Chemisorption Of Benzene Research Articles

Comparative study of interzeolite transformed metallosilicates for the chemisorption of benzene, toluene, and xylenes

Zeolitic metallosilicates with Lewis acidity are effective in the chemisorption of benzene, toluene, and xylenes (BTX) because of the strong π-interactions between the Lewis acid sites and BTX adsorbates. In this study, we prepared boro-, nickel-, and zincosilicate delMWW (two-dimensional delaminated MWW) and beta zeolite (*BEA) adsorbents by using the interzeolite transformation method and compared them with their mother MWWs for BTX chemisorption. Different structural properties, framework-atom species, and hydroxyl and Lewis acid site concentrations were systematically characterized by using various analytical methods. Among the studied adsorbents, nickel silicate beta zeolite (Ni-BEA) exhibited the highest concentration of Lewis acid sites and a hydrophobic character because of the stable framework-bound Ni species. In addition, its advantageous textural properties, large pore size, high surface area, and nano-sized crystals, induced the highest chemisorption content and a strong chemisorption ability of BTX at high-temperature (120 °C) adsorption under humid conditions.

Density functional theory and experimental study on the chemisorption and catalytic decomposition of benzene over exposed bio-char surface: The influence of unsaturated carbon atoms and potassium

Catalytic decomposition over char surface is an effective approach for tar removal. In this work, the density functional theory (DFT) and experiments were used to study the complicated mechanism of the interaction between specific sites on char surface and tar compound at molecular level. Two kinds of bio-char models were used and compared. Three molecular reaction paths were built and corresponding species were optimized. The characteristics of electron density of key components and reaction potential energy were calculated and analyzed. Results show that heterogeneous adsorption of benzene on the active sites of char surface could effectively enhance the decomposition of benzene due to the electron shift caused by chemisorption on char surface. The decomposition temperature is largely reduced from 1100℃ to 900℃ or lower. The band gaps indicate that benzene tends to be more likely excited by model char without potassium, which has more dangling bond on char surface. Potential energy results of three reaction paths show that activation energies of about 208.64 kJ/mole and 163.32 kJ/mole are needed for ring breakage of benzene in Path-1 and Path-2, which are much lower than that of thermal decomposition. The energy barrier for benzene decomposition over char-K is 270.04 kJ/mole. Potassium plays a negative effect on the cracking of tar due to the pore polycondensation and occupation of active sites. Kinetic analysis reveals that the char-tar interactions are largely affected by temperature and higher temperature benefits the breakage of aromatic ring. Experimental results indicate that improving the porosity and surface area of char, especially the active surface area with unsaturated carbon atoms, is crucial for the activation and decomposition of aromatic tar compounds.

Experimental and density functional theory investigations on the oxidation of typical aromatics over the intermetallic compounds-derived AuMn/meso-Fe2O3 catalysts

Aromatic volatile organic compounds (VOCs) are usually pollutants to the atmosphere environment and human health. In the present work, we first adopted the co-reduction and KIT-6-templating strategies to prepare the AuxMny intermetallic compounds and mesoporous iron oxide (meso-Fe2O3), respectively, and then used the impregnation method to generate the AuxMny/meso-Fe2O3 catalysts. Catalytic performance of these materials was evaluated for benzene, toluene or o-xylene (typical aromatics) oxidation. It is found that the Au5Mn2/meso-Fe2O3 catalyst exhibited the best activity: the temperatures at benzene conversions of 50 and 90 % were 237 and 254 °C at a space velocity of 20,000 mL/(g h), with the TOFAu values and specific reaction rates at 210 and 230 °C being 1.08 and 2.29 s−1, and 6.92 and 11.58 mmol/(gAu s), respectively. Such excellent performance of Au5Mn2/meso-Fe2O3 was related to its well dispersed Au5Mn2 nanoparticles, high adsorbed oxygen species concentration, good low-temperature reducibility, high benzene adsorption capacity, and strong benzene adsorption. The benzene chemisorption mechanism and adsorbed oxygen species could greatly influence the oxidation of benzene. According to the first-principles calculations, we find that benzene adsorption on the surface of Au in Au5Mn2/meso-Fe2O3 was in the form of a π bond, in which the Au could obtain electrons from benzene ring in the presence of benzene adsorption, resulting in a stronger adsorption of benzene and thus increasing the catalytic activity for benzene oxidation. The mechanism of benzene oxidation might take place via the route of benzene → phenol → benzoquinone → maleate (acetate) → carbon dioxide and water.

