Hydroxyl Coverage Research Articles

Kinetics and Thermodynamics of H2O Dissociation on Reduced CeO2(111)

We use density functional theory to investigate the reaction between reduced CeO2–x(111) and water. H2O dissociation to hydroxyl is facile on surface vacancies and lattice oxygen, while subsequent decomposition of hydroxyl into H2 has a high barrier, which results in reversible adsorption of H2O under ultra-high-vacuum conditions. The barrier to H2 formation through hydroxyl decomposition decreases by 0.2 eV, while H2O formation becomes more difficult at high hydroxyl coverage. However, on isolated oxygen vacancies on a hydroxyl covered surface, H2 may be produced through a CeH intermediate with a 1.14 eV barrier. Oxygen vacancies are found to be more stable in the subsurface than in the surface layer at all vacancy coverages and for hydroxyl coverages less than 25–50%. The competition of H2O desorption and vacancy diffusion from the subsurface to the surface may prevent formation of hydroxyl from H2O dosing at low temperature, while the highly stable hydroxyl phase may provide a thermodynamic driving for...

Infrared spectroscopy study of adsorption and photodecomposition of formic acid on reduced and defective rutile TiO2 (110) surfaces

Adsorption and photodecomposition of formic acid on rutile TiO2 (110) have been investigated with infrared reflection–absorption spectroscopy (IRRAS) employing p- and s-polarized light along the [001] and [11¯0] crystal directions. The single crystal surfaces were prepared either by sputtering and annealing in ultrahigh vacuum (UHV) to obtain a reduced surface (r-TiO2), or by sputtering without annealing to create a rough, highly defective surface (sp-TiO2). Results are compared with corresponding measurements on rutile nanocrystals performed in synthetic air. IRRAS spectra obtained on r-TiO2 and rutile nanocrystals are very similar, and show that in both cases formic acid dissociates and is predominately adsorbed as a bridging bidentate formate species, and that the formate adsorption structure on the nanocrystals is dominated by interactions with majority (110) surfaces. In contrast, the IRRAS spectra on sp-TiO2 are different, with only minor spectral features associated with (110) surfaces and lost azimuthal symmetry, both of which imply changed adsorption geometry due to bonding to low-coordinated Ti atoms with lower valences. The UV-induced rate of formate photodecomposition is about 30 times higher on rutile nanocrystals in synthetic air compared with sp-TiO2 under UHV conditions, and even larger than on r-TiO2. These differences are explained by the lack of oxygen and limited hydroxyl coverage under UHV conditions. The difference in reactivity between the r-TiO2 and sp-TiO2 surfaces is attributed to a high concentration of strongly bonded bridging bidentate formate species on the (110) surface, which lowers its reactivity. The results point to a pressure gap where the availability of molecular oxygen and the hydroxyl concentration limit the photoreactivity in UHV leading to an almost 20-fold decrease of the formate degradation rate in UHV. In contrast, the structure represented by the single crystal (110) surface is shown to capture the essential structural properties, which dictates the formic acid adsorption and adsorption structure of rutile nanocrystals.

Effect of surface hydroxyl coverage on platinum nanoparticles in the oxygen reduction reaction: All-electron density functional theory analysis

Catalysts in commercial proton exchange membrane fuel cells must be optimized for activity and cost, but improvements are hampered for Pt nanoparticles by low activity resulting from extensive OH adsorption. A model system clearly elucidating the impact of OH coverage on Pt nanoparticles was therefore developed. Using first principles calculations, free energies due to changing OH coverage in the oxygen reduction reaction were predicted as functions of electrode potential and local surface structure. Free energy contour plots that considered surface OH coverage were developed. Therefrom, we theoretically predicted the optimum range of operating potentials, yielding design guidelines for catalytic surfaces.

Surface reactions of dimethyl ether on γ-Al2O3: 1. Adsorption and thermal effects

Reactions of dimethyl ether (DME) over γ-Al2O3 at 250°C have been investigated in a flow catalytic reactor. The main products of the interaction between DME and alumina are methanol and water. Heat evolution is observed as DME is passed over alumina, and replacing DME with nitrogen gives way to heat absorption. Calcination of alumina before the reaction considerably strengthens the exotherm, which is due to DME adsorption, while the endotherm is due to the desorption of weakly bound DME. The role of the hydroxyl groups of γ-Al2O3 in methanol and water formation has been elucidated. Treating alumina with water vapor after bringing it into contact with DME completely restores the hydroxyl cover and replaces strongly adsorbed DME with hydroxyl groups.

