Surface Hydroxyl Research Articles

The Roles of Surface Hydrogen and Hydroxyl in Alkaline Hydrogen Oxidation on Ni-based Electrocatalysts.

One important target for anion exchange membrane fuel cells (AEMFCs) is to enable the application of anode non-precious metal hydrogen oxidation reaction (HOR) catalyst. Nickel presents a promising candidate for alkaline HOR; yet, its practical application is hampered by the intrinsically sluggish activity and poor stability. Herein, a series of Ni-based metals (Ni5Mo, Ni25Co, Ni14W and Ni) are electrodeposited as model catalysts to systematically explore the alkaline HOR by considering the role of adsorbed hydroxyl (OHad). Spectroscopic studies together with density functional theory calculations shed light on the beneficial effect of transition metal M (M=Mo, Co, W) alloying/doping on HOR by introducing the charge transfer from M to Ni and down shifting Ni 3d band center. The HOR specific activities on Ni-based catalysts reveal a volcano-type relationship with the hydrogen binding energy (HBE). The strongly adsorbed OHad is proven to induce deactivation for Ni active sites, and the deactivation potential is OHad binding energy (OHBE) dependent. This study adds new insight into the HOR mechanism and stability of Ni-based electrocatalysts, providing a new avenue for the rational design of highly efficient and robust alkaline HOR catalysts.

Surface Hydroxyl Groups Functionalized Porous CeO2 for Enhanced Selective Adsorption of As(III).

The adsorption technique has been considered as one of the promising methods to remove arsenic ions in aqueous systems. However, the adsorbents are usually poorly selective and have low capacity. Herein, a kind of porous CeO2 containing surface hydroxyl group which synthesized facilely possesses the great performance of selective adsorption of As(III) with 98.57% removal against 14 other coexisting metal ions. The results showed that the processes of As(III) and As(V) uptake were heterogeneous monolayer adsorptions and included external and intraparticle diffusions. The adsorption capacities at pH 2.5 reached 111.24 and 56.89 mg/g for As(III) and As(V), respectively. In addition, it also showed that As(III) could be oxidized to As(V) in the adsorption process. Density functional theory calculations revealed that the OH group in H3AsO30 and the As atom in H3AsO40 have affinity with lattice oxygen (O2-) in CeO2, while the O atom in H2AsO4- preferred the Ce atom in CeO2. This study provides a novel porous CeO2 containing hydroxyl groups for selective and efficient removal of arsenic and elucidates the mechanism of As(III) and As(V) adsorptions.

Tuning the interaction between Cu and surface -OH in Cu/Ti3C2 MXene derived catalyst for enhanced semi-hydrogenation of dimethyl oxalate

Methyl glycolate (MG), a crucial chemical raw material and fine chemical intermediate, holds immense potential in the production of the next-generation biodegradable material, polyglycolic acid, via polycondensation reactions. In this study, layered Ti3C2 MXene was used as support precursor to construct Cu based catalysts for the semi-hydrogenation of dimethyl oxalate to MG. By fine-tuning the loading method of Cu species, the interaction between Cu species and surface hydroxyl groups (-OH) was modulated, and the effect on the semi-hydrogenation performance of Cu-based catalysts were thereby explored. These catalysts underwent rigorous characterization using BET, SEM-EDS, XRD, FT-IR, H2-TPR and XPS techniques. The results revealed that varying loading methods significantly influenced the interaction between Cu species and surface -OH on Ti3C2 MXene during preparation process, altering the content of Ti-O-Cu as well as the Cu0 ratio of Cu/Ti3C2-derived catalysts. The Cu/Ti3C2 MXene-I catalyst displayed remarkable performance, achieving a MG selectivity of 88.85 % and stability of up to 120 h in the DMO hydrogenation reaction due to its high Cu0 ratio and large Cu particles.