Using strain to control molecule chemisorption on silicene.

The strain dependence of benzene chemisorption on a silicene freestanding layer has been studied by means of density functional theory calculations. It appears that the molecule, which is adsorbed via a [4+2] pseudo-cycloaddition on the substrate, is more stable when adsorbed on strained than on unstrained silicene since the adsorption energy increases (in absolute value) with tensile or compressive strain. These results, which were not easily predictable, are interpreted in terms of strain-induced reinforcement of the Si-C bonds, formation of a pz-like atomic orbital at the silicene atoms, which augments the silicene reactivity and, for compressive or large tensile strains, increasing of the sp3 character of the Si-Si bonds.

Comprehensive Kinetic and Mechanistic Analysis of TiO2 Photocatalytic Reactions According to the Direct–Indirect Model: (II) Experimental Validation

As a continuation of the Direct–Indirect (D-I) model theoretical approach presented in Part I of this publication, concerning the photocatalytic oxidation of organic molecules in contact with TiO2 dispersions, a comparative photooxidation kinetic analysis of three model organic molecules, benzene (BZ) dissolved in acetonitrile (ACN), phenol (PhOH) dissolved in either water or acetonitrile, and formic acid (FA) dissolved in water, is presented to test the applicability of the D-I model under both equilibrium and nonequilibrium adsorption–desorption conditions. A previous analysis involving diffuse reflectance ultraviolet–visible (DRUVS) and Fourier transform infrared (FTIR) spectroscopy, combined with adsorption isotherm plots, shows that BZ chemisorption on the TiO2 surface is not allowed, physisorption being in this case the only possible adsorption mode. In line with D-I model predictions, BZ photooxidation is observed to take place via an adiabatic indirect transfer (IT) mechanism, with the participati...

Resonant photoactivation of cadmium sulfide and its effect on the surface chemical activity

Photo-enhanced surface chemical activity of cadmium sulfide gives rise to a wide class of surface-dependent phenomena, such as heterogeneous photocatalysis, chemoresistivity, and chemiluminescence, which have several technological and scientific applications. In this work, the photochemical properties of nanostructured cadmium sulfide films are investigated by means of electrical conductance measurements in controlled atmosphere, while irradiated by light of wavelengths ranging from 400 to 645 nm. Chemisorption of benzene, carbon monoxide, methane, ethanol, and hydrogen sulfide onto CdS surface has been analyzed as a function of the wavelength, in a gas concentration range of the order of parts per million. It resulted that the increase of photoconductance with gas adsorption is resonant with the bandgap energy. It turns out that this resonant enhancement of the surface chemical activity can be of advantage for all the optical and chemical mechanisms that depend upon it. An interpretation of these results, in terms of electronic optical transitions and Fermi level shift induced by light, is proposed.

Adsorption of Benzene on 4H-SiC (0001) Surface: First Principles Calculations

We study adsorption of the benzene molecule on the Si terminated (0001) surface of 4H-SiC by performing rst principles calculations in the framework of density functional theory. We nd out that chemical reaction leading to the chemisorption of benzene on the surface has endothermic character. The adsorbed benzene molecule is bounded to two surface Si atoms and it does not lose its integrity, however, it undergoes strong deformations and causes distortion of the substrate. We analyze also changes in the electronic structure caused by benzene adsorption.

A HREELS and DFT Study of the Adsorption of Aromatic Hydrocarbons on Diamond (111)

Ultrathin layers of organic molecules can be assembled on group IV (e.g., silicon, germanium, diamond) semiconductor surfaces using surface analogues of cycloaddition reactions. We present a study of the chemisorption of benzene, toluene, and styrene on the Pandey chain of C(111) using high resolution electron energy loss spectroscopy and density functional theory calculations. Two cycloaddition reactions, namely, the [4 + 2] and [2 + 2], were examined. The [4 + 2] reaction is found to be thermodynamically unfavorable on C(111), while the [2 + 2] reaction involving the ring is slightly exothermic. In the case of aromatic molecules with an external unsaturated functional group, the reaction can proceed via the external functionality, thereby preserving the aromatic ring and providing further stability. Different reactivity patterns to the C(100) surface are rationalized on the basis of steric effects imposed by the geometrical structure of the Pandey chain. Our study demonstrates the potential of employing the Pandey chain as a template for assembling one-dimensional molecular structures on the diamond surface.

