Metallic nature and surface diffusion of CO adsorbed on Ru nanoparticles in aqueous media: A 13C NMR study

We report the first observation of the 13C nuclear magnetic resonance spectroscopy (NMR) of 13CO, adsorbed from 13CO saturated 0.5 M sulfuric acid solutions, onto the surfaces of commercial Ru-black nanoparticles. The 13C NMR spectra consist of a symmetrically broadened peak having a large isotropic shift as compared to CO adsorbed onto supported Ru catalysts. The variation of the spin-lattice relaxation rate follows Korringa behavior, indicating the metallic nature of adsorbed CO, in addition to varying across the spectrum in a Korringa-like manner. Motional narrowing of the NMR spectrum at higher temperatures, together with an additional contribution to the spin-lattice relaxation rate, indicate that adsorbed CO undergoes rapid diffusion on the particle surfaces. A two-band model analysis of the NMR results indicates that the CO adsorption bond is weaker on Ru as compared to either Pt or Pd. This is also supported by a reduction in the activation energy for CO diffusion on Ru vs either Pt or Pd nanoparticles.     

Kinetic studies of adsorbed CO electrochemical oxidation on Pt(335) at full and sub-saturation coverages

Electrochemical measurements were performed to characterize the kinetics of adsorbed CO oxidation on the surface of the stepped Pt(s)-[4(111)x(100)][triple bond, length half m-dash]Pt(335) single crystal electrode. For CO adsorbed to full coverage at 0.1 V (versus the reversible hydrogen electrode, RHE) in 0.5 M H(2)SO(4) at ambient temperature (23 degrees C), oxidation of the layer gave 7.6 x 10(14) +/- 0.3 CO/cm(2) as the saturation CO coverage, just below the average value reported for CO on Pt(335) in ultra high vacuum (8.3 x 10(14) +/- 0.6 CO/cm(2)). In potential step measurements carried out between 0.75 and 0.9 V, the peak region in the current-time transient was consistent with the surface reaction between adsorbed CO and adsorbed oxide as rate limiting. Plotting the log of the rate constant for the surface reaction versus potential gave a Tafel slope of 79 mV per decade, consistent with responses for CO electrochemical oxidation on structurally related stepped Pt electrodes. For CO coverages below saturation, current-time transients were more stable in 0.05 M H(2)SO(4) than in the higher concentration electrolyte. Numerically solving the rate equations to the Langmuir-Hinshelwood model of adsorbed CO electrochemical oxidation reproduced the main features in current-time transients measured at 0.7 V in 0.05 M H(2)SO(4) for sub-saturation CO coverages. The results provide new insights into CO oxidation on Pt at sub-saturation coverage and confirm that anions play a role in CO surface chemistry.     

Adsorption of H2O, CO2 and Xe on Soft Surfaces

The interactions of water, carbon dioxide, and Xe with octadecanethiol (C(18)H(37)SH, ODT) self-assembled monolayers (SAMs) were studied under ultrahigh vacuum conditions employing temperature-programmed desorption and optical diffraction measurements. The ODT layer was grown on a 1 nm thick gold film deposited over a Ru(001) single-crystal substrate. The gases used in this report differ in their lateral interactions while adsorbed on ODT-SAM being either repulsive (Xe) or attractive (H(2)O, CO(2)). The activation energies for desorption of the first layer from ODT are E(a) = 3.6 +/- 0.9, 4.1 +/- 0.5, and 8.5 +/- 0.9 kcal/mol for Xe, CO(2), and H(2)O, respectively. Sticking probabilities of the three gases on the soft ODT surface are S(0) = 0.7 +/- 0.1, 0.8 +/- 0.1, and 0.95 +/- 0.05 for xenon, CO(2), and water, respectively, derived from the respective adsorption curves. Optical diffraction studies from multilayer coverage grating of Xe on ODT-SAM have demonstrated that sublimation is a thermodynamically more favorable process over diffusion and wetting. The significantly lower binding energy of the first layers of H(2)O and CO(2) adsorbed on the soft surface of ODT compared to that on clean metals and oxides, reflects generally weak (CO(2)) and hydrophobic (H(2)O) interactions that are important for understanding the behavior of these molecules on interfaces that are found in biological systems.     

