Carbon nanotubes supported Pt-Ni catalysts and their properties for the liquid phase hydrogenation of cinnamaldehyde to hydrocinnamaldehyde

The Pt-Ni catalysts supported on CNTs have been prepared by wet impregnation and the selective hydrogenation of cinnamaldehyde (CMA) to the corresponding hydrocinnamaldehyde (HCMA) over the catalysts has been studied in ethanol at different reaction conditions. The results show that Pt-0.34 wt% Ni/CNTs catalyst exhibits the highest activity and selectivity at a reaction temperature of 70 °C under a pressure of around 2.0 MPa, and 98.6% for the conversion of CMA and 88.2% for the selectivity of CMA to HCMA, respectively. The selective hydrogenation for the CC bond in CMA would be improved as increasing the reaction temperature, and the hydrogenation for the CO bond in CMA is enhanced as increasing the H₂ pressure. In addition, these catalysts have also been characterized using TEM-EDS, XPS, H₂-TPR and H₂-TPD techniques. The results show that Pt particles are dispersed more homogeneously on the outer surface of the nanotubes, while the strong interaction between Pt and Ni would improve the increasing of activated hydrogen number because of the hydrogen spillover from reduced Pt⁰ onto CNTs and increase the catalytic activity and selectivity of CMA to HCMA.     

A selective [4 + 2]-like cycloaddition of α, β-unsaturated ketone on Si(1 1 1)-7 × 7

The interaction of ethyl vinyl ketone (EVK) with Si(1 1 1)-7 × 7 has been investigated using high-resolution electron energy loss spectroscopy (HREELS), X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) calculations. The disappearance of both stretching vibrations of CH₂ (3099 cm⁻¹) and CO (1684 cm⁻¹) coupled with the appearance of new CC stretching mode (1660 cm⁻¹) in the HREELS spectra of chemisorbed EVK clearly demonstrates the direct involvement of conjugated CC and CO bonds to form a SiC¹H₂C²HC³(C⁴H₂C⁵H₃)OSi surface species via [4 + 2]-like cycloaddition in a highly selective manner. In addition, XPS studies show that the C1s binding energies of C¹/C² and C³ upon chemisorption display chemical downshifts of 0.8 eV and 2.2 eV, respectively, further confirming the proposed [4 + 2]-like cycloaddition reaction for the EVK/Si(1 1 1)-7 × 7 system. DFT theoretical calculations suggest that the proposed [4 + 2]-like cycloadduct is thermodynamically most favorable.     

Chemisorption of 1,1-dichloroethene on the Si(1 1 1)-7 × 7 surface

Chemisorption of 1,1-dichloroethene (Cl₂CCH₂) to a Si(1 1 1)-7 × 7 surface was studied by means of X-ray photoelectron spectroscopy using synchrotron radiation, recording chlorine 2p and carbon 1s spectra. For carbon 1s, spectral assignment of the chemisorbed species is based on quantum chemical calculations of chemical shifts in model compounds.The results confirm the identity of covalently bonded 1-chlorovinyl (-CClCH₂) and vinylidene (CCH₂) adspecies. Upon chemisorption at room temperature it was found that about one-third of the molecules break one C-Cl bond while about two-thirds of the adsorbates break two C-Cl bonds. We do not, however, find evidence for isomerization of CCH₂ to di-bonded vinylene (-CHCH-).     

Reaction of acetonitrile with the silicon(0 0 1) surface: A combined XPS and FTIR study

The adsorption of acetonitrile on the Si(0 0 1) surface has been investigated using X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared spectroscopy (FTIR). XPS and FTIR spectra indicate that adsorbed acetonitrile forms two correlated binding configurations, a CN species with a strong FTIR absorption at 1540 cm⁻¹ and a CCN (ketenimine) species that has a very strong FTIR absorption at 1952 cm⁻¹. The CCN FTIR peak at 1952 cm⁻¹ shows a striking polarization dependence, with the infrared transition dipole almost entirely in the plane of the sample and parallel to the SiSi dimer axis. Our data suggests that the primary CCN structure results from cleavage of two C-H bonds, forming a structure in which the N and terminal C atom are both linked to the surface. Temperature-dependent experiments help to elucidate the complicated reaction mechanism for acetonitrile adsorbing onto the Si(0 0 1) surface. Dosing at higher temperature increases the amount of CCN relative to CN species while heating leads to direct transformation of the CN to the CCN species. Our results indicate that previous studies, which considered only products formed by cleavage of a single C-H bond, have misidentified the primary ketenimine product. A reinterpretation of the earlier results, combined with data presented here, sheds new light onto the products and mechanism of interaction of acetonitrile with Si(0 0 1).     

