Adsorption Of Methyl Chloride Research Articles

Adsorption of Methyl Chloride on Molecular Sieves, Silica Gels, and Activated Carbon

AbstractAdsorption of methyl chloride (CH3Cl or MeCl) on five different types of adsorbents was investigated experimentally at increasing pressures and room temperature. Prior to adsorption, all adsorbents were analyzed to assess their physical and chemical characteristics. The experimental data was then used to determine the adsorption isotherms, heats of adsorption, adsorption rates, and their respective theoretical models. The MeCl adsorption capacity was found to reasonably correlate with the adsorbent's surface area. The MeCl adsorption isotherm and adsorption rates were fitted for the first time to a Freundlich isotherm model and pseudo first‐/second‐order kinetic models, respectively. The range of heat of adsorption indicated a physisorption type of bonding; hence, the investigated adsorbents can potentially be regenerated for cyclic adsorption.

Interaction of coadsorbed CH3Cl and D2O layers on Pd(111) studied by sum frequency generation

Adsorption of methyl chloride and coadsorption of CH3Cl and D2O on Pd(111) surfaces at T=100 K have been studied under ultrahigh-vacuum conditions using femtosecond sum frequency generation (SFG) spectroscopy in the spectral regions of CH and OD bands. On the bare Pd(111) substrate, the CH3Cl coverage dependence of the resonant SFG signal is consistent with a progressive molecular rearrangement starting at half saturation followed by the growth of two ordered monolayers in which the molecular axes are perpendicular to the surface. When CH3Cl is adsorbed on top of predeposited D2O on Pd(111), the SFG signals as a function of the CH3Cl exposure indicate that methyl chloride is adsorbed onto D2O through hydrogen bonding. On the contrary when the adsorption order is reversed the strong decrease of the CH3 signal as a function of the D2O exposure is explained by assuming that water molecules penetrate inside the CH3Cl layers, leading to the formation of disordered CH3Cl clusters. In all cases a nonresonant contribution due to molecular adsorption is observed and it shows a dependence upon surface structure and coverage significantly different from that of the resonant vibrational bands.

Dissociative adsorption of methyl chloride on Si(001) studied by scanning tunneling microscopy

Chemical reactions of methyl chloride (CH3Cl) on a clean Si(001) surface at approximately 300 K are studied by means of scanning tunneling microscopy (STM) under ultra-high-vacuum conditions. The features appearing in the STM images are identified with the possible products of dissociated CH3 and Cl, and their distribution is also evaluated as well as the development of their distribution with increasing CH(3)Cl doses. The amount of Cl atoms found on the surface is approximately twice as large as that of the CH3 molecules. This leads to the conclusion that dissociative adsorption of CH3Cl on Si occurs in different processes: CH3Cl(precursor) --> CH3(ad) + Cl(ad) and CH3Cl(precursor) --> CH3(gas) + Cl(ad).

Methyl Chloride Adsorption on Si(001)−Electronic Structure

The adsorption of methyl chloride (CH3Cl) on Si(001) surfaces is studied using first-principles calculations based on gradient-corrected density-functional theory (DFT-GGA) and ultrasoft pseudopote...

Adsorption of methylchloride on Si(100) from first principles

The chemisorption of methylchloride (CH3Cl) on Si(100) is studied from first principles. We find that, among a number of possible adsorption configurations, the lowest-energy structure is one in which the methylchloride molecule is dissociated into CH3 and Cl fragments which are bound to the two Si atoms of the same surface dimer. Our calculations show that dissociative chemisorption of methylchloride on Si(100) may proceed along different reaction paths characterized by different energy barriers that the system must overcome: some dissociation processes are mediated by a molecular precursor state and, at least in one case, we find that the dissociation process is nonactivated, in agreement with recent experimental findings. We have also generated, for many possible adsorption structures, theoretical scanning tunneling microscopy images which could facilitate the interpretation of experimental measurements.

The interaction of methyl chloride with a silicon(111) surface

AbstractIn the technical Müller‐Rochow synthesis, dimethyl dichlorsilane (DMD) is fabricated from methyl chloride which interacts with a ‘contact mass’ consisting of elemental Si and Cu (Cu being a catalyst). In order to better understand some of the elementary steps of this reaction, we have followed the adsorption of methyl chloride on a (7×7) reconstructed Si(111) surface in the temperature range between 100 and 1000 K by means of LEED, Auger electron spectroscopy (AES), temperature‐programmed thermal desorption (TPD), vibrational loss spectroscopy (HREELS), and work function change (Δϕ) measurements. CH3C1 adsorbs effectively in a single weakly chemisorbed state. The adsorption energy is strongly coverage‐dependent and decreases from initially ˜ 40 kJ/mol to ˜ 30 kJ/mol near the monolayer. During adsorption, the work function of the Si(111) surface first increases sharply by 0.36 eV and saturates, after an intermediate maximum of ˜ 40 eV near the monolayer, at ˜ 0.61 eV. CH3C1‐derived vibrational losses appear at 88, 176, 340, and 370 meV and can be associated with known gas phase vibrations of CH3C1. An additional band at 260 meV is tentatively ascribed to the Si‐H stretching vibration, pointing to competing dissociation of adsorbed CH3C1 into hydrogen and methylene chloride CH2C1. Otherwise, CH3C1 does not react with Si(111) under our UHV conditions (CH3C1 partial pressures ≤10−5 mbar, 100 ≤T≤300 K), as the absence of any C1‐ and/or Si‐containing mass fragments in the TPD spectra proves. We deduce from our investigation that the combined chlorination and methylation of Si in the technical Müller‐Rochow process is dictated by processes which run only at much larger (atmospheric) pressures and/or elevated temperatures.