In situ ATR-IR spectroscopic and reaction kinetics studies of water-gas shift and methanol reforming on Pt/Al2O3 catalysts in vapor and liquid phases

Reaction kinetics measurements of the water-gas shift reaction were carried out at 373 K on Pt/Al2O3 in vapor phase to investigate the effects of CO, H2, and H2O partial pressures. Results of in situ ATR-IR studies conducted in vapor phase under similar conditions suggest that the Pt surface coverage by adsorbed CO is high (approximately 90% of the saturation coverage), leading to a negligible effect of the CO pressures on the rate of reaction. The negative reaction order with respect to the H2 pressure is caused by the increased coverage of adsorbed H atoms, and the fractional positive order with respect to the water pressure is consistent with non-equilibrated H2O dissociation on Pt. Results of in situ ATR-IR studies carried out at 373 K show that the presence of liquid water leads to a slight decrease in the Pt surface coverage by adsorbed CO (approximately 80% of the saturation coverage) when the CO partial pressure is the same as in the vapor-phase studies. The rate of the WGS reaction in the presence of liquid water is comparable to the rate under complete vaporization conditions when other factors (such as CO partial pressure) are held constant. Reaction kinetics measurements of methanol reforming were carried out at 423 K over a total pressure range of 1.36-5.84 bar. In situ ATR-IR studies were conducted at 423 K to determine the Pt surface coverage by adsorbed CO in completely vaporized methanol feeds and in aqueous methanol solutions. The decomposition of methanol is found to be slower during the reforming of methanol in liquid phase than in vapor phase, which leads to a lower rate of hydrogen production in liquid phase (0.08 min(-1) at 4.88 bar) than in vapor phase (0.23 min(-1) at 4.46 bar). The lower reaction order with respect to methanol concentration observed for vapor-phase versus liquid-phase methanol reforming (0.2 versus 0.8, respectively) is due to the higher extent of CO poisoning on Pt for reforming in vapor phase than in liquid phase, based on the higher coverage by adsorbed CO observed in completely vaporized methanol feeds (55-60% of the saturation coverage) than in aqueous methanol feed solutions (29-40% of the saturation coverage).     

In-situ IR spectroscopy with a gas diffusion cell CO adsorption and oxidation on Pt

A gas diffusion cell allowing ready access of reactant to the electrode through the thin electrolyte layer has been used to obtain in-situ infra-red spectra from adsorbed CO on Pt under conditions of sustained reaction. The potential dependence of CO oxidation and adsorption and the inhibition of CO oxidation by adsorbed CO have been investigated.     

Direct observation of the kinetically relevant site of CO hydrogenation on supported Ru catalyst at 700 K by time-resolved FT-IR spectroscopy

Time-resolved FT-IR spectra of carbon monoxide hydrogenation over alumina-supported ruthenium particles were recorded on the millisecond time scale at 700 K using pulsed release of CO and a continuous flow of H(2)-N(2) (ratio 0.067 or 0.15, 1 atm total pressure). Adsorbed carbon monoxide was detected along with gas phase products methane (3016 and 1306 cm(-1)), water (1900-1300 cm(-1)), and carbon dioxide (2348 cm(-1)). Aside from adsorbed CO, no other surface species were observed. The rate of formation of methane is 2.5 +/- 0.4 s(-1) and coincides with the rate of carbon dioxide growth (3.4 +/- 0.6 s(-1)), thus indicating that CH(4) and CO(2) originate from a common intermediate. The broad band of adsorbed carbon monoxide has a maximum at 2010 cm(-1) at early times (36 ms) that shifts gradually to 1960 cm(-1) over a period of 3 s as a result of the decreasing surface concentration of CO. Kinetic analysis of the adsorbed carbon monoxide reveals that surface sites absorbing at the high frequency end of the infrared band are temporally linked to gas phase product growth. Specifically, a (linear) CO site at 2026 cm(-1) decays with a rate constant of 2.9 +/- 0.1 s(-1), which coincides with the rise constant of CH(4). This demonstrates that the linear CO site at 2026 cm(-1) is the kinetically most relevant one for the rate-determining CO dissociation step under reaction conditions at 700 K.     