Site-blocking effects of preadsorbed H on Pt(1 1 1) probed by 1,3-butadiene adsorption and reaction

The influence of hydrogen coadsorption on hydrocarbon chemistry on transition metal surfaces is a key aspect to an improved understanding of catalytic selective hydrogenation. We have investigated the effects of H preadsorption on adsorption and reaction of 1,3-butadiene (H₂CCHCHCH₂, C₄H₆) on Pt(1 1 1) surfaces by using temperature-programmed desorption (TPD) and Auger electron spectroscopy (AES). Preadsorbed hydrogen adatoms decrease the amount of 1,3-butadiene chemisorbed on the surface and chemisorption is completely blocked by the hydrogen monolayer (saturation) coverage (θ_H = 0.92 ML). No hydrogenation products of reactions between coadsorbed H adatoms and 1,3-butadiene were observed to desorb in TPD experiments over the range of θ_H investigated (θ_H = 0.6-0.9 ML). This is in strong contrast to the copious evolution of ethane (CH₃CH₃, C₂H₆) from coadsorbed hydrogen and ethylene (CH₂CH₂, C₂H₄) on Pt(1 1 1). Hydrogen adatoms effectively (in a 1:1 stoichiometry) remove sites from interaction with chemisorbed 1,3-butadiene, but do not affect adjacent sites. The adsorption energy of coadsorbed 1,3-butadiene is not affected by the presence of hydrogen on Pt(1 1 1). The chemisorbed 1,3-butadiene on hydrogen preadsorbed Pt(1 1 1) completely dehydrogenates to H₂ and surface carbon upon heating without any molecular desorption detected, which is identical to that observed on clean Pt(1 1 1). In addition to revealing aspects of site blocking that should have broad implications for hydrogen coadsorption with hydrocarbon molecules on transition metal surfaces in general, these results also provide additional basic information on the surface science of selective catalytic hydrogenation of butadiene in butadiene-butene mixtures.     

A theoretical study of c-C5H8 adsorption on Ge (0 0 1)-2 × 1 and on dimer vacancies on the surface: Electronic structure and bonding

In this work we analyzed the geometry and the chemical interactions for c-C₅H₈ adsorption on Ge (0 0 1), using density functional theory calculations (DFT). We examined the changes in the atomic interactions using a slab model. We considered two cases, the cyclopentene adsorption on Ge(0 0 1) and on dimer vacancies on the surface. We found an average distance H-Ge, -C-Ge and C-Ge of 1.50, 1.70 and 1.65 Å, respectively, on dimer vacancies; and an average C-Ge distance of 2.05 Å on Ge-Ge dimer. We also computed the density of states (DOS) and the DOS weighted overlap populations (OPDOS) corresponding to C-C, C-Ge, C-H, and Ge-Ge bonds. During adsorption the main contribution are the CC double bond in both cases, and the next C and the H's belonging to this bonds in the case of adsorption on dimer vacancies. The orbital contribution includes participation of the 2p_y and 2p_z orbitals corresponding to unsaturated C atoms, 2p_z corresponding to side saturated C, and the 4p orbitals of Ge for the adsorption on dimer vacancies; 2s and 2p_z orbitals corresponding to double bond C atoms, 4s and 4p_z orbitals of Ge for the adsorption on Ge(0 0 1).     

Selective surface chemistry of allyl alcohol and allyl aldehyde on Si(1 0 0)2×1: Competition of [2 + 2] CC cycloaddition with O-H dissociation and with [2 + 2] CO cycloaddition in bifunctional molecules

Competition between the CC functional group with the OH group in allyl alcohol and with the CO group in allyl aldehyde in the adsorption and thermal chemistry on Si(1 0 0)2×1 has been studied by X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD), as well as density-functional theory (DFT) calculations. The similarities found in the C 1s and O 1s spectra for both molecules indicate that the O-H dissociation product for allyl alcohol and [2 + 2] CO cycloaddition product for allyl aldehyde are preferred over the corresponding [2 + 2] CC cycloaddition products. Temperature-dependent XPS and TPD studies further show that thermal evolution of these molecules gives rise to the formation of ethylene, acetylene, and propene on Si(1 0 0)2×1, with additional CO evolution only from allyl alcohol. The formation of these desorption products also supports that the [2 + 2] CC cycloaddition reaction does not occur. In addition, the formation of SiC at 1090 K is observed for both allyl alcohol and allyl aldehyde. We propose plausible surface-mediated reaction pathways for the formation of these thermal evolution products. The present work illustrates the crucial role of the Si(1 0 0)2×1 surface in selective reactions of the Si dimers with the O−H group in allyl alcohol and with the CO group in allyl aldehyde over the CC functional group common to both molecules.     

