Reactivity Of Surface Species Research Articles

A general method for studying reactive surface species, CT-SKAn: Charge-transfer spectrokinetic analysis

A spectrokinetic methodology was developed to determine reactive surface species by combining operando ultraviolet–visible spectroscopy and charge transfer kinetic models. The methodology consisted of three general steps: 1) concomitant measurement of reaction rates and charge transfer via ultraviolet–visible spectroscopy; 2) development of rate expressions from kinetic models involving the adsorbed species of interest. These rate expressions relate reaction rates, charge transfer, and partial pressures of gas phase species; and 3) evaluation of the goodness of fit of the rate expressions to the experimental data. The species whose rate expressions show the best fit are the more likely reactive surface species for the studied reaction. An example is presented for the determination of reactive oxygen species during ethanol oxidation over Au/TiO2. The charge transfer spectrokinetic analysis showed that surface hydroperoxyl, hydroxyl, and atomic oxygen species were reactive surface intermediates, whereas surface molecular oxygen was not.

Reaction with Water Vapor Defines Surface Reconstruction and Membranolytic Activity of Quartz Milled in Different Molecular Environments.

Industrial processing of quartz (SiO2) and quartz-containing materials produces toxic dust. Fracturing quartz crystals opens the Si‒O bond and produces highly reactive surface species which mainly react with molecules like water and oxygen. This surface-reconstruction process forms silanol (Si‒OH) on the quartz surface, which can damage biological membranes under specific configurations. To comprehend the impact of the quartz surface restructuring on membranolytic activity, the formation and reactivity of quartz radicals produced in four distinct molecular environments with electron paramagnetic resonance (EPR) spectroscopy are evaluated and their membranolytic activity is measured through in vitro hemolysis test. The four molecular environments are formulated with and without molecular water vapor and oxygen (±H2O/±O2). The absence of water favored the formation of surface radical species. In water-rich environments, diamagnetic species prevailed due to radical recombination. Quartz milled in -H2O/±O2 acquired membranolytic activity when exposed to water vapor, unlike quartz milled in +H2O/±O2. After being stabilized by reaction with water vapor, the membranolytic activity of quartz is maintained over time. It is demonstrated that the type and the reactivity of radical sites on quartz are modulated by the outer molecular environment, ultimately determining the biological activity of milled quartz dust.

SiGeSn buffer layer for the growth of GeSn films

Inclusion of Si atoms to the growth surface during the molecular beam epitaxy of Ge and Sn to form a SiGeSn alloy was identified as a reactive surface species and as a means to compensate strain, which allowed for the subsequent growth of GeSn alloys with high Sn content. The development of a SiGeSn virtual substrate having a 15% Sn concentration and lattice parameter larger than 5.72 Å is demonstrated, using atomic force microscopy, x-ray reciprocal space mapping, and transmission electron microscopy, as a method for the direct growth of thick (>500 nm) fully relaxed GeSn alloys with greater than 10% Sn. This buffer layer enables the monolithic integration of GeSn with silicon for optoelectronic applications, as the SiGeSn virtual substrate allows for selective chemical etching of GeSn, which is important for device fabrication.

Deep Learning-Assisted Investigation of Electric Field-Dipole Effects on Catalytic Ammonia Synthesis.

External electric fields can modify binding energies of reactive surface species and enhance catalytic performance of heterogeneously catalyzed reactions. In this work, we used density functional theory (DFT) calculations-assisted and accelerated by a deep learning algorithm-to investigate the extent to which ruthenium-catalyzed ammonia synthesis would benefit from application of such external electric fields. This strategy allows us to determine which electronic properties control a molecule's degree of interaction with external electric fields. Our results show that (1) field-dependent adsorption/reaction energies are closely correlated to the dipole moments of intermediates over the surface, (2) a positive field promotes ammonia synthesis by lowering the overall energetics and decreasing the activation barriers of the potential rate-limiting steps (e.g., NH2 hydrogenation) over Ru, (3) a positive field (>0.6 V/Å) favors the reaction mechanism by avoiding kinetically unfavorable N≡N bond dissociation over Ru(1013), and (4) local adsorption environments (i.e., dipole moments of the intermediates in the gas phase, surface defects, and surface coverage of intermediates) influence the resulting surface adsorbates' dipole moments and further modify field-dependent reaction energetics. The deep learning algorithm developed here accelerates field-dependent energy predictions with acceptable accuracies by five orders of magnitudes compared to DFT alone and has the capacity of transferability, which can predict field-dependent energetics of other catalytic surfaces with high-quality performance using little training data.

Transition state stabilization depends on solvent identity, pore size, and hydrophilicity for epoxidations in zeolites

Ti-silicates activate H2O2 to form Ti-hydroperoxo and Ti-peroxo intermediates that can react with alkenes to form epoxide products. Comparisons of kinetics for 1-octene epoxidation with H2O2 on Ti-BEA and Ti-MFI catalysts with different hydrophilicities in methanol (CH3OH) or acetonitrile (CH3CN) solvents show the significance of the solvent for stabilizing catalytically-relevant species and the complex interdependencies between solvent, catalyst topology, and hydrophilicity. Epoxidation turnover rates are higher in CH3CN than CH3OH for Ti-BEA, but the opposite trend is observed for Ti-MFI. Ti-silicates with greater silanol densities, however, give greater epoxidation turnover rates than their hydrophobic counterparts in both solvents. Kinetic, spectroscopic, and thermodynamic analyses show that differences in turnover rates mainly arise from changes in the stabilization of reactive surface species by solvent mediated interactions, because the mechanism of the reaction and stability of the fluid-phase reactants remain similar in CH3CN and CH3OH. Specifically, apparent activation free energy values (ΔGApp‡) indicate that surface intermediates responsible for alkene epoxidation are stabilized to a greater extent in CH3CN on Ti-BEA and in CH3OH on Ti-MFI. Hydrophilic Ti-silicates present lower ΔGApp‡ values regardless of solvent identity, which suggests that these differences correspond to the number of hydrogen-bonding solvent molecules found near reactive species bound to Ti active sites. Taken together, these findings demonstrate the role of solvent molecules in allowing reactive intermediates to recognize the properties of active sites beyond the length-scale of covalent bonds, which carry implications for epoxidation but also other reactions within solvent-filled pores of microporous materials.

Green synthesis of cavity-containing manganese oxides with superior catalytic performance in toluene oxidation

Manganese oxide prepared by a simple hydrothermal method is highly active and stable in both lab and quasi industrial conditions for the total oxidation of toluene, achieving total conversion at only 175 °C in the absence of water and at 150 °C in the presence of 4 mol % water. Moreover, this performance is stable for at least 6 cycles. The enhanced catalytic performance has been related to the presence of highly reactive oxygen surface species. This species are probably promoted by Mn3O4 nanoparticles with Mn3+ at the tetragonal distorted cationic sites, likely compensated by structural water at the anionic sites. The presence of structural water can be related to the formation of internal cavities/vesicles in the Mn3O4 crystalline structure.

Effect of Controlled-Atmosphere Storage and Ethanol Rinsing on NaNi0.5Mn0.5O2 for Sodium-Ion Batteries.

NaNi0.5Mn0.5O2 is a promising sodium-ion battery cathode material that has been extensively studied. However, the air sensitivity of this material limits practical application and is not well understood. Here, we present a detailed study of NaNi0.5Mn0.5O2 powders stored in different atmospheres (oxygen, argon, and carbon dioxide), either dry or wet. X-ray diffraction and Fourier transform infrared measurements were used to characterize the materials and their surface species before and after controlled-atmosphere storage. It was determined that the exposure of untreated NaNi0.5Mn0.5O2 powders to moisture results in desodiation and material degradation, leading to poor cycling. This effect was found to be caused by reactive surface species. From these results, a simple ethanol washing method was found to significantly reduce the air-reactivity and improve the electrochemical performance of NaNi0.5Mn0.5O2 by removing surface impurities formed by air exposure.

Ni-doped ceria nanorods for the WGS reaction: Effect of Ni distribution in methane suppression

• Ni-doped CeO 2 nanorods system exhibits great activity and selectivity toward WGS reaction. • Structure provides Ni δ + ( δ ≤ 2) into the (110) reactive ceria surface and well-dispersed Ni species interacting with the former. • The Ni distribution in the system enables the formation of formate intermediate species suppressing the non-desirable methanation route. Behavior of Ni-doped CeO 2 nanorods (NR-CeNiX) in WGS reaction is evaluated. The proposed novel system simultaneously exhibits reactive (110)-ceria facets and Ni substitution. Catalytic runs for WGS reaction show greater stability, activity and selectivity towards the shift reaction regarding the traditional Ni-supported (110)-ceria system. Analysis of the surface phenomena during the catalytic reaction, followed by in-situ DRIFTS, indicates a formate via mechanism where ceria-nickel interaction plays a key role in the suppression of methane formation. Further surface characterization by X-Ray photoelectron spectroscopy evidences the existence of low-coordination Ni at the ceria surface suggesting that Ni location is the reason for the improvement attained.

Probing the Activity of Different Oxygen Species in the CO Oxidation over RuO2(110) by Combining Transient Reflection–Absorption Infrared Spectroscopy with Kinetic Monte Carlo Simulations

Transient spectroscopic surface-chemistry experiments in combination with spatially resolved kinetic Monte Carlo (KMC) simulations offer great potential to gain a wealth of molecular information on the kinetics of catalytic surface reactions as exemplified by the CO oxidation reaction over RuO2(110). This approach surpasses the common problem that in the steady-state reactions, the prevailing species detectable by in operando surface-sensitive spectroscopy are frequently spectator species, thereby obscuring the reactive surface species. Our experiment is sensitive to the relative activity of different oxygen species by saturating the surface with loosely bound oxygen, leaving only single vacancies where CO can adsorb and recombine with oxygen. With in situ reflection–absorption infrared spectroscopy (RAIRS) in combination with ab initio based KMC simulations, we follow the time evolution toward steady state (transient experiment). In this way, we are able to resolve a long-standing controversy about the a...

Solute-mediated interactions between active droplets.

Concentration gradients play a critical role in embryogenesis, bacterial locomotion, as well as the motility of active particles. Particles develop concentration profiles around them by dissolution, adsorption, or the reactivity of surface species. These gradients change the surface energy of the particles, driving both their self-propulsion and governing their interactions. Here, we uncover a regime in which solute gradients mediate interactions between slowly dissolving droplets without causing autophoresis. This decoupling allows us to directly measure the steady-state, repulsive force, which scales with interparticle distance as F∼1/r^{2}. Our results show that the dissolution process is diffusion rather than reaction rate limited, and the theoretical model captures the dependence of the interactions on droplet size and solute concentration, using a single fit parameter, l=16±3nm, which corresponds to the length scale of a swollen micelle. Our results shed light on the out-of-equilibrium behavior of particles with surface reactivity.

High‐Energy‐Resolution X‐ray Absorption Spectroscopy for Identification of Reactive Surface Species on Supported Single‐Site Iridium Catalysts

AbstractInvited for the cover of this issue are the groups of David A. Dixon at the University of Alabama, Simon R. Bare at Stanford Synchrotron Radiation Lightsource and Bruce C. Gates at the University of California, Davis. The image depicts a visual representation of the key conclusion of the work. Read the full text of the article at 10.1002/chem.201701459.

Front Cover: High‐Energy‐Resolution X‐ray Absorption Spectroscopy for Identification of Reactive Surface Species on Supported Single‐Site Iridium Catalysts (Chem. Eur. J. 59/2017)

High-energy-resolution fluorescence detection (HERFD) X-ray absorption spectroscopy at SSRL is combined with density functional theory and FEFF simulations ascribing spectral features to ethylene and CO ligands adsorbed on catalytically active iridium centers isolated on zeolite HY and on MgO, pushing detection limits below those of conventional infrared spectroscopy, and allowing XAS to identify reactionary intermediates. More information can be found in the Full Paper by D. A. Dixon, S. R. Bare, B. C. Gates et al. on page 14760. High-energy-resolution fluorescence detection (HERFD) X-ray absorption spectroscopy at SSRL is combined with density functional theory and FEFF simulations ascribing spectral features to ethylene and CO ligands adsorbed on catalytically active iridium centers isolated on zeolite HY and on MgO, pushing detection limits below those of conventional infrared spectroscopy, and allowing XAS to identify reactionary intermediates. More information can be found in the Full Paper by D. A. Dixon, S. R. Bare, B. C. Gates et al. on page 14760.

High-Energy-Resolution X-ray Absorption Spectroscopy for Identification of Reactive Surface Species on Supported Single-Site Iridium Catalysts.

We report high-energy-resolution X-ray absorption spectroscopy detection of ethylene and CO ligands adsorbed on catalytically active iridium centers isolated on zeolite HY and on MgO supports. The data are supported by density functional theory and FEFF X-ray absorption near-edge modelling, together with infrared (IR) spectra. The results demonstrate that high-energy-resolution X-ray absorption spectra near the iridium LIII (2p3/2 ) edge provide clearly ascribable, distinctive signatures of the ethylene and CO ligands and illustrate effects of supports and other ligands. This X-ray absorption technique is markedly more sensitive than conventional IR spectroscopy for characterizing surface intermediates, and it is applicable to samples having low metal loadings and in reactive atmospheres and is expected to have an increasing role in catalysis research by facilitating the determination of mechanisms of solid-catalyzed reactions through identification of reaction intermediates in working catalysts.

Oxidation State Discrimination in the Atomic Layer Deposition of Vanadium Oxides

We describe the use of a vanadium 3+ precursor for atomic layer deposition (ALD) of thin films that span the common oxidation states of vanadium oxides. Self-limiting surface synthesis of V2O3, VO2, and V2O5 are realized through four distinct reaction mechanisms accessed via judicious choice of oxygen ALD partners. In situ quartz crystal microbalance and quadrupole mass spectrometry were used to study the reaction mechanism of the vanadium precursor with O3, H2O2, H2O/O2, and H2O2/H2. A clear distinction between nonoxidative protic ligand exchange and metal oxidation is demonstrated through sequential surface reactions with different nonmetal precursors. This synergistic effect provides greater control of the resultant metal species in the film, as well as reactive surface species during growth. In an extension of this approach, we introduce oxidation state control through reducing equivalents of H2 gas. When H2 is dosed after H2O2 during growth, amorphous films of VO2 are deposited that are readily cryst...

Role of Spatial Constraints of Brønsted Acid Sites for Adsorption and Surface Reactions of Linear Pentenes.

The Brønsted acid sites of H-ZSM-5 and ferrierite reversibly adsborb linear pentenes via hydrogen bonding, rapidly isomerizing the double bond. On H-ZSM-5, dimerization of adsorbed pentenes occurs at a slower rate and leads to pentyl ester covalently bound to the surface. Pentene adsorbed on zeolites with narrower pores, such as ferrierite, remained stable in a hydrogen-bonded state even up to 423 K. Comparing the differential heat of adsorption of 2-pentene on silicalite and ferrierite allowed for the determination of the enthalpy difference between physically adsorbed pentene in ZSM-5 and the localized hydrogen-bonded π-complex at Brønsted acid sites, -36 kJ/mol. The activation energy (35 kJ/mol) for dimerization is almost identical to this enthalpy difference, suggesting that the rate-determining step is associated either with the mobilization of π-bonded 2-pentene or with the equally large activation barrier to form an alkoxy group via a carbenium-ion transition state. In a closed system, the dimerization rate is first order in the concentration of the π-complex that is both in equilibrium with the mobile pentene phase and in production of the carbenium ion that reacts with the mobile pentene. Overall, the alkoxy group is -41 ± 7 kJ/mol more stable than physisorbed pentene, establishing a series of energetically well-separated groups of reactive surface species.

Reactive surface organometallic complexes observed using dynamic nuclear polarization surface enhanced NMR spectroscopy.

Dynamic Nuclear Polarization Surface Enhanced NMR Spectroscopy (DNP SENS) is an emerging technique that allows access to high-sensitivity NMR spectra from surfaces. However, DNP SENS usually requires the use of radicals as an exogenous source of polarization, which has so far limited applications for organometallic surface species to those that do not react with the radicals. Here we show that reactive surface species can be studied if they are immobilized inside porous materials with suitably small windows, and if bulky nitroxide bi-radicals (here TEKPol) are used as the polarization source and which cannot enter the pores. The method is demonstrated by obtaining significant DNP enhancements from highly reactive complelxes [([triple bond, length as m-dash]Si-O-)W(Me)5] supported on MCM-41, and effects of pore size (6.0, 3.0 and 2.5 nm) on the performance are discussed.

Mechanism of the selective catalytic reduction of NO x with NH3 over W-doped Fe/TiO2 catalyst

The W-doped Fe/TiO2 catalyst prepared by an impregnation method exhibited a good NH3-selective catalytic reduction(SCR) activity and N2 selectivity with broad operation temperature window. The interaction between Fe and W could increase the amount of surface chemisorbed oxygen, and thus enhances the low temperature SCR activity by facilitating the fast SCR of 2NH3+NO+NO2→2N2+3H2O. The NH3-SCR reaction mechanism over the W-Fe/TiO2 was fully investigated via in situ diffuse reflectance infrared Fourier transform spectroscopy(in situ DRIFTS). In the low temperature range( 250 °C), the reactive surface species were mainly NH2, and the SCR mainly followed the Eley-Rideal mechanism, during which the formation of NH2NO intermediate species after H-abstraction of NH3 was the rate-determining step.

Charge inhomogeneity of graphene on SiO2: dispersion-corrected density functional theory study on the effect of reactive surface sites

Reactive surface species present on SiO2 in a mixture with inert ones is one likely origin of charge puddles observed in supported graphenes.

Recent Advances in Porous Silicon Technology for Drug Delivery

Porous silicon (pSi) is a nanostructured carrier system that has received considerable attention over the past 10 years, for use in a wide variety of biomedical applications, including biosensing, biomedical imaging, tissue scaffolds and drug delivery. This interest is due to several key features of pSi, including excellent in vivo biocompatibility, the ease of surface chemistry modification and the control over its 3D porous network structure. With control of these physical parameters pSi has successfully been used for the delivery of a variety of therapeutics, ranging from small-molecule drugs to larger peptide/protein-type therapeutics. In this review, the authors provide a brief overview of pSi fabrication methods, particularly with regard to the need to passivate the highly reactive Si-Hx surface species of native pSi, typically via thermal oxidation, hydrocarbonization or hydrosilylation. This surface modification, in turn, controls both the loading and release of therapeutics. The authors will then report on specific case studies of leading examples on the use of pSi as a therapeutic-delivery system. Specifically, the first reported in vivo study that demonstrated the use of pSi to improve the delivery of a Biopharmaceutical Classification System Class 2 poorly soluble drug (indomethacin), by using thermally oxidized pSi, is discussed, as well as highlighting a study that determined the biodistribution of (18)F-radiolabeled thermally hydrocarbonized pSi after oral dosing. The authors also report on the development of composite pSi-poly(D,L-lactide-co-glycolide) microparticles for the controlled delivery of protein therapeutics. Finally, the use of pSi in the delivery of bioactives, such as the successful use of thermally carbonized pSi to deliver Melanotan II, an unspecific agonist for the melanocortin receptors that are involved in controlling fluid uptake is discussed. With a growing body of literature reporting the successful use of pSi to deliver a range of therapeutics, we are entering what may be a golden age for this drug-delivery system, which may finally see the long-held promises finally achieved.

ChemInform Abstract: Reactivity of Surface Species in Heterogeneous Catalysts Probed by In Situ X‐Ray Absorption Techniques

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