Traditionally, the Raman spectrum of ethanol in the C-H vibrational stretching region between 2800 cm -1 and 3100 cm -1 had been assigned as three bands of the -CH 2 symmetric stretching, the -CH 3 symmetric stretching, and the -CH 3 antisymmetric stretching. In this report, new Raman spectral features were observed for ethanol and two deuterated ethanols, that is, CD 3 CH 2 OH and CH 3 CD 2 OH, in both gaseous and liquid phases with polarized photoacoustic Raman spectroscopy (PARS) as well as normal Raman spectroscopy. With the aid of depolarization ratio measurements and quantum chemical calculations, different assignments were presented to the complex spectral features in the ethanol Raman spectra. In the gaseous ethanol spectra, the band at ∼2882 cm -1 was assigned to the overlapping symmetric stretching vibrational modes of both -CH 2 and -CH 3 ; the band at ∼2938 cm -1 was assigned to the two symmetric -CH 3 Fermi resonance modes and the weak -CH 2 antisymmetric stretching mode; and the band at ∼2983 cm -1 was assigned to the symmetric -CH 2 Fermi resonance mode and the weak -CH 3 antisymmetric stretching mode. The liquid ethanol spectral features are similar to the gaseous spectra with a much stronger -CH 3 antisymmetric stretching mode and a red shift of about 10 cm -1 , which can be attributed to the effects of solvent interactions. The new assignments of both the gaseous and liquid ethanol spectra not only confirmed the recent results from the sum-frequency generation vibrational spectroscopy studies of the ethanol molecules at the air/liquid interface but the differences in the gaseous and liquid phases, as well as at the interfaces, can also provide detailed experimental evidence in understanding of the molecular interactions and dynamics of the ethanol molecule in different chemical environments.

Syngas (CO/H 2 ) produced from coal, natural gas, or biomass has attracted much attention as alternative to petroleumderived fuels and chemicals. Syngas can be selectively converted to oxygenates, such as alcohols, aldehydes, and carboxylic acids, or hydrocarbons by Fischer-Tropsch synthesis (FTS). [1] Industrially, rhodium- [2] and cobalt-based [3] catalysts are often used for production of C 2 oxygenates and hydrocarbons. Despite numerous studies, the exact mechanism remains in debate, and represents a major challenge in catalysis. [4] Formyl, formed by CO hydrogenation, has been implicated as of the key reactive intermediates in syngas conversion, [4e, 5] and it was proposed that hydrogenation of HCO followed by C = O bond scission leads to the formation of a CH x monomer. Then chain growth proceeds by CO insertion into CH x , by carbene coupling, or by condensation of C 1 oxygenates with elimination of water, with formation of C n (n ! 2) oxygenates or hydrocarbons. However, the short lifetime of HCO prevents its characterization, which typically requires elevated pressures, and identification of its role in syngas conversion. [6] Recently, direct evidence for HCO as the key intermediate for CO methanation was obtained by in situ spectroscopic experiments on supported Ru catalysts. [7] Herein we report on the use of DFT calculations (for computational details, see Methods) to explore the role of HCO in syngas conversion and its dependence on the catalyst. Insertion of HCO was revealed to be an efficient alternative for chain growth on Rh(111) and Co(0001) surfaces in syngas conversion for the first time. Since HCO was proposed to be a prerequisite for the formation of a CH x monomer (the key intermediate involved in chain growth), there should be a sufficiently high concentration of HCO for the formation of C 2 oxygenates and hydrocarbons. The results were compared to reaction pathways of CO insertion and carbene coupling. This work offers a mechanistic understanding of syngas chemistry, by achieving fundamental insight that could be used to design and develop improved catalysts for these and other important reactions of technological interest.We first investigated competitive CO versus HCO insertion into CH x (x = 1-3) on Rh(111), as shown in Figure 1 a. The calculated activation energy barriers for CO insertion into CH, CH 2 , and CH 3 of 1.34, 1.25, and 1.55 eV, respectively, are significantly higher than the corresponding barriers for HCO insertion (0.89, 0.75, and 1.02 eV). Compared to the most commonly studied CO insertion pathway, the kinetic preference for the HCO insertion pathway is immediately apparent. Moreover, HCO insertion into CH x is slightly endothermic or exothermic, with reaction energies of 0.27, À0.10, and À0.04 eV, whereas CO insertion is endothermic by 1.11, 0.69, and 0.35 eV, respectively. Therefore, the HCO insertion pathway is preferred on thermochemical grounds. Regardless of the pathway, insertion into CH 2 (CH 3 ) is the most kinetically favorable (unfavorable) step among al...