Anharmonic force field and equilibrium structure of nitric acid

The quadratic, cubic and semi-diagonal quartic force field of nitric acid has been calculated at the CCSD(T) level of theory employing a basis set of triple-ζ quality. A semi-experimental equilibrium structure has been derived from experimental ground state rotational constants and rovibrational interaction parameters calculated from the ab initio force field. It is found that the A and B semi-experimental equilibrium rotational constants of the ¹⁸O isotopologues (for which the rotation of principal axes is large) cannot be accurately reproduced. This problem is discussed and a remedy is proposed. Finally, the semi-experimental structure is in agreement with the ab initio structure calculated at the CCSD(T) level of theory using a basis set of at least quadruple-ζ quality and a core correlation correction, except for the long NO single bond for which the CCSD(T) value is too short due to inadequate treatment of electron correlation. The empirical structures are also determined and their accuracy is discussed. The best equilibrium structure is: r_e(NO_syn) = 1.209(1) Å, r_e(NO_anti) = 1.194(1) Å, r_e(NO) = 1.397(1) Å, r_e(OH) = 0.968(1) Å, ∠(ONO_syn) = 115.8(1)°, ∠(ONO_anti) = 114.2(1)° and ∠(NOH) = 102.2(1)°.     

Chemistry of l-proline on Pd(1 1 1): Temperature-programmed desorption and X-ray photoelectron spectroscopic study

The surface chemistry of proline is explored on Pd(1 1 1) using a combination of temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy. Proline adsorbs on Pd(1 1 1) at temperatures of 250 K and below into second and subsequent layers prior to the saturation of the first layer, where approximately 70% of the adsorbed proline is present in its zwitterionic form. Molecular proline desorbs between ∼315 K and ∼333 K depending on coverage. When adsorbed at ∼300 K, only the first monolayer is formed, and the proline is present as zwitterions, oriented such that all of the carbons are detected equally by XPS. Proline decomposes by scission of the C-COO bond, where the carboxylate moiety desorbs as carbon monoxide and carbon dioxide, while the nitrogen-containing moiety desorbs as to HCN, and evolves pyrrole at ∼390 K, pyrrolidine at ∼410 K, and final species that desorbs at ∼450 K that cannot be unequivocally assigned but may be 2-butenenitrile (CH₃-CHCH-CN), 3-butenenitrile (CH₂CH-CH₂-CN), 2-methyl-2-propenenitrile (CH₂C(CH₃)-CN) or cyclopropanecarbonitrile.     

On the synthesis and optical absorption studies of nano-size magnesium oxide powder

Magnesium oxide (MgO) nano-size powder is synthesized using magnesium nitrate hexahydrate and oxalic acid as precursors with ethanol as a solvent. The process involves gel formation, drying at 100 °C for 24 h to form magnesium oxalate dihydrate [α-MgC₂O₄·2H₂O] and its decomposition at 500, 600, 800, and 1000 °C for 2 h to yield MgO powder (average crystallite size ∼6.5-73.5 nm). The sol-gel products at various stages of synthesis are characterized for their thermal behaviour, phase, microstructure, optical absorption, and presence of hydroxyl and other groups like OCO, CO, C-C, etc. MgO powder is shown to possess an f.c.c. (NaCl-type) structure with lattice parameter increasing with decrease in crystallite size (t_av); typical value being ∼4.222(2) Å for t_av∼6.5 nm as against the bulk value of 4.211 Å. Infrared absorption has shown MgO to be highly reactive with water. Also, a variety of F- and M-defect centres found in MgO produce energy levels within the band gap (7.8 eV), which make it attractive for application in plasma displays for increasing secondary electron emission and reducing flickering effects. The possible application of the intermediate sol-gel products, viz., α-MgC₂O₄·2H₂O and anhydrous magnesium oxalate (MgC₂O₄) in understanding the plants and ESR dosimetry, respectively, has also been suggested.     

