Molecular species adsorbed on soot particles issued from low sooting methane and acetylene laminar flames: A laser-based experiment

Recent advances in the field of laser desorption/laser ionization mass spectrometry (LD/LI/MS) have renewed interest in these separation methods for fast analysis of chemical species adsorbed on soot particles. These techniques provide mass-separation of the desorbed phase with high selectivity and sensitivity and require very small soot samples. Combining LD/LI/MS with in situ measurements of soot and gaseous species is very promising for a better understanding of the early stage of soot growth in flames. In this work, three lightly sooting laminar jet flames (a methane diffusion flame and two premixed acetylene flames of equivalence ratio (?) = 2.9 and 3.5) were investigated by combining prompt and 50 ns-delayed laser-induced incandescence (LII) for spatially resolved measurements of soot volume fraction (f_v) and laser-induced fluorescence (LIF) of polycyclic aromatic hydrocarbons (PAH). Soot and PAH calibration is performed by two-colour cavity ring-down spectroscopy (CRDS) at 1064 and 532 nm. Soot particles were sampled in the flames and analysed by LD/LI/Time-of-flight- MS. Soot samples are cooled to −170 °C to avoid adsorbed phase sublimation (under high vacuum in the TOF-MS). Our set-up is novel because of its ability to measure very low concentration of soot and PAH together with the ability to identify a large mass range of PAHs adsorbed on soot, especially volatile two-rings and three-rings PAHs. Studied flames exhibited a peak f_v ranging from 15 ppb (acetylene, ? = 2.9) to 470 ppb (acetylene, ? = 3.5). Different mass spectra were found in the three flames, each exhibiting one predominant PAH mass; 202 amu (4-rings) in methane, 178 amu (3-rings) in acetylene,? = 2.9 and 128 amu (2-rings) in acetylene, ? = 3.5. These variations with flame condition contrasts with other recent studies and is discussed. The other PAH masses ranged from 102 (C₈H₆) to 424 amu (C₃₄H₁₆) and are well predicted by the stabilomer grid of Stein and Farr.     

Effect of ceria nanoparticles on soot inception and growth in toluene-oxygen-argon mixtures

Soot formation from the combustion of toluene (C₆H₅CH₃) and of two concentrations of nano-sized-ceria-laden toluene was monitored using a shock tube to observe the effect of the organometallic additive on the formation of soot from its point of inception. Two concentrations of ceria, of chemical composition CeO_1.63, were employed to examine the effect on soot production of toluene over the range of temperature 1588-2370 K using two levels of inert gas dilution in which reflected-shock pressure was maintained near 1.5 atm. The ceria nanoparticles were synthesized using a microemulsion technique which employs sodium dioctyl sulfosuccinate (AOT), a surfactant, to retard agglomeration. Introduction of the nanoparticles into the shock tube is achieved using a novel, two-stage injection procedure. Soot yield measurements reveal that the presence of ceria has no direct implications on peak soot concentration near 1950 K. A shift in the parabolic soot profile of toluene in the direction of increased temperature was observed for each concentration of ceria with a larger shift occurring for increased concentration of ceria, although the same effect was exhibited for the toluene-AOT mixtures in absence of ceria, supporting an inefficaciousness of ceria on soot suppression on kinetic timescales. It is evidenced in measured soot delay times that the presence of the surfactant in absence of ceria significantly slows the rate of soot growth for T < 2000 K, while the presence of ceria has a relatively negligible impact. Under conditions of higher fuel concentration, a remarkable decrease in soot accumulation on the shock tube walls was observed in experiments using the ceria-toluene mixtures over that yielded by pure toluene combustion. In the present paper, the authors report the first measurements of nanoparticle-influenced combustion of a hydrocarbon as performed in a shock tube.     