Electro-oxidation of carbon monoxide on well-ordered Pt(111)/Sn surface alloys

The electro-oxidation of CO on model platinum-tin alloy catalysts has been studied by ex-situ electrochemical measurements following the preparation of the Pt(111)/Sn(2x2) and Pt(111)/Sn(radical3 x radical3)R30 degrees surfaces. A surface redox couple, which is associated with the adsorption/desorption of hydroxide on the Sn sites, is observed at 0.28 V(RHE)/0.15 V(RHE) in H(2)SO(4) electrolyte on both surfaces. Evidence that it is associated with the adsorption of OH comes from ex-situ photoemission measurements, which indicate that the Sn atoms are in a metallic state at potentials below 0.15 V(RHE) and an oxidized state at potentials above 0.28 V(RHE). Specific adsorption of sulfate anions is not associated with the surface process since there is no evidence from photoemission of sulfate adsorption, and the same surface couple is observed in the HClO(4) electrolyte. CO is adsorbed from solution at 300 K, with saturation coverages of 0.37 +/- 0.05 and 0.2 +/- 0.05 ML, respectively. The adsorbed CO is oxidatively stripped at the potential coincident with the adsorption of hydroxide on the tin sites, viz., 0.28 V(RHE). This strong promotional effect is unambiguously associated with the bifunctional mechanism. The Sn-induced activation of water, and promotion of CO electro-oxidation, is sustained as long as the alloy structure remains intact, in the potential range below 0.5 V(RHE). The results are discussed in the light of the requirements for CO-tolerant platinum-based electrodes in hydrogen fuel cell anode catalysts and catalysts for direct methanol electro-oxidation.     

Identification of defect sites on MgO(100) thin films by decoration with Pd atoms and studying CO adsorption properties.

CO adsorption on Pd atoms deposited on MgO(100) thin films has been studied by means of thermal desorption (TDS) and Fourier transform infrared (FTIR) spectroscopies. CO desorbs from the adsorbed Pd atoms at a temperature of about 250 K, which corresponds to a binding energy, E(b), of about 0.7 +/- 0.1 eV. FTIR spectra suggest that at saturation two different sites for CO adsorption exist on a single Pd atom. The vibrational frequency of the most stable, singly adsorbed CO molecule is 2055 cm(-)(1). Density functional cluster model calculations have been used to model possible defect sites at the MgO surface where the Pd atoms are likely to be adsorbed. CO/Pd complexes located at regular or low-coordinated O anions of the surface exhibit considerably stronger binding energies, E(b) = 2-2.5 eV, and larger vibrational shifts than were observed in the experiment. CO/Pd complexes located at oxygen vacancies (F or F(+) centers) are characterized by much smaller binding energies, E(b) = 0.5 +/- 0.2 or 0.7 +/- 0.2 eV, which are in agreement with the experimental value. CO/Pd complexes located at the paramagnetic F(+) centers show vibrational frequencies in closest agreement with the experimental data. These comparisons therefore suggest that the Pd atoms are mainly adsorbed at oxygen vacancies.     

Dynamics of CO in mesoporous silica monitored by time-resolved step-scan and rapid-scan FT-IR spectroscopy

Carbon monoxide molecules generated in the channels of mesoporous MCM-41 silica sieve from a precursor (diphenyl cyclopropenone) by photodissociation with a nanosecond laser pulse were monitored by time-resolved Fourier transform infrared (FTIR) spectroscopy using the step-scan and rapid-scan methods. A very broad absorption of CO is observed in the region 2200-2080 cm(-1) at room temperature that decays in a biphasic mode. Two-thirds of the band intensity decays on the hundreds of microsecond scale (lifetime 344 +/- 70 micros). The process represents the escape of the molecules through the mesopores into the surrounding gas phase, and a diffusion constant of 1.5 x 10(-9) m(2)/s is derived (assuming control by intra-MCM-41 particle diffusion). The broad profile of the absorption is attributed to contact of the random hopping CO with siloxane and silanol groups of the pore surface. Measurements using MCM-41 with the silanols partially capped by trimethyl silyl groups gave further insight into the nature of the IR band profile. These are the first observations on the diffusion behavior of carbon monoxide in a mesoporous material at room temperature. The residual carbon monoxide remains much longer in the pores and features distinct peaks at 2167 and 2105 cm(-1) characteristic for CO adsorbed on SiOH groups C end on and O end on, respectively. The bands decrease with time constants of 113 +/- 3 ms (2167 cm(-1)) and 155 +/- 15 ms (2105 cm(-1)) suggesting that CO in these sites is additionally trapped by surrounding diphenyl acetylene coproduct and/or precursor molecules.     

