Scientific Publications 2010
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2010. "Hydrogen on and in Selected Overlayer Near-Surface Alloys and the Effect of Subsurface Hydrogen on the Reactivity of Alloy Surfaces." Topics in Catalysis 53(5-6):384-392. doi:10.1007/s11244-010-9444-5 Abstract The interaction of hydrogen with the closepacked facets of seventeen transition metals overlaid with 1 ML of five transition metals (Au, Ag, Cu, Pt, and Pd) has been studied using periodic self-consistent (GGA-PW91) density functional theory (DFT) calculations. For noble metal overlayers (Au, Ag, and Cu), hydrogen at the hostmetal/ overlayer interface (subsurface hydrogen) is more stable than subsurface hydrogen in the pure host. For certain Au and Ag overlayers, subsurface hydrogen is more stable than surface hydrogen in the same system. The presence of subsurface hydrogen was found to have a significant effect on the electronic structure of the overlayer, resulting in its modified surface reactivity.
2010. "A graphene-based electrochemical sensor for sensitive detection of paracetamol ." Talanta 81(3):754-759. Abstract An electrochemical sensor based on the electrocatalytic activity of functionalized graphene for sensitive detection of paracetamol is presented. The electrochemical behaviors of paracetamol on graphene-modified glassy carbon electrodes (GCEs) were investigated by cyclic voltammetry and square-wave voltammetry. The results showed that the graphene-modified electrode exhibited excellent electrocatalytic activity to paracetamol. A quasi-reversible redox process of paracetamol at the modified electrode was obtained, and the over-potential of paracetamol decreased significantly compared with that at the bare GCE. Such electrocatalytic behavior of graphene is attributed to its unique physical and chemical properties, e.g., subtle electronic characteristics, attractive π–π interaction, and strong adsorptive capability. The sensor shows great promise for simple, sensitive, and quantitative detection of paracetamol.
2010. "Synthesis Dependent Core Level Binding Energy Shift in the Oxidation State ofPlatinum Coated on Ceria–Titania and its Effect on Catalytic Decomposition ofMethanol." Applied Catalysis. A, General 388(1-2):262-271. doi:10.1016/j.apcata.2010.08.060 Abstract Synergistic interaction of catalyst and support has attracted the interest of the catalytic community for several decades. The decomposition/oxidation of alcohols for the production of hydrogen as a source of fuel requires such support catalyst interaction. Recent studies have suggested the active role of oxide based supports on the catalytic ability of noble metals such as gold, platinum and palladium. Herein, we report the effect of synthesis technique on the catalytic activity of platinum coated on mixed ceria–titania support system. Wet impregnation technique followed by calcination was compared with the chemical reduction of platinum during the coating over oxide support. Methanol decomposition studied using an in-house built catalytic reactor coupled to a mass spectrometer showed that catalyst prepared by thermal reduction of platinum demonstrated better catalytic ability than the catalyst prepared by chemical reduction of platinum. Transmission electron microscopy revealed that the size of both platinum and ceria–titania particles remained unchanged, while the X-ray photoelectron spectroscopy (XPS) revealed that the oxidation state of platinum was modified by different coating procedures. A shift in the core level binding energy of the Pt 4f towards lower binding energy was observed with chemical reduction. Based on the XPS data it was found that platinum (on ceria–titania supports) in mixed oxidation state outperformed the Pt in reduced metallic state. Results from catalysis and in situ Fourier transform infra red spectroscopy are presented and discussed.
