Ms Contact Research Articles

Molecular Beam Chemistry: Formation of Benzene and Other Higher Hydrocarbons from Small Alkanes and Alkenes in a Catalytic Supersonic Nozzle

Large yields of higher hydrocarbons are produced by flowing C2, C3, or C4 alkanes or alkenes, typically at 80 Torr, 1000 °C, and 10 ms contact time, through a supersonic nozzle made of nickel or molybdenum. In the mass spectra of the products, CnHm, the most prominent peaks (extending to n = 12) contain even numbers of carbon atoms, but there are also substantial peaks with odd carbon atom numbers. The largest peaks have m ∼ n hydrogen atoms, but many others also appear. For n = 6 the mass spectrum indicates benzene is essentially the sole product. The total conversion of the parent beam is roughly 40−60%, of which typically 15−25% is benzene; the yields increase as C2 → C4 and are appreciably higher for alkenes. Under the same conditions, methane does not form higher hydrocarbons. Experiments with mixtures of CD4 and C2H6 show, however, that D atoms from methane pervade the higher hydrocarbon products.

Facile production of higher hydrocarbons from ethane in a catalytic supersonic nozzle

Flowing ethane, typically at 80 Torr, 1000°C, and 10 ms contact time, through a supersonic nozzle made of nickel or molybdenum converts roughly 40% to higher hydrocarbons. In the mass spectra of the products, C n H m , the most prominent peaks contain even numbers of carbon atoms ( n = 4, …, 12), but there are also substantial peaks with odd carbon atom numbers ( n = 3, …, 11). The largest peaks have m ≈ n hydrogen atoms, but many others also appear. For n = 6 the mass spectrum indicates benzene is probably the sole product, with yield up to 15%. Under the same conditions methane does not form higher hydrocarbons.

Partial oxidation of CH 4 and C 2H 6 over noble metalcoated monoliths

The direct partial oxidation of hydrocarbons offers promising alternatives to chemical synthesis. By replacing endothermic processes such as steam reforming and steam cracking, fast and exothermic oxidation reactions should require much smaller and simpler reactors. However, direct oxidation reactions are much more difficult to manage because of potential heat release in total oxidation and hazardous because of the possibility of homogeneous reactions which are nonselective and can produce flames and explosions. We describe experiments in which monolith catalysts are used for partial oxidation of CH 4 and C 2H 6 to produce synthesis gas or alkenes by direct oxidation at or above atmospheric pressure in pure O 2 in nearly adiabatic reactors operating at 1000°C with very high flowrates (space velocities of 10 6h −1 and residence times of 10 −3 s). With methane oxidation we obtain over 90% selectivities to synthesis gas (a 2:1 H 2:CO mixture) with > 90% conversion of the methane and complete conversion of O 2 on Rh coated ceramic monoliths with contact times of 10 −3 s. With Pt catalysts under the same conditions, the H 2 selectivity drops to 70%; while with Pd, the catalyst rapidly forms carbon. This process appears to be primarily a surface reaction in which CH4 pyrolyzes on the hot Rh surface and the H atoms dimerize and the carbon is oxidized to CO. A model has been constructed which accurately predicts the conversions and selectivities and the variations between Rh and Pt. With higher alkanes, synthesis gas is produced on Rh with comparable selectivities and conversions on metal-coated monoliths. However, with Pt we observe up to 70% selectivity to alkenes with 80% conversion of alkanes at adiabatic temperatures near 1000°C with approximately 5 ms contact times. These results can be explained as occurring by predominantly surface reactions in which the alkane adsorbs to form the alkyl by H abstraction with adsorbed O atoms. Then the adsorbed alkyls undergo primarily β-elimination reactions on Pt to produce alkenes. These products are therefore far from thermodynamic equilibrium at these very short contact times, even though the temperatures are very high. The use of very short contact times and high temperatures promises to provide new routes to production of partial oxidation products with very small adiabatic reactors and thus opens up new types of reactions and reactors for chemical synthesis.

Characterization of rectifying contacts on natural type IIb diamond

Metal-semiconducting diamond (MS) and metal-insulating diamond-semiconducting diamond (MIS) contacts were fabricated on the same type IIb single crystal diamond. Direct comparisons of MS and MIS structure characteristics were made by analysis of current-voltage and differential capacitance-voltage (C-V) data. Both the MS and MIS contacts exhibited good rectifying characteristics, with a 5 V rectification ratio ≳106. The depletion layer uncompensated acceptor concentration measured in both structures by C-V analysis was ∼1.8×1016 cm−3.

Side-chain entropy and packing in proteins.

