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At least 883 records · Page 49
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Hydrogen and methoxy coadsorption in the computation of the catalytic conversion of methanol on the ceria (111) surface

Methanol decomposition to formaldehyde catalyzed by the ceria (111) surface has been investigated using the DFT + U method. Our results rationalize experimental temperature programmed desorption experiments on the fully oxidized surface. Particular attention has been paid to the model correctness of methoxy with coadsorbed hydrogen on the surface. This issue has been raised by the experimental observation of water desorption at low temperature removing hydrogen from the system. Our investigation also includes hydrogen diffusion as a means of hydrogen removal. We find that the presence of methoxy reduces the reaction energy for water formation considerably. In addition, the preference of the electron to locate at the methoxy oxygen results in a dehydrated surface that does not contain Ce 3+ ions, despite the existence of a vacancy.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Hydrogen Adsorption and Reaction on RuO2(110)

Ruthenium dioxide is used extensively as a co-catalyst in a wide range of photocatalytic water splitting systems. As such, understanding how it promotes the evolution of hydrogen and oxygen is of high importance. In this study, we employ scanning tunneling microscopy to study the adsorption and reaction of hydrogen on a model RuO2(110) surface. We find that at low coverages H2 dissociates heterolytically to hydride-hydroxyl pairs even at 5 K. The onset temperature for the low coverage H2 dissociation is surprising as it is not expected based on prior temperature programmed desorption studies. At higher coverages, H2 chemisorbs molecularly on bare ruthenium sites that are neighboring the hydride-hydroxyl pairs. Upon annealing, the chemisorbed H2 disappears by 120 K, primarily due to H2 desorption from the surface. Further annealing between 120 K and 295 K leads to the conversion of hydride species to bridging hydroxyls. The bridging hydroxyls ultimately recombine between 450 and 600 K to form water yielding clean RuO2(110) with oxygen vacancies.

Dahal, Arjun P.↗
DOE OSTI2018

Weakening the strength of CO binding on subsurface alloyed Pt(111)

The interaction of CO with Pt/3d/Pt(111) subsurface alloys (3d: Ni, Co, Fe) was investigated by combination of vibrational spectroscopy and temperature programmed desorption. The binding strength of CO is significantly reduced on Pt/Fe/Pt(111) and Pt/Co/Pt(111) and is sorted in the following order: Pt>Ni>Co≈Fe. Our analysis shows that the 3d admetal is stable in the subsurface region and CO is linearly bonded to the atop sites of the Pt(111) surface. At high CO coverage, compressed structures are obtained. At low CO coverage, the desorption activation energy of CO (~143 kJ/mol for Pt(111)) drops to 124 kJ/mol for Pt/Ni/Pt(111), and goes further down to 111 and 110 kJ/mol for Pt/Co/Pt(111) and Pt/Fe/Pt(111), respectively. Here, the enhancement in the surface activity is attributed to the modified backdonation to the antibonding states of adsorbed CO due to the downshifted 5d-band center of Pt upon embedding 3d metals in the subsurface region.

36 MATERIALS SCIENCE↗
DOE OSTI2021

CO adsorption on MnO(100): Experimental benchmarks compared to DFT

In this study, CO adsorption on the MnO(100) surface was studied using temperature programmed desorption (TPD) and density functional theory (DFT). TPD results show that CO is weakly-bound on MnO(100), with an experimental adsorption energy of -35.6 ± 2.1 kJ/mol at terrace sites in the low coverage limit. PBE simulations suggest that CO adsorption causes an implausible (2 × 2) surface reconstruction. PBE+U simulations show no signs of surface reconstruction, and provide an accurate estimate of the adsorption energy (-36.4 kJ/mol) when combined with the DFT-D3 method with Becke-Jonson damping to correct for van der Waals interactions. This simulation also shows that CO adsorbs C-down onto the Mn 2+ terrace site in a tilted geometry, which is also observed experimentally and computationally on the similarly-structured NiO(100) transition metal oxide surface. TPD results for large doses show a plateauing of the coverage at about 0.4 monolayers of CO at 85 K, with a defect coverage equivalent to 0.08 monolayers. Adsorption associated with defect sites is indicated by a high-temperature desorption tail which is not satisfactorily explained by DFT simulations of simple step or oxygen vacancy defects.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2012

Immobilization of U(VI) from Oxic Groundwater by Hanford 300 Area Sediments and Effects of Columbia River Water

Regions within the U.S. Department of Energy Hanford 300 Area (300 A) site experience periodic hydrologic influences from the nearby Columbia River as a result of changing river stage, which causes changes in groundwater elevation, flow direction and water chemistry. An important question is the extent to which the mixing of Columbia River water and groundwater impacts the speciation and mobility of uranium (U). In this study, we designed experiments to mimic interactions among U, oxic groundwater or Columbia River water, and 300 A sediments in the subsurface environment of Hanford 300 A. The goals were to investigate mechanisms of: 1) U immobilization in 300 A sediments under bulk oxic conditions and 2) U remobilization from U-immobilized 300 A sediments exposed to oxic Columbia River water. Initially, 300 A sediments in column reactors were fed with U(VI)-containing oxic 1) synthetic groundwater (SGW), 2) organic-amended SGW (OA-SGW), and 3) de-ionized (DI) water to investigate U immobilization processes. After that, the sediments were exposed to oxic Columbia River water for U remobilization studies. The results reveal that U was immobilized by 300 A sediments predominantly through reduction (80-85%) when the column reactor was fed with oxic OA-SGW. However, U was immobilized by 300 A sediments through adsorption (100%) when the column reactors were fed with oxic SGW or DI water. The reduced U in the 300 A sediments fed with OA-SGW was relatively resistant to remobilization by oxic Columbia River water. Oxic Columbia River water resulted in U remobilization (~7%) through desorption, and most of the U that remained in the 300 A sediments fed with OA-SGW (~93%) was in the form of uraninite nanoparticles. These results reveal that: 1) the reductive immobilization of U through OA-SGW stimulation of indigenous 300 A sediment microorganisms may be viable in the relatively oxic Hanford 300 A subsurface environments and 2) with the intrusion of Columbia River water, desorption may be the primary process resulting in U remobilization from OA-SGW-stimulated 300 A sediments at the subsurface of the Hanford 300 A site.

Ahmed, B.↗
DOE OSTI2016

The adsorption of rare earth ions using carbonized polydopamine nano shells

Herein we report the structure effects of nano carbon shells prepared by carbonized polydopamine for rare earth elements (REEs) adsorption for the first time. The solid carbon sphere, 60 nm carbon shell and 500 nm carbon shell were prepared and investigated for adsorption and desorption of REEs. The adsorption of carbon shells for REEs was found to be better than the solid carbon sphere. The effect of acidities on the adsorption and desorption properties was discussed in this study. The good adsorption performance of carbon shells can be attributed to their porous structure, large specific surface area, amine group and carbonyl group of dopamine.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Imaging Fluorescence-Correlation Spectroscopy for Measuring Fast Surface Diffusion at Liquid/Solid Interfaces

The development of techniques to probe interfacial molecular transport is important for understanding and optimizing surface-based analytical methods including surface-enhanced spectroscopies, biological assays, and chemical separations. Single-molecule-fluorescence imaging and tracking has been used to measure lateral diffusion rates of fluorescent molecules at surfaces, but the technique is limited to the study of slower diffusion, where molecules must remain relatively stationary during acquisition of an image in order to build up sufficient intensity in a spot to detect and localize the molecule. Although faster time resolution can be achieved by fluorescence-correlation spectroscopy (FCS), where intensity fluctuations in a small spot are related to the motions of molecules on the surface, long-lived adsorption events arising from surface inhomogeneity can overwhelm the correlation measurement and mask the surface diffusion of the moving population. Here, we exploit a combination of these two techniques, imaging-FCS, for measurement of fast interfacial transport at a model chromatographic surface. This is accomplished by rapid imaging of the surface using an electron-multiplied-charged-coupled-device (CCD) camera, while limiting the acquisition to a small area on the camera to allow fast framing rates. The total intensity from the sampled region is autocorrelated to determine surface diffusion rates of molecules with millisecond time resolution. Here, the technique allows electronic control over the acquisition region, which can be used to avoid strong adsorption sites and thus minimize their contribution to the measured autocorrelation decay and to vary the acquisition area to resolve surface diffusion from adsorption and desorption kinetics. As proof of concept, imaging-FCS was used to measure surface diffusion rates, interfacial populations, and adsorption–desorption rates of 1,1'-dioctadecyl-3,3,3'3'-tetramethylindocarbocyanine (DiI) on planar C18- and C1-modified surfaces.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Autocorrelation and Multifractal Detrended Fluctuation Analyses Reveal Superdiffusive Mass Transport in Solvent-Filled Nanoporous Media

Fluorescence fluctuation spectroscopy experiments were conducted to better understand the complex mass transport dynamics of organic molecules in liquid-filled nanoporous media. Anodic aluminum oxide (AAO) membranes incorporating 10 and 20 nm diameter cylindrical pores were employed as model materials. Nile red (NR) dye was used as a fluorescent tracer. The dye was dissolved separately in ethanol and toluene at a concentration of 20 nM and used to fill the membrane nanopores. Confocal fluorescence microscopy was employed to capture photon intensity time series data reflecting apparent diffusion of the dye within the pores. Autocorrelation of these data revealed that NR diffusion within the membranes occurred over a broad range of time scales. The autocorrelation decays were fit to a model for one-dimensional diffusion incorporating both fast and slow components having apparent diffusion coefficients, D f and D s , differing by a factor of ∼100. The fast mechanism was attributed to hindered bulk-like diffusion in the central pore cavity, while slow diffusion likely involved absorption of the dye to the pore surfaces. Unfortunately, important evidence of diffusion anomalies is lost in the broad autocorrelation decays obtained. The method of multifractal detrended fluctuation analysis (MF-DFA) was applied to the same data as a means to overcome this limitation. MF-DFA revealed that time series acquired from within the nanopores were multifractal and exhibited evidence of anomalous superdiffusion, likely resulting from the participation of a desorption-mediated diffusion mechanism. Monte Carlo simulations of time series modeling desorption-mediated diffusion in cylindrical nanopores provided support for this assignment. Here, the new knowledge gained affords an improved understanding of hydrocarbon dynamics within nanoporous oil and gas shales.

Diffusion↗
DOE OSTI2018

Detection of Inorganic Salt-Based Homemade Explosives (HME) by Atmospheric Flow Tube–Mass Spectrometry

Using a commercial mass spectrometer interfaced with an atmospheric flow tube (AFT) allowed detection of a variety of inorganic compounds used as oxidizers in homemade explosives at picogram levels. The AFT provides reaction times between 3-5 seconds with flows of 6 L/min, enabling detection levels, after thermal desorption, similar to those previously demonstrated for RDX vapors in the low parts-per-quadrillion range. The thermal desorption of chlorate and perchlorate salts resulted in the production of the corresponding anions which have higher electron affinities than the nitrate reactant ions. A dielectric barrier discharge, used as the ionization source, produced the nitrate reactant ions. In some instances, the molecular salt formed adducts with the nitrate, chlorate, and/or perchlorate anions, giving insight into the original identity of the salt cation. Urea nitrate, guanidine nitrate, and potassium nitrate were also detected as adducts with the nitrate reactant ion. Direct room-temperature vapor detection of urea nitrate and hydrogen peroxide, which have relatively high vapor pressures compared to the other salts in this study, is also demonstrated. Room-temperature vapor detection of chlorate and perchlorate salts is possible by the addition of a dilute acid which converts the salt into a more volatile acidic form. A discussion of the instrumentation, methods used, and the ionization chemistry is provided.

chlorate, perchlorate, nitrate, detection, hydroge↗
DOE OSTI2021

Degradation of Adsorbed Bisphenol A by Soluble Mn(III)

Bisphenol A (BPA), a high production volume chemical and potential endocrine disruptor, is found to be associated with sediments and soils due to its hydrophobicity (log K OW of 3.42). Here, we used superfine powdered activated carbon (SPAC) with a particle size of 1.38 ± 0.03 μm as a BPA sorbent and assessed degradation of BPA by oxidized manganese (Mn) species. SPAC strongly sorbed BPA, and desorption required organic solvents. No degradation of adsorbed BPA (278.7 ± 0.6 mg BPA g –1 SPAC) was observed with synthetic, solid α-MnO 2 with a particle size of 15.41 ± 1.35 μm; however, 89% mass reduction occurred following the addition of 0.5 mM soluble Mn(III). Small-angle neutron scattering data suggested that both adsorption and degradation of BPA occurred in SPAC pores. The findings demonstrate that Mn(III) mediates oxidative transformation of dissolved and adsorbed BPA, the latter observation challenging the paradigm that contaminant desorption and diffusion out of pore structures are required steps for degradation. Soluble Mn(III) is abundant near oxic-anoxic interfaces, and the observation that adsorbed BPA is susceptible to degradation has implications for predicting, and possibly managing, the fate and longevity of BPA in environmental systems.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Irreversible Trace Metal Binding to Goethite Controlled by the Ion Size

The dynamics of trace metals at mineral surfaces influence their fate and bioaccessibility in the environment. Trace metals on iron (oxyhydr)oxide surfaces display adsorption–desorption hysteresis, suggesting entrapment after aging. However, desorption experiments may perturb the coordination environment of adsorbed metals, the distribution of labile Fe(III), and mineral aggregation properties, influencing the interpretation of labile metal fractions. Here, in this study, we investigated irreversible binding of nickel, zinc, and cadmium to goethite after aging times of 2–120 days using isotope exchange. Dissolved and adsorbed metal pools exchange rapidly, with half times <90 min, but all metals display a solid-associated fraction inaccessible to isotope exchange. The size of this nonlabile pool is the largest for nickel, with the smallest ionic radius, and the smallest for cadmium, with the largest ionic radius. Spectroscopy and extractions suggest that the irreversibly bound metals are incorporated in the goethite structure. Rapid exchange of labile solid-associated metals with solution demonstrates that adsorbed metals can sustain the dissolved pool in response to biological uptake or fluid flow. Trace metal fractions that irreversibly bind following adsorption provide a contaminant sequestration pathway, limit the availability of micronutrients, and record metal isotope signatures of environmental processes.

58 GEOSCIENCES↗
DOE OSTI2026

CO 2 Sorption in Moisture Swing Anion Exchange Resins for Direct Air Capture: Experimental Isotherm Determination and Modeling

Moisture swing (MS) sorption is a promising direct air capture (DAC) technology to achieve negative CO 2 emissions and counter global warming. Here, in this study, MS CO 2 sorption in a model MS sorbent, IRA900, was investigated. IRA900 is a macroporous commercial strong-base anion exchange resin (AER) with quaternary ammonium functional groups. A rigorous CO 2 desorption process was developed to desorb all CO 2 from the sample and obtain CO 2 sorption isotherms as a function of relative humidity, CO 2 partial pressure, and temperature. CO 2 sorption increased with decreasing relative humidity, decreasing temperature, and increasing CO 2 partial pressure. Remarkably, the total CO 2 uptake from the sorption isotherms matched the ion exchange capacity (IEC) of the material, suggesting a stoichiometry of one CO 2 molecule reacting per active site. An isotherm model for CO 2 sorption starting with empty sorption sites (i.e., with the AER in the OH – form) described the experimental data well, supporting the hypothesis that the bicarbonate-loaded AER fully unloaded to the OH – state following rigorous CO 2 desorption. CO 2 sorption kinetics were studied, and carbon diffusion coefficients were estimated as a function of %RH and CO 2 partial pressure. Overall, this study provides a roadmap for the systematic evaluation of MS CO 2 DAC materials.

32 ENERGY CONSERVATION, CONSUMPTION, AND UTILIZATI↗
DOE OSTI2020

Water Uptake by Silica Nanopores: Impacts of Surface Hydrophilicity and Pore Size

Water uptake at 298 K by two mesoporous silicas with different pore sizes was studied using volumetric vapor sorption. Here, through variation of sample pretreatment temperature and time, the number of surface hydroxyl groups was varied, leading to marked changes in the water sorption behavior. With increasing surface hydroxyl density, the adsorption of water to the pore surfaces increases, leading to formation of a thicker and denser water sorption layer. The BET model was used to measure surface hydroxyl density from low pressure parts of water adsorption isotherms. Adsorption of water molecules scales with the number density of surface hydroxyl groups, and monolayer coverage is reached at reduced pressure of ca. 0.3. The onset of adsorption pore condensation of water shifts to lower reduced pressure with increasing surface hydroxyl density, indicating growing thickness of adsorption layers. However, the water pore condensation step of the desorption branch shifts to smaller reduced pressure with increasing surface hydroxylation, reducing the adsorption-desorption hysteresis width. An adsorbed phase model is utilized to calculate the distribution of pore water molecules between adsorbed and pore condensation phases, and characterize the density and thickness of the water sorption phase. In the smaller pores, the adsorbed phase forms a sparse monolayer, while approximately a bilayer is formed in the wider pores. The previously reported silica nanopore underfilling by water is confirmed and rationalized by a significantly reduced water density in the adsorbed phase with respect to bulk, while the pore core is filled with water of approximately bulk density.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Binding of Formic Acid on Anatase TiO2(101)

The adsorption of formic acid (FA) and the formation of formate species on metal oxide surfaces are of great interest in catalysis. Formic acid is used to probe the adsorption properties of surface sites and formates are common intermediates in many catalytic reactions. Here we focus on the interaction of FA with a prototypical anatase TiO2(101) surface by using a combination of scanning tunneling microscopy (STM), infrared reflection absorption spectroscopy (IRAS), electron stimulated desorption (ESD), and density functional theory (DFT) to assess the coverage dependent evolution of different FA-derived surface species at low temperatures (80–240 K). We find that isolated FA adsorbs at 80 K in both monodentate (MD) and bidentate (BD) configurations on top of undercoordinated Ti sites (Ti5c). The MD form is likely deprotonated and readily converts upon annealing to bridging BD on two neighboring Ti5c sites. DFT calculations show that molecularly-bound MD FA is metastable and readily converts into a deprotonated state in the proximity of subsurface oxygen vacancy. The stability of the MD species increases as the availability of the paired Ti5c sites for BD formation diminishes at high coverages. Upon surface saturation, a mixed configuration of alternating bidentate and monodentate species with coverage of 2/3 FA/Ti5c represents the most favorable configuration. This is in contrast with the adsorption of FA on rutile TiO2(110) and many other oxides, where bidentate formate species dominate. Y.W., A.D., G.A.K., R.R., N.G.P., and Z.D. were supported by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences (BES), Division of Chemical Sciences, Geosciences and Biosciences (CSGB) under FWP 47319. B.W. and A.S. acknowledge the support of DOE BES, CSGB Division under Award DESC0007347. The experimental studies were performed in EMSL, a national scientific user facility sponsored by the Department of Energy’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory (PNNL). PNNL is a multiprogram national laboratory operated for DOE by Battelle. The authors also acknowledge computational resources from the TIGRESS high-performance computer center at Princeton University.

Wang, Yang↗
DOE OSTI2021

Formation of Gas-Phase Allyl Radicals from Glycerol on Rutile TiO2(110)

The reaction pathways of glycerol on partially reduced TiO2(110) have been studied by a combination of molecular beam dosing and temperature-programmed desorption (TPD). Glycerol is found to bind more strongly to TiO2(110) than glycols due to an additional hydroxyl group in the molecule. When the Ti surface sites are saturated by glycerol, the majority of glycerol molecules (~ 95%) undergo further reactions to yield carbon-free (water and hydrogen) and carbon-containing products that are desorbed during the TPD ramp. In contrast to glycols for which both water and hydrogen arise exclusively from the cleavage of glycol O-H bonds, we identify two distinct reaction channels resulting from the cleavage of O-H and C-H bonds, respectively, for both water and hydrogen evolution from glycerol on TiO2(110). Quantitative determination of the desorption yields of water and hydrogen reveals that the C-H bond scission channel which occurs at higher temperature than the O-H bond scission channel, is a minor reaction channel for water formation, while it is a major reaction channel responsible for hydrogen evolution. For the carbon-containing products, we exclude both carbon deposition and carbon oxides (CO and CO2) formation, and identify the allyl radical as the major carbon-containing product. The formation of the allyl radical is accompanied by the formation of various minor carbon-containing products including propylene, acrolein, allyl alcohol, acetone, formaldehyde and ethylene. The different surface chemistries observed between glycerol and simpler alcohols and glycols on TiO2(110), suggest that smaller oxygenates cannot serve as models for larger oxygenates in probing their reaction pathways on oxide surfaces. The new reaction channels observed for glycerol on TiO2(110) may open up a new way to convert bio-renewable glycerol directly to pure hydrogen and hydrocarbons on TiO2. This work was supported by the US Department of Energy, Office of Sciences, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences, and performed in EMSL, a national scientific user facility sponsored by the Department of Energy’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory (PNNL). PNNL is a multiprogram national laboratory operated for the DOE by Battelle.

Chen, Long↗
DOE OSTI2022

Electron-Stimulated Formation and Release of Molecular Hydrogen and Oxygen from Boehmite Nanoplatelet Films

Here, the production and release of H 2 and O 2 during and after electron (100–1000 eV) irradiation of boehmite nanoplatelet films was studied. H 2 and O 2 produced during irradiation likely correspond to electron-induced dissociation of the hydroxyls primarily in the terminal surface layers of the boehmite followed by the H· + H· → H 2 (g) and O + O → O 2 (g) combinative desorption reactions as well as the abstraction reactions H· + >OH – → H 2 (g) + >O·̅. Postirradiation temperature-programmed desorption shows a temperature-dependent release of H 2 but no additional release of O 2 . This process likely involves diffusion and recombination of H· atoms initially trapped between the γ-AlOOH planes followed by recombination at edge and surface sites. H 2 production upon thermal-annealing-irradiated material shows no dependence on incident electron flux, suggesting the precursor species (H·) trapped within the bulk are long-lived.

Jones, Brant M.↗
DOE OSTI2024

Assessment of Long-Term Degradation of Adsorbents for Direct Air Capture by Ozonolysis

Porous adsorbents are a promising class of materials for the direct air capture of CO 2 (DAC). Practical implementation of adsorption-based DAC requires adsorbents that can be used for thousands of adsorption–desorption cycles without significant degradation. We examined the potential degradation of adsorbents by a mechanism that appears to have not been considered previously, namely, ozonolysis by trace levels of ozone from ambient air. We focused on amine-appended metal–organic frameworks, specifically amine-functionalized Mg 2 (dobpdc), as a representative DAC adsorbent. Estimates based on the number of amine sites in these adsorbents and the ozone concentration in air suggest that degradation by ozone may be relevant over thousands of adsorption–desorption cycles if reactions with adsorbed ozone are fast. We used density functional theory calculations to estimate reaction rates for amine groups and carbon–carbon double bonds in amine-functionalized Mg 2 (dobpdc).

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