Electrochemical quartz crystal microbalance studies of the rectified quantized charging of gold nanoparticle multilayers

Electrochemical quartz crystal microbalance (EQCM) was employed to investigate the dynamics of rectified quantized charging of gold nanoparticle multilayers by in situ monitoring of the interfacial mass changes in aqueous solutions with varied electrolytes. EQCM measurements showed that interfacial mass changes only occurred at potentials more positive than the potential of zero charge (PZC), where nanoparticle quantized charging was well-defined, whereas in the negative potential regime where only featureless voltammetric responses were observed, the QCM frequency remained virtually invariant. This was ascribed to the fact that nanoparticle quantized charging was induced by the formation of ion-pairs between hydrophobic electrolyte anions (PF6-, ClO4-, BF4-, and NO3-) and positively charged gold nanoparticles. Based on the total frequency changes and the number of electrolyte anions adsorbed onto the particle layers, the number of water molecules that were involved in the ion-pairing processes was then quantitatively estimated at varied particle charge states, which was found to increase with increasing hydrophobicity of the anions. Additionally, the electron-transfer dynamics of the gold particle multilayers were also evaluated by electrochemical impedance measurements. It was found that the particle electron-transfer rate was about an order of magnitude slower than that of the ion diffusion and binding.     

Cluster size analysis of two-dimensional order in colloidal gold nanoparticle arrays

A protocol for cluster size distribution analysis was developed in order to parametrize local two-dimensional (2D) order in a quantitative manner, using mean cluster sizes and fractional hcp cluster formation (fhcp). Cluster size analysis was performed on 2D arrays of Au nanoparticles encapsulated in resorcinarene tetrathiol, which were organized into close-packed films at aqueous interfaces. The degree of monolayer formation and 2D order within the self-assembled nanoparticle arrays was observed to be strongly dependent on the amount and type of electrolyte (chloride and/or citrate) adsorbed on the nanoparticle surface, prior to encapsulation and extraction to the solvent interface. Increasing the concentration of adsorbed electrolyte could promote monoparticulate film formation but had a variable effect on local 2D order.     

Specific ion effects on nanoparticle stability and organocation-particle interactions in tetraalkylammonium-silica mixtures

Tetramethylammonium (TMA)- and tetrapropylammonium (TPA)-silica mixtures containing monovalent salts were studied to determine how salt impacts nanoparticle stability and organocation-silica interactions. Dynamic light scattering (DLS) results show that salt concentrations as low as 5 mM can induce nanoparticle aggregation. The extent of aggregation increases with the ionic size of the alkali-metal cations, consistent with the Hoffmeister series. Thus specific ion effects are observed in these mixtures. Pulsed-field gradient (PFG) NMR shows a more obvious increase in the self-diffusion coefficient of TPA than TMA in the presence of salt, indicating TPA is more easily displaced from the nanoparticle surface due to the background electrolyte. A two-site model is used to describe the exchange between tetraalkylammonuim (TAA) adsorbed on the nanoparticles and TAA in solution, from which the binding isotherms of the organocations at low electrolyte concentration was obtained and analyzed using the Langmuir formalism. This analysis also shows specific-ion effects, with the amount of TPA adsorbed to be much smaller than TMA and also much more sensitive to the presence of salt. In the context of the oriented aggregation mechanism proposed previously in the literature, the current work suggests one route for tuning the organocation-particle interaction and thus a route to controlling the rates of some steps in the mechanism.     