2010. "A Comparative Study between Co and Rh for Steam Reforming of Ethanol." Applied Catalysis. B, Environmental 96(3-4):441-448. doi:10.1016/j.apcatb.2010.02.041 Abstract Rh and Co-based catalyst performance was compared for steam reforming of ethanol under conditions suitable for industrial hydrogen production. The reaction conditions were varied to elucidate the differences in reaction pathways on both catalysts. On Co/ZnO, CH4 is a secondary product formed through the methanation reaction, while it is produced directly by ethanol decomposition on Rh. The difference in the reaction mechanism is shown to favor Co-based catalysts for selective hydrogen production under elevated system pressures (up to 15 bar) of industrial importance. The carbon deposition rate was also studied, and we show that Co is more prone to coking and catalyst failure. However, the Co/ZnO catalyst can be regenerated, by mild oxidation, despite the high carbon deposition rate. We conclude that Co/ZnO is a more suitable catalyst system for steam reforming of ethanol due to the low methane selectivity, low cost and possibility of regeneration with mild oxidation.
2010. "Liquid Chromatography Mass Spectrometry-Based Proteomics: Biological and Technological Aspects." The Annals of Applied Statistics 4(4):1797-1823. Abstract Mass spectrometry-based proteomics has become the tool of choice for identifying and quantifying the proteome of an organism. Though recent years have seen a tremendous improvement in instrument performance and the computational tools used, significant challenges remain, and there are many opportunities for statisticians to make important contributions. In the most widely used "bottom-up" approach to proteomics, complex mixtures of proteins are first subjected to enzymatic cleavage, the resulting peptide products are separated based on chemical or physical properties and analyzed using a mass spectrometer. The two fundamental challenges in the analysis of bottom-up MS-based proteomics are: (1) Identifying the proteins that are present in a sample, and (2) Quantifying the abundance levels of the identified proteins. Both of these challenges require knowledge of the biological and technological context that gives rise to observed data, as well as the application of sound statistical principles for estimation and inference. We present an overview of bottom-up proteomics and outline the key statistical issues that arise in protein identification and quantification.
2010. "Atomic oxygen flux determined by mixed-phase Ag/Ag2O deposition." Thin Solid Films 519(2):635-640. doi:10.1016/j.tsf.2010.08.081 Abstract The flux of atomic oxygen generated in a electron cyclotron resonance (ECR) microwave plasma source was quantified by two different methods. The commonly applied approach of monitoring the frequency change of a silver-coated quartz crystal microbalance (QCM) deposition rate monitor as the silver is oxidized was found to underestimate the atomic oxygen flux by an order of magnitude compared to a more direct deposition approach. In the mixed-phase Ag/Ag2O deposition method, silver films were deposited in the presence of the plasma such that the films were partially oxidized to Ag2O; x-ray photoelectron spectroscopy (XPS) was utilized for quantification of the oxidized fraction. The inaccuracy of the QCM oxidation method was tentatively attributed to efficient catalytic recombination of O atoms on the silver surface.
2010. "Spectroscopic evidence for Ag(III) in highly oxidized silver films by x-ray photoelectron spectroscopy." Journal of Physical Chemistry C 114(49):21562–21571. doi:10.1021/jp107914e Abstract In situ x-ray photoelectron spectroscopy (XPS) was utilized to identify the chemical state of silver in a range of silver oxide thin films obtained by co-deposition of silver and atomic oxygen. A highly oxidized silver species was observed at an unexpectedly low Ag 3d5/2 binding energy (BE) of 366.8 eV with an associated broad satellite at 368.2 eV; this species was assigned as Ag(III). It was found to be highly unstable in vacuum, but could be regenerated by further exposure to atomic oxygen. Both BE shifts and intensity changes of the O 1s peak were found to correlate with changes in the silver oxidation state. Theoretical calculations of the expected XPS of high spin Ag(III) provide insight into the significance of satellite structure and shake features in the Ag 3d spectra.
2010. "Retrieval of Aerosol Optical Depth in Vicinity of Broken Clouds from Reflectance Ratios: Case Study." Atmospheric Measurement Techniques 3:1333-1349. doi:10.5194/amt-3-1333-2010 Abstract A recently developed reflectance ratio (RR) method for the retrieval of aerosol optical depth (AOD) is evaluated using extensive airborne and ground-based data sets collected during the Cloud and Land Surface Interaction Campaign (CLASIC) and the Cumulus Humilis Aerosol Processing Study (CHAPS), which took place in June 2007 over the U.S. Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) Southern Great Plains site. A detailed case study is performed for a field of single-layer shallow cumuli observed on June 12, 2007. The RR method is applied to retrieve the spectral values of AOD from the reflectance ratios measured by the MODIS Airborne Simulator (MAS) for two pairs of wavelengths (660 and 470 nm and 870 and 470 nm) collected at a spatial resolution of 0.05 km. The retrieval is compared with an independent AOD estimate from three ground-based Multi-filter Rotating Shadowband Radiometers (MFRSRs). The interpolation algorithm that is used to project MFRSR point measurements onto the aircraft flight tracks is tested using AOD derived from NASA Langley High Spectral Resolution Lidar (HSRL). The RR AOD estimates are in a good agreement (within 5%) with the MFRSR-derived AOD values for the 660-nm wavelength. The AODs obtained from MAS reflectance ratios overestimate those derived from MFRSR measurements by 15-30% for the 470-nm wavelength and underestimate the 870-nm AOD by the same amount.
2010. "Molecular Simulation of the Diffusion of Uranyl Carbonate Species in Aqueous Solution." Geochimica et Cosmochimica Acta 74(17):4937-4952. doi:10.1016/j.gca.2010.06.007 Abstract Molecular dynamics simulations of aqueous uranyl carbonate species were carried out with two different potential models to gain molecular-level insight into the hydration properties of these species and evaluate the ability of the two models to reproduce published ab initio and experimental data. The simulation results were used to estimate the self-diffusion coefficients of uranyl carbonate species that often dominate uranyl speciation in groundwater systems. The first potential model was based on a series of shell models developed by Parker and co-workers (including (DE LEEUW and PARKER, 1998; KERISIT and PARKER, 2004; PAVESE et al., 1996). The second potential model was a rigid-ion model based on the flexible SPC water model (TELEMAN et al., 1987), the uranyl model of Guilbaud and Wipff (GUILBAUD and WIPFF, 1996), and the parameters for the carbonate ion given by Greathouse and co-workers (GREATHOUSE and CYGAN, 2005; GREATHOUSE et al., 2002). Analysis of structural (mean interatomic distances and coordination numbers) and dynamical (water residence times in hydration shell and self-diffusion coefficients) properties showed that, overall, the first potential model performed best when compared to published data, although the only major discrepancy with the second model was a misrepresentation of the configuration adopted by the alkaline-earth uranyl carbonate ions. The diffusion coefficients obtained for the alkaline-earth cations and the uranyl ion were compared with three variants of the Stokes-Einstein (SE) equation and it was found that none of the three SE models were able to reproduce both the absolute values and the overall trend determined from the molecular dynamics simulations. However, as would be expected based on the SE equation, a plot of the diffusion coefficients of the uranyl carbonate complexes as a function of the inverse of the equivalent spherical radius showed a general linear dependence with the two models yielding almost identical gradients. The nature of the alkaline-earth cation in the uranyl carbonate complexes was found to have only a small effect on the ion’s diffusion coefficient.
2010. "Phosphine oxide derivatives as hosts for blue phosphors: A joint theoretical and experimental study of their electronic structure ." Chemistry of Materials 22(1):247-254. Abstract We report on a joint theoretical and experimental investigation of the electronic structure of a series of bis(diphenylphosphine oxide) derivatives containing a central aromatic core with high triplet energy. Such molecules can serve as host material in the emissive layer of blue electro-phosphorescent organic devices. The aromatic cores considered in the theoretical study consist of biphenyl, fluorene, dibenzofuran, dibenzothiophene, dibenzothiophenesulfone or carbazole, linked to the two phosphoryl groups in either para or meta positions. With respect to the isolated core molecules, it is found that addition of the diphenylphosphine oxide moieties has hardly any impact on the core geometry and only slightly reduces the energy of the lowest triplet state (by at most ~0.2 eV). However, the diphenylphosphine oxide functionalities significantly impact the ionization potential and electron affinity values, in a way that is different for para and meta substitutions. Excellent comparison is obtained between the experimental UPS and IPES spectra of the para biphenyl and meta dibenzothiophene and dibenzothiophenesulfone compounds and the simulated spectra. In general, the phosphine oxide derivatives present triplet energies that are calculated to be at least 0.2 eV higher than those of currently widely used blue phosphorescent emitters.
2010. "Formaldehyde Polymerization on (WO3)3/TiO2(110) Model Catalyst." Journal of Physical Chemistry C 114(40):17017-17022. Abstract Polymerization of formaldehyde, H2CO, was studied under ultrahigh vacuum conditions on a model catalyst consisting of monodispersed (WO3)3 clusters anchored on TiO2(110. Formaldehyde oligomers, (H2CO)n, desorbing from the polymer that formed on the catalyst surface are detected between 250 and 325 K in temperature programmed desorption experiments. At least two monolayers (ML) of H2CO are required on the surface to observe (H2CO)n desorption and the amount saturates for H2CO coverages in excess of ~30 ML. The presence of H2CO multilayers is required for the polymerization to take place indicating that it had to occur below 100 K. The saturation amount increases with increasing coverage of (WO3)3 clusters with the highest amount of ~13 ML observed on 1.2 (WO3)3/nm2 . No (H2CO)n desorption was observed on the bare TiO2(110) surface.
2010. "Nanobiocatalysis for protein digestion in proteomic analysis ." Proteomics 10(4 SP ISS):687-699. Abstract The process of protein digestion is a critical step for successful protein identification in the bottom-up proteomic analysis. To substitute the present practice of in-solution protein digestion, which is long, tedious, and difficult to automate, a lot of efforts have been dedicated for the development of a rapid, recyclable and automated digestion system. Recent advances of nanobiocatalytic approaches have improved the performance of protein digestion by using various nanomaterials such as nanoporous materials, magnetic nanoparticles, and polymer nanofibers. Especially, the unprecedented success of trypsin stabilization in the form of trypsin-coated nanofibers, showing no activity decrease under repeated uses for one year and retaining good resistance to proteolysis, has demonstrated its great potential to be employed in the development of automated, high-throughput, and on-line digestion systems. This review discusses recent developments of nanobiocatalytic approaches for the improved performance of protein digestion in speed, detection sensitivity, recyclability, and trypsin stability. In addition, we also introduce the protein digestions under unconventional energy inputs for protein denaturation and the development of microfluidic enzyme reactors that can benefit from recent successes of these nanobiocatalytic approaches.
2010. "Photophysical pathways of cytosine in aqueous solution." Physical Chemistry Chemical Physics. PCCP 12(19):5024-5031. doi:10.1039/b926125g Abstract The following manuscript was reported to EMSL in accordance with the DOE Non-Proprietary User Agreement. The effects of aqueous solvation on the photophysical pathways involving the S1 excited state in cytosine have been studied with a mean-field QM/MM approach. Two main pathways with small barriers were found previously in isolated cytosine, using multireference configuration interaction (MRCI) methods, that facilitate radiationless decay to the ground state. These pathways are examined in solvated cytosine using a mean-field QM/MM combined with MRCI, and it is found that barriers in each direction increase moderately. The barriers in the presence of the solvent are 0.23 eV and 0.31 eV for the two different pathways compared to 0.15 eV and 0.14 eV in the gas phase, indicating that the aqueous environment does not make one of the two directions much more preferable.
2010. "Heterogeneous Nucleation of Ice on Anthropogenic Organic Particles Collected in Mexico City." Geophysical Research Letters 37(11):Article No. L11803. doi:10.1029/2010GL043362 Abstract This study reports on heterogeneous ice nucleation activity of predominantly organic or coated with organic material anthropogenic particles sampled within and around the polluted environment of Mexico City. The onset of heterogeneous ice nucleation was observed as a function of particle temperature (Tp), relative humidity (RH), nucleation mode, and chemical composition of particles influenced by their photochemical atmospheric aging. Particle analyses was conducted using computer controlled scanning electron microscopy with energy dispersive analysis of X-rays (CCSEM/EDX) and scanning transmission X-ray microscopy with near edge X-ray absorption fine structure spectroscopy (STXM/NEXAFS). In contrast to the majority of laboratory studies employing proxies of organic aerosol, we show that anthropogenic organic particles collected in Mexico City have can potentially induce ice nucleation at experimental conditions relevant to cirrus formation. The reported results suggest a new paradigm for the potential impact of organic particles on ice cloud formation and climate.
2010. "Reduction of FeO/Pt(1 1 1) Thin Films by Exposure to Atomic Hydrogen." Surface Science 604(1):11-20. doi:10.1016/j.susc.2009.10.008 Abstract Using scanning tunneling microscopy (STM), X-ray photoemission spectroscopy (XPS) and density functional theory (DFT) calculations we have studied the reduction of ultra-thin films of FeO(1 1 1) grown on Pt(1 1 1) after exposure to atomic hydrogen at room temperature. A number of new ordered, partlyreduced FeOx structures are identified and as a general trend we reveal that all the reduced FeOx structures incorporate 2-fold coordinated Fe atoms as opposed to the original 3-fold coordinated Fe atoms in the FeO film. We find that when all the Fe atoms are 2-fold O-coordinated the FeOx surface structure is resistant to further reduction at room temperature. We observe that water easily dissociates on the most heavily reduced FeOx, structure in contrast to the initially inert FeO film, and reveal that it is possible to partially re-oxidize the FeOx film by heating the surface slightly in the presence of water.
2010. "Synthesis and application of 1,3,4,5,7,8-Hexafluorotetracyanonaphtoquinodimethane (F6-TNAP): A Conductivity Dopant for Organic Light-Emitting Devices." Chemistry of Materials 22(13):3926-3932. Abstract We report the synthesis, photophysical and organic light-emitting device (OLED) properties of an organic molecular p-dopant 1,3,4,5,7,8-hexafluorotetracyanonaphthoquinodimethane (F6-TNAP). F6-TNAP was obtained in a three step 2 pot synthesis from commercially available octafluoronaphthalene. Doping effect of F6-TNAP was evaluated using films of 1-5% F6-TNAP with N,N'-di-1-naphthyl-N,N'-diphenyl-1,1'-biphenyl-4,4'diamine-NPD) co-evaporated on quartz. UV-vis analysis of these films showed an absorption peak at 950 nm corresponding to the charge transfer complex resulting from electron transfer from-NPD to F6-TNAP. Hole only devices using NPD as the hole transport layer (HTL) doped with F6-TNAP show greater than 2V decrease in operating voltage compared to the undoped device. A decrease in operating voltage was also demonstrated in blue OLED devices using F6-TNAP doped HTL, with a slight decrease in external quantum efficiency (EQE), thus resulting in a net improvement in power efficiency.
2010. "Clinical Microfluidics for Neutrophil Genomics and Proteomics." Nature Medicine 16(9):1042-1048. doi:10.1038/nm.2205 Abstract Neutrophils play critical roles in modulating the immune response. We present a robust methodology for rapidly isolating neutrophils directly from whole blood and develop ‘on-chip’ processing for mRNA and protein isolation for genomics and proteomics. We validate this device with an ex vivo stimulation experiment and by comparison with standard bulk isolation methodologies. Lastly, we implement this tool as part of a near patient blood processing system within a multi-center clinical study of the immune response to severe trauma and burn injury. The preliminary results from a small cohort of patients in our study and healthy controls show a unique temporal gene expression pattern clearly demonstrating the ability of this tool to discriminate temporal transcription of neutrophils in a clinical setting.