C 2 oxygenate (acetaldehyde, ethanol, etc.) formation from syngas (CO + H 2 ) is an important industrial process for the production of clean liquid energy fuels and valuable chemical feedstocks that are catalyzed industrially by Rh modified with Mn and Fe, etc. In an effort to identify catalysts based on less expensive metals and higher C 2 oxygenate selectivity, density functional theory (DFT) calculations were performed to tune the relative activity of the selectivity-determining steps, i.e., CO insertion in CH x (x = 1, 2, 3) versus CH x hydrogenation by changing composition and structure of material. We find that the Rh-decorated Cu alloy catalyst has significantly lower CO insertion barriers compared to pristine Rh(111) and vicinal Rh(553) surfaces, whereas the variation of CH x hydrogenation barriers on the three surfaces is modest. A semiquantitative kinetic analysis based on DFT calculations shows that the C 2 oxygenate selectivity on RhCu( 111) is substantially improved, with the production rate of C 2 oxygenates slightly higher than CH 4 under experimental conditions, compared with Rh(111) and Rh(553) that are highly selective to CH 4 . Our calculations suggest that the improved C 2 oxygenate selectivity on the RhCu alloy is primarily due to the fact that CO insertion is rather sensitive, whereas hydrogenation is insensitive to the ensemble effect. Furthermore, the Rh-decorated Cu alloy has stronger resistance toward coking and lower constituent cost compared to pure Rh catalysts and is thus a promising candidate for an improved C 2 oxygenate synthesis catalyst.

Articles you may be interested inCommunication: Vacuum ultraviolet laser photodissociation studies of small molecules by the vacuum ultraviolet laser photoionization time-sliced velocity-mapped ion imaging method J. Chem. Phys. 135, 071101 (2011); 10.1063/1.3626867 Photodissociation dynamics of 3-bromo-1,1,1-trifluoro-2-propanol and 2-(bromomethyl) hexafluoro-2-propanol at 234 nm: Resonance-enhanced multiphoton ionization detection of Br (2 P j ) Photodissociation and photoionization of 2,5-dihydroxybenzoic acid at 193 and 355 nm J. Chem. Phys. 133, 244309 (2010); 10.1063/1.3518709 Br ( P j 2 ) atom formation dynamics in ultraviolet photodissociation of tert-butyl bromide and iso-butyl bromideThe photodissociation dynamics of diiodomethane molecules has been investigated in the wavelength range of 277-305 nm by an ion imaging spectrometer operated under optimal conditions for velocity mapping, where the ions were generated from ͑2ϩ1͒ multiphoton ionization of I( 2 P 3/2 ) and I*( 2 P 1/2 ) fragments with the same laser as that to dissociate the parent molecules. The speed and angular distributions of I* and I fragments were determined from the images. The translational energy distribution of I*( 2 P 1/2 ) fragment consists of a single Gaussian component ͑named G*͒, while that of I( 2 P 3/2 ) consists of two Gaussian components ͑named G1 and G2͒. It was found that the component G* and G2 show similar angular distributions and similar fragmentation energy partitioning ratios, indicating that these two components originate from dissociation at the same electronically excited state, while the component G1 is from another state. Three fragmentation pathways were employed to account for the experimental observations, the adiabatic dissociation from the 1B 1 state to form I( 2 P 3/2 ) with component G1, the adiabatic dissociation from the 2B 1 state to form I*( 2 P 1/2 ) with component G*, and the nonadiabatic dissociation from the 2B 1 state caused by coupling with the higher 2A 1 state to form I( 2 P 3/2 ) with component G2.