Photo-electrochemical decomplexation of cobalt complexes and enhanced cobalt recovery through electrified membranes

The increasing presence of cobalt-chelated species in semiconductor manufacturing poses significant challenges for wastewater treatment because of their structural stability and high solubility. Efficient decomplexation of these metal–ligand complexes is critical for mitigating heavy metal pollution and enabling resource recovery. This study investigates the (photo)electrochemical decomplexation of cobalt-ethylenediaminetetraacetic acid (Co-EDTA) complexes using a TiO2-encapsulated electrified carbon membrane, with a focus on cobalt recovery. Compared to electrochemical oxidation alone, the photoelectrochemical (PEC) system with 365 nm UV-LED illumination demonstrated better performance, achieving over 90 % EDTA-Na2 removal within 20 min and approximately 50 % Co-EDTA decomposition within 180 min. Kinetic analysis confirmed enhanced reaction rates under PEC conditions, and system robustness was validated across varying applied voltages and electrolyte concentrations, as well as real-world semiconductor wastewater. Mechanistic studies revealed that the PEC process is driven by the generation of hydroxyl radicals (•OH), formed through the interaction of photoinduced electron-hole pairs with surface hydroxyl groups on the TiO2 membrane. These radicals facilitate stepwise decomplexation, breaking down Co-EDTA complexes and releasing cobalt ions. This work highlights the potential of TiO2 electrified membrane-based photo-electrochemical systems for efficient decomplexation and cobalt recovery, offering a sustainable solution for advanced wastewater treatment and resource recovery.

Unusually high selectivity (100 %) of photocatalytic C[sbnd]C coupling achieved by instant reverse reduction of byproducts

The photocatalytic CC coupling of benzyl alcohol (BA) into hydrobenzoin (HB), is appealing to obtain high-value chemicals. However, the selectivity of HB is still low due to the inevitable formation of benzaldehyde. Herein, we report In(OH)3-ZnS photocatalyst for CC coupling of BA into HB with very high selectivity (∼100 %). The introduction of In(OH)3 onto ZnS with stable interaction facilitates light harvesting and separation of photo-excited charges. As a result, BA conversion on optimized In(0.1)-ZnS catalyst (73 %) is much higher than ZnS (29 %). Besides, the surface hydroxyl groups derived from In(OH)3 enables the facile desorption of CH(OH)Ph radical. Therefore, the over oxidation of CH(OH)Ph radical into by-product of benzaldehyde can be effectively inhibited. More significantly, in-situ FTIR spectra and reduction of by-product manifest the instant reverse reduction process of benzaldehyde into CH(OH)Ph radical during CC coupling of BA, which is the key to realizing satisfied HB selectivity (100 %). Theoretical simulations reveal that the weak adsorption of CH(OH)Ph radical over catalyst and the high energy barrier of over-oxidation of CH(OH)Ph into benzaldehyde contributes to the formation of highly selective coupling products. This work will inspire new insights to design rational photoredox systems for organic transformations with high selectivity

Efficiently enhancing stability for a low concentration HCHO degradation using hexagonal prism MnCe-MOFs catalysts

The efficient and stable removal of indoor formaldehyde (HCHO) a low concentration, as required by the national standard (≤ 0.08 mg/m3), remains a significant challenge. In this study, we investigated a novel hexagonal prism structure of MnCe-MOFs catalysts, synthesized via the hydrothermal method using DMF (N, N-Dimethylformamide), for the complete degradation of HCHO at ambient temperature. Monometallic and bimetallic MOFs materials, as well as the effects of calcination temperature and the stability of these high-performance catalysts in degrading HCHO were investigated and characterized using EDS, XPS, H2-TPR, IR and ESR. Results demonstrated that HCHO degradation using MnCe-MOFs achieved 97.2% efficiency within 48 hours, maintaining above 96% efficiency after three experimental cycles, exhibiting the highest activity and stability. The high performance was attributed to the columnar structure with abundant pores and a high specific surface area, lower initial reduction temperature, and an abundant presence of surface hydroxyl groups. Moreover, the spherical nanoparticles of MnCe-MOFs were uniformly distributed, and the highly dispersed CeOx provided plentiful active sites, contributing to the catalysts’ high activity and stability.