2010. "Active-space completely-renormalized equation-of-motion coupled-clusterformalism: Excited-state studies of green fluorescent protein, free-base porphyrin, and oligoporphyrin dimer." Journal of Chemical Physics 132(15):Article Number: 154103. Abstract The development of efficient parallel implementations of electronic structure methods enables not only the study of excited states of large molecular systems but also a unique opportunity to assess the role of various correlation effects in describing excitation energies for systems composed of hundreds of electrons. In this article, we discuss the impact of triply excited configurations in Equation-of-Motion Coupled Cluster (EOMCC) formalism. As a benchmark system we chose the fused porphyrin dimer, which is described by the basis set composed of 942 functions and where 270 electrons were correlated in the EOMCC calculations.
2010. "Excited-State Energies and Electronic Couplings of DNA Base Dimers." Journal of Physical Chemistry B 114(4):1674-1683. doi:10.1021/jp9072697 Abstract The singlet excited electronic states of two π-stacked thymine molecules and their splittings due to electronic coupling have been investigated with a variety of computational methods. Focus has been given on the effect of intermolecular distance on these energies and couplings. Single-reference methods, CIS, CIS(2), EOMCCSD, TDDFT, and the multireference method CASSCF, have been used, and their performance has been compared. It is found that the excited-state energies are very sensitive to the applied method but the couplings are not as sensitive. Inclusion of diffuse functions in the basis set also affects the excitation energies significantly but not the couplings. TDDFT is inadequate in describing the states and their coupling, while CIS(2) gives results very similar to EOM-CCSD. Excited states of cytosine and adenine π-stacked dimers were also obtained and compared with those of thymine dimers to gain a more general picture of excited states in π-stacked DNA base dimers. The coupling is very sensitive to the relative position and orientation of the bases, indicating great variation in the degree of delocalization of the excited states between stacked bases in natural DNA as it fluctuates.
2010. "Effect of Extent of Natural Subsurface Bioreduction on Fe-mineralogy of Subsurface Sediments." Journal of Physics: Conference Series 217(1):012047 1-8. doi:10.1088/1742-6596/217/1/012047 Abstract Naturally bioreduced zones with considerable sorbed U were recently identified at a former U mining and processing site at Rifle, CO, USA. Most of the sorbed U appears to be associated with Fe minerals. Variably reduced sediment samples were analyzed by suite of techniques, primarily by room temperature Mössbauer spectroscopy. Fe-oxides of different types and crystallinity, and Fe(II)/Fe(III)-containing clays are dominant in all the sediments. The amounts of poorly crystalline Fe(III)-oxide, however, was lower in the reduced samples. In addition, framboidal pyrites with sorbed U were common in the highly reduced sediments. Overall, the information gained from this work may help develop design field strategies for immobilization and stabilization of U(VI) in contaminated subsurface environments.
2010. "Trends in Aqueous Hydration Across the 4f Period Assessed by Reliable Computational Methods." Inorganic Chemistry 49(17):7808-7817. doi:10.1021/ic100623y Abstract The geometric and electronic structures, as well as the thermodynamic properties of trivalent lanthanide hydrates {Ln(H₂O)₈,₉ ³໋ and Ln(H₂O)₈,₉(H₂O)₁₂,₁₄ ³໋, Ln = La – Lu} have been examined using unrestricted density functional theory (UDFT), unrestricted M€oller-Plesset perturbation theory (UMP2), and multiconfigurational self-consistent field methods (MCSCF). While Ln-hydrates with 2-5 unpaired f-electrons have some multiconfigurational character, the correlation energy lies within 5-7 kcal/mol across the period and for varying coordination numbers. As such DFT yields structural parameters and thermodynamic data quite close to experimental values. Both UDFT and UMP2 predict free energies of water addition to the Ln(H₂O)₈ ³໋ species to become less favorable across the period; however, it is a non-linear function of the surface charge density of the ion. UDFT further predicts that the symmetry of the metal-water bond lengths is sensitive to the specific f-electron configuration, presumably because of repulsive interactions between filled f-orbitals and water lone-pairs. Within the Ln(H₂O)₈,₉(H₂O)₁₂,₁₄ ³໋clusters, interactions between solvation shells overrides this orbital effect, increasing the accuracy of the geometric parameters and calculated vibrational frequencies. Calculated atomic charges indicate that the water ligands each donate 0.1 to 0.2 electrons to the Ln(III) metals, with increasing electron donation across the period. Significant polarization and charge transfer between solvation shells is also observed. The relationship between empirical effective charges and calculated atomic charges is discussed with suggestions for reconciling the trends across the period.