DFT calculations of XPS/NEXAFS and IR spectra to elucidate the reaction products of acetonitrile with Si(0 0 1)-2 × 1

NEXAFS data [S. Rangan et al., Phys. Rev. B 71 (2005) 165319] and FTIR data [M.P. Schwartz, R.J. Hamers, Surf. Sci. 601 (2007) 945] apparently do not converge in the identification of the reaction products of acetonitrile (CH₃CN) with Si(0 0 1)-2 × 1 at room temperature. Using DFT calculations of core-excited/core-ionized spectra and of IR vibrational frequencies and intensities, we show the consistency of the body of experimental data. Three species are present on the surface in equivalent amounts, a CN moiety, a pendent CN and a CCN ketenimine submitted to a strong twist imposed by the Si bond directionality. More generally, the paper shows the usefulness of spectroscopic data simulations in the elucidation of complex surface chemistry problems.     

Positive and negative ionic desorption from condensed formic acid photoexcited around the O 1s-edge: Relevance to cometary and planetary surfaces

Photon stimulated ion desorption (PSID) studies have been performed in condensed formic acid using oxygen 1s-edge synchrotron radiation from the Brazilian synchrotron light source (LNLS), operating in a single-bunch mode. Ion formation was discussed in terms of the Auger stimulated ion desorption (ASID) and X-ray induced electron stimulated desorption (XESD) mechanisms. It is found that O 1s(C-OH) → π*(CO) and O 1s(CO) → 3s/σ*(HCO) transitions favored the production of C⁺, CH⁺, O⁺, O⁻ and H⁻ ions. The hydroxyl anion has not been observed while the hydroxyl cation showed low intensity or was absent. Some anion formation routes from dissociative reactions are suggested taking into account the positive ion yields.     

FTIR spectra and rovibrational analysis of the ν11 band of trans-ClHCCHF and ν10 of trans-ClHCCDF

High-resolution Fourier transform infrared spectra of natural trans-ClHCCHF and of its isotopologue trans-ClHCCDF have been recorded in the region between 700 and 1150 cm⁻¹ with the purpose to analyze the ν₁₁ fundamental of the main species and the ν₁₀ of its deuterated compound. Both bands, of symmetry species A″, present c-type envelope absorptions. Beside the expected features, the K structure of the P(J), Q(J), and R(J) manifolds was resolved and identified; the assignment of the rovibrational transitions was extended up to J = 92 and K_a = 13 for the trans-³⁵ClHCCHF and up to J = 86 and K_a = 10 for trans-³⁵ClHCCDF. More than 2900 and 2700 lines for the main and deuterated species, respectively, were analyzed by a least-squares procedure and reliable spectroscopic molecular parameters were determined for both isotopologues.     

Hydrogen related point defects in silicon based layers: Si(·)H and SiOOH

Layers prepared by pulsed TEA CO₂ pulsed laser ablation (PLA) of SiO and SiO₂ targets in helium were exposed to hydrogen and deuterium atmosphere up to several kPa. The deposited layers were investigated by FTIR, EPR and XP spectroscopy. Among various Si species silyl radical Si(·)H (Si(·)D) at 2166 (1568) cm⁻¹—H(I) center—and silyl hydroperoxide SiOOH (SiOOD) at 3587 (2648) cm⁻¹ were identified in FTIR spectra. Chemical pathways for production of these species are discussed. Experimental results are supported by quantum chemical calculations.     

S180 cell growth on low ion energy plasma treated TiO2 thin films

X-ray photoelectron spectroscopy (XPS) was used to characterise the effects of low energy (<2 eV) argon ion plasma surface modification of TiO₂ thin films deposited by radio frequency (RF) magnetron sputter system. The low energy argon ion plasma surface modification of TiO₂ in a two-stage hybrid system had increased the proportion of surface states of TiO₂ as Ti³⁺. The proportion of carbon atoms as alcohol/ether (COX) was decreased with increase the RF power and carbon atoms as carbonyl (CO) functionality had increased for low RF power treatment. The proportion of C(O)OX functionality at the surface was decreased at low power and further increase in power has showed an increase in its relive proportion at the surface. The growth of S180 cells was observed and it seems that cells are uniformly spreads on tissue culture polystyrene surface and untreated TiO₂ surfaces whereas small-localised cell free area can be seen on plasma treated TiO₂ surfaces which may be due to decrease in C(O)OX, increase in CO and active sites at the surface. A relatively large variation in the surface functionalities with no change in the surface roughness was achieved by different RF plasma treatments of TiO₂ surface whereas no significant change in S180 cell growth with different plasma treatments. This may be because cell growth on TiO₂ was mainly influenced by nano-surface characteristics of oxide films rather than surface chemistry.     