Influence of hydroxyls on Pd atom mobility and clustering on rutile TiO(2)(011)-2 × 1.

Understanding agglomeration of late transition metal atoms, such as Pd, on metal oxide supports, such as TiO2, is critical for designing heterogeneous catalysts as well as for controlling metal/oxide interfaces in general. One approach for reducing particle sintering is to modify the metal oxide surface with hydroxyls that decrease adatom mobility. We study by scanning tunneling microscopy experiments, density functional theory (DFT) calculations, and Monte Carlo (MC) computer simulations the atomistic processes of Pd sintering on a hydroxyl-modified TiO2(011)-2 × 1 surface. The formation of small 1-3 atom clusters that are stable at room temperature is achieved on the hydroxylated surface, while much larger clusters are formed under the same conditions on a hydroxyl-free surface. DFT shows that this is a consequence of stronger binding of Pd atoms adjacent to hydroxyls and increased surface diffusion barriers for Pd atoms on the hydroxylated surface. DFT, kinetic MC, and ReaxFF-based NVT-MC simulations show that Pd clusters larger than single Pd monomers can adsorb the hydrogen from the oxide surface and form Pd hydrides. This depletes the surface hydroxyl coverage, thus allowing Pd to more freely diffuse and agglomerate at room temperature. Experimentally, this causes a bimodal cluster size distribution with 1-3 atom clusters prevalent at low Pd coverage, while significantly larger clusters become dominant at higher Pd concentrations. This study demonstrates that hydroxylated oxide surfaces can significantly reduce Pd cluster sizes, thus enabling the preparation of surfaces populated with metal clusters composed of single to few atoms.

Electro-oxidation of formate-based solutions on Au/Pd core–shell nanoparticles – Experiment and simulation

Au/Pd core–shell nanoparticles (NPs) were employed to study electro-oxidation mechanism of formate-based solutions. A series of combined studies including Raman spectroscopy, electrochemical stripping analysis, and computational simulations were conducted to understand electro-oxidation of formate-based solutions on the Au/Pd NPs. Compared to the commercial Pd black, the Au/Pd NPs showed superior catalytic activities, especially when using concentrated formate solutions. CO stripping test in conjunction with in situ Raman studies showed a lower oxidation potential (0.6 V vs. Ag/AgCl) of CO on the Au/Pd NPs than the Pd black (>0.6 V). This indicated a strong electronic coupling between Au and Pd in the core–shell NPs which could enhance the oxidation of formate species. In addition, theoretical studies suggested lower formate and hydroxyl coverages at the same applied potential for the Au/Pd NPs than the Pd black in the formate-based solutions, shedding lights on their superior formate-oxidizing ability.

Impact of surface roughness of Au core in Au/Pd core–shell nanoparticles toward formic acid oxidation – Experiment and simulation

The Au/Pd core–shell nanoparticles (NPs) were synthesized via galvanic replacement of Cu by Pd on hollow Au cores by adding different concentrations of Na2SO3 solution. It was found that the higher concentration of Na2SO3 that was used, the rougher the Au nanospheres became. However, the rougher Au surface may cause more defects in the Pd layers and decrease the catalytic abilities. The Au/Pd NPs synthesized using 0 M Na2SO3 (denoted as 0 M-Au/Pd NPs) have the smoothest Pd surface and demonstrate higher formic acid oxidation (FAO) activity (0.714 mA cm−2, normalized to the surface area of Pd) than other Au/Pd NPs and commercial Pd black (0.47 mA cm−2). Additional electrochemical characterization of the 0 M-Au/Pd NPs also demonstrated lower CO-stripping onset and peak potentials, higher stability (8× improvement in stabilized oxidation current), and superior durability (by 1.6×) than the Pd black. In addition, a simple simulation of FAO was adopted to predict the anodic curve by including reaction intermediates of formate and hydroxyl. The 0 M-Au/Pd NPs were found to show higher formate and lower hydroxyl coverage than the Pd black.