The phase diagram of the lyotropic nematic mesophase in the TTAB/NaBr/water system

The phase diagram of the nematic mesophase present in the tetradecyltrimethylammonium bromide/sodium bromide/water ternary system was determined. A calamitic nematic mesophase (N_C) was observed which extends to very high concentrations of electrolyte. The order parameters of the surfactant head group in the mesophases were studied by the NMR quadrupolar splitting of the deuterated surfactant. On increasing the temperature of nematic mesophases with low electrolyte concentrations, a phase separation occurs with the formation of a more highly ordered hexagonal phase and an isotropic phase. Diffusion measurements of the isotropic micellar solution by the NMR PFG method were used to estimate hydrodynamic radii at low surfactant concentrations and to study micelle diffusion as the concentration of the surfactant was increased to the liquid crystalline region. At higher surfactant concentrations, the diffusion coefficient reached a limiting value. The calamitic nematic mesophase in this surfactant/electrolyte/water system appears to be formed by long wormlike micelles.     

Modeling nonclassical heterogeneous bubble nucleation from cellulose fibers: application to bubbling in carbonated beverages

In this paper, the kinetics of CO(2) bubble nucleation from tiny gas pockets trapped inside cellulose fibers immersed in a glass of champagne were investigated, in situ, from high-speed video recordings. Taking into account the diffusion of CO(2)-dissolved molecules from the liquid bulk to the gas pocket, a model was derived which enabled us to connect the kinetics of bubble nucleation with both fiber and liquid parameters. Convection was found to play a major role in this process. The boundary layer around the gas pocket where a gradient of CO(2)-dissolved molecules exists was also indirectly approached and found to be in the order of 10-20 mum. Because most of the particles adsorbed on the wall of a container or vessel free from any particular treatment are also believed to be cellulose fibers coming from the surrounding air, the results of this paper could be indeed extended to the more general field of nonclassical heterogeneous bubble nucleation from supersaturated liquids.     

Phase field modeling of CH4 hydrate conversion into CO2 hydrate in the presence of liquid CO2

We present phase field simulations to estimate the conversion rate of CH(4) hydrate to CO(2) hydrate in the presence of liquid CO(2) under conditions typical for underwater gas hydrate reservoirs. In the computations, all model parameters are evaluated from physical properties taken from experiment or molecular dynamics simulations. It has been found that hydrate conversion is a diffusion controlled process, as after a short transient, the displacement of the conversion front scales with t(1/2). Assuming a diffusion coefficient of D(s) = 1.1 x 10(-11) m(2) s(-1) in the hydrate phase, the predicted time dependent conversion rate is in reasonable agreement with results from magnetic resonance imaging experiments. This value of the diffusion coefficient is higher than expected for the bulk hydrate phase, probably due to liquid inclusions remaining in the porous sample used in the experiment.     

Structural and kinetic studies on uranyl(V) carbonate complex using 13C NMR spectroscopy

We have measured 13C NMR spectra of uranyl(V) carbonate complex in D2O solution containing 1.003 M Na2(13)CO3 at various temperatures. Two singlet signals corresponding to free and coordinated CO3(2-) were observed at 169.13 and 106.70 ppm, respectively. From the peak area ratio, the structure of the uranyl(V) carbonate complex was determined as [U(V)O2(CO3)3]5-. Furthermore, kinetic analyses of the exchange reaction of free and coordinated CO3(2-) in [U(V)O2(CO3)3]5- were carried out using 13C NMR line-broadening. As a result, the first-order rate constant at 298 K and the activation parameters for CO3(2-) exchange reaction in [U(V)O2(CO3)3]5- were evaluated as 1.13 x 10(3) s(-1) and deltaH(double dagger) = 62.0 +/- 0.7 kJ x mol(-1), deltaS(double dagger) = 22 +/- 3 J x mol(-1) x K(-1), respectively. We suggest that the exchange follows a dissociative mechanism as in the corresponding [U(VI)O2(CO3)3]4- complex.     