Molecular modelling investigations on the possibility of phenanthrene dimers to be the primary nuclei of soot

Pyrene dimerisation was successfully used to model the beginning of soot nucleation in some simulation models. However, the quantum mechanics (QM) calculations proved that the binding energy of a PAH dimer with three six-member rings was similar to that of a pyrene dimer. Meanwhile, the high concentration of phenanthrene at flame conditions indicated high probability of collisions among them. The small difference of the binding energy and high concentration indicated that PAHs structurally smaller than pyrene also could be involved in soot inception. Hence, binary collisions of phenanthrene were simulated to find out whether phenanthrene dimers can serve as soot primary nuclei or not by using non-equilibrium molecular dynamics (MD). Three temperatures, six collision orientations and 155 initial translational velocities (ITVs) were considered. The results indicated that the number of dimers with lifetime over 10 ps which can serve as soot nuclei decreased from 52 at 1000 K to 17 at 1600 K, and further to 6 at 2400 K, which means that low temperature was more favourable for phenanthrene to form soot nuclei. Meanwhile, no soot nuclei were formed at the high velocity region (HVR), compared to 43 and 9 at low and middle velocity regions (LVR and MVR), respectively, when temperature was 1000 K. Also, no soot nuclei were formed at HVR when the temperature was raised to 1600 K and 2400 K. This indicated that HVR was unfavourable for phenanthrene to form soot nuclei. The results computationally further illustrated that small PAHs such as phenanthrene could serve as soot primary nuclei, since they have similar mole fractions in some flames. This may be useful for future soot simulation models.     

Effects of potassium on the reaction pathway of C3H7 species over Mo2C/Mo (1 0 0)

The adsorption and surface reactions of propyl iodide on clean and potassium-modified Mo₂C/Mo(1 0 0) surfaces have been investigated by thermal desorption spectroscopy (TPD), X-ray photoelectron spectroscopy (XPS) and high resolution electron energy loss spectroscopy (HREELS) in the 100-1200 K temperature range. This work is strongly related to the better understanding of the catalytic effect of Mo₂C in the conversion of hydrocarbons. Potassium was found to be an effective promoter: it induced the rupture of C-I bond in the adsorbed C₃H₇I even at 100 K. The extent of C-I bond scission varied approximately linearly with the concentration of K coverage at the adsorption temperature of 100 K. As revealed by HREELS and TPD measurements the primary products of the dissociation are C₃H₇ and I. The former one was stabilized by potassium and underwent dehydrogenation and hydrogenation to give propene and propane. The desorption of both compounds is reaction-limited process. A fraction of propyl groups was converted into di-σ-bonded propene, which was stable up to ∼380 K. The coupling reaction of propyl species was also facilitated by potassium and resulted in the formation of hexane and hexene with T_p ∼ 230-250 K. Hydrogen was released with T_p = 390 K, indicative of a desorption limited process. The effect of potassium was explained by the extended electron donation to adsorbed propyl iodide in one hand, and by the direct interaction between potassium and I on the other hand. This was reflected by the shift of the desorption of potassium from the coadsorbed layer at and above 1.0 ML to higher temperature, and by the coincidal T_p values (∼700 K) of potassium and iodine. The formation of KI was also supported by the appearance of a loss feature at 650 cm⁻¹ in the HREEL spectra attributed to a phonon mode of KI.     

Heat release of carbon particle formation from hydrogen-free precursors behind shock waves

The process of heat release during carbon particle formation and growth after pyrolysis of carbon suboxide C₃O₂ behind shock waves was investigated. For this goal, temperature and optical density of gas-particle mixtures initially consisting of 3% C₃O₂ + 5% CO₂ in Ar were measured as a function of time. The temperature was determined by two-channel emission-absorption spectroscopy at λ = 2.7 ± 0.4 μm, corresponding to the CO₂ (1,0,1) vibrational band. In the range of initial temperatures behind the shock waves from 1600 up to 2200 K a significant heating of the mixture during particle formation and growth was observed that increased towards higher temperatures. The analysis of the obtained data in combination with previous results about the temperature dependence of the particle size shows a decrease of the heat release of condensation from ∼200 kJ/mol per atom for particles containing ∼1000 atoms to ∼50 kJ/mol per atom for particle containing ∼10⁶ atoms.     

