Elastic Theory of Defects in Toroidal Crystals

We report a comprehensive analysis of the ground state properties of axisymmetric toroidal crystals based on the elastic theory of defects on curved substrates. The ground state is analyzed as a function of the aspect ratio of the torus, which provides a non-local measure of the underlying Gaussian curvature, and the ratio of the defect core-energy to the Young modulus. Several structural features are discussed,including a spectacular example of curvature-driven amorphization in the limit of the aspect ratio approaching one. The outcome of the elastic theory is then compared with the results of a numerical study of a system of point-like particles constrained on the surface of a torus and interacting via a short range potential.Comment: 24 pages, 24 figure

A critical analysis of the experimental evidence for and against a formate mechanism for high activity water-gas shift catalysts (invited review)

International audienceAn in-depth analysis of the evidence for and against a formate-based mechanism for the water gas shift reaction has shown that with very few exceptions the published results cannot be used to provide any mechanistic information either for or against a formate model. Where reliable quantitative data are available, these show unequivocally that for all high activity catalysts the contribution of IR-observable formate to the production of CO2 is only of minor importance. It is found that the formates seen by IR typically account for less than about 10-15% of the total WGS reaction products. The formates observed by IR were potentially the main reaction intermediates only in the case of very low activity catalysts. For the high activity catalysts that are of interest for current fuel cell applications it is clear that the major contribution of a formate mechanism is yet to be proven. The current published in situ/operando FTIR data relating to formates cannot be used to elucidate the main reaction mechanism nor for deriving improved catalytic formulations

A critical analysis of the experimental evidence for and against a formate mechanism for high activity water-gas shift catalysts (invited review)

International audienceAn in-depth analysis of the evidence for and against a formate-based mechanism for the water gas shift reaction has shown that with very few exceptions the published results cannot be used to provide any mechanistic information either for or against a formate model. Where reliable quantitative data are available, these show unequivocally that for all high activity catalysts the contribution of IR-observable formate to the production of CO2 is only of minor importance. It is found that the formates seen by IR typically account for less than about 10-15% of the total WGS reaction products. The formates observed by IR were potentially the main reaction intermediates only in the case of very low activity catalysts. For the high activity catalysts that are of interest for current fuel cell applications it is clear that the major contribution of a formate mechanism is yet to be proven. The current published in situ/operando FTIR data relating to formates cannot be used to elucidate the main reaction mechanism nor for deriