-doped H2TiO3 (Nd-HTO-1 %) was synthesized. The crystallinity, morphology, hydrophilicity, adsorption property and mechanism of Nd-HTO-1 % were systematically characterized by a series of techniques. Interestingly, Nd-doping promotes the generation of oxygen vacancies, thereby increasing the density of surface hydroxyl groups and making the surface high-hydrophilic and particle size decreased. After 1 % Nd-doping, high-hydrophilic HTO was obtained with a contact angle of 16.5°. The adsorption capacity of Nd-HTO-1 % increased by 12.5 % compared with HTO, and the equilibrium time of adsorption reduced by half to 4 h. After eight cycles, Nd-HTO-1 % still showed a considerable adsorption capacity of 39.69 mg·g−1. In the meantime, the dissolution loss of titanium is merely 0.14 %, showing excellent cycling stability. Combined with the considerable selectivity exhibiting in Qarhan salt lake brine, it demonstrates potential in utilization of Nd-doped H2TiO3 for lithium extraction from salt lake brine.

Formation of extracellular polymeric substances corona on TiO2 nanoparticles: Roles of crystalline phase and exposed facets

Nanoparticles (NPs) in the environment can interact with macromolecules in the surrounding environment to form eco-corona on their surfaces, which in turn affects the environmental fate and toxicity of nanoparticles. Wastewater treatment plants containing large amounts of microbial extracellular polymeric substances (EPS) are an important source of NPs into the environment, where the formation of EPS coronas on NPs is critical. However, it remains unclear how the crystalline phase and exposed facets, which are intrinsic properties of NPs, affect the formation of EPS coronas on NPs. This study investigated the formation of EPS corona on three TiO2 NPs (representing the most widely used engineered NPs) with different crystalline phases and exposed facets. The protein type and abundance in EPS coronas on TiO2 NPs varied depending on the crystalline phase and exposed facets. Anatase with {101} facets and {001} facets preferred to adsorb proteins with lower molecular weights and higher H-bonding relevant amino acids, respectively, while EPS corona on rutile with {110} facets had proteins with higher hydrophobicity. In addition, the selective adsorption of proteins was primarily determined by steric hindrance, hydrogen bonding, and hydrophobic interaction between TiO2 NPs and proteins, which were affected by changes in aggregation state, surface hydroxyl density, and hydrophobicity of TiO2 NPs induced by crystalline phase and exposed facets. Moreover, crystalline phase and exposed facets-induced EPS corona changes altered the aggregation state and oxidation potential of TiO2–EPS corona complexes. These findings emphasize the important role of crystalline phase and exposed facets in the environmental behavior of nanoparticles and may provide insights into the safe design of nanoparticles.

Influence of heat treatment on the microstructure and surface groups of Stöber silica

Heat treatment is routinely utilized in preparing nanoporous materials (including Stöber silica), and can substantially affect their performance in diverse application fields. However, the effects of heat treatment at different temperatures on the structure and surface properties of Stöber silica have not been systematically investigated before. In this work, Stöber silica (washed with water or ethanol) was calcined at different temperatures (from 250 °C to 1000 °C), and the heat-treated samples were characterized through nitrogen adsorption at 77 K, scanning electron microscopy, simultaneous thermal analysis, elemental analysis, and Fourier-transform infrared spectroscopy. The results show that there is no significant difference in the morphology and particle size of the calcined samples. The internal micropores almost collapse after calcination at 500 °C, and the pores with a smaller diameter are the first to shrink during calcination. The variation in the number of the surface hydroxyl groups and ethoxyl groups with the calcination temperature is discussed in detail. The carbon content analysis and differential scanning calorimetry curves reveal that the surface ethoxyl groups (for the samples washed with ethanol) are completely removed after calcination at 500 °C. After calcination at temperatures above 800 °C, the hydroxyl groups almost completely condense into siloxanes. The specific surface area calculated according to the thermogravimetric mass loss and surface hydroxyl density is found to be significantly different from the measured Brunauer–Emmett–Teller specific surface area. Our results may offer practical guidance for the application of Stöber silica subjected to similar heat-treatment processes.

Cu-doped CoOOH activates peroxymonosulfate to generate high-valent cobalt-oxo species to degrade organic pollutants in saline environments

A Cu doped CoOOH-activated peroxymonosulfate (CoCu10/PMS) process was developed to enable the generation of Co(IV)=O for efficient oxidation of pollutants in saline water. The formation of Co(IV)=O was demonstrated through phenylmethyl sulfoxide (PMSO) probe and 18O-isotope-labeling tests. CoCu10 was synthesized using a simple and scalable co-precipitation method, enabling efficient synthesis at laboratory scale for large quantities (gram level). Cu doping increased the surface hydroxyl density of CoOOH, thereby improving its activation performance. Coordinating with PMS through surface hydroxyls, CoCu10 formed catalyst-PMS complexes, then donated electrons to induce heterolytic cleavage of O–O bonds in complexes, resulting in the formation of Co(IV)=O. At a concentration of 0.2 g/L catalyst and 0.2 mM PMS, 97.7% of tetracycline was degraded within 10 min. CoCu10 maintained stable performance over eight cycles. Thanks to Co(IV)=O, the CoCu10/PMS system efficiently operates under different concentrations of Cl−, HCO3−, NO3−, SO42−, H2PO4− and mixed salt.