Surface morphology of Mn implanted Ge(1 0 0): A systematic investigation as a function of the implantation substrate temperature

Ge (1 0 0) wafers were implanted with 100 keV Mn⁺ ions with a dose of 2 × 10¹⁶ ions/cm² at different temperatures, ranging from 300 to 573 K. The surface morphology of implanted samples, analyzed with scanning electron microscopy and atomic force microscopy measurements, reveals for the 300-463 K implant temperature range the formation of a surface swelled and porous film, containing sponge-like structures. On the contrary, samples implanted in the 513-573 K temperature range present an atomically flat surface, with a roughness less than 1 nm, indicating that crystalline order has been preserved. X-ray photoemission spectroscopy depth profiling measurements indicate the presence of adsorbed oxygen in the porous layer of lower-temperature implanted samples, as well the presence of a large Mn concentration below the expected end of range for impinging ions. Mn and O concentrations at anomalously great depths are maximum in the 413 K implanted sample, indicating that the phenomenon of ion beam induced porosity is best favored at a well defined temperature.     

Thermal isomerization and cyclization of 1-naphthylacetylene: Experimental results, quantum chemical and transition state theory calculations

The isomerization of 1-naphthylacetylene diluted in argon was studied behind reflected shock waves in a 2 in i.d. single pulse shock tube over the temperature range 1000-1250 K and overall densities of ∼3 × 10⁻⁵ mol/cm³. The only reaction product found in the post shock mixtures was acenaphthylene. The first order rate constant of the isomerization was found to be k = 2.08 × 10¹² exp(−54.2 × 10³/RT) s⁻¹, where R is expressed in units of cal/K mol. Potential energy surfaces of the cyclization reaction 1-naphthylacetylene → acenaphthylene and the isomerization 1-naphthylacetylene → 2-naphthylacetylene were calculated using the Becke three-parameter hybrid method with Lee-Yang-Parr correlation functional approximation (B3LYP). Structure, energy and frequency calculations were carried out with the Dunning correlation consistent polarized double ζ (cc-pVDZ) basis set. The rate constant (k_∞) for the 1-naphthylacetylene → acenaphthylene cyclization was calculated using transition state theory, the value obtained is k_∞ = 3.52 × 10¹² exp(−55.9 × 10³/RT) s⁻¹, where R is expressed in units of cal/K mol. The agreement between the experiment and the calculations is very good. RRKM calculations were done to transfer k_∞ to the pressure of the single pulse shock tube experiments. In view of high temperature and the large molecule involved the deviation from k_∞ is very small. The isomerization 1-naphthylacetylene → 2-naphthylacetylene proceeds via the formation of an unstable intermediate 1,2-naphthalenocyclobutene and has a high barrier of ∼83.5 kcal/mol. In view of this high barrier, the isomerization cannot compete with the cyclization that proceeds with a barrier of ∼56 kcal/mol.     

Laser induced thermal desorption of hydrogen from Zr(0 0 0 1): Relationship to water dissociation and hydrogen dissolution

On metals such as Zr, during hydrogen exposure, dissolution competes with desorption; this competition can be probed by thermal desorption at different heating rates. In the case of desorption from preadsorbed hydrogen, only ∼1% of the hydrogen can be desorbed even at heating rates of >10¹⁰ K s⁻¹. Recent measurements of the dynamics of hydrogen released by water dissociation on Zr(0 0 0 1) [G. Bussière, M. Musa, P.R. Norton, K. Griffiths, A.G. Brolo, J.W. Hepburn, J. Chem. Phys. 124 (2006) 124704] have shown that the desorbing hydrogen originates from the recombinative desorption of adsorbed H-atoms and that over 25% of the water collisions lead to hydrogen desorption. To gain further insight into the desorption and dissolution of hydrogen and in an attempt to resolve the paradox of the different desorption yields from H₂ vs. H₂O exposures, we report new measurements of the laser induced thermal desorption (LITD) of hydrogen from Zr(0 0 0 1) at initial temperatures down to 90 K. The low temperature was chosen because work function measurements suggested that hydrogen adsorbed into only the outermost (surface site) of the two available adsorption sites (surface and subsurface), from which we postulated much more efficient desorption at high heating rates compared to desorption from the sub-surface sites. However, hydrogen desorption by LITD from Zr(0 0 0 1) at 90 K still only accounts for 1% of the adsorbed species, the remainder dissolving into the bulk at LITD heating rates. The different yields alluded to above remain unexplained (Bussière, 2006).     

