Methane oxidation mechanism on Pt(111): a cluster model DFT study

The electronic energy barriers of surface reactions pertaining to the mechanism of the electrooxidation of methane on Pt (111) were estimated with density functional theory calculations on a 10-atom Pt cluster, using both the B3LYP and PW91 functionals. Optimizations of initial and transition states were performed for elementary steps that involve the conversion of CH(4) to adsorbed CO at the Pt/vacuum interface. As a first approximation we do not include electrolyte effects in our model. The reactions include the dissociative chemisorption of CH(4) on Pt, dehydrogenation reactions of adsorbed intermediates (*CH(x) --> *CH(x-1) + *H and *CH(x)O --> *CH(x-1)O + *H), and oxygenation reactions of adsorbed CH(x) species (*CH(x) + *OH --> *CH(x)OH). Many pathways were investigated and it was found that the main reaction pathway is CH(4) --> *CH(3) --> *CH(2) --> *CH --> *CHOH --> *CHO --> *CO. Frequency analysis and transition-state theory were employed to show that the methane chemisorption elementary step is rate-limiting in the above mechanism. This conclusion is in agreement with published experimental electrochemical studies of methane oxidation on platinum catalysts that have shown the absence of an organic adlayer at electrode potentials that allow the oxidation of adsorbed CO. The mechanism of the electrooxidation of methane on Pt is discussed.     

Monte Carlo free energy calculations using electronic structure methods

The molecular mechanics-based importance sampling function (MMBIF) algorithm [R. Iftimie, D. Salahub, D. Wei, and J. Schofield, J. Chem. Phys. 113, 4852 (2000)] is extended to incorporate semiempirical electronic structure methods in the secondary Markov chain, creating a fully quantum mechanical Monte Carlo sampling method for simulations of reactive chemical systems which, unlike the MMBIF algorithm, does not require the generation of a system-specific force field. The algorithm is applied to calculating the potential of mean force for the isomerization reaction of HCN using thermodynamic integration. Constraints are implemented in the sampling using a modification of the SHAKE algorithm, including that of a fixed, arbitrary reaction coordinate. Simulation results show that sampling efficiency with the semiempirical secondary potential is often comparable in quality to force fields constructed using the methods suggested in the original MMBIF work. The semiempirical based importance sampling method presented here is a useful alternative to MMBIF sampling as it can be applied to systems for which no suitable MM force field can be constructed.     

Calculation of normal frequencies of adsorbed molecules by the LCGTO-MCP-LSD method

Density functional calculations have been performed to determine the equilibrium geometry and normal frequencies of the Pd₃(CCH₃) cluster (ethylidyne on Pd₃). We find promising agreement between the theoretically obtained frequencies and those of experimental studies for CCH₃ on the Pd(111) surface. The effect of the metal cluster relaxation on the calculated equilibrium geometry and normal frequencies is examined.     

Multi-harmonic active structural acoustic control of a helicopter main transmission noise using the principal component analysis

This paper evaluates the feasibility and effectiveness, under controlled conditions, of active structural acoustic control of a helicopter main transmission in order to attenuate the vibration of the receiving roof panel and its sound radiation into the cabin. The vibroacoustic analysis of a typical helicopter drive train is conducted to extract the dominant tones generated by the various transmission stages. A finite element model of a Bell 407 transmission and simplified roof structure is developed in order to investigate various active control arrangements using piezoceramic actuators to control wave propagation in the gearbox supporting struts or receiving panel. The principal component analysis is used to extract the most significant control paths and reduce the control effort. A multiple frequency principal component least mean square (PC-LMS) algorithm is implemented on a laboratory setup and is used to successfully reduce up to seven gearbox tones simultaneously.     

