The rate-limiting step for alkane dehydrogenation in zeolite H-ZSM-5.

We have carried out a computational study of protolytic cracking, dehydrogenation, and H/D exchange of ethane, propane, and butane using a cluster model of H-ZSM-5. Our previous work has demonstrated that quantum-chemical techniques can give quantitatively accurate activation energies for alkane cracking in zeolites [1]. Experimental kinetic studies have shown that the apparent activation energies for cracking and H/D exchange decrease with n-alkane chain length, while for dehydrogenation the energies increase [2,3]. Our goal is to study the dependence of the activation energy on the alkane chain length in these reactions and to understand why the dehydrogenation reaction behaves so differently

Sample size determination: A comparison of attribute and continuous variable methods

96-100<span style="font-size:12.0pt;line-height: 115%;font-family:" times="" new="" roman";mso-fareast-font-family:"times="" roman";="" mso-ansi-language:en-us;mso-fareast-language:en-us;mso-bidi-language:ar-sa"="" lang="EN-US">Each of the two methods of sample size determination - the Attribute and the Continuous Variable Method has its use in the investigation of social science problems. The former allows the computation of sample size with reference to any parameters of the variable and, therefore, can substitute for the Continuous Variable Method, but, with and probable increase in sample size. The later is very useful when data are collected in ratio form. However, it demands estimates of dispersion from the mean which may be primary purpose of the research in the first place. The Attribute Method is highly recommended for library and information science since it can be substituted for Continuous Variable Method.</span

Variable-temperature Magic-angle-spinning Technique for Studies of Mobile Species in Solid-state NMR

The utility of variable-temperature (VT) magic-angle-spinning (MAS) Fourier transform nuclear magnetic resonance (FT-NMR) spectroscopy is demonstrated for the study of mobile species in solids, especially when quadrupolar nuclei are being observed A new VT-MAS probe is described for use in the temperature range 150-400 K, using spectrometers with high-field superconducting magnets. It has been utilized in studies of the bonding and dynamics of alkali-metal cations in hydrated zeolites at an applied field of 7.05 T. Results are presented for the cases of Cs/Na-A and Cs/Li-A zeolites, in which the Cs+ ions exchange rapidly, at 293 K, between six-ring and eight-ring sites in the large cage, so that on the time scale of the NMR measurements the ions are indistinguishable and a single 133Cs resonance is observed. Below 250 K, two well-resolved signals are observed in the Cs/Na-A zeolite, 107 ppm apart. These are easily assigned to the Cs ions in the two sites, the large difference in chemical shifts being attributed to the stronger bonding of Cs+ ions at the six-ring sites. Three sites are distinguished in the Cs/Li-A zeolite, the third site being assigned to a position near a four-ring in the large cage. © 1989 American Chemical Society

Crystal structure, short-range oxygen defects, and water adsorption in La- and Nd-modified ZrO{sub 2}

Doping Rare-earth (RE) elements to ZrO{sub 2} helps stabilize the cubic and tetragonal phases and improves resistance to thermal shock and sintering at high temperatures. Since a RE ion has a lower valency (3{sup +}) than Zr ion (4{sup +}), oxygen vacancies are formed to preserve electroneutrality. We have studied the crystal structure of La{sub 0.1}Zr{sub 0.9}O{sub 1.95} and Nd{sub 0.1}Zr{sub 0.9}O{sub 1.95} by neutron diffraction and examined the associated oxygen defects by a Fourier transform of the filtered residual diffuse scattering. The hydration process was investigated by inelastic neutron-scattering measurements of the hydrogen vibrational density of states of the surface hydroxyl groups and physisorbed water on these fine powders. We compare the O-H stretch vibrations for samples from with only surface hydroxyl groups to multilayer coverage of water molecules. The decreasing energies and increasing widths of the O-H stretch bands with increasing H{sub 2}O coverage indicate the influence of hydrogen bonding on the motion of water molecules. Similar elastic and inelastic experiments were also performed on a high surface-area pure ZrO{sub 2} powder

Alternative catalyst and exhaust gas sensor work at Argonne National Laboratory

Research programs at Argonne National Laboratory in the areas of automobile emissions monitoring and control are described. The mandate to improve automobile efficiency while reducing Pollution requires the development of new catalysts for exhaust emissions control that are capable of functioning efficiently under lean-burn engine operating conditions. It is also desirable that the use of expensive noble metal catalysts be avoided. NO{sub x} emissions will not be efficiently controlled by the current three-way, supported noble metal catalysts under lean-burn conditions. New catalysts are being sought that could effect the selective catalytic reduction (SCR) of NO{sub x} by exhaust hydrocarbons in the presence of oxygen. Molecular sieve zeolites of the ZSM-5 and ferrierite types, ion-exchanged with copper ions, are the best of the catalysts known to effect this chemistry, but the mechanism of the SCR is still not understood. In this project the authors will first undertake the investigation of the SCR of NO using model reactions to test postulated mechanistic pathways. Initial experiments have been devised to investigate the possible participation of metal alkyl complexes, metal oxime complexes, N-alkyl-N-nitroso-alkylaminato-metal complexes, and metal nitrile complexes in the zeolites. ANL will also develop microsensors, based on surface acoustic wave (SAW) chemical sensing techniques, and a micro mass-spectrometer (MS) for tailpipe or engine-out emission monitoring. The sensor configurations and sensing techniques of the proposed SAW and micro-MS are described