Shock tube measurements of ignition delay times and OH time-histories in dimethyl ether oxidation

Ignition delay times and OH concentration time-histories were measured in DME/O₂/Ar mixtures behind reflected shock waves. Initial reflected shock conditions covered temperatures (T₅) from 1175 to 1900 K, pressures (P₅) from 1.6 to 6.6 bar, and equivalence ratios (?) from 0.5 to 3.0. Ignition delay times were measured by collecting OH^∗ emission near 307 nm, while OH time-histories were measured using laser absorption of the R₁(5) line of the A-X(0,0) transition at 306.7 nm. The ignition delay times extended the available experimental database of DME to a greater range of equivalence ratios and pressures. Measured ignition delay times were compared to simulations based on DME oxidation mechanisms by Fischer et al. [7] and Zhao et al. [9]. Both mechanisms predict the magnitude of ignition delay times well. OH time-histories were also compared to simulations based on both mechanisms. Despite predicting ignition delay times well, neither mechanism agrees with the measured OH time-histories. OH Sensitivity analysis was applied and the reactions DME ↔ CH₃O + CH₃ and H + O₂ ↔ OH + O were found to be most important. Previous measurements of DME ↔ CH₃O + CH₃ are not available above 1220 K, so the rate was directly measured in this work using the OH diagnostic. The rate expression k[1/s] =  1.61 × 10⁷⁹T^−18.4 exp(−58600/T), valid at pressures near 1.5 bar, was inferred based on previous pyrolysis measurements and the current study. This rate accurately describes a broad range of experimental work at temperatures from 680 to 1750 K, but is most accurate near the temperature range of the study, 1350-1750 K. When this rate is used in both the Fischer et al. and Zhao et al. mechanisms, agreement between measured OH and the model predictions is significantly improved at all temperatures.     

Methanol adsorption on a CeO2(1 1 1)/Cu(1 1 1) thin film model catalyst

Adsorption and desorption of methanol on a CeO₂(1 1 1)/Cu(1 1 1) thin film surface was investigated by XPS and soft X-ray synchrotron radiation PES. Resonance PES was used to determine the occupancy of the Ce 4f states with high sensitivity. Methanol adsorbed at 110 K formed adsorbate multilayers, which were partially desorbed at 140 K. Low temperature desorption was accompanied by formation of chemisorbed methoxy groups. Methanol strongly reduced cerium oxide by forming hydroxyl groups at first, which with increasing temperature was followed by creation of oxygen vacancies in the topmost cerium oxide layer due to water desorption. Dissociative methanol adsorption and creation of oxygen vacancies was observed as a Ce⁴⁺ → Ce³⁺ transition and an increase of the Ce 4f electronic state occupancy.     