Influence of natural adsorbates of magnesium oxide on its reactivity in basic catalysis

Solid materials possessing basic properties are naturally covered by carbonates and hydroxyl groups. Those natural adsorbates modify their chemical reactivity. This article aims to specifically evidence the role of surface carbonates and hydroxyls in basic heterogeneous catalysis on MgO. It compares the catalytic behaviors of hydroxylated or carbonated MgO surfaces for two types of reactions: one alkene isomerization and one alcohol conversion (hept-1-ene isomerization and 2-methyl-3-butyn-2-ol conversion). Catalysis experiments showed that carbon dioxide adsorption poisons the catalyst surface and the DRIFT-DFT combination showed that the nature of active sites in the two reactions differs. On the reverse, partial hydroxylation of the surface enhances activity for both reactions. Interestingly hept-1-ene isomerization gives a volcano curve for the conversion as a function of hydroxyl coverage. Calculations of the electronic structure of magnesium oxide surfaces show that neither Lewis basicity nor Brønsted basicity of the surface defects (steps for example) are enhanced by hydroxylation. Meanwhile CO2 adsorption followed by IR spectroscopy shows that (110) and (111) unstable planes are strongly basic and are stabilized by partial surface hydroxylation. These results could explain the volcano curve obtained for the evolution of alkene isomerisation as a function of hydroxyl coverage.

Highly‐Tunable Nickel Cobalt Oxide as a Low‐Temperature P‐Type Contact in Organic Photovoltaic Devices

AbstractWe report on the investigation of nickel cobalt oxide (NixCo3−xO4) thin films grown by pulsed laser deposition as hole‐transport interlayers (HTL) in organic photovoltaic (OPV) devices. Films of 7 nm thickness were grown under various oxygen deposition pressures (pO2) in the range of 2–200 mTorr. We explore both bulk and surface properties of these thin films. The workfunction (ϕ) for each of the films was statistically similar (∼4.7 eV), regardless of pO2. There was not a strong dependence of the power conversion efficiency (η) on the conductivities of the HTLs varying between 0.009 ‐ 10 S/cm. The observed differences in OPV efficiencies (ranging from 1.16 to 2.46%) were correlated to the near surface chemical composition of the NixCo3−xO4 HTL, as observed by differences in the relative surface hydroxyl concentration. The critical role of the near‐surface composition of the HTL at the HTL/organic interface was further explored by modifying the hydroxyl concentration using an oxygen plasma treatment. This treatment mitigated the impact of surface hydroxyl coverage, demonstrating either identical or increased values for ϕ and η, regardless of initial pO2 in the creation of the NixCo3−xO4 HTL. To further explore this we also employed a phosphonic acid surface modification agent on the HTL, increasing ϕ to 5.2 eV producing the best η value of 3.4%, equivalent to the PEDOT:PSS control devices. These results indicate that nickel cobalt oxide is a promising p‐type oxide for carrier‐selective interlayers in organic solar cells; however, for this to be fully realized the specific surface chemistry at the oxide/polymer interface must be controlled to increase ϕ and optimize device performance.

In situ MAS NMR investigation of the hydrogenation of acrylonitrile on Pt- and Rh-containing zeolites Y