2010. "Excellent activity and selectivity of Cu-SSZ-13 in the selective catalytic reduction of NOx with NH3." Journal of Catalysis 275(2):187-190. doi:10.1016/j.jcat.2010.07.031 Abstract Superior activity and selectivity of a Cu ion-exchanged SSZ-13 zeolite in the selective catalytic reduction (SCR) of NOx with NH3 were observed, in comparison to Cu-beta and Cu-ZSM-5 zeolites. Cu-SSZ-13 was not only more active in the NOx SCR reaction over the entire temperature range studied (up to 550 °C), but also more selective toward nitrogen formation, resulting in significantly lower amounts of NOx by-products (i.e., NO2 and N2O) than the other two zeolites. In addition, Cu-SSZ-13 demonstrated the highest activity and N2 formation selectivity in the oxidation of NH3. The results of this study strongly suggest that Cu-SSZ-13 is a promising candidate as a catalyst for NOx SCR with great potential in after-treatment systems for either mobile or stationary sources.
2010. "Metal Carbonation of Forsterite in Supercritical CO2 and H2O Using Solid State 29Si, 13C NMR Spectroscop." Journal of Physical Chemistry C 114(9):4126 - 4134. doi:10.1021/jp1001308 Abstract Ex situ solid state NMR was used for the first time to study fundamental mineral carbonation processes and reaction extent relevant to geologic carbon sequestration (GCS) using a model silicate mineral forsterite (Mg2SiO4)+supercriticalCO2 with and without H2O. Run conditions were 80 C and 96 atm. 29Si NMR clearly shows that in the absence of CO2, the role of H2O is to hydrolyze surface Mg-O-Si bonds to produce dissolved Mg2+, and mono- and oligomeric hydroxylated silica species. Surface hydrolysis products contain only Q0 (Si(OH)4) and Q1(Si(OH)3OSi) species. An equilibrium between Q0, Q1 and Mg2+ with a saturated concentration equivalent to less than 3.2% of the Mg2SiO4 conversion is obtained at a reaction time of up to 7 days. Using scCO2 without H2O, no reaction is observed within 7 days. Using both scCO2 and H2O, the surface reaction products for silica are mainly Q3 (SiOH(OSi)3) species accompanied by a lesser amount of Q2 (Si(OH)2(OSi)2) and Q4 (Si(OSi)4). However, no Q0 and Q1 were detected, indicating the carbonic acid formation/deprotonation and magnesite (MgCO3) precipitation reactions are faster than the forsterite hydrolysis process. Thus it can be concluded that the Mg2SiO4 hydrolysis process is the rate limiting step of the overall mineral carbonation process. 29Si NMR combined with XRD, TEM, SAED and EDX further reveal that the reaction is a surface reaction with the Mg2SiO4 crystallite in the core and with condensed Q2-Q4 species forming amorphous surface layers. 13C MAS NMR identified a possible reaction intermediate as (MgCO3)4-Mg(OH)2-5H2O. However, at long reaction times only crystallite magnesite MgCO3 products are observed.