Air-activated carbons from almond tree pruning: Preparation and characterization

In this work the results obtained in the preparation and characterization of carbons made from almond tree pruning by non-catalytic and catalytic gasification (using K and Co) with air are analyzed and discussed. The main aim was to obtain high quality activated carbons at the lowest possible cost. The variables studied have been the temperature (190-260 °C) and the time (1-10 h) in non-catalytic gasification and the influence of the catalyst type (K and Co, 1 wt.% referred to cation, at 190 °C and 1 h) and the time (1-4 h) in catalytic gasification with Co at 190 °C. The air flow rate used in all the series was 167 cm³ min⁻¹. In non-catalytic gasification the reaction normalized rate versus the conversion degree was maintained until a conversion value of 10% for the experiment made at 260 °C since, at lower temperatures, this rate drops quickly for low conversion values. The N₂ adsorption isotherms for the carbons of this series resemble type I, although there is an increase of N₂ adsorbed volume at relatively high pressures. A temperature rise produced an increase of the carbon porosity and BET specific surface (116-469 m² g⁻¹). The activation time has a positive effect on the N₂ volume adsorbed by the carbons. The isotherms shapes were similar to those previously commented. A concentration equal to 1 wt.% was used to study the influence of the catalyst type. Under the studied experimental conditions, Co drives to a bigger porosity development than K, although with both catalysts a very similar pore size distribution is obtained. The activation time, in the gasifications catalyzed with Co, gives rise to a very important porosity development in the carbons. This produces a strong increase of the carbon specific surface area with very high values in the 4 h experiment, in which a BET specific surface of 959 m² g⁻¹ was obtained.     

Low temperature adsorption of oxygen on reduced V2O3(0 0 0 1) surfaces

Well ordered V₂O₃(0 0 0 1) films were prepared on Au(1 1 1) and W(1 1 0) substrates. These films are terminated by a layer of vanadyl groups under typical UHV conditions. Reduction by electron bombardment may remove the oxygen atoms of the vanadyl layer, leading to a surface terminated by vanadium atoms. The interaction of oxygen with the reduced V₂O₃(0 0 0 1) surface has been studied in the temperature range from 80 to 610 K. Thermal desorption spectroscopy (TDS), infrared reflection absorption spectroscopy (IRAS), high resolution electron energy loss spectroscopy (HREELS), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) were used to study the adsorbed oxygen species. Low temperature adsorption of oxygen on reduced V₂O₃(0 0 0 1) occurs both dissociatively and molecularly. At 90 K a negatively charged molecular oxygen species is observed. Upon annealing the adsorbed oxygen species dissociates, re-oxidizing the reduced surface by the formation of vanadyl species. Density functional theory was employed to calculate the structure and the vibrational frequencies of the O₂ species on the surface. Using both cluster and periodic models, the surface species could be identified as η²-peroxo () lying flat on surface, bonded to the surface vanadium atoms. Although the O-O vibrational normal mode involves motions almost parallel to the surface, it can be detected by infrared spectroscopy because it is connected with a change of the dipole moment perpendicular to the surface.     

STM studies of cyclohexene hydrogenation/dehydrogenation and its poisoning by carbon monoxide on Pt(1 1 1)

A scanning tunneling microscope that can be operated in ultra high vacuum (<10⁻⁹ Torr) as well as at high pressures (1 − 10³ Torr) has been utilized to study the structures formed by cyclic C₆ hydrocarbons adsorbed on a platinum (1 1 1) crystal surface. Catalytic reactions of cyclohexene were also studied in the presence of hydrogen at pressures (up to 200 mTorr) and 300 K-350 K temperature range. Cyclohexane and cyclohexene produced the same adsorbed structure, which is attributed to the partially dehydrogenated π-allyl (C₆H₉). 1,3-Cyclohexadiene produced structures similar to those produced by benzene. In contrast 1,4-cyclohexadiene forms a structure that we attribute to intact molecular 1,4-cyclohexadiene. During reaction the STM images appear disordered, indicative of rapid diffusion of surface species. Addition of 5 mTorr of CO stops the catalytic activity and forms an ordered structure on the surface.     

