F. Barreca

Solid acids with SO3H groups and tunable surface properties: versatile catalysts for biomass conversion

Acid catalysis plays an important role in biomass conversion processes for producing chemicals and fuels. We report a relatively simple procedure for synthesizing versatile, strong acid catalysts based on carbon and carbon–silica composites with sulfonic acid groups. The process involves chemical activation of a sulfonic acid organic precursor at low temperature. The synthesis conditions can be modified to tune the surface composition, texture, and the acid properties of the materials towards superior catalytic performances. Molecular level insights into the nature and strength of the acid sites were gained by combining high resolution XPS and 1H-decoupled 31P MAS NMR spectroscopy of adsorbed triethylphosphine oxide. These materials are effective acid catalysts for the conversion of different biomass-derived chemicals to useful bio products such as furanic ethers and levulinate esters.

Solid acids with SO3H groups and tunable surface properties

Acid catalysis plays an important role in biomass conversion processes for producing chemicals and fuels. We report a relatively simple procedure for synthesizing versatile, strong acid catalysts based on carbon and carbon–silica composites with sulfonic acid groups. The process involves chemical activation of a sulfonic acid organic precursor at low temperature. The synthesis conditions can be modified to tune the surface composition, texture, and the acid properties of the materials towards superior catalytic performances. Molecular level insights into the nature and strength of the acid sites were gained by combining high resolution XPS and 1H-decoupled 31P MAS NMR spectroscopy of adsorbed triethylphosphine oxide. These materials are effective acid catalysts for the conversion of different biomass-derived chemicals to useful bio products such as furanic ethers and levulinate esters.

Time resolved imaging studies of the plasma produced by laser ablation of silicon in O 2 0Ar atmosphere

The dynamics of the expansion plasma produced by excimer laser ablation of a silicon target into oxygen and mixed O20Ar atmosphere were studied by means of time-resolved imaging of the expanding plume. Experiments were performed in pure oxygen, ranging between 0.13 and 13.33 Pa, and at different O20Ar ratios at a fixed total pressure of 13.33 Pa. The occurrence of a shock wave ~SW! generated by the supersonic expansion of the plasma was observed at high pressure values. The presence of the SW had a strong influence on the structure of SiOx thin films. In fact, silicon dioxidethinfilmswerealwaysobtainedinpresenceoftheSW,irrespectiveoftheoxygencontentinthegaseousmixture. On the contrary, suboxide thin films were obtained when the expansion occurred at lower pressure values ~no SW presence!. The temperature rise following the developing of the SW, is supposed to enhance the oxygen molecules dissociation by increasing the efficiency of the silicon oxidation reaction.

Mesoporous carbon–silica solid acid catalysts for producing useful bio-products within the sugar-platform of biorefineries

Useful bio-products are obtainable via the catalytic conversion of biomass or derived intermediates as renewable carbon sources. In particular, furanic ethers and levulinate esters (denoted bioEs) have wide application profiles and can be synthesised via acid-catalysed reactions of intermediates such as fructose, 5-hydroxymethyl-2-furaldehyde (HMF) and furfuryl alcohol (FA) with ethanol. Solid acid catalysts are preferred for producing the bioEs with environmental benefits. Furthermore, the versatility of the catalyst in obtaining the bioEs from different intermediates is attractive for process economics, and in the case of porous catalysts, large pore sizes can be beneficial for operating in the kinetic regime. Carbon-based materials are attractive acid catalysts due to their modifiable surface, e.g. with relatively strong sulfonic acid groups (SO3H). Considering these aspects, here, we report the preparation of mesoporous (SO3H)-functionalised-carbon/silica (C/S) composites with large pores and high amounts of acid sites (up to 2.3 mmol g−1), and their application as versatile solid acid catalysts for producing bioEs from fructose, HMF and FA. The mesoporous composites were prepared by activation of an organic compound deposited on the ordered mesoporous silicas MCF (mesostructured cellular foam) and SBA-15, where the organic compound (p-toluenesulfonic acid) acted simultaneously as the carbon and SO3H source. The atomic-level characterisation of the acid nature and strengths was performed by 31P solid-state NMR studies of an adsorbed base probe, in combination with FT-IR and XPS. Comparative catalytic studies showed that the C/S composites are interesting catalysts for obtaining bioEs in high yields, in comparison with classical solid acid catalysts such as sulfonic acid resin Amberlyst™-15 and nanocrystalline (large pore) zeolite H-beta.

Raman microscopy study of pulsed laser ablation deposited silicon carbide films

Silicon carbide films have been deposited by pulsed laser ablation. The sample microstructure was studied by means of SEM imaging and spatially resolved Raman spectroscopy. Some inhomogeneities, on an otherwise structureless sample surfaces, were evident in the SEM images. A detailed Raman imaging study was carried out over a properly selected area including some inhomogeneous spots. Analysis of the spectral features relative to phonon modes revealed a variety of structural configurations. In the homogeneous region, the amorphous phases of silicon carbide, graphitic carbon and silicon were identified. On the other hand the inhomogeneous spots contained predominantly microcrystalline phases of both silicon and graphitic and/or tetrahedral carbon species. Micro-Raman spectroscopy provided an excellent tool, in giving local structural information by selectively probing a microscopic scattering volume.

