William E. Farneth

The influence of oxonium ion and carbenium ion stabilities on the Alcohol/H-ZSM-5 interaction☆

Abstract We have examined the adsorption of methanol, ethanol, 1-propanol, 1-butanol, 2-propanol, and 2-methyl-2-propanol on H-ZSM-5 using temperature-programmed desorption (TPD) and thermogravimetrie analysis (TGA). For each of the alcohols except 1-butanol, there exists a clearly defined desorption state corresponding to one alcohol molecule per Al atom in the zeolite. For 2-methyl-2-propanol and 2-propanol, this molecule at the Al site completely reacts to olefin products and water; however, reaction occurs at 295 K for 2-methyl-2-propanol, while 2-propanol does not react until 360 K. Most of the methanol, ethanol, and 1-propanol adsorbed at the Al sites at this coverage desorbs unreacted. A substantial amount of the 1-butanol adsorbed near the Al sites reacts, but differs from the other alcohols in that it reacts through a dibutyl ether intermediate. These results can be understood in terms of the stabilities of the carbenium ion intermediates which can be formed during reaction. A potential energy scheme, based on gas-phase proton affinity, is developed. This scheme provides a formalism for explaining relative reactivities of alcohols on H-ZSM-5 in TPD.

An infrared spectroscopy study of simple alcohols adsorbed on H-ZSM-5

Abstract We have examined the adsorption of methanol, ethanol, 1-propanol, 1-butanol, 2-propanol, and 2-methyl-2-propanol on H-ZSM-5 using transmission infrared spectroscopy. In agreement with previous results from temperature-programmed desorption (TPD) and thermogravimetric analysis [M. T. Aronson, R. J. Gorte, and W. E. Farneth, J. Catal.98, 434 (1986)], each of the alcohols adsorbs in the vicinity of the Al atoms, as evidenced by the disappearance of the hydroxyl band associated with the hydrogen cation. For 2-methyl-2-propanol adsorbed at 295 K and 2-propanol at 360 K, changes in the IR spectral features and isotopic scrambling following exposure to D2O indicate that these molecules dehydrate to form carbenium ion-like intermediates prior to desorbing as olefin products. Methanol, ethanol, and 1-propanol remain unreacted up to their desorption temperatures. While 1-butanol is shown to be very reactive in TPD, no isotopic scrambling was observed during exposure to D2O and reaction appears to occur through a bimolecular mechanism. The results of this study are explained in terms of a model which assumes that the primary interaction between the alcohols and the zeolite is due to proton transfer at the Al sites. The model pictures the adsorbed species as ion pairs consisting of oxonium ions or carbenium ions coupled with the anionic zeolite framework.

Stoichiometric adsorption complexes in H-ZSM-5

Abstract We have examined the adsorption of methanol, 2-propanol, and 2-methyl-2-propanol on H-ZSM-5 samples with silica/alumina ratios of 38, 70, and 520 using temperature-programmed desorption (TPD) and thermogravimetric analysis (TGA) to examine the effect of Al concentration on the strength of the acid sites. In agreement with the results of a previous study [M. T. Aronson, R. J. Gorte, and W. E. Farneth, J. Catal.98, 434 (1986)], a clearly defined adsorption state corresponding to a coverage of one molecule per Al atom could be identified for each of the alcohols on each of the zeolites, with the possible exception of methanol on HZSM-5(520). Furthermore, the unimolecular reactivities of the molecules in these 1:1 adsorption complexes are unaffected by the Al concentration. Only secondary reactions, such as the extent of the oligomerization of olefin products formed from 2-propanol and 2-methyl-2-propanol, are found to be different for the different samples. Adsorption complexes with a 1:1 stoichiometry are also observed following adsorption and evacuation of 2-propanamine and 2-propanone on several zeolite samples, showing that molecules with other functional groups also form adsorption complexes at the Al sites. These results imply that the acid sites in H-ZSM-5 are identical and are present at a concentration of one per Al atom.

Quantitative determination of the number of active surface sites and the turnover frequencies for methanol oxidation over metal oxide catalysts: I. Fundamentals of the methanol chemisorption technique and application to monolayer supported molybdenum oxide catalysts

Abstract A detailed study of the methanol chemisorption and oxidation processes on oxide surfaces allowed the development of a method to quantify the number of surface active sites (Ns) of metal oxide catalysts. In situ infrared analysis during methanol adsorption showed that molecular methanol and surface methoxy species are co-adsorbed on an oxide surface at room temperature, but only surface methoxy species are formed at 100°C. Thermal stability and products of decomposition of the adsorbed species were determined with temperature programmed reaction spectroscopy (TPRS) experiments. Controlled adsorption with methanol doses resulted in a stable monolayer of surface methoxy species on the oxide surfaces. The stoichiometry of methanol chemisorption resulted in one surface methoxy adsorbed per three Mo atoms for polymerized surface molybdenum oxide structures, regardless of surface molybdenum oxide coordination. The activity of the catalysts per surface active sites (turnover frequencies — TOF) was calculated in order to quantitatively compare the reactivity of a series of monolayer supported molybdenum oxide catalysts. The TOF value trends reflect the influence of the bridging Mo–O–Support bond and the electronegativity of the metal cation of the oxide support.