Study of surface-bulk mass transport and phase transformation in nano-TiO2 using hyperfine interaction technique

Phase transition from anatase to rutile for the 70 nm TiO₂ crystallite has been investigated by the time differential perturbed angular correlation (TDPAC) technique. The study involved the annealing of the TiO₂ nanocrystals, adsorbed with the nuclear probe (¹⁸¹Hf/¹⁸¹Ta) at trace level, at different temperatures for different durations. The TDPAC measurement was also supported by XRD measurement where the width of the peaks increases with the increase in annealing temperature indicating a crystal growth. The samples annealed up to 823 K for 4 h showed no phase transition, except for the growth of the crystallites. However, it showed phase transition at the same temperature (823 K), when annealed for longer duration, indicating the slower kinetics of the phase transition process. Further the sample, when annealed at 1123 K for 4 h, showed phase transition. It has also been observed that the ¹⁸¹Hf tracer, adsorbed on 70 nm anatase TiO₂, diffuses from surface to bulk during the phase transition process and the extent of diffusion in anatase differs from that in rutile phase. However, surface to bulk mass-transfer is found to play a significant role in the phase transition process.     

A shock tube investigation of H atom production from the thermal dissociation of ortho-benzyne radicals

The thermal dissociation of ortho-benzyne (o-C₆H₄) has been studied behind reflected shock waves under very isolated conditions. In the shock tube experiments 1,2-diiodobenzene was employed as a thermal source for the o-C₆H₄ radical. For different series of experiments the temperature ranged from 1600 to 2400 K at pressures between 1.4 and 2 bar. Very low initial concentrations of the radical precursor, 0.5-4 ppm diluted in argon, were used. In situ atomic resonance absorption measurement of iodine atoms formed during the thermal dissociation of the radical precursor molecule provides for a precise determination of the initial 1,2-diiodobenzene concentration. ARAS (atomic resonance absorption spectroscopy) was also used to record the absorption profiles of hydrogen atoms obtained during the pyrolysis of ortho-benzyne. From the measured H atom absorption profiles and by taking into account recent results from literature, it is shown that the thermal dissociation of ortho-benzyne occurs via two pathways: besides the molecular route:

(R1a)     

Laser shock wave consolidation of nanodiamond powders on aluminum 319

A novel coating approach, based on laser shock wave generation, was employed to induce compressive pressures up to 5 GPa and compact nanodiamond (ND) powders (4-8 nm) on aluminum 319 substrate. Raman scattering indicated that the coating consisted of amorphous carbon and nanocrystalline graphite with peaks at 1360 cm⁻¹ and 1600 cm⁻¹ respectively. Scanning electron microscopy revealed a wavy, non-uniform coating with an average thickness of 40 μm and absence of thermal effect on the surrounding material. The phase transition from nanodiamond to other phases of carbon is responsible for the increased coating thickness. Vicker's microhardness test showed hardness in excess of 1000 kg_f/mm² (10 GPa) while nanoindentation test indicated much lower hardness in the range of 20 MPa to 2 GPa. Optical surface profilometry traces displayed slightly uneven surfaces compared to the bare aluminum with an average surface roughness (R_a) in the range of 1.5-4 μm depending on the shock wave pressure and type of confining medium. Ball-on-disc tribometer tests showed that the coefficient of friction and wear rate were substantially lower than the smoother, bare aluminum sample. Laser shock wave process has thus aided in the generation of a strong, wear resistant, durable carbon composite coating on aluminum 319 substrate.     

Raman spectroscopy of SWNTs produced by a XeCl excimer laser ablation at high temperatures

Synthesis of single-wall carbon nanotubes (SWNTs) was carried out by an ablation method using a XeCl excimer laser. It was irradiated onto a graphite target containing Co and Ni at the temperatures of 1073, 1173, 1273, 1373, 1473, 1523 and 1623 K under the atmosphere (0.1 MPa) of Ar gas with the flow rate of 12 ml/min. The measurement by a scanning/transmission electron microscope and Raman spectroscopy found the formation of SWNTs with the diameter of about 1.3 nm and the length of about 2 μm in ablated carbonaceous soot. The ratio of peak intensity of 1590 cm⁻¹ (G band) to that of 1335 cm⁻¹ (D band) in the high frequency Raman spectra increased with increasing the ambient temperature. The radial breathing mode (RBM) in the low frequency Raman spectra shows that the mean diameter of SWNTs increased with increasing the ambient temperature.     