Adsorption features of pigments onto iron oxides: Co-effect of the molecular structure of adsorbates and the varied surface properties of minerals

The knowledge about the adsorption properties of pigments onto different iron oxide minerals is still limited. The adsorption behaviors of three typical pigments, including amaranth (AR), tartrazine (TA), and brilliant blue (BB), onto three iron oxides (ferrihydrite, goethite, and hematite) were investigated. The primary mechanism of their interactions was the electrostatic attraction. The affinities of iron oxides toward any pigment were generally in the sequence of ferrihydrite > goethite > hematite (e.g., the maximum adsorption amounts (qmax) of AR onto the three iron oxides were 35.5 mg/g, 11.6 mg/g, and 4.2 mg/g, respectively), which was related to the differences in physicochemical properties of minerals (e.g., specific surface area, surface hydroxyl density, and crystal structure). Interestingly, for a given iron oxide, the pigment-binding capacity of the mineral was in the order of AR > TA > BB. This effect was attributed to variations in pigment molecular sizes and electrostatic interactions between pigments and minerals. The pigment-binding capabilities of iron oxides changed as the solution pH varied. Hydrogen-bonding and electrostatic interactions were the main mechanisms below and above pH > pHPZC (pH value at zero point charge), respectively. Furthermore, adding Cu2+ facilitated pigment adsorption, mainly stemming from the surface-bridging effect. Pigments also enhanced Cu2+ adsorption onto iron oxides due to the decreasing electrostatic repulsion between Cu2+ and particles as well as the complexion between Cu2+ with pigment molecules adsorbed on mineral surfaces. The insightful information demonstrates that ubiquitous iron oxide minerals can affect the fate of pigments in natural environments.

Ethanol Dehydration to Ethylene over High-Energy Facets Exposed Gamma Alumina

Highly efficient and stable catalysts are among the key factors in industrial ethanol dehydration to ethylene. Among the widely studied catalysts, alumina is the most suitable for industrial application. In this study, novel gamma alumina was synthesized by solvent protection and a hydrothermal procedure. HRTEM, XRD, FT-IR, NH3-TPD, H-D exchange, and 29Si MAS NMR were employed to compare the difference in physicochemical properties between the novel gamma alumina and commercial alumina. Characterization results show that the as-synthesized novel gamma alumina mainly exposes the high-energy crystal plane (111) while the commercial alumina mainly exposes the thermostatically stable (110) crystal plane. The dominating (111) plane, according to the characterizations, endows the novel gamma alumina with a higher density of surface hydroxyl groups, higher acid content, and higher surface energy compared to the commercial alumina. The catalytic performance of the two catalysts for industrial ethanol dehydration to ethylene was studied. The novel (111) plane-exposed alumina showed a higher yield of ethylene than commercial alumina under the same reaction conditions. This could be related to the difference in atomic arrangement and the unsaturated aluminum coordination of different crystal planes. Stability testing under severe reaction conditions (450 °C, 1 MPa, 4 h−1) indicates that novel gamma alumina shows better stability (catalyst life cycle increased by 50%) and produces less acetaldehyde as a byproduct. The effects of steam treatment on the catalytic performance were further investigated. The surface acidity and the catalytic performance of novel gamma alumina present a volcanic curve with the increase in steam treatment temperature. Under the optimal water vapor treatment temperature of 650 °C, the conversion of ethanol and selectivity of ethylene were both higher than 99%.

Deep Photocatalytic Oxidation of Aromatic VOCs on ZnSn LDH: Promoting Role of Electron Enrichment of Surface Hydroxyl

Photocatalytic oxidation is a very promising technology for air purification, but its wide application is greatly limited by poor mineralization capacity and catalytic stability. Herein, we successfully tailored and identified the deep oxidation of aromatic VOCs including toluene, styrene, and chlorobenzene by electron enrichment of surface hydroxyl (OH) over ZnSn LDH photocatalyst. By means of regulation of electron donation of Sn atom to OH, the electron density of OH is effectively increased. The electron-rich OH greatly enhanced the interaction between the aromatic VOCs and the photocatalyst, concurrently promoting the reactive oxygen species formation including •OH and •O2–, which allowed rapid opening of the aromatic ring and deep oxidizing into CO2. 100% of toluene removal and mineralization efficiency was attained on ZnSn LDH at a high weight hourly space velocity (WHSV) of 60 000 mL gcat–1 h–1. Moreover, ZnSn LDH exhibited impressive adaptability for varying humidity and concentration, as well as satisfactory durability for long-term reaction. This work provides a simple and effective strategy for regulating the electron density of surface hydroxyls, and it provides an approach for purifying refractory pollutants.

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.