ABSTRACT
The analysis of four different biodiesel blends, as well as homemade biodiesel prepared from vegetable oil, has been performed using gas chromatography-mass spectrometry. The identification of methyl esters within the biodiesel along with any background components is made possible by recognizing their mass spectral fragmentation patterns. These fuels were subjected to typical fire scene environments, specifically weathering and microbial degradation, to investigate how these environments affect the analysis. A matrix study was also performed on wood, carpet, and clothing in order to identify any interferences from these substrates. The data obtained herein will provide the forensic science community with the data needed to help recognize these increasingly common ignitable liquids.
ABSTRACT
A method for the catalytic formation of electroauxiliaries and subsequent anodic oxidation has been developed. The process interfaces N-heterocyclic carbene-based organocatalysis with electro-organic synthesis to achieve direct oxidation of catalytically generated electroactive intermediates. We demonstrate the applicability of this method as a one-pot conversion of aldehydes to esters for a broad range of aldehyde and alcohol substrates. Furthermore, the anodic oxidation reactions are very clean, producing only H(2) gas as a result of cathodic reduction.
ABSTRACT
A mechanism-based equation for the size of a forming transition-metal nanocluster vs time has been derived based on the Finke-Watzky two-step mechanism for transition-metal nanocluster nucleation (A --> B, rate constant k1) and autocatalytic growth (A + B --> 2B, rate constant k2), where A is the nanocluster precursor and B is the growing nanocluster. The resultant equation expresses nanocluster diameter as a function of time, D(t), in terms of k1, k2, the initial concentration of the nanocluster precursor complex, [A]0, and the number of catalytically effective nuclei derived from either (i) the final nanocluster size, D(f), or (ii) the number of atoms in the average catalytically effective nucleus, N*, and the induction period time, t ind (N* being by definition the number of atoms present in the average size nucleus at the end of the induction period and when observable catalysis begins). By fitting experimentally determined nanocluster size vs time data using this equation, evidence for the validity of the equation is obtained for Ir(0) nanoclusters formed from the well-studied system of H2 reduction of the precursor [(1,5-COD)Ir x P2W15Nb3O62](8-). The D(t) equation is then used to determine N* for nine prior Ir(0) nanocluster preparations from five different [(1,5-COD)Ir(+)]n [anion(n-)] precursors. Also given is a relationship allowing one to interconvert between nanocluster size data and nanocluster precursor concentration data, again when the two-step nucleation and growth mechanism has been shown to apply. Some of the key experimental factors that are known to affect the kinetics of nanocluster formation, and therefore nanocluster size, are also summarized. A look ahead to needed future work is also provided.
ABSTRACT
A review of the literature of kinetic and mechanistic studies of transition-metal nanocluster nucleation and growth is presented; the focus is on nucleation processes. A brief survey of nucleation theory is given first, with an emphasis on classical nucleation theory, as this is the logical starting point of transition-metal nanocluster nucleation and growth studies. The main experimental methods for following nanocluster formation are examined next--dynamic light scattering, UV-visible spectroscopy, electron microscopy, and X-ray spectroscopies--with special attention paid to their strengths and weaknesses. Several specific examples of transition-metal nanocluster formation are then given, beginning with LaMer's classic sulfur sol system and including the Finke-Watzky mechanism of slow continuous nucleation A-->B followed by fast autocatalytic surface growth A+B-->2B. Finally, brief overviews of semiconductor nanoparticle preparations, solid-state nucleation studies-emanating from Avrami's work--and protein agglomeration mechanistic studies are also provided, as these processes are relevant, conceptually and in a general sense, to the field of transition-metal nanocluster nucleation and growth mechanisms.
ABSTRACT
The four-step mechanism by which transition-metal nanoclusters or bulk-metal films self-assemble from metal salts under reductive conditions has been discovered. The presence of two autocatalytic steps in the same reaction scheme--double autocatalysis--is the key to the sharp "turn-on" feature after an induction period observed in the signature kinetic curves. Predictions of the new mechanism that are tested experimentally include the following: that low concentrations and high temperatures will favor nanoclusters over bulk-metal film formation; that bulk-metal is formed in some, if not many, literature syntheses reporting only Pt(0) nanoclusters; and that added ligands are one key to turning on the new mechanism. Particle-size-dependent metal-ligand bond dissociation energies are another implication from this mechanistic work.