Surface reactions of 2-iodopropane on GaAs(1 0 0)

The surface reactions of 2-iodopropane ((CH₃)₂CHI) on gallium-rich GaAs(1 0 0)-(4 × 1), was studied by temperature programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). CH₃CHICH₃ adsorbs molecularly at 120 K but dissociates below room temperature to form chemisorbed 2-propyl ((CH₃)₂CH) and iodide (I) species. Thermal activation causes desorption of the molecular species at 240 K, and this occurs in competition with the further reactions of the (CH₃)₂CH and I chemisorbed species. Self-coupling of the (CH₃)₂CH results in the formation of 2,3-dimethylbutane ((CH₃)₂CH-CH(CH₃)₂) at 290 K. β-Hydride elimination in (CH₃)₂CH yields gaseous propene (CH₃CHCH₂) at 550 K while reductive elimination reactions of (CH₃)₂CH with surface hydrogen yields propane (CH₃CH₂CH₃) at 560 K. Recombinative desorption of the adsorbed hydrogen as H₂ also occurs at 560 K. We observe that the activation barrier to carbon-carbon bond formation with 2-propyls on GaAs(1 0 0) is much lower than that in our previous investigations involving ethyl and 1,1,1-trifluoroethyl species where the β-elimination process was more facile. The difference in the surface chemistry in the case of 2-propyl species is attributable to its rigid structure resulting from the bonding to the surface via the second carbon atom, which causes the methyl groups to be further away from the surface than in the case of linear ethyl and 1,1,1-trifluoroethyl species. The β-hydride and reductive elimination processes in the adsorbed 2-propyl species thus occurs at higher temperatures, and a consequence of this is that GaI desorption, which is expected to occur in the temperature range 550-560 K becomes suppressed, and the chemisorbed iodine leaves the surface as atomic iodine.     

The catalytic reduction of NO by H2 on Ru(0 0 0 1): Observation of NHads species

Adsorption of NO and the reaction between NO and H₂ were investigated on the Ru(0 0 0 1) surface by X-ray photoelectron spectroscopy (XPS). Surface composition was measured after NO adsorption and after the selective catalytic reduction of nitric oxide with hydrogen in steady-state conditions at 320 K and 390 K in a 30:1 mixture of H₂ and NO (total pressure = 10⁻⁴ mbar). After steady-state NO reduction, molecularly adsorbed NO in both the linear on-top and threefold coordinations, NH_ads and N_ads species were identified by XPS. The coverage of the NH_ads and N_ads species was higher after the reaction at 390 K than the corresponding values at 320 K. Strong destabilisation of N_ads by O_ads was detected. A possible reaction mechanism is discussed.     

Adsorption and reaction of methanol on thin-film cerium oxide

Methanol adsorption and reaction has been studied on cerium oxide thin films that were vapor deposited on Ru(0 0 0 1). The methanol behavior was examined as a function of temperature and Ce oxidation state. Methanol reacts at low temperatures with fully oxidized CeO₂ to produce water at 200 K while formaldehyde and methanol desorb near 560 K. This leads to the reduction of the ceria. On reduced ceria, more methanol can be adsorbed and it undergoes more extensive decomposition producing CO and H₂ near 640 K in addition to formaldehyde and water. As the degree of ceria reduction increases, more H₂ and less H₂O are produced. TPD experiments using isotopically labeled CH₃OD show that deuterated water is produced from the oxidized surface at low temperatures, whereas the deuterium is stabilized on the reduced surface and is incorporated into the dihydrogen that desorbs near 600 K. High resolution C 1s and O 1s XPS and C k-edge NEXAFS measurements were performed to quantify the amount of methanol adsorbed and to identify the adsorbed species.