The surface chemistry of methyl and methylene radicals adsorbed on Cu(111)

The interactions of methyl and methylene radicals on Cu(111) were investigated with XPS, AES and HREELS under various exposure conditions. The CH₂ and CH₃ radicals are generated through a hot nozzle source with ketene and azomethane gases. It is shown that with substrate at 300 K, the impinging CH₃ radicals are trapped mainly as CH₃(ads), while a part of the adsorbate decomposes to form CH₂(ads) and H(ads). H atoms are found to desorb at about 380 K, while the chemisorbed hydrocarbon adspecies desorb at about 420 K. In drastic contrast, exposing the clean Cu surface to methylene radicals results not only in the trapping of CH₂(ads), but also in the formation of complex aromatic species. The adlayer is sensitive to annealing at elevated temperatures. Desorption and partial conversion to methylidyne take place at around 420 K. The CH(ads) species can survive up to 700 K and then decomposes to form residual carbon above 800 K. In both radical-Cu(111) systems, surface coverage appears to saturate near one monolayer. The relative concentrations of different surface species in the adlayer, however, depend on the amount of radical exposure. The reaction properties of the two systems are compared and discussed.     

Interaction of atomic H and CO with Cu(1 1 0) and bimetallic Ni/Cu(1 1 0)

Co-adsorption of hydrogen and CO on Cu(1 1 0) and on a bimetallic Ni/Cu(1 1 0) surface was studied by thermal desorption spectroscopy. Hydrogen was exposed in atomic form as generated in a hot tungsten tube. The Ni/Cu surface alloy was prepared by physical vapor deposition of nickel. It turned out that extended exposure of atomic hydrogen leads not only to adsorption at surface and sub-surface sites, but also to a roughening of the Cu(1 1 0) surface, which results in a decrease of the desorption temperature for surface hydrogen. Exposure of a CO saturated Cu(1 1 0) surface to atomic H leads to a removal of the more strongly bonded on-top CO (α₁ peak) only, whereas the more weakly adsorbed CO molecules in the pseudo threefold hollow sites (α₂ peak) are hardly influenced. No reaction between CO and H could be observed. The modification of the Cu(1 1 0) surface with Ni has a strong influence on CO adsorption, leading to three new, distinct desorption peaks, but has little influence on hydrogen desorption. Co-adsorption of H and CO on the Ni/Cu(1 1 0) bimetallic surface leads to desorption of CO and H₂ in the same temperature regime, but again no reaction between the two species is observed.     

TiO2(110)-(1 × 1) supported Cu particles: An FT-RAIRS investigation

The morphology of TiO₂(110)-(1 × 1) supported Cu particles has been investigated by Fourier Transform Reflection Absorption Infrared Spectroscopy (FT-RAIRS), employing adsorbed CO as a probe molecule sensitive to local surface structure. For Cu coverage (deposited at 300 K) less than 2.85 MLE nucleated Cu particles in the range 2 nm–4 nm are formed, as indicated by a final state shift in the core level Cu(2p_3/2) binding energy and by the existence of only transmission bands in the FT-RAIRS spectra for adsorbed CO. ν_S(CO) indicates that these small particles expose sites similar to those of the stepped Cu surfaces Cu(211), Cu(311), and Cu(755). At Cu coverages in the range of 6 MLE and above, corresponding to particle sizes above 4.6 nm, ν_S(CO) indicates the predominance of (110), (100) and (111) adsorption sites. Annealing the Cu layers to 650 K results in the slight growth of the particle sizes, and transformation of the CO adsorption sites corresponding to the close packed facets. The transformation of the local dielectric from that of titania to that dominated by the Cu particle is shown to take place between 3.7 and 4.2 nm, and this change is also to a smaller extent sensitive to the dispersion of the particles.     

Ethylene adsorption on regularly stepped copper surface: C2H4 on Cu(210)

Ethylene adsorption on regularly stepped Cu(210) surface was investigated with infrared reflection–adsorption spectroscopy and temperature programmed desorption. At 90 K, π-bonded ethylene was adsorbed on Cu(210) molecularly and all species were desorbed below 160 K. There were three types of π-bonded ethylene on the surface. Recent experimental studies have suggested that ethylene is dehydrogenated on Cu(410) due to the regular step [Kravchuk et al., J. Phys. Chem. C, 113 (2009) 20881]. However, neither the formation of di-σ-bonded ethylene nor dehydrogenation occurred on Cu(210).     

Adsorption and dissociation of water on LaB6(100) investigated by surface vibrational spectroscopy

The chemisorption of water (H₂O and D₂O) on a LaB₆(100) surface was studied with reflection absorption infrared spectroscopy (RAIRS) and high resolution electron energy loss spectroscopy (HREELS). The clean surface was exposed to H₂O and D₂O at temperatures from 90 K to room temperature, and spectra were acquired after heating to temperatures as high as 1200 K. It was found that water molecularly adsorbs on the surface at 90 K as a monomer at low coverages and as amorphous solid water at higher coverages. Water adsorbs dissociatively at room temperature to produce surface hydroxyl species as indicated by OH/OD stretch peaks at 3676/2701 cm^?1. Room temperature adsorption also reveals low frequency loss features in HREEL spectra near 300 cm^?1 that are quite similar to results obtained following the dissociative adsorption of O₂. In the latter case, the loss features were attributed to the LaO stretch of O atoms bridge-bonded between two La atoms. In the case of dissociative adsorption of H₂O, the low frequency loss features could be due to either the LaO vibrations of adsorbed O or of adsorbed OH.     

Multi-species time-history measurements during high-temperature acetone and 2-butanone pyrolysis

High-temperature acetone and 2-butanone pyrolysis studies were conducted behind reflected shock waves using five species time-history measurements (ketone, CO, CH₃, CH₄ and C₂H₄). Experimental conditions covered temperatures of 1100–1600 K at 1.6 atm, for mixtures of 0.25–1.5% ketone in argon. During acetone pyrolysis, the CO concentration time-history was found to be strongly sensitive to the acetone dissociation rate constant k₁ (CH₃COCH₃ → CH₃ + CH₃CO), and this could be directly determined from the CO time-histories, yielding k₁(1.6 atm) = 2.46 × 10¹⁴ exp(?69.3 [kcal/mol]/RT) s^?1 with an uncertainty of ±25%. This rate constant is in good agreement with previous shock tube studies from Sato and Hidaka (2000) [3] and Saxena et al. (2009) [4] (within 30%) at temperatures above 1450 K, but is at least three times faster than the evaluation from Sato and Hidaka at temperatures below 1250 K. Using this revised k₁ value with the recent mechanism of Pichon et al. (2009) [5], the simulated profiles during acetone pyrolysis show excellent agreement with all five species time-history measurements. Similarly, the overall 2-butanone decomposition rate constant k_tot was inferred from measured 2-butanone time-histories, yielding k_tot(1.5 atm) = 6.08 × 10¹³ exp(?63.1 [kcal/mol]/RT) s^?1 with an uncertainty of ±35%. This rate constant is approximately 30% faster than that proposed by Serinyel et al. (2010) [11] at 1119 K, and approximately 100% faster at 1412 K. Using the measured 2-butanone and CO time-histories and an O-atom balance analysis, a missing removal pathway for methyl ketene was identified. The rate constant for the decomposition of methyl ketene was assumed to be the same as the value for the ketene decomposition reaction. Using the revised k_tot value and adding the methyl ketene decomposition reaction to the Serinyel et al. mechanism, the simulated profiles during 2-butanone pyrolysis show good agreement with the measurements for all five species.     