An accurate method for the determination of <Superscript>226</Superscript>Ra activity concentrations in soil

A quantitative method to determine the activity concentration of ²²⁶Ra in soil samples was established using high performance environmental gamma-ray spectrometry. In this method, a semi-empirical calibration procedure was developed for full energy peak efficiency calculation utilizing the elemental composition of the soil sample. Aatami software was used to deconvolute the ²³⁵U and ²²⁶Ra doublet at 185.7 keV and 186.2 keV, respectively, and to fit the baseline of the soil gamma-spectrum for the determination of ²²⁶Ra activity. The results indicated that the Aatami doublet deconvolution procedure provides a rapid and accurate analysis of a complicated spectrum in comparison with other cumbersome spectral interference correction methods. The study also compared the results with those obtained by radon progeny (²¹⁴Pb, or ²¹⁴Bi) measurements and found that the deconvolution method provided a more accurate ²²⁶Ra activity as it is independent of the error caused by radon diffusion. This error can be quite large since the amount of escaped radon gas through the sample container walls and sealing cannot be accurately quantified.     

Independent environmental monitoring and public dose assessment around the Canadian Nuclear Power Plants

Journal of Radioanalytical and Nuclear Chemistry - Radiological impact to members of the public living around the Canadian Nuclear Power Plants (NPPs) was assessed using environmental monitoring...     

A new bomb-combustion system for tritium extraction

Quantitative extraction of tritium from a sample matrix is critical to efficient measurement of the low-energy pure beta emitter. Oxidative pyrolysis using a tube furnace (Pyrolyser) has been adopted as an industry standard approach for the liberation of tritium (Warwick et al. in Anal Chim Acta 676:93–102, 2010) however pyrolysis of organic-rich materials can be problematic. Practically, the mass of organic rich sample combusted is typically limited to <1 g to minimise the possibility of incomplete combustion. This can have an impact on both the limit of detection that can be achieved and how representative the subsample is of the bulk material, particularly in the case of heterogeneous soft waste. Raddec International Ltd (Southampton, UK), in conjunction with GAU-Radioanalytical, has developed a new high-capacity oxygen combustion bomb (the Hyperbaric Oxidiser; HBO₂) to address this challenge. The system is capable of quantitatively combusting samples of 20–30 g under an excess of oxygen, facilitating rapid extraction of total tritium from a wide range sample types.     

Molecular structure of mono- and dicarbonyls of rhodium and palladium

Summary Density Functional Theory has been applied to the study of the molecular structure of neutral and positively charged mono- and dicarbonyls of rhodium and palladium. The calculated optimized geometries, dissociation energies and normal frequencies are reported for the MCO, MCO⁺, M(CO)₂ and M(CO) ₂ ⁺ systems (where M=Rh and Pd), and the trends are discussed in detail. For neutral carbonyls, we interpret the M–C bond strength in terms of repulsion, which must be avoided, and attraction. These are related to the metal atom properties, such as the atomic splittings and the atomic ionization energies. In ionic carbonyls, the bonding is characterized by electrostatic attraction and repulsion. The rhodium carbonyls are generally found to be more stable than the corresponding palladium carbonyls. The palladium dicarbonyls are found to be linear, while both linear and bent structures are stable for rhodium dicarbonyls. An interpretation of these trends is made.     

A scalable divide-and-conquer algorithm combining coarse and fine-grain parallelization

We describe an efficient algorithm for carrying out a “divide-and-conquer” fit of a molecule's electronic density on massively parallel computers. Near linear speedups are achieved with up to 48 processors on a Cray T3E, and our results indicate that similar efficiencies could be attained on an even greater number of processors. To achieve optimum efficiency, the algorithm combines coarse and fine-grain parallelization and adapts itself to the existing ratio of processors to subsystems. The subsystems employed in our divide-and-conquer approach can also be made smaller or bigger, depending on the number of processors available. This allows us to further reduce the wallclock time and improve the method's overall efficiency. The strategies implemented in this paper can be extended to any other divide-and-conquer method used within an ab initio, density functional, or semi-empirical quantum mechanical program. Received: 15 September 1997 / Accepted: 21 January 1998