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The mechanism for ethylene oligomerization by (acac)NiH has been studied using density functional theory (DFT). The transition states for chain propagation and chain termination were optimized and the related reaction barriers calculated. Several possible mechanisms were considered for the chain termination step. Chain termination by beta-hydrogen elimination was found to be energetically unfavorable, and is not likely to be important. Instead, beta-hydrogen transfer to the incoming ethylene unit seems to be operative. The most favorable beta-hydrogen transfer pathway has two transition states. The first leads from a weak pi-complex between an incoming ethylene unit and (acac)NiCH(2)CH(2)R to an intermediate in which the two olefins C(2)H(4) and H(2)CCHR both are strongly pi-complexed to the nickel hydride (acac)NiH. The second barrier takes the intermediate to another weak pi-complex between (acac)NiCH(2)CH(3) and H(2)C=CHR from which the oligomer H(2)C=CHR can be released and the catalyst (acac)NiCH(2)CH(3) regenerated. Due to the mechanism of chain termination, the actual catalyst is proposed to be (acac)NiCH(2)CH(3) whereas (acac)NiH serves as a precursor or precatalyst.

A family of various cerium oxide-based catalysts were synthesized by adopting flame aerosol (FSP), coprecipitation, wet impregnation, and hydrothermal synthesis techniques. The resulting catalysts were explored for the selective catalytic reduction (SCR) of NO x using NH3 as reductant. In our studies, both the preparation method and the Ce/W ratios were found to be critical variables for successful catalyst promotion. For the industrial realization, we have scaled up the SCR activity tests. The microreactor catalytic formulations at simulated diesel engine exhaust conditions revealed that the Ce–W (1:1 atomic ratio) and Ce–W/TiO2 catalysts showed high deNO x activity, while the other catalysts’ activity was found to be rather low. Of interest is the finding that the Ce–W/TiO2/cordierite and Ce–W (1:1 atomic ratio)/cordierite formulations show impressive deNO x performance and high N2 selectivity with respect to a commercial vanadia based reference currently used for mobile applications. To gain fundamental insights which may acquaint further improvements to the promoted Ce-based catalysts, X-ray photoelectron spectroscopy and other characterizations were executed to study the relationship between catalyst surface and NO x reduction activity. Our XRD results indicate smaller lattice parameters of prepared catalysts compared to that of CeO2 (0.540 nm). The crystal lattice contraction is attributed to the lesser ionic radius of relevant foreign metal ions (W6+ = 0.067 nm and Ti4+ = 0.074 nm) in relation to Ce4+ (0.092 nm) in the host lattice. This lattice shrinkage elucidates the formation of solid solutions. These results illustrate that the synthesis technique and various promoters could indeed influence the lattice structures and electronic state of the active components. The XPS results illustrate the higher atomic ratios of Ce3+/(Ce3+ + Ce4+) 0.30 and 0.29 in Ce–W/TiO2 and Ce–W (1:1) coprecipitation catalysts, respectively, compared to other samples. The higher surface Ce3+/Ce4+ ratio in Ce–W (1:1) coprecipitation and Ce–W/TiO2 samples indicate the enrichment in surface oxygen vacancies, which results in activation of reactive molecules and enhanced adsorption of oxygen species in SCR reaction. Interestingly, the surface atomic ratio of Ce3+/Ce4+ and Ce3+/Ce n+ are interrelated to the SCR activity of the individual catalysts.

The crystal and molecular structure of bis(trimesitylphosphine)silver(I) hexafluorophosphate [Ag(P(mesityl)3)2]+PF6" have been determined. Crystals are trigonal, space group 3,21 (D4 5No. 152), with a = 15.378 (2) A, c = 19.945 (4) A, and 3 formula units in the unit cell. The structure was solved by the heavy-atom method and refined by full-matrix least-squares calculations with three-dimensional diffractometer data to a final discrepancy index RF = 0.070 for 922 reflections with I > 3 ( ). This is the first structure determination of a two-coordinate silver cation with phosphine ligands. Both cation and anion lie on separate crystallographic twofold axes, the anion being additionally disordered over two sites. In the cation, the bulk of the P(mes)3 (mes = mesityl) ligands ensures an almost linear P-Ag-P moiety (179.4°), a regular propeller conformation with dihedral angles between the Ag-P-C and mesityl planes in the 45-51 °range, a relatively long Ag-P distance 2.461 (6) Á and irregular angles at phosphorus (Ag-P-C 107.3-109.3 (9)°, C-P-C 108-113(1)°). Cone angle calculations are made for the P(mes)3 ligand ( = 203°).

Density functional calculations have been carried out on the thermochemical aspects of catalytic ethylene dimerization by the d8 hydride (propanedialato(l-))Ni-(I), where the propanedialate(l-) anion served as a model for the chelate acac ligand, acetylacetonate(l-). The hydride (I) was found to have a low-spin d8 configuration with a square planar structure where one site is vacated cis to hydrogen. It was shown that ethylene inserts readily into the Ni-H bond of (I) with an exothermicity of 44.6 kcal/mol. The resulting ethyl complex (III) has a strong agostic interaction between nickel and a /3-hydrogen. It is suggested that the ethyl complex is the actual catalyst in the dimerization cycle. The agostic interaction in is estimated to have a strength of 10 kcal/mol. The next insertion of ethylene into the Ni-ethyl bond of III leads to the butyl complex (IV). The insertion is exothermic by 25 kcal/mol. The reaction between (IV) and ethylene leads finally to the release of 1-butene and the regeneration of the ethyl complex (III). This step is nearly thermoneutral with a reaction heat of 0.1 kcal/mol. It is suggested that the elimination of 1-butene takes place via a transition state in which both ethylene and 1-butene are -complexed to I. The internal barrier of activation for the final step is calculated to be 17 kcal/mol. The substitution of hydrogen by CH3 or CF3 groups on the propanedialate(l-) ligand were also considered. It was found that such substitutions only had a minor effect on the thermochemistry of the insertion processes in the dimerization cycle.