Temperature-dependent coordination of phosphine to five-coordinate alkenylruthenium complexes

The red, five-coordinate complexes Ru(CO)Cl(PPh(3))2(CH=CHPh) and [Ru(CO)Cl(PPh(3))2]2(mu-CH=CHC(6)H(4)CH=CH) undergo reversible coordination of PPh(3) at low temperature to produce the pale yellow, six-coordinate complexes Ru(CO)Cl(PPh(3))3(CH=CHPh) and [Ru(CO)Cl(PPh(3))3]2(mu-CH=CHC(6)H(4)CH=CH). X-ray crystal structures of the latter complex and of the hydride complex RuH(CO)Cl(PPh(3))3 were obtained. 1H and 31P NMR spectra between 20 and -70 degrees C exhibit large changes in both equilibrium constants and dynamic effects. Thermodynamic parameters, DeltaH = -17.5 +/- 2.0 kcal/mol and DeltaS = -57.5 +/- 7.6 eu, were obtained for PPh(3) coordination to the monoruthenium complex, and activation parameters, DeltaH = 20.6 +/- 0.7 kcal/mol and DeltaS = 41.6 +/- 2.0 eu, were obtained for the reverse decoordination. Coordination of PPh(3) was not observed upon cooling of the shorter bridged complex, [Ru(CO)Cl(PPh(3))2]2(mu-CH=CHCH=CH).     

Photoinduced surface dynamics of CO adsorbed on a platinum electrode

The surface dynamics of adsorbed CO molecules formed by dissociative adsorption of HCHO at a polycrystalline Pt electrode/electrolyte solution interface was studied by picosecond time-resolved sum-frequency generation (TR-SFG) spectroscopy. A SFG peak at 2050-2060 cm(-1) was observed at the Pt electrode in HClO(4) solution containing HCHO at 0-300 mV (vs Ag/AgCl), indicating the formation of adsorbed CO at an atop site of the Pt surface as a result of dissociative adsorption of HCHO. The peak position varied with potential by approximately 33 cm(-1)/V, as previously found in an infrared reflection absorption spectroscopy (IRAS) study. Irradiation of an intense picosecond visible pulse (25 ps, 532 nm) caused an instant intensity decrease and broadening of the CO peak accompanied by the emergence of a new broad peak at approximately 1980 cm(-1) within the time resolution of the system. These results suggest a decrease and increase in the populations of CO adsorbed on atop and bridge sites, respectively, upon visible pump pulse irradiation.     

Lateral diffusion coefficients of an eicosanyl-based bisglycerophosphocholine determined by PFG-NMR and FRAP

We report the lateral diffusion properties of 2,2'-di-O-decyl-3,3'-di-O-(eicosanyl)-bis-(rac-glycero)-1,1'-diphosphocholine (C20BAS) using pulsed-field gradient NMR (PFG-NMR) and fluorescence recovery after photobleaching (FRAP). C20BAS membranes display a melting transition at Tm = 15.7 degrees C, as determined by differential scanning calorimetry and 31P NMR chemical shift anisotropy. The lateral diffusion coefficient of C20BAS, as determined by PFG-NMR and FRAP, at 25 degrees C, were DPFG-NMR = 1.9 +/- 0.6 x 10(-8) cm2/s and DFRAP C20BAS = 1.2 +/- 0.1 x 10(-8) cm2/s, respectively. In comparison, the lateral diffusion coefficient of the monopolar phospholipid, 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), was 1.8 +/- 0.9 x 10(-8) and 2.5 +/- 0.9 x 10(-8) cm2/s using PFG-NMR and FRAP, respectively.     

Deuterium NMR of the TGBA* phase in chiral liquid crystals

A deuterium NMR (DNMR) study of the TGBA* (twist grain boundary smectic A*) phase in an NMR magnetic field of 9.4 T for the chiral compound 4-[4'-(1-methyl heptyloxy)] biphenyl 4-(10-undecenyloxy) benzoate (11EB1M7) is reported. The deuterium two-dimensional (2D) exchange spectra were observed for the first time in this phase. The present study allows us to learn how the helicoidal structures arrange in an external magnetic field. To derive quantitative kinetic parameters of this novel phase, both 1D and 2D experimental spectra were simulated by means of a jump diffusion model. By comparing the experimental and simulated spectra, an accurate determination of the dynamic parameters in the TGBA* phase was obtained. Furthermore, the twist angle between two neighboring smectic A blocks is found as 26 +/- 10 degrees, which is consistent with the X-ray results for similar chiral liquid crystals. The diffusion constant (D(parallel)) is estimated to be 3.2 x 10(-12) m(2)/s at 379.5 K.