Synergistic mechanism of disparate surface hydroxyls and oxygen vacancies towards peroxymonosulfate activation for durable water decontamination

Synergistic mechanism of disparate surface hydroxyls and oxygen vacancies towards peroxymonosulfate activation for durable water decontamination

Application of Surface Complexation Modeling to Investigate the Mechanism of Cu2+ Adsorption on TiO2, Al2O3, and SiO2 Under High Surface Coverage

We have shown that the adsorption of Cu2+ ions on various metal oxides, depending on the pH of the solution, can be described assuming the formation of only two surface complexes with surface hydroxyl groups SOH: SOCu(OH) and SOCu+. Using an ion-selective electrode for Cu2+, we determined the adsorption edges, i.e., the dependence of the amount of adsorbed metal expressed as a percentage depending on the solution pH for three oxides: TiO2, Al2O3, and SiO2. The measurements were carried out with high surface coverage where the ratio of the adsorption sites/copper ions in the system were from 2 to 3, depending on the oxide. Simultaneously, with the adsorption edge, the hydrogen surface charge density and the electrokinetic potential of the oxide particles were measured as a function of pH. These three types of experimental data were fitted all together using the surface complexation model (2-pK TLM). In modeling, it was not necessary to consider the precipitation of Cu(OH)2 on the oxide surface to obtain good agreement with the data. Additionally, it was shown that the presence of charged surface species SOCu+ (about 10% of total adsorbed copper) was crucial to fit the data for zeta potential.

CO induces the reduction of Mo in the glass liquid to molybdenum carbide to form glass defects in the hybrid furnace

In this work, a ceramic platinum continuous melting optical glass furnace containing oxygen combustion was taken as an example to analyze the causes of a unique glass defect. It was finally determined that the defect in the glass was composed of the top condensate, the brick skeleton, and the Mo2C generated in the glass liquid. It was determined that the monoclinic Al2O3 and ZrO2 phase in the brick desorbed the surface hydroxyl and water molecules under the high temperature environment inside the furnace, forming Lewis acid sites with the ability to adsorb electrons. CO was adsorbed on the surface of Al2O3 and ZrO2 by Lewis acid sites and fell into the glass liquid with the top condensate and the brick skeleton.CO then reduced Mo ions in the glass melt to form Mo2C, causing the formation of the defects in the glass.

Surface hydroxyls of spinel oxides: A double-edged sword in the peroxymonosulfate activation process

The sulfate-based advanced oxidation process (SR-AOP) mediated by spinel oxides has shown great potential in pollutant degradation. However, the role of surface hydroxyls in the reaction process remains unclear, especially in real environments containing salts. Here, we investigated the role of surface hydroxyls of spinel oxides in activating peroxymonosulfate (PMS). By adjusting the intrinsic composition of the metal elements, we synthesized a CoFeMnO4 spinel with high surface hydroxyls using a simple and scalable co-precipitation method. CoFeMnO4 demonstrated unparalleled activation performance and a unique non-radical activation pathway (dominated by high-valence metal-oxo species and singlet oxygen), achieving 100 % sulfamethoxazole degradation within 4 min with a low concentration of PMS. Surface hydroxyls of CoFeMnO4 played a crucial role in the activation process, serving as adsorption sites for PMS and facilitating its activation. Interestingly, we discovered that several common anions (Cl−, HCO3−, SO42− and NO3−) could undergo ligand exchange with surface hydroxyls of CoFeMnO4, occupying some of sites intended for PMS adsorption, which significantly inhibited the PMS decomposition on the catalyst surface. Further, it was finally discovered that the inhibition of the reaction system by these anions was primarily due to their competition for surface hydroxyls, rather than the commonly believed the consumption of reactive oxygen species by anions. Given the inevitable presence of various anions in real water bodies, this finding highlights the dual nature of surface hydroxyls as active sites in SR-AOPs: While they aid in activation process, the competition from anions for them would diminish the overall system performance.