Methyl chloride and trichlorosilane adsorption on Cu(110)

The adsorption of methyl chloride (CH 3Cl) and trichlorosilane (HSiCl 3) on Cu(110) was studied by temperature-programmed desorption (TPD), high resolution electron energy loss spectroscopy (HREELS), and ultraviolet photoelectron spectroscopy (UPS). Methyl chloride interacts weakly with Cu(110) at 100 K, and we estimate from TPD that the adsorption energy is only 8–10 kcal/mol. It adsorbs nondissociatively and reversibly on Cu(110) under these conditions; no evidence of thermal decomposition was observed. HREELS indicates that chemisorbed CH 3C1 adopts a bonding geometry with a symmetry lower than C 3v, presumably bonded to the surface through the Cl atom with the methyl group tipped over with respect to the surface normal. In contrast, HSiCl 3 interacts strongly with Cu(110) at 100 K. Trichlorosilane is initially dissociatively adsorbed, but most of the HSiCl 3 in the chemisorbed monolayer adsorbs molecularly and reversibly due to the strong poisoning of the Cu(110) surface by Cl adatoms. Thermal decomposition, which leaves some Si-Cl and Si-H bonds intact, occurs readily at 140–300 K. Silicon deposited on the Cu(110) surface during HSiCl 3 decomposition is in a metallic bonding environment, directly bonded to the Cu. Low coverages of HSiCl 3 form an H 2 desorption state near 300 K that is attributed to the decomposition of an SiH surface species with an activation energy for decomposition of 18 kcal/mol. This H 2 desorption state is not observed for the thermal decomposition of large coverages of HSiCl 3 on Cu(110) nor for pre-chlorinated Cu(110) surfaces. On pre-chlorinated Cu(110), HSiCl 3 is only weakly adsorbed with an adsorption energy of 8 kcal/mol and is adsorbed nondissociatively with complete reversibility. The importance of these results for understanding the Cu-catalyzed direct synthesis reaction between CH 3Cl and Si to form methylchlorosilanes is discussed.

Adsorption of CH 3Cl on clean and Cl-dosed Pd(100) surfaces

The adsorption of methyl chloride on a Pd(100) surface has been investigated by ultraviolet photoelectron spectroscopy (UPS), electron energy loss spectroscopy (in the electronic range, EELS), temperature-programmed desorption (TPD) and work function change. CH 3Cl adsorbs with high sticking probability at 80–100 K. UPS and TDS spectra suggest that the adsorption of CH 3Cl is molecular at 100 K, with a little distortion of the corresponding gas-phase molecular electronic structure. No dissociation of CH 3Cl was observed even up to 550 K. By means of TPD, we distinguished two adsorption states with desorption energies of 46.9 and 33.4 kJ/mol. The formation of a condensed layer at 105–110 K was also observed. Adsorption of CH 3Cl caused a significant work function decrease, Δϕ = −0.91 eV, indicating a dipole with positive end pointed away from the surface. The effects of electronegative additives, preadsorbed Cl and O were also examined. Preadsorbed Cl caused a slight destabilization of adsorbed CH 3Cl at lower concentration, prevented the adsorption of CH 3Cl at higher concentration and facilitated the formation of a condensed layer. No such effect was experienced in the presence of preadsorbed O.

Reactions of methyl chloride and of methylene chloride at metal surfaces: II. Reactions over evaporated films of titanium and other metals

The reaction of methyl chloride and hydrogen, and of methylene chloride and hydrogen has been studied at the surface of a number of evaporated metal films in a static system. Adsorption of alkyl chloride resulted in rupture of all carbon-chlorine bonds before any carbon-hydrogen bonds were broken; thus in the reaction with deuterium, virtually no deuteroparent was formed. By the use of 13C and 35Cl it was shown that in the adsorption of methyl chloride on titanium, carbon-chlorine bond rupture was totally irreversible. Over nickel, tungsten, copper, platinum, cobalt, manganese, aluminum and silver, methane was the only hydrocarbon product. Over palladium and titanium higher hydrocarbons were formed. In the reaction of methylene chloride over titanium, polymer was also formed, and olefins were the main C 2 and C 3 products. The reaction of methyl chloride on titanium was shown to be first order in both methyl chloride and hydrogen pressures. Adsorption measurements with the alkyl chlorides on the above metals gave surface hydrogen/carbon ratios which, in most cases, suggested the average surface hydrocarbon residue to be more extensively dehydrogenated than the group in the original alkyl chloride: the extent of dehydrogenation increased with increasing temperature. The exception to this was titanium where adsorption of methyl chloride and methylene chloride gave approximately CH 3 and CH 2 surfaces residues, respectively. From the influence of hydrogen on the reaction product distribution, and from the results from reactions with deuterium it has been concluded that on palladium, chemisorbed hydrogen was directly involved in the process of product desorption, but on titanium this was not so, and inter-residue hydrogen transfer occurred in desorption. Evidence has been adduced that carbon-carbon bond formation probably occurred by reaction of an alkyl chloride molecule with a surface CH 2 group. Activation energies and frequency factors have been evaluated for the reactions of both alkyl chlorides on titanium.