Bonding configurations and optical band gap for nitrogenated amorphous silicon carbide films prepared by pulsed laser ablation

SiCxNy thin films have been deposited by ablating a sintered silicon carbide target in a controlled nitrogen atmosphere. The structural and the optical properties of the films were investigated by x-ray photoelectron spectroscopy (XPS) and spectroscopic ellipsometry. The nitrogen content was found to be dependent on the nitrogen partial pressure and did not exceed 7.5%. XPS measurements of the N 1s, C 1s, and Si 2p photoelectron peaks pointed out the nitrogen preferential bonding to silicon atoms with a concurrent decrease of the Si–C and Si–Si bond density. A slight increase of sp3 hybridized carbon bonds has been also observed. The optical band gap Eg values, obtained from fitting the ellipsometric data with an appropriate multiple layers model, were found to increase up to 2.4 eV starting from a value of 1.6 eV for a non-nitrogenated sample. This finding was related to progressive substitution of the weaker Si–Si bonds by the stronger Si–C and Si–N ones which, presumably, involves the removal of electr...

Excimer laser ablation of silicon carbide ceramic targets

Abstract The ablation mechanisms of ceramic silicon carbide targets were investigated for laser fluences up to 12 J/cm2. Two different regimes were identified: the vaporization was the dominant process but, above approximately 3 J/cm2, the plasma shielding of the laser beam took place and decreased the efficiency of the process. Moreover, after a few laser shots, the decomposition of the target material took place with the segregation of a carbon phase, as resulted from Raman microscopy measurements. Laser induced periodic structures were also evidenced on the inner laser spots at different fluences. The effects of the ablation mechanisms were correlated with the morphological properties of the films, in particular with the size and the number of the particulates.

Investigation of a nanocrystalline silicon phase embedded in SiOx thin films grown by pulsed laser deposition

The formation of nanocrystalline silicon clusters in SiOx thin films has been investigated by means of micro-Raman and Fourier transform infrared spectroscopic techniques. The samples were deposited, by means of pulsed laser ablation of a silicon target in a controlled oxygen gas environment, on substrates heated up to 883 K. Experimental results show that, by appropriately varying the deposition parameters, it is possible to achieve a fully coordinated silicon dioxide phase with the contemporary development of a nanometer-sized crystalline silicon phase and/or an amorphous one. Comparison between the effect of a relatively high substrate temperature and of a post-deposition annealing treatment have been exploited. From the line-shape analysis of the silicon TO vibrational mode Raman band, crystalline silicon volume fractions up to 90% have been estimated, while the nanocrystalline cluster sizes remained almost constant around 3.5 nm. A good agreement between our results and the predictions of silicon nan...

Influence of the deposition parameters on the electronic and structural properties of pulsed laser ablation prepared Si1−xCx thin films

Si1−xCx thin films have been deposited by pulsed laser ablation of a polycrystalline silicon carbide target in vacuum. The influence of the deposition parameters on the optical and structural properties of the samples was investigated by means of Fourier transform IR, Raman, ex situ ellipsometric, x-ray photoelectron, and x-ray absorption near edge structure spectroscopies. Both deposition temperature and laser fluence were increased up to 1150K and 14J∕cm2, respectively. Increasing the two parameters, a better quality of the thin films was observed due to the existence of a crystalline order on a nanometric scale. Nevertheless, at higher deposition temperatures, a graphitic phase occurred inducing a degradation of the optical properties. Such a phase was not observed increasing the laser fluence. The film formation mechanism has also been qualitatively discussed on the basis of the subplantation model proposed by Lifshitz et al. [Phys. Rev. Lett. 62, 1290 (1989); Phys. Rev. B 41, 1046 (1990)].

Characterization of pulsed laser deposited a-C films by means of reflection electron energy loss spectroscopy

Abstract Amorphous carbon films have been deposited by ablating a high purity graphite target using a KrF excimer pulsed laser. Both the laser energy density (up to 15 J/cm 2 ) and the substrate temperature (up to 900 K) were varied. Reflection electron energy loss spectra have been carried out in an ultra high vacuum surface analysis apparatus. The loss spectra were processed by removing the elastic peak contribution and the secondary electron background. The optical functions were deduced using a procedure based on the assumption that surface effects are negligible with respect to the bulk ones. The loss spectra are dominated by the presence of the π and π+σ plasma resonances, whose energy positions and shapes are significantly affected by the different preparation conditions. In particular, it has been made possible to determine, on the basis of reflection electron energy loss (REEL) data only, the sp 2 /sp 3 bonding ratio. Overall, these results are also relevant for the correlation between the details of the preparation method and the material electronic properties.