This work presents first ex situ and in situ MAS NMR spectroscopic studies of the effect of noble metal reduction on the hydroxyl coverage of zeolites 0.9[Pt]Na-Y and 0.4[Rh]Na-Y and the hydrogenation of a double-bond-containing organic reactant, acrylonitrile. By 1H MAS NMR spectroscopy, it was found that the number of Brønsted acidic bridging OH groups (SiOHAl) formed upon noble metal reduction under flowing hydrogen nearly agrees with the number of the initially introduced complexes of bivalent metal cations. Hence, about 50% of the bivalent metal cations formed two SiOHAl groups per metal site upon reduction. After adsorption of reactants on the reduced metal-containing catalysts, no effect of the noble metals on the chemical shieldings of 13C nuclei of these molecules occurred. On the other hand, the 1H MAS NMR signals of reactant molecules adsorbed on zeolite 0.9[Pt]Na-Y have shown a significantly larger residual line width compared with those on zeolite 0.4[Rh]Na-Y, which indicates a higher mobility of reactants adsorbed on the latter zeolite. By in situ1H MAS NMR spectroscopy of the acrylonitrile hydrogenation carried out under semi-batch conditions at 295K, a reaction velocity on zeolite 0.4[Rh]Na-Y (r=7.0×10−3mmol/s) was determined, which is more than a factor six higher in comparison with that on zeolite 0.9[Pt]Na-Y (r=1.1×10−3mmol/s). During the initial period (t=12–24s) of the acrylonitrile hydrogenation on zeolite 0.4[Rh]Na-Y, weak 1H MAS NMR signals of temporary intermediates were observed. These temporary signals may be due to half-hydrogenated intermediates with short lifetime.

Oxidation of Au by Surface OH: Nucleation and Electronic Structure of Gold on Hydroxylated MgO(001)

The nucleation and electronic structure of vapor-deposited Au on hydroxylated MgO(001) surfaces has been investigated under ultrahigh vacuum conditions. Hydroxylated MgO(001) surfaces with two different hydroxyl coverages, 0.4 and 1 monolayer, respectively, were prepared by exposure to water (D(2)O) at room temperature. Scanning tunneling microscopy experiments show significantly higher gold particle densities and smaller particle sizes on the hydroxylated MgO surface as compared to gold deposited on clean MgO(001). Infrared spectroscopy and X-ray photoelectron spectroscopy experiments were performed to reveal details about the initial nucleation of gold. Gold atoms are found to chemically interact with a specific type of hydroxyl groups on the MgO surface, leading to the formation of oxidized gold particles. The enhanced adhesion of Au particles, which is due to the formation of strong Au-O interfacial bonds, is responsible for the observed higher stability of small Au clusters toward thermal sintering on hydroxylated MgO surfaces. The results are compared to similar studies on Au/TiO(2)(110) model systems and powder samples prepared by the deposition-precipitation route.

Comparison of Nanostructured Titania Matrices Obtained by Carbon Template and Sol–Gel Methods for Controlled Release of Fluoxetine

Two types of titania matrix were investigated as a support for the prolonged drug delivery of the antidepressant fluoxetine. Sample MT was synthesized using carbon template and consisted of titania microtubes on which then fluoxetine was adsorbed. Sample SG was synthesized by the sol-gel method when the drug was added during the reaction. The morphology of the powder surfaces was found to be different: nanotubes versus almost spherical particles with much larger surface area of SG and smaller pores. The relative degrees of hydroxyl coverage of the surface were studied by FTIR-spectroscopy and were found to be much larger for the sol-gel complex. Theoretical modeling was applied to consider possible interactions between the drug and the matrices. The liberation of the drug was proved to be faster from complex MT and was attributed to weaker drug-matrix interactions in combination with larger pore size.

Polyurethane-based bulk nanocomposites from 1-thioglycerol-stabilized CdTe quantum dots with enhanced luminescence

1-Thioglycerol (TG)-stabilized quantum dots (QDs) are good candidate for performing a stepwise polymerization reaction with polyurethane (PU) prepolymers, but their use is limited by their worse photoluminescence (PL) in comparison to QDs stabilized by other mercapto-ligands. To overcome this problem, TG-stabilized CdTe QDs with enhanced PL were synthesized in water through a N2H4-promoted growth approach. The current synthesis significantly shortened the duration of the size evolution of QDs, particularly for obtaining samples with orange and red emissions. It also permitted QD growth at low temperatures, such as room temperature, which avoided the decomposition of TG and subsequent embedment of sulfur into the QDs as occurs in the conventional synthesis. Most importantly, the TG ligand endowed the QDs with hydroxyl coverage, making the QDs miscible with PU prepolymers in dimethyl sulfoxide, and therefore overcoming the main problem in fabricating PU-based nanocomposites of eliminating water. The hydroxyl coverage further allowed for the linkage of PU on the surface of the QDs through the reaction between –OH and –NCO, producing CdTe QD–PU bulk nanocomposites. Systematic characterization indicated that the QDs were well dispersed in the PU medium, and the size-dependent PL of QDs was also maintained.