Resonant Auger spectroscopy study of charge transfer phenomena in N 1s core-excited acetonitrile adsorbates on Si(0 0 1)-2 × 1

Acetonitrile (CH₃CN) adsorbs on Si(0 0 1)-2 × 1 at room temperature under two forms, a cycloaddition-like adduct (Si-CN-Si) and a pendent cyano (Si-CH₂-CN) resulting from the decomposition of the molecule. Resonant Auger spectroscopy has been used to study the excited-state-dependent electron transfer from the N 1s core-excited molecular adsorbate to the silicon substrate, using the core-hole lifetime (∼6 fs) as an internal clock. It is shown that the π_CN^∗ NEXAFS state lies within the silicon bandgap because of a core-excitonic effect. Therefore no charge transfer of the excited electron to the substrate is observed. On the other hand the π_CN^∗ NEXAFS state is placed within the silicon conduction band. Excitation to this orbital leads to valence/Auger spectra in which both resonant and normal Auger contributions are observed. Therefore there is evidence for a charge transfer from the pendent CN to the silicon surface, on a timescale estimated to tens of femtoseconds.     

Theoretical and experimental studies of Ag-Pt interactions for supported Ag-Pt bimetallic catalysts

The electronic structure and chemical properties of catalysts prepared by the electroless deposition (ED) of Ag onto Pt/SiO₂ were studied using a combination of X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) calculations. XPS studies revealed a negative shift (up to −0.75 eV) in the Ag 3d binding energy (BE) relative to bulk Ag. Both the magnitude and direction of the shift are consistent with DFT calculations of model Ag/Pt(1 1 1) surfaces. DFT calculations have also been employed to study the adsorption of two probe molecules, carbon monoxide and 1-epoxy-3-butene (EpB), on the model surfaces. Combined with previously published reports, the results presented here suggest that (1) the AgPt/SiO₂ catalysts that are most active for hydrogenation of the EpB olefin function consist of an adlayer of Ag on Pt rather than a surface or bulk alloy and that (2) the higher activity and selectivity of ED-prepared Ag-Pt/SiO₂ catalysts for CC hydrogenation of EpB to 1-epoxybutane are consistent with computed electronic (ligand) and bifunctional effects.     

The chemistry of C2 perfluoroalkyl iodide on the Cu(1 1 1) single crystal surface

The thermal chemistry of perfluoroethyl iodide (C₂F₅I) adsorbed on Cu(1 1 1) has been investigated by temperature-programmed reaction/desorption (TPR/D), reflection-absorption infrared spectroscopy (RAIRS), and X-ray photoelectron spectroscopy (XPS). I 4d and F 1s XPS spectra show that dissociative adsorption of C₂F₅I to form the surface-bound perfluroethyl (Cu-C₂F₅) moieties occurs at very low temperature (T < 90 K), while the C-F bond cleavage in adsorbed perfluroethyl (Cu-C₂F₅) begins at ca. 300 K. XPS and TPR/D studies further reveal that the reactions of ^βCF₃^αCF_2(ad) on Cu(1 1 1) are strongly dependent on the surface coverage. At high coverages (?0.16 L exposure), the adsorbed perfluroethyl (Cu-C₂F₅) evolves, via α-F elimination, into the surface-bound tetrafluoroethylidene moieties (CuCF-CF₃) followed by a dimerization step to form octafluoro-2-butene (CF₃CFCFCF₃) at 315 K as gas product. The surface-bound (Cu-C₂F₅) decomposes preferentially, at low coverages (?0.04 L), via consecutive α-F abstraction to afford intermediate, trifluoroethylidyne (CuCCF₃), resulting in the final coupling reaction to yield hexafluoro-2-butyne (CF₃CCCF₃) at 425 K. However, at middle coverages (ca. 0.08-0.16 L exposure), the adsorbed perfluroethyl (Cu-C₂F₅) first experiences an α-F elimination and then prefers to loss the second F from β position to yield the intermediate of Cu-CF₂-CFCu (μ-η,η-perfluorovinyl), which may further evolve into hexafluorocyclobutene (CF₂CFCFCF₂) at 350 K through cyclodimerization reaction. Our results have also shown that the surface reactions to yield the products, CF₃CFCFCF₃ and CF₃CCCF₃, obey first-order kinetics, whereas the formation of CF₂CFCFCF₂ follows second-order kinetics.