Stoichiometry and structural effects in alcohol chemisorption temperature-programmed desorption on molybdenum trioxide.

The mechanism of alcohol oxidation over MoO/sub 3/ has been examined by using temperature-programmed desorption spectroscopy (TPD) with simultaneous microbalance and mass spectral detection. Two types of experiments are reported. The alcohol structure has been varied over the sequence methyl, ethyl, 2-propyl, tert-butyl, and systematic changes in the amount of chemisorption, the peak desorption temperature, and the nature of the products have been observed. The amount of water produced during chemisorption of ethyl alcohol on MoO/sub 3/ at room temperature has been measured. This determination makes it possible to estimate the coverage of alkoxy groups after the chemisorption stage. The fate of these ethoxy groups during subsequent TPD can be followed. A stoichiometric accounting of both acetaldehyde production and catalyst reduction can be made.

A simple solution-based route to high-efficiency CZTSSe thin-film solar cells

Solution-based Cu2ZnSn(S,Se)4 (CZTSSe) thin film deposition routes focusing on chemistries with attractive scalability and handling characteristics, while delivering high efficiency devices are promising technologies for low-cost solar cells. In this paper we describe a new approach for the fabrication of CZTSSe thin films using inks comprised of mixtures of binary and ternary metal-chalcogenide nanocrystals dispersed in simple organic solvents. The resulting blended inks can be coated under ambient conditions to give precursor films which are in turn converted to device-quality CZTSSe absorbers via thermal annealing in a selenium atmosphere. Using this approach we have demonstrated CZTSSe solar cells with total area efficiencies in excess of 8.5% (or 9.6% per active area) under 1 Sun AM1.5 illumination.

Infrared active phonons in (Pr2-xCex)CuO4

Abstract We report infrared (IR) reflectivity measurements of polycrystals of the electron-doped superconductor (Pr2-xCex)CuO4. To understand the complex results, we utilized conducting ceramics in which the ab-plane phonons are screened, allowing observation of the A2u phonons (motion parallel to the c-axis). To obtain the Eu modes (motion in the ab-plane), we utilized small ab-plane single crystals. With these approaches, all of the allowed IR phonons (3A2u + 4Eu) were observed. We have assigned all of the modes to specific atomic motion.

Characterization and understanding of performance losses in a highly efficient solution-processed CZTSSe thin-film solar cell

We present results on the characterization of a highly efficient CZTSSe solar cell fabricated using a solution-based process, aiming to gain a better understanding of its efficiency-limiting causes. Under red light illumination, we observed a red-kink in the current-density versus voltage (J-V) curve, likely due to a persistent photoconductivity in the buffer layer. Temperature-dependent J-V analysis suggests that interface recombination is the dominant loss mechanism. Defect analysis using admittance spectroscopy (AS) shows a single bulk defect level at ~63 meV and may be attributed to copper vacancy (VCu). The carrier concentration of the device determined using drive-level capacitance profiling (DLCP) is ~2.5×1016 cm-3.

Ferroelectricity and superconductivity in YBa2Cu3O7-Δ

Abstract If a crystalline material possesses a center of symmetry, it cannot be ferroelectric. Also, its Raman active modes must occur at different frequencies from its infrared (IR) active modes; this is the Exclusion Principle. By studying single crystals and ceramics, we have determined the Raman and IR active phonon frequencles for atomic motion along the c-axis in YBa2Cu3O7-[ddot], both in the superconducting and semiconducting phases. The measured phonon frequencies are consistent with the Exclusion Principle, hence this material does not appear to be ferroelectric in either the superconducting or semiconducting phases.

Far Infrared Spectroscopy of RBa 2 Cu 3 O x with Variations in R and x

We report far infrared reflectance measurements of RBa 2 Cu 3 O x , where R-Y, Er and Nd and 6 2 Cu 3 O x . Based on these measurements we assign the observed phonon features. In particular, one vibration is clearly identified as an R vibration, while the lowest energy mode (near 151 cm -1 ) is associated with the linear Cu-O chains.