Polyoxometalate grafting onto silica: stability diagrams of H3PMo12O40 on {001}, {101}, and {111} β-cristobalite surfaces analyzed by DFT

The process of grafting H(3)PMo(12)O(40) onto silica surfaces is studied using periodic density functional theory methods. For surfaces with a high hydroxyl coverage, the hydroxyl groups are consumed by the polyoxometalate protons, resulting in water formation and the creation of a covalent bond between the polyoxometalate and the surface, and mostly no remaining acidic proton on the polyoxometalate. When the surfaces are partially dehydroxylated and more hydrophobic, after temperature pretreatment, less covalent and hydrogen bonds are formed and the polyoxometalate tends to retain surface hydroxyl groups, while at least one acidic proton remains. Hence the hydroxylation of the surface has a great impact on the chemical properties of the grafted polyoxometalate. In return, the polyoxometalate species affects the compared stability of the partially hydroxylated silica surfaces in comparison with the bare silica case.

Reactive Molecular Dynamics Studies of DMMP Adsorption and Reactivity on Amorphous Silica Surfaces

By using molecular dynamics (MD) computer simulations in conjunction with the ReaxFF reactive force fields, the interaction of dimethyl methylphosphonate (DMMP) with amorphous silica as a function of surface hydration was examined. Surface hydroxylation densities of 2.0, 3.0, 4.0, and 4.5 hydroxyl/nm2 were modeled. The amorphous silica surface used in our simulations is quantified structurally and compares well to experimental findings. At the higher OH densities, binding of DMMP to the hydroxylated silica was found to occur through a combination of van der Waals interactions and hydrogen bonding. In addition to these types of interactions, at the lower OH surface coverages, strong covalent bonding between the phosphonyl (P═O) oxygen of DMMP and 3-coordinate Si defects on the surface was observed. Finally, at extremely low hydroxyl coverages (2.0 nm−2), DMMP fragmentation was found to occur. The binding energy of DMMP on amorphous silica with a hydroxyl density of 4.5 OH/nm2 was calculated to be −4.7 kcal/mol. Addition of a water layer to the silica-supported DMMP system showed that water can displace and/or hydrolyze the adsorbed DMMP molecules. To validate the ReaxFF/MD findings, we performed MP2 and DFT quantum chemical studies of reactions predicted by the MD/ReaxFF by using small silica clusters. The quantum chemistry results support the MD/ReaxFF results, providing further verification of our findings and indicating the viability of ReaxFF/MD to study complex surface chemistry.

Adhesion at Al-hydroxide-polymer interfaces: Influence of chemistry and evidence for microscopic self-pinning

We present a combined chemical (auger electron spectroscopy) and microscopic (optical microscopy, scanning electron microscopy and scanning probe microscopy) study of the work of adhesion and delamination mechanisms at interfaces between a glassy polymer (glycol-modified polyethylene terephthalate) and Al covered with different types of surface hydroxides. A clear correlation between the measured work of adhesion and the chemical nature of the Al surface, specifically the hydroxyl coverage and the iso-electric point is found. The magnitude of the work of adhesion points to important contributions from plastic deformation in the glassy polymer for some cases. Delamination is shown to be accompanied by the formation of microscopic shear bands at such interfaces. The non-monotonous stress–strain behaviour of the glassy polymer that gives rise to the shear bands is also shown to lead to peculiar pinning events at the crack front. Evidence indicates that the occurrence of protrusions in the crack front deriving for example from the presence of stress concentrators and crack initiation sites ahead of the front, combined with mode and rate dependence of the local energy release rate along the front may pin the front at positions adjacent to a protrusion. It is believed such microscopic mode-dependent pinning phenomena may be relevant for the adhesion on patterned interfaces.

