Infrared and NMR Spectroscopic Studies of n-Alkanethiols Chemically Grafted on Dimethylzinc-Modified Silica Surfaces

Abstract

Transmission infrared and solid-state nuclear magnetic resonance (NMR) spectroscopies are used to study the surface species formed by sequential reaction of hydroxylated, high-surface-area silica with dimethylzinc (DMZ) and n-alkanethiols of various chain lengths. Reaction of DMZ forms mainly surface methylzinc species. These convert to Zn-bound ethanethiolate surface species by reaction with ethane thiol, evolving nearly identical amounts of methane at each step. Temperature-dependent infrared spectra of these surface thiolate species reveal that they are stable in air and aqueous environments and up to about 400 K in vacuo. The major gas-phase products identified during annealing are diethyl sulfide and ethylene. Longer-chain n-alkanethiols (up to C16) are reacted in benzene solutions and the modified silica powders are characterized by transmission infrared and solid-state NMR spectroscopies. The spectroscopic data demonstrate that the alkanethiolate chains retain their integrity and bind to the surface via the sulfur atom with their “tails” away from the surface. High thiolate coverages (similar to those of well-characterized self-assembled monolayers of thiols on gold and trichlorosilanes on silicon) are obtained in the case of shorter hydrocarbon chains. Carbon−hydrogen−nitrogen analyses and electron microprobe elemental analyses reveal that the density of the thiolate layer decreases with increasing chain length. This is attributed to the highly irregular surface geometry and may be avoided by using planar substrates, thus providing a novel strategy for self-assembly of organic molecules on various hydroxylated surfaces.

High temperature stability of n-decanethiol by adsorption on nickel powders used as reforming catalysts in solid oxide fuel cells (SOFC)

“…Table 1 enables a comparison of spectra observed at different temperatures, along with their assignments. In large agreement with the literature data, [19,20,21,24,26] we are able to draw the following conclusions: DRIFTS spectra of n-decanethiol adsorbed on Ni recorded at four different temperatures, 23°, 700°, 1030°, and 1050°C a) at high frequency and b) at low frequency.…”

“…(2) The spectrum recorded of the samples treated at 700˚C (exposure of 13 hours at this temperature) and 1030°C shows, besides bands observed at 23°C, the appearance of new bands corresponding to chemical species containing the groups C=C, -C=C=CH 2 , >C=C=CH 2 , and aromatic ring [20,21,24,25 ]. When these surfaces are thermally treated under Ar carrier gas at 1050°C over 6 hours, all previous bands characteristic of the adsorption of n-decanethiol on Ni disappear, and other bands, corresponding to characteristic groups in olefinic or aromatics, or both, compounds [20,21,24,26] appear. In a previous study [19] the authors have shown that the same type nickel powder, impregnated with n-decanethiol, is inactive for the steam reforming of methane, and this deactivation was found to be due to carbon deposition of the "unsaturated" type.…”

Fluorescence Anisotropy in Studies of Solute Interactions with Covalently Modified Colloidal Silica Nanoparticles

“…A number of methods have been used to study modified silica surfaces, such as solid-state NMR spectroscopy, − atomic force microscopy, Fourier transform infrared spectroscopy, photon correlation spectrometry, thermogravimetric analysis, nitrogen adsorption, X-ray powder diffraction, and rheological measurements . A key disadvantage of many of these methods is the need for large sample sizes or samples with high surface areas, particularly when monolayer or lower coverage of a modifier is present on a silica surface.…”

Regeneration of an n-decanethiol-poisoned nickel catalyst

“…An analysis of the possible assignment of the individual absorption bands to the corresponding bond configuration was undertaken. At low frequencies, the spectrum recorded at 23 • C shows the presence of bands characteristic of n-decanethiol adsorption on Ni (ν(C S), δ(CH 2 ) and δ(CH 3 )), and the presence of sulfonates [37][38][39][40][41]. The IR spectra of the samples tested for methane steam reforming at 700 • C (exposure of 13 h at this temperature) show, in addition to the bands observed at 23 • C, the appearance of new bands, corresponding to characteristic groups of unsaturated compounds [38][39][40][41].…”

“…At low frequencies, the spectrum recorded at 23 • C shows the presence of bands characteristic of n-decanethiol adsorption on Ni (ν(C S), δ(CH 2 ) and δ(CH 3 )), and the presence of sulfonates [37][38][39][40][41]. The IR spectra of the samples tested for methane steam reforming at 700 • C (exposure of 13 h at this temperature) show, in addition to the bands observed at 23 • C, the appearance of new bands, corresponding to characteristic groups of unsaturated compounds [38][39][40][41]. At high frequencies, the spectrum recorded at 700 • C shows the presence of bands ν(CH 2 ) and ν(CH 3 ) also present at 25 • C. The presence of the last two bands at 700 • C can be explained as follows: the heating of the decanethiol-impregnated Ni powder up to 700 • C during the reaction of steam reforming is characterized by the partial pyrolysis of the adsorbed layers.…”