Partial hydrogenation of benzene over the metallic Zn modified Ru-based catalyst

From the adsorption point of view, partial hydrogenation of benzene to cyclohexene over the metallic Zn modified Ru-based catalyst was experimentally and theoretically investigated. A decreased hydrogenation activity but increased selectivity to cyclohexene over the prepared Ru-Zn/ZrO 2 catalyst was observed in the partial hydrogenation of benzene. Theoretical calculations suggest that the above phenomena are mainly resulted from the depression of the chemisorption of benzene and cyclohexene on the modified catalyst, especially for the latter. Undesired deep hydrogenation from cyclohexene to cyclohexane in the middle and late reaction stage therefore is effectively retarded, by which an improved cyclohexene yield is guaranteed. An optimal Zn content of 2.72 wt.% in the Ru-based catalyst was proposed by both the experiment and calculation for the partial hydrogenation of benzene, and a cyclohexene yield up to 44% was obtained over Ru-Zn/ZrO 2 catalyst.

Vibrational characterization of different benzene phases on flat and vicinal Si(100) surfaces

Based on high-resolution electron energy loss spectroscopy and temperature-programmable desorption, benzene chemisorption on vicinal and nominally flat Si(100) surfaces has been studied for various adsorption, annealing, and site blocking treatments. Three different chemisorbed benzene (C6H6 and C6D6) phases with distinct thermal desorption characteristics and different vibrational spectra have been separated and characterized on both substrates. All three phases are identified as 1,4-cyclohexadiene-like structures with butterfly geometry. Whereas the dominant phase is di-sigma bonded to the two Si atoms of a single Si-Si dimer, the benzene orientation (double bond orientation) in the other phases is rotated. Di-sigma bonding to Si atoms of adjacent Si-Si dimer for the latter cases is most likely. Coverage and temperature dependent conversions between the different phases have been addressed by vibrational spectroscopy.

Structural Analysis of Chemisorbed Benzene and Pyrazine on Silicon Surfaces by using Photoelectron Diffraction

We have studied the chemisorption of benzene and pyrazine on Si(001)-2×1 at room temperature using photoelectron diffraction (PED). For benzene, comparison between the experimental and simulated PED patterns with R-factor analysis leads to the conclusion that a di-σ-bonded configuration is mainly formed at the saturation coverage. For pyrazine, the PED results show that the double dimer bridging configuration is formed, in which pyrazine molecule forms a bridging between the dimer rows with the two Si-N bonds.

Adsorption and reaction of cyclohexene and 1-hexene on nascent gold surface formed by friction

Adsorption and reaction of cyclohexene, 1-hexene and benzene on nascent gold surface formed by friction were investigated using an adsorption test apparatus in high vacuum at room temperature. Chemisorption of cyclohexene, 1-hexene and benzene was observed during the sliding processes by a quadrupole mass spectrometer. It is found that nascent gold surface has catalytic activity for hydrogenation of cyclohexene and 1-hexene, and for dehydrogenation of the olefins and benzene. The adsorbed cyclohexene and 1-hexene molecules were hydrogenated on the surface to form cyclohexane and hexane, respectively. As dehydrogenation products, hydrogen from all the sample gases, and benzene from cyclohexene desorbed. Adsorption rate of the olefins was higher than that of benzene, and hydrogenation activity of 1-hexene is higher than that of cyclohexene. However, on nascent Pd–Ag alloy surface, the desorption of hydrogenation products was not detected, although adsorption of the sample gases and desorption of the dehydrogenation products were also observed.

Chemisorption of benzene and STM dehydrogenation products on Cu(100)

Modification of individual chemisorbed benzene molecules on Cu(100) has recently been performed in atomic manipulation experiments [J. Phys. Chem. A. 104, 2463 (2000); Surf. Sci 451, 219 (2000)]. Benzene dissociates under controlled voltage pulses in a scanning tunneling microscope (STM) junction. The reaction is characterized as a dehydrogenation process and the fragments are identified as benzyne, ${\mathrm{C}}_{6}{\mathrm{H}}_{4}.$ Here we present a density functional theory investigation of the chemisorption of benzene on the Cu(100) surface, the nature of the bonding and its effect on the STM images. The fragments phenyl and benzyne formed after one-fold and two-fold dehydrogenation of chemisorbed benzene are studied in the same manner. The stability of the fragments is explored via their chemisorption energy, their electronic structure on the surface and their affinity for hydrogen. Benzyne fragments seem to be the most stable, in agreement with the conclusion of the aforementioned STM experiments.