Facet-Dependent Photocatalytic CO2 Reduction on TiO2 Crystals in the Presence of SO2: Role of Surface Hydroxyl.

Photocatalytic CO2 reduction to solar energy makes great sense to mitigate the greenhouse effect caused by CO2, and great efforts have been made to promote CO2 conversion efficiency. However, the effect of impurities in the photoreduction of CO2 has received relatively little attention. Here, the different CO2 photoreduction behaviors of TiO2 exposed (101) facet (TiO2-101) and (001) facet (TiO2-001) with an SO2 impurity were investigated. On TiO2-101, SO2 accelerates the deactivation of the catalyst for CO2 photoreduction activity, since SO2 mainly binds to the OHb site of TiO2-101. This site is highly susceptible to the transfer of photogenerated holes, resulting in the rapid generation of SO42-, which occupies the active site and poisons the catalyst. For TiO2-001, SO2 has relatively little negative effect on stability, as SO2 mainly binds to the weak binding site (OHt) of TiO2-001, preventing it from being oxidized to SO42-, which alleviates catalyst deactivation and ensures the continuity of CO2 reduction. This paper provides further insights into the role of SO2 in CO2 photoreduction over distinct TiO2 facets and reveals the importance of facet engineering in the photocatalytic reduction of CO2 from industrial flue gas.

Metal heteroatoms significantly enhance efficacy of TiO2 nanomaterials in promoting hydrolysis of organophosphates: Implications for mitigating pollution of plastic additives

Organophosphate esters (OPEs) are prevalent pollutants in the aquatic environment. OPEs are released from many sources, particularly, from the breakdown and weathering of plastic wastes, as OPEs are commonly used plastic additives. Metal oxide mineral nanoparticles play critical roles in the hydrolytic transformation of OPEs. While natural minerals often contain metal impurities, it is unclear how metal heteroatoms affect the efficiency of mineral nanoparticles in mediating hydrolysis reactions. Herein, we show that transition metal-doped anatase titanium dioxide (TiO2) nanomaterials are more effective in catalyzing the hydrolysis of 4-nitrophenyl phosphate (pNPP), a model OPE compound, with the relative effects of the heteroatoms following the order of Fe > Cr > Mn ≈ Ni > Co > Cu. With multiple lines of evidence based on spectroscopic analysis, kinetics modeling, and theoretical calculations, we show that metal doping increases the Lewis acidity of the TiO2 nanomaterials by increasing the oxidation state of surface Ti atoms, inducing oxygen vacancies, and creating additional Lewis acid sites with stronger acidities. Moreover, the increased amounts of surface hydroxyl groups due to metal doping enhance inner-sphere complexation of pNPP through ligand exchange. The interesting observation that Fe-doped TiO2 exhibited the highest catalytic efficiency may have important implications for reducing the risks of organophosphate plastic additives, as iron is the most common heteroatom of naturally occurring TiO2 (nano)materials.

Increasing hydrophobicity of ceramic membranes by post-deposition nitrogen annealing of molecular layer deposition grown hybrid layers

Molecular layer deposition is increasingly used to functionalize the surface of planar and 3D porous substrates. This study explores functionalized ‘alucone’ hybrid layers grown on ceramic substrates to enhance their surface hydrophobicity. The layers were made from trimethyl-aluminum as a precursor with five different alcohols as a co-reactant: aliphatic (ethane-1,2-diol, 2,2-oxydiethanol, 1,6-hexanediol), and aromatic (4-aminophenol, benzene-1,4-diol). After post-deposition annealing in N2 atmosphere at 250 and 350 °C, the changes in the physical properties and the chemical affinity of the layers were studied and compared to those of the as-deposited hybrid layers. Differential thermal analysis of the hybrid layers showed higher decomposition temperatures forthe layers grown from aromatic alcohols than for those grown from aliphatic alcohols. Additionally, Fourier-transform infrared and X-ray photoelectron spectroscopy showed that an increase in annealing temperature decreased the concentrations of surface hydroxyl-groups and caused changes in the carbon-related groups. These results could be correlated to the hydrophobicity of the layers: higher water contact angles (>90°) were measured for annealed samples, compared to those (<90°) of the as-deposited samples grown at 150 °C. These findings confirm that the proposed method of MLD functionalization and post-deposition annealing can be used to tune the surface hydrophobicity of ceramic membranes.