A vibrational and TDS study of sulfur adsorbates on Cu(100): Evidence for CH3S species

Vibrational (EELS) and TDS data for methyl mercaptan (CH₃SH), dimethyl sulfide (CH₃)₂S and dimethyl disulfide (CH₃S)₂ are analyzed to determine the nature of the adsorption states on Cu(100). Dimethyl sulfide is reversibly adsorbed on Cu(100); no dissociation (CS bond breaking) was found. By contrast, methyl mercaptan and dimethyl disulfide dissociate below 300 K to form adsorbed CH₃S (methyl mercaptide) species. Depending on the coverage, two orientations of methyl mercaptide are found: linear and bent. The two different orientations can be distinguished via the surface dipole selection rule by different intensities of the methyl rocking and deformation vibrations. By contrast with the methoxy species, which on Cu(100) decomposes to formaldehyde, no H₂C=S is liberated during decomposition of CH₃S. The mercaptide is stable to ∼ 350 K, but decomposes at higher temperatures to form adsorbed sulfur and recombinant methane, hydrogen and ethane. The methane appears to be formed by methyl-hydrogen recombination when the C-S bond scission occurs. TDS results show that sulfur released from the decomposition poisons the surface toward further adsorption. In addition, the selectivity toward methane versus ethane can be altered by pre-titrating the adsorbed hydrogen with oxygen, thereby changing the relative methyl-hydrogen and methyl-methyl recombination probabilities.     

H2S dissociation on the Fe(1 0 0) surface: An ab initio molecular dynamics study

The adsorption of H₂S on Fe(1 0 0) is examined using ab initio molecular dynamics at 298 and 1808 K. Dissociation of H₂S occurs at both temperatures simulated, to leave adsorbed S and two H atoms. The dissociation occurs via a two step process and the mechanism is found to be different depending on the temperature of the reaction. At 1808 K, diffusion of the dissociated H atoms into the subsurface region is also observed.     

An investigation of the effect of pre-dosed O atoms on the adsorption of NO on Pt{2 1 1}

Reflection absorption infrared spectroscopy (RAIRS) and temperature programmed desorption (TPD) have been used to investigate the effect of pre-dosed O atoms on the adsorption of NO on Pt{2 1 1} at room temperature. RAIRS experiments show that no new species are formed when NO is adsorbed onto a Pt{2 1 1} surface that has been pre-dosed with oxygen and no species are lost from the spectra, compared to spectra recorded for NO adsorption on the clean Pt{2 1 1} surface. However pre-dosed oxygen atoms do influence the frequency and intensity of several of the observed infrared bands. In stark contrast, pre-dosed O has a large effect on the TPD spectra. In particular N₂ and N₂O desorption, seen following NO adsorption on the clean Pt{2 1 1} surface, is completely inhibited. This effect has been assigned to the blocking of NO dissociation by the pre-adsorbed O atoms. A new NO desorption peak, not seen for NO adsorption on the clean Pt{2 1 1} surface, is also observed in TPD spectra recorded following NO adsorption on an oxygen pre-dosed Pt{2 1 1} surface.     

Adsorption of small molecules on a (2 × 1) PdZn surface alloy on Pd(1 1 1)

The adsorption of methanol, formaldehyde, methoxy, carbon monoxide and water on a (2 × 1) PdZn surface alloy on Pd(1 1 1) has been studied using DFT calculations. The most stable adsorption structures of all species have been investigated with respect to the structure and the electronic properties. It was found that methanol is only weakly bound to the surface. The adsorption energy only increases with higher methanol coverage, where chain structures with hydrogen bonds between the methanol molecules are formed. The highest adsorption energy was found for the formate species followed by the methoxy species. The formaldehyde species shows quite some electronic interaction with the surface, however the stable η₂ formaldehyde has only an adsorption energy of about 0.49 eV. The calculated IR spectra of the different species fit quite well to the experimental values available in the literature.     

Vibrational spectroscopy of oxygen on the (100) and (111) surfaces of lanthanum hexaboride

Reflection absorption infrared spectroscopy (RAIRS) and high resolution electron energy loss spectroscopy (HREELS) have been used to study the adsorption of oxygen on the (100) and (111) surfaces of lanthanum hexaboride. Exposure of the surface at temperatures of 95 K and above to O₂ produces atomic oxygen on the surface and yields vibrational peaks in good agreement with those observed in previous HREELS studies. On the La-terminated (100) surface, RAIRS peaks correspond to vibrations of the boron lattice that gain intensity due to a decrease in screening of surface dipoles that accompanies oxygen adsorption. A sharp peak at ～ 734 cm^?1 in the HREEL spectrum shows isotopic splitting with RAIRS into two components at 717 and 740 cm^?1 with full widths at half maxima of only 12 cm^?1. The sharpness of this mode is consistent with its interpretation as a surface phonon that is well separated from both the bulk phonons and other surface phonons of LaB₆. On the boron-terminated LaB₆(111) surface, broad and weak features are assigned to both vibrations of the boron lattice and of boron oxide. On the (100) surface, oxygen blocks the adsorption sites for CO, and adsorbed CO prevents the dissociative adsorption of O₂.     

CO adsorption on clean and oxidized Pt3Ti(111)

CO adsorption on clean and oxidized Pt₃Ti(111) surfaces has been investigated by means of Auger Electron Spectroscopy (AES), Thermal Desorption Spectroscopy (TDS), Low Energy Electron Diffraction (LEED) and High Resolution Electron Energy Loss Spectroscopy (HREELS). On clean Pt₃Ti(111) the LEED patterns after CO adsorption exhibit either a diffuse or a sharp c(4 × 2) structure (stable up to 300 K) depending on the adsorption temperature. Remarkably, the adsorption/desorption behavior of CO on clean Pt₃Ti(111) is similar to that on Pt(111) except that partial CO decomposition on Ti sites and partial CO oxidation have also been evidenced. Therefore, the clean surface cannot be terminated by a pure Pt plane. Partially oxidized Pt₃Ti(111) surfaces (< 135 L O₂ exposure at 1000 K) exhibit a CO adsorption/desorption behavior rather similar to that of the clean surface, showing again a c(4 × 2) structure (stable up to 250 K). Only the oxidation of CO is not detectable any more. These results indicate that some areas of the substrate remain non-oxidized upon low oxygen exposures. Heavily oxidized Pt₃Ti(111) surfaces (> 220 L O₂ exposure at 1000 K) allow no CO adsorption indicating that the titanium oxide film prepared under these conditions is completely closed.     

Application of low energy ion blocking for adsorption site determination of Na Atoms on a Cu(111) surface

Ion blocking in the low keV energy range is demonstrated to be a sensitive method for probing surface adsorption sites by means of the technique of time-of-flight scattering and recoiling spectroscopy (TOF-SARS). Adsorbed atoms can block the nearly isotropic backscattering of primary ions from surface atoms in the outmost layers of a crystal. The relative adsorption site position can be derived unambiguously by simple geometrical constructs between the adsorbed atom site and the surface atom sites. Classical ion trajectory simulations using the scattering and recoiling imaging code (SARIC) and molecular dynamics (MD) simulations provide the detailed ion trajectories. Herein we present a quantitative analysis of the blocking effects produced by sub-monolayer Na adsorbed on a Cu(111) surface at room temperature. The results show that the Na adsorption site preferences are different at different Na coverages. At a coverage θ = 0.25 monolayer, Na atoms preferentially populate the fcc threefold surface sites with a height of 2.7 ± 0.1 Å above the 1st layer Cu atoms. At a lower coverage of θ = 0.10 monolayer, there is no adsorption site preference for the Na atoms on the Cu(111) surface.     

Visible light induced photodesorption of NO from the α-Cr2O3(0001) surface

Nitric oxide chemistry and photochemistry on the Cr-terminated surface of α-Cr₂O₃(0001) were examined using temperature programmed desorption (TPD), sticking coefficient measurements and photodesorption. NO exposed to α-Cr₂O₃(0001) at 100 K binds at surface Cr cation sites forming a strongly bound surface species that thermally desorbs at 320–340 K, depending on coverage. No thermal decomposition was detected in TPD in agreement with previous results in the literature. Sticking probability measurements at 100 K indicated near unity sticking for NO up to coverages of ~ 1.3 ML, with additional adsorption with higher exposures at decreased sticking probability. These results suggest that some Cr cation sites on the α-Cr₂O₃(0001) surface were capable of binding more than one NO molecule, although it is unclear whether this was as separate NO molecules or as dimers. Photodesorption of adsorbed NO was examined for surface coverages below the 1 ML point. Both visible and UV light were shown to photodesorb NO without detectable NO photodecomposition. Visible light photodesorption of NO occurred with a greater cross section than estimated using UV light. The visible light photodesorption event was not associated with bandgap excitation in α-Cr₂O₃(0001), but instead was linked to excitation of a surface Cr^3 +–NO^? charge transfer complex. These results illustrate that localized photoabsorption events at surface sites with unique optical properties (relative to the bulk) can result in unexpected surface photochemistry.     

Low temperature oxidation of Fe2+ surface sites on the (2 × 1) reconstructed surface of α-Fe2O3(011­2)

Temperature programmed desorption (TPD), electron energy loss spectroscopy (ELS) and low energy electron diffraction (LEED) were used to study the interaction of molecular oxygen with the (2 × 1) reconstructed surface of hematite α-Fe₂O₃(011­2) under UHV conditions. The (2 × 1) surface is formed from vacuum annealing of the ‘ideal’ (1 × 1) surface and possesses Fe²⁺ surface sites based on ELS. While O₂ does not stick to the (1 × 1) surface at 120 K, the amount of O₂ that can be reversibly adsorbed at 120 K on the (2 × 1) surface was estimated to be ～ 0.5 ML (where 1 ML is defined as the Fe³⁺ surface coverage on the ideal (1 × 1) surface), with additional O₂ that is irreversibly adsorbed based on subsequent H₂O TPD. Molecularly and dissociatively adsorbed O₂ modifies the surface chemistry of H₂O both in terms of enhanced OH stability (relative to either the (1 × 1) or (2 × 1) surfaces) and in the blocking of H₂O adsorption sites. While O₂ adsorption at 120 to 300 K does not transform the (2 × 1) surface into the (1 × 1) surface, the influence of O₂ on the (2 × 1) surface involves both charge transfer from surface Fe²⁺ sites and formation of an ordered c(2 × 2) structure resulting from O₂ dissociation.     

The thermal decomposition of 2,5-dimethylfuran

The thermal decomposition of 2,5-dimethylfuran is studied in a bench-scale pyrolysis set-up equipped with a dedicated on-line analysis section including a GC × GC-FID/(TOF-MS). This analysis section enables both qualitative and quantitative on-line analysis of the entire reactor effluent with an unseen level of detail. The reactor temperature was varied from 873 K to 1098 K at a fixed pressure of 1.7 bar and a residence time of 300–400 ms, covering the complete conversion range of 2,5-dimethylfuran.The main products at low conversions are hydrogen, CO, methane, phenol, 2-methylfuran, 1,3-cyclopentadiene and a C₇H₁₀O isomer. At higher conversions increasing amounts of mono- and poly-aromatics such as benzene, toluene, indene and naphthalene are formed. These species appear to be secondary reaction products of 1,3-cyclopentadiene. At the highest temperatures more than 10 mol% of 2,5-dimethylfuran is converted into mono-, di-, tri- and tetra-aromatic products, which are known soot precursors. This high tendency to form molecular weight growths species even under diluted conditions can pose a threat for the use of 2,5-dimethylfuran as a fuel.     

Adsorption of thiophene on silica-supported Mo clusters

The adsorption/decomposition kinetics/dynamics of thiophene has been studied on silica-supported Mo and MoS_x clusters. Two-dimensional cluster formation at small Mo exposures and three-dimensional cluster growth at larger exposures would be consistent with the Auger electron spectroscopy (AES) data. Thermal desorption spectroscopy (TDS) indicates two reaction pathways. H₄C₄S desorbs molecularly at 190–400 K. Two TDS features were evident and could be assigned to molecularly on Mo sites, and S sites adsorbed thiophene. Assuming a standard preexponential factor (ν = 1 × 10¹³/s) for first-order kinetics, the binding energies for adsorption on Mo (sulfur) sites amount to 90 (65) kJ/mol for 0.4 ML Mo exposure and 76 (63) kJ/mol for 2 ML Mo. Thus, smaller clusters are more reactive than larger clusters for molecular adsorption of H₄C₄S. The second reaction pathway, the decomposition of thiophene, starts at 250 K. Utilizing multimass TDS, H₂, H₂S, and mostly alkynes are detected in the gas phase as decomposition products. H₄C₄S bond activation results in partially sulfided Mo clusters as well as S and C residuals on the surface. S and C poison the catalyst. As a result, with an increasing number of H₄C₄S adsorption/desorption cycles, the uptake of molecular thiophene decreases as well as the H₂ and H₂S production ceases. Thus, silica-supported sulfided Mo clusters are less reactive than metallic clusters. The poisoned catalyst can be partially reactivated by annealing in O₂. However, Mo oxides also appear to form, which passivate the catalyst further. On the other hand, while annealing a used catalyst in H/H₂, it is poisoned even more (i.e., the S AES signal increases). By means of adsorption transients, the initial adsorption probability, S₀, of C₄H₄S has been determined. At thermal impact energies (E_i = 0.04 eV), S₀ for molecular adsorption amounts to 0.43 ± 0.03 for a surface temperature of 200 K. S₀ increases with Mo cluster size, obeying the capture zone model. The temperature dependence of S₀(T_s) consists of two regions consistent with molecular adsorption of thiophene at low temperatures and its decomposition above 250 K. Fitting S₀(T_s) curves allows one to determine the bond activation energy for the first elementary decomposition step of C₄H₄S, which amounts to (79 ± 2) kJ/mol and (52 ± 4) kJ/mol for small and large Mo clusters, respectively. Thus, larger clusters are more active for decomposing C₄H₄S than are smaller clusters.     

Observation of hydrogen adsorption on 6H-SiC(0 0 0 1) surface

We have used coaxial impact-collision ion scattering spectroscopy (CAICISS) and time-of-flight elastic recoil detection analysis (TOF-ERDA) to investigate the adsorption of atomic hydrogen on the 6H-SiC(0 0 0 1)√3×√3 surface. It has been found that the saturation coverage of hydrogen on the 6H-SiC(0 0 0 1)√3×√3 surface is about 1.7 ML. Upon saturated adsorption of atomic hydrogen, the √3×√3 surface structure changes to the 1×1 structure. The data of the CAICISS measurements have indicated that as a result of the hydrogen adsorption, Si adatoms on the √3×√3 surface move from T₄ to on-top sites.     

DFT study of self-coupling reaction of CF2(ads) coadsorbed on Cu(111) surface for forming CF2=CF2(g)

Total energy calculations based on the density functional theory (DFT) with ultrasoft pseudopotential, generalized gradient spin-polarized approximation and the partial structural constraint path minimization (PSCPM) method were carried out to establish the energetically more favorable reaction pathways for the self-coupling reaction of coadsorbed CF_2(ads) leading to the formation of CF₂=CF_2(ads) on the Cu(111) surface. In addition, the calculated electronic properties, namely partial density of states (PDOS), suggest that the initial breaking of the Cu(111)–CF_2(ads) bond associating with the electron delocalization on the Cu(111) surface and the electron transfer from Cu(111) to both units of CF_2(ads) are factors controlling the energy barrier for self-coupling reaction. Finally, the calculated energy barrier (0.310 eV) for the self-coupling reaction of CF_2(ads) coadsorbed on the Cu(111) surface in comparison with that (0.204 eV) for the single α-fluoride elimination of adsorbed CF_3(ads) on the Cu(111) surface qualitatively manifests that the formation of CF₂ = CF_2(g) at 250 K is limited by the self-coupling reaction of coadsorbed CF_2(ads) instead of the single α-fluoride elimination of adsorbed CF_3(ads).     

Optical characterization of methanol adsorption on the bare and oxygen precovered Cu(1 1 0) surface

We have studied the adsorption and reaction of methanol on the bare and oxygen precovered Cu(1 1 0) surface at 200 K using reflectance difference spectroscopy (RDS). On the bare and fully oxygen covered surface, the sticking coefficient is close to zero. In contrast, on the partially oxygen covered surface, a sticking coefficient close to unity is obtained. This observation suggests a high mobility of methanol on both bare and oxygen covered Cu(1 1 0) and of methoxy on Cu(1 1 0). Two reaction regimes, an oxygen supply limited and an adsorption site limited regime are identified. The transition between these two regimes occurs for an oxygen coverage of about 0.2.