Reconstruction and stability of β-cristobalite 001, 101, and 111 surfaces during dehydroxylation

We analysed the dehydroxylation of 001, 101, and 111 β-cristobalite surfaces using the periodic density functional theory method and established the OH density stability diagrams of these surfaces as a function of temperature and water partial pressure. Our calculations suggest that important surface reconstructions, involving SiO(2) unit migrations, are required to reach the experimentally measured values for hydroxyl coverage. Our thermochemical data, i.e., 3.7-5.2 OH nm(-2) in standard conditions and 1.4-2.6 OH nm(-2) at P = 10(-10) atm and T = 800 K, agree with the experimental values for amorphous silica and explain the trends observed, although some topological differences obviously exist between our periodic models and amorphous silica surfaces.

Quartz Crystal Microbalance Studies of Al2O3 Atomic Layer Deposition Using Trimethylaluminum and Water at 125 °C

Al(2)O(3) atomic layer deposition (ALD) growth with Al(CH(3))(3) (trimethylaluminum (TMA)) and H(2)O as the reactants was examined at the relatively low temperature of 125 degrees C using quartz crystal microbalance (QCM) measurements. The total Al(2)O(3) ALD mass gain per cycle (MGPC) and MGPCs during the individual TMA and H(2)O reactions were measured versus TMA and H(2)O exposures. The Al(2)O(3) MGPC increased with increasing H(2)O and TMA exposures at fixed TMA and H(2)O exposures, respectively. However, the TMA and H(2)O reactions were not completely self-limiting. The slower surface reaction kinetics at lower temperature may require very long exposures for the reactions to reach completion. The Al(2)O(3) MGPCs increased quickly versus H(2)O exposure and slowly reached limiting values that were only weakly dependent on the TMA doses. Small TMA exposures were also sufficient for the Al(2)O(3) MGPCs to reach different limiting values for different H(2)O doses. The TMA MGPCs increased for higher TMA exposures at all H(2)O exposures. In contrast, the H(2)O MGPCs decreased for higher TMA exposures at all H(2)O exposures. This decrease may occur from more dehydroxylation at larger hydroxyl coverages after the H(2)O exposures. The hydroxyl coverage after the H(2)O exposure was dependent only on the H(2)O exposure. The Al(2)O(3) MGPC was also linearly dependent on the hydroxyl coverage after the H(2)O dose. Both the observed hydroxyl coverage versus H(2)O exposure and the Al(2)O(3) ALD growth versus H(2)O and TMA exposures were fit using modified Langmuir adsorption isotherm expressions where the pressures are replaced with exposures. These results should be useful for understanding low-temperature Al(2)O(3) ALD, which is important for coating organic, polymeric, and biological substrates.

Stabilization of the (1 1 0) tetragonal zirconia surface by hydroxyl chemical transformation

Different hydroxyl coverages on the (1 1 0) and (1 0 1) surfaces of tetragonal zirconia have been studied with periodic density functional theory. The tetragonal zirconia (1 1 0) surface is polar and intrinsically unstable. It is however very efficiently stabilized by hydroxyl formation which decreases the effective charge of surface oxygen atoms and hence avoids the electrostatic instability. The hydroxylation induces a strong stabilization of the (1 1 0) surface with respect to the non-polar (1 0 1) termination, and explains why the (1 1 0) surface of ZrO 2 can be found in some catalytic preparations. Surface chemical transformation appears to be a more efficient way to stabilize the (1 1 0) surface in comparison with the surface reconstruction processes.

<I>In-Situ</I> Fourier Transform Infrared Spectroscopy Study on CO Oxidation Over Au/Titanate and Au/Titania Nanocatalysts

Surface charge state and surface hydroxyl coverage of the catalyst support were reported to have great impact on the CO catalytic oxidation activity. Protonic layered titanate nanotubes are featured with cation-vacancy defects and full surface Ti-OH group coverage, whereas titania nanoparticles are featured with anion-vacancy defects and partly Ti-OH surface group coverage. In order to explore the influence on the activity from these differences, Fourier transform infrared spectra study of CO adsorption and oxidation over the Au/titanate and Au/titania catalysts is performed and the results indicate that (i) both Au/titanate and Au/titania have strong adsorption for CO molecules, and (ii) the Au/titania sample has a higher reactive activity for CO oxidation than the Au/titanate sample. This phenomenon is reasonably explained combining with a traditional strong metal-support interaction theory known in CO catalytic oxidation reactions.