Photocatalytic ammonia synthesis from nitrogen in water using iron oxides: Comparative efficiency of goethite, magnetite, and hematite

Photocatalytic ammonia synthesis from nitrogen and water presents a promising pathway for decentralized sustainable ammonia production, leveraging the abundant solar energy. In this study, we explore the efficacy of three iron oxide polymorphs – goethite (α-FeO(OH)), magnetite (Fe3O4), and hematite (α-Fe2O3) – as photocatalysts for nitrogen reduction under ultraviolet (UV) light. The materials were synthesized using hydrothermal and polymeric precursor methods, characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), UV–Vis spectroscopy, photoluminescence spectroscopy, and thermal analysis to understand their structural, surface, and optoelectronic properties. Among the materials tested, goethite demonstrated the highest ammonia production rate (20.6 µmol g−1h−1), which we attribute to its larger specific surface area and the stability of its surface hydroxyl groups, which play a critical role in facilitating the protonation and electron transfer necessary for nitrogen reduction. Curiously, magnetite also displayed some activity (10.3 µmol g−1h−1), likely due to the formation of a heterojunction with the co-occurring goethite phase. Hematite showed the fastest area-based production rate (1.05 µmol m−2h−1), suggesting it is the polymorph with highest density of active sites for N2 reduction. This work contributes to the ongoing search for greener and lower-cost alternatives to the Haber-Bosch process, with implications for both agriculture and energy storage.

Hydroxylation surfaces dominantly enhanced xylene sensing dynamics in CuCo2O4/CuO/Cu heterostructures

The surface hydroxylation is a feasible strategy for optimizing the performance of functional materials, but is rarely applied in gas sensing field and the parallel gas sensing improvement mechanism has not been clearly revealed. Herein, sodium borohydride (NaBH4) induced reduction treatment is proposed to engineer the surfaces of CuCo2O4/CuO composite sensing materials. The surface metal hydroxyl groups (M-OH) or hydroxyl groups (-OH) of as-prepared CCO-0.3 materials (treating CuCo2O4/CuO by 0.3 mmol/L NaBH4 solution) specifically adsorb xylene gas via methyl groups through noncovalent interactions. Thus, hydroxylated surfaces promote xylene gas recognition and signal conversion, and bring significantly enhanced xylene response and sensing selectivity. This reduction strategy holds great promise for improving the performances of various gas sensing materials.

Dynamically Confined Active Silver Nanoclusters with Ultrawide Operating Temperature Window in CO oxidation.

Supported metal nanoclusters are often highly active in many catalytic reactions but less stable particularly under harsh reaction conditions. Here, we demonstrate that this activity-stability trade-off can be efficiently broken through rational design of surrounding microenvironment of the supported nanocatalyst including gas adsorbate overlayer and underneath support surface chemistry. Our studies reveal that chemisorbed oxygen species on Ag surface and surface hydroxyl groups on oxide support, which are dynamically consumed during reaction but sustained by reaction environment (O2 and H2O vapor), drive spontaneous redispersion of Ag particles and stabilization of highly active Ag nanoclusters. Such a dynamic confinement effect from gas-catalyst-support interaction enables the Ag nanoclusters to exhibit complete catalytic oxidation of CO over a wide temperature window of 25 - 500 °C under dry conditions and 200 - 800 °C under wet conditions as well as remarkable stability at 300 °C over 1000 h.

Dynamically Confined Active Silver Nanoclusters with Ultrawide Operating Temperature Window in CO oxidation

AbstractSupported metal nanoclusters are often highly active in many catalytic reactions but less stable particularly under harsh reaction conditions. Here, we demonstrate that this activity‐stability trade‐off can be efficiently broken through rational design of surrounding microenvironment of the supported nanocatalyst including gas adsorbate overlayer and underneath support surface chemistry. Our studies reveal that chemisorbed oxygen species on Ag surface and surface hydroxyl groups on oxide support, which are dynamically consumed during reaction but sustained by reaction environment (O2 and H2O vapor), drive spontaneous redispersion of Ag particles and stabilization of highly active Ag nanoclusters. Such a dynamic confinement effect from gas‐catalyst‐support interaction enables the Ag nanoclusters to exhibit complete catalytic oxidation of CO over a wide temperature window of 25‐500 °C under dry conditions and 200‐800 °C under wet conditions as well as remarkable stability at 300 °C over 1000 h.

Formation of Both Free Hydroxyl Radicals and Surface Oxygen During Catalytic Ozonation by Single-Atom Iron: An Overlooked Pollutant-Dependent Oxidation Mechanism

Formation of Both Free Hydroxyl Radicals and Surface Oxygen During Catalytic Ozonation by Single-Atom Iron: An Overlooked Pollutant-Dependent Oxidation Mechanism

Singlet Oxygen Photocatalytic Generation by Silanized TiO2 Nanoparticles.

A commercial TiO2 sample, used as received or hydrothermally treated to increase surface hydroxylation, has been functionalized by surface modification with hexadecyltrimethoxysilane. The anchoring of the silane has been characterized by means of FTIR and solid-state NMR spectroscopies, and the grafting density was determined by thermogravimetric and N2 physisorption analyses. The silane moieties induce a partial decrease of the shielding of the valence electrons of the Ti ions at the surface, and a local modification of their crystal field, as demonstrated by XPS and UV/Vis spectroscopy, respectively. The changes in coordination and the produced oxygen vacancies result in the formation of Ti3+ defects localized in the sub-surface region, as revealed by EPR spectroscopy. These paramagnetic centers are stabilized in the silanized samples, as the electron transfer to O2 is efficiently inhibited even under UV irradiation. However, the amount of Ti3+ centers appears to be correlated with the singlet oxygen (1O2) formation rate. Accordingly, epoxidation of limonene under UV light, chosen as a model photocatalytic reaction triggered by 1O2, occurred with higher selectivity when TiO2 was silanized and upon simultaneous NIR irradiation. These evidences suggest that in the silanized sample 1O2 may be generated through Förster-type energy transfer from excited sub-surface Ti3+ centers.

Singlet Oxygen Photocatalytic Generation by Silanized TiO2 Nanoparticles

AbstractA commercial TiO2 sample, used as received or hydrothermally treated to increase surface hydroxylation, has been functionalized by surface modification with hexadecyltrimethoxysilane. The anchoring of the silane has been characterized by means of FTIR and solid‐state NMR spectroscopies, and the grafting density was determined by thermogravimetric and N2 physisorption analyses. The silane moieties induce a partial decrease of the shielding of the valence electrons of the Ti ions at the surface, and a local modification of their crystal field, as demonstrated by XPS and UV/Vis spectroscopy, respectively. The changes in coordination and the produced oxygen vacancies result in the formation of Ti3+ defects localized in the sub‐surface region, as revealed by EPR spectroscopy. These paramagnetic centers are stabilized in the silanized samples, as the electron transfer to O2 is efficiently inhibited even under UV irradiation. However, the amount of Ti3+ centers appears to be correlated with the singlet oxygen (1O2) formation rate. Accordingly, epoxidation of limonene under UV light, chosen as a model photocatalytic reaction triggered by 1O2, occurred with higher selectivity when TiO2 was silanized and upon simultaneous NIR irradiation. These evidences suggest that in the silanized sample 1O2 may be generated through Förster‐type energy transfer from excited sub‐surface Ti3+ centers.