What role does side-chain packing play in protein stability and structure? To address this question, we compare a lattice model with side chains (SCM) to a linear lattice model without side chains (LCM). Self-avoiding configurations are enumerated in 2 and 3 dimensions exhaustively for short chains and by Monte Carlo sampling for chains up to 50 main-chain monomers long. This comparison shows that (1) side-chain degrees of freedom increase the entropy of open conformations, but side-chain steric exclusion decreases the entropy of compact conformations, thus producing a substantial entropy that opposes folding; (2) there is side-chain "freezing" or ordering, i.e., a sharp decrease in entropy, near maximum compactness; and (3) the different types of contacts among side chains (s) and main-chain elements (m) have different frequencies, and the frequencies have different dependencies on compactness. mm contacts contribute significantly only at high densities, suggesting that main-chain hydrogen bonding in proteins may be promoted by compactness. The distributions of mm, ms, and ss contacts in compact SCM configurations are similar to the distributions in protein structures in the Brookhaven Protein Data Bank. We propose that packing in proteins is more like the packing of nuts and bolts in a jar than like the pairwise matching of jigsaw puzzle pieces.

Characterization of MS, MiS, and Mos Contacts to Type IIB Diamond by Capacitance-Voltage and Current-Volt Age

ABSTRACTThree types of electrical contacts on natural type lib single crystal diamonds have been studied by current-voltage (IV) and capacitance-voltage (CV) measurements. Vertical structures were fabricated with the metal-diamond (MS), metal-undoped diamond-doped diamond (MiS), or metal-SiO2-doped diamond (MOS) contacts and ohmic contacts on the opposite face of the diamond. The MS contact was rectifying and both the MiS and MOS were blocking contacts. While very high leakage currents were measured on the MiS structure in forward bias (MO6times the reverse bias), an accumulation region was observed in the MiS CV data. The uncompensated acceptor concentration values calculated from the CV data were in good agreement for different structures fabricated on the same samples and with the secondary ion mass spectroscopy (SIMS) measurements of the bulk boron concentration. Extreme stretch out was observed in the CV of the MiS and MOS due to interface traps. The density of interface traps was estimated to be in the 1012 cm-2 eV-1 range for these structures.

Effect of MS contact on the electrical behaviour of solar cells

The electrical behaviour of the AlSi contact and its effect on the performance of solar cells has been studied. When the sheet resistance ϱ s of the n-layer is greater than about 200 Ω/□ the contact will function as a rectifier and the dark I– V characteristics of the solar cell is mainly limited by the reverse biased MS contact. This accounts for the large ideality factor (> 2) observed experimentally. In addition the illuminated I– V curve of the solar cell in the fourth quadrant has an S-shape with an inflection around zero cell current, and the conversion efficiency is considerably reduced. In fact this inflection is a diagnostic tool for a rectifying MS contact. A circuit model has been developed to describe the behaviour of a solar cell with any internal construction and under all operating conditions. This model is confirmed by the electrical behaviour. Finally fabrication methods are proposed to overcome the contact problems associated with low doping concentration at the metal-semiconductor interface.

Effects of variations in contact times and copper contents in a 13C CPMAS NMR study of samples of four organic soils

Contact time, T c, was varied from 0.5 to 4 ms in a 13C CPMAS NMR study of samples of four organic soils differing only in copper content and degree of decomposition. Initial polarization was rapid, with maximum intensity reached at or before 0.5 ms contact time and a large decrease in intensity by 4 ms contact time. The distribution of intensity was determined by dividing the spectra into four regions, corresponding approximately to aliphatic, carbohydrate, aromatic and carboxyl carbons. Three samples, with up to 1150 p.p.m. Cu, showed small increases in aromaticity with increasing T c, with the greatest effect between 0.5 and 1 ms, and fairly constant intensity distributions between 1 and 4 ms. The sample with 2920 p.p.m. Cu showed much greater increase in aromaticity with increasing T c and a decrease in the 50–110 p.p.m. region, which is dominated by carbohydrate signals. T 1 ϱ H was negatively correlated with increasing Cu content for all four regions of the spectra, but the effect was much larger and the correlation coefficient much higher for the carbohydrate region. A T c of 1 ms appears suitable for organic soil samples; however, caution must be exercised in drawing conclusions from intensity data, especially where samples contain high concentrations of metals. These data also indicate a possible preferential localization of Cu in the carbohydrate or hydrophilic portions of the organic matter; 13C CPMAS NMR may provide a novel, non-perturbing method to study metal binding in organic soils.

Inelastic Electron Tunneling Spectroscopy in MIS and MS Tunnel Junctions

Calculation of the tunneling conductance for inelastic tunneling process with excitation energy \( \hbar \omega \) are carried out, and it is clarified that inelastic electron tunneling spectroscopy (IETS) can be a useful tool for investigation of electronic states in semiconductors. The electronic states of the semiconductor are related to the inelastic tunneling current in the bias range from \( \hbar \omega/e\) to \( \hbar \omega /e+V_{\text{F}}\). This bias increase of \( \hbar \omega/e\) brings some advantage such as separation of the electronic structure from zero bias anomaly and from other structure due to excitation in the barrier. In experiments for MS contacts of InSrTiO 3- x , concaves appeared in the d V /d J curves via inelastic tunneling process, and they were confirmed to be due to the electronic states of the SrTiO 3- x .