Molecular adsorption at well-defined electrode surfaces: benzene on Pd(1 1 1) studied by EC-STM and HREELS

The chemisorption of benzene at well-defined Pd(111) electrode surfaces in 0.1 M HClO4 has been studied by a combination of electrochemistry, molecular resolution scanning tunneling microscopy, and high-resolution electron energy loss spectroscopy. At potentials within the double-layer region, a well-ordered Pd(111)-c(2√3×3)-rect-C6H6 adlattice was formed in which the adsorbed benzene molecules are slightly tilted with respect to the metal surface. When the potential was decreased to values just inside the hydrogen evolution region, a third of the chemisorbed molecules were desorbed and a new ordered adlayer, Pd(111)-(3×3)-C6H6, was generated; in this less-densely packed phase, the coadsorption of water was indicated, and the aromatic molecules were chemisorbed more closely parallel to the surface. The potential-dependent structural and compositional transitions were found to be reversible. Structural models of the c(2√3×3)-rect and (3×3) adlattices are proposed.

Reaction of Benzene Molecule on Size-Selected Nickel Cluster Ions

Absolute cross sections for the reaction of a benzene molecule with a nickel cluster cation with the sizes of 3−11 were measured in a beam-gas cell geometry at collision energies less than 1 eV in a tandem mass spectrometer. Three different reaction processes were found to proceed: benzene chemisorption with and without Ni release and dissociative chemisorption with C2H2 release. The size-dependent reaction cross sections were explained in terms of a statistical model.

Tuning the chemistry of metal surfaces: II. Acetylene cyclotrimerization on ultrathin Pd films on Ta(1 1 0)

Chemisorption of acetylene (C 2H 2) and benzene (C 6H 6) on ultrathin Pd films (monolayer, bilayer, trilayer, etc.) deposited on Ta(1 1 0) is weaker than on bulk-terminated Pd surfaces, but it is not as strongly perturbed as was seen for H 2, CO, NO, and C 2H 4. Thus, the well-known alterations of the chemical properties of ultrathin metal films deposited on refractory metal substrates can be modified by strongly adsorbed species. C 2H 2 is predominately reversibly adsorbed on the Pd monolayer ( θ Pd=1) at 91 K, with thermal desorption peaks at 180 and 265 K. C 6H 6 is formed via cyclotrimerization of C 2H 2 and desorbs in a single peak at 407 K, but the yield is small (1% of an adsorbed benzene monolayer on that surface). Bilayer and trilayer Pd films ( θ Pd=2–3) have similar C 2H 2 chemistry to that from thicker films. On a thick film ( θ Pd=5) at 175 K, C 2H 2 adsorption leads to C 2H 2 desorption in a very broad peak near 330 K, and to benzene desorption at 250 and 500 K, with a yield of twice that on the Pd monolayer. Benzene chemisorption on the Pd monolayer is only slightly weaker than that on a thick Pd film ( θ Pd=5) or on Pd(1 1 1), (a reduction of 6 kcal/mol) but is completely reversibly adsorbed.

Chemisorption of benzene on metal dimer anions: A study by photoelectron detachment spectroscopy

Photoelectron detachment spectra of M2(C6H6)− (M=Pt, Pd, Pb) have been measured in the gas phase using photon energies of a Nd:YAG laser. The vibrationally resolved ground state transition from the anion to the neutral reveals an adiabatic electron affinity of (2.01±0.05) eV and (0.88±0.05) eV for Pt2(C6H6) and Pd2(C6H6), respectively. A ground state vibrational energy of (24.2±1) meV has been resolved for Pt2(C6H6). The corresponding vibrational energy of Pt2(C6H6)− amounts to (19.0±1.0) meV. The ground state vibrational energies of Pd2(C6H6) and Pd2(C6H6)− are (20.3±1.0) meV and (18.0±2.0) meV, respectively. The small vibrational frequencies suggest a perpendicular coordination (C6v-symmetry) of the benzene-adsorbed transition metal dimers. Pb2, on the other hand, is bound parallel to the benzene plane (C2v-symmetry). A closed shell ground state electron configuration is postulated for Pb2(C6H6) in contrast to the triplet ground state of unreacted Pb2. The vertical electron affinity of Pb2(C6H6) is (1.95±0.05) eV.

Modeling Chemisorption of Benzene and Its Hydrogenation on Platinum Surfaces. 1. Complexes of Benzene with Pt and Pt2

The potential energy surfaces (PESs) of the interaction of benzene with Pt and Pt2 are thoroughly studied at the density functional B3LYP computational level using the Hay−Wadt relativistic core potential. The PES of the benzene−Pt complexation reveals two stable structures, the traditional di-π complex and a σ one which appears to be more stable than the former one by 9.5 kcal/mol. The optimized geometry and vibrational modes of the σ complex are reported in the present work for the first time although it has been predicted experimentally by Gland and Somorjai nearly 30 years ago. We show that the di-π ⇔ σ transition is governed by the transition structure with the three-center two-electron bond C···H···Pt. A sliding motion of the Pt atom over the benzene ring to the di-π complex involves a transition structure whose activation barrier amounts 4.7 kcal/mol. The PES of the interaction of bare benzene with Pt2 consists of three stable structures referred to as di-π, π−σ, and di-σ types. The latter one cons...

Symmetry and Electronic Structure of Benzene Adsorbed on Single-Domain Ge(100)-(2×1) and Ge/Si(100)-(2×1)

The electronic structure of benzene adsorbed on single-domain Ge(100)-(2×1) and on a single-domain Ge monolayer grown on Si(100)-(2×1) has been investigated by angle-resolved ultraviolet photoemission spectroscopy (ARUPS) using linearly polarized synchrotron radiation and by temperature-programmed desorption (TPD) spectroscopy. For benzene chemisorption on both surfaces C2v adsorption symmetry is found on the basis of the ARUPS data. The detailed analysis indicates a flat-lying molecule which is di-σ-bonded to a Ge−Ge dimer and which exhibits a 1,4-cyclohexadiene-like electronic structure. The similarity of the ARUP spectra for benzene on Ge(100), on Ge/Si(100), and on Si(100) indicates nearly identical electronic structures. Benzene TPD spectra on a Ge monolayer on Si(100) show two desorption peaks at 311 and 369 K which are interpreted as desorption from terrace and step sites, respectively. Similarly, benzene desorption on Ge(100) leads to desorption peaks at about 230 and 250 K, again interpreted as desorption of chemisorbed benzene from terrace and step sites. Additionally, a weakly bound benzene species is found on Ge(100) which desorbs between 155 and 220 K and which is absent on the other surfaces. ARUPS data indicate C1 adsorption symmetry and an electronic structure similar to that of gas-phase benzene for this species.

The adsorption of benzene on H/W(110) and CO/W(110) layers

The adsorption of benzene (Bz) on partial and full hydrogen and CO layers on W(110) indicates that H or CO block benzene chemisorption. The Bz vs. CO coverage curve shows two linear regimes, with different extrapolated maximum Bz coverages, suggesting that for CO/W>0.3 Bz is adsorbed (at least in part) tilted rather than flat on the surface. The adsorption of Bz on H/W(110) layers suggests that the effective blocking area of H atoms is 5×10 −16 cm 2, i.e. 0.69 of the area per W atom. Bz chemisorption decreases the work function by 1.5 eV at maximum coverage, 2.5×10 14 molecules/cm 2 on clean W(110). In the presence of CO |Δ φ| values are smaller for the same Bz coverage than on clean W(110), suggesting electronic CO–Bz interaction. In the case of Bz/H/W(110) there is also a decrease in |Δ φ| but only at high H coverages. Bz physisorption is different on saturated H/W(110) from that on saturated CO/W(110) layers. In the former case the various thermal desorption peaks seen for physisorbed Bz on chemisorbed Bz on W(110) or on O/W(110) are present, while on CO/W(110) the so-called α 1 peak at 160 K is absent, indicating that physisorbed Bz is more weakly bound on CO 0.7/W(110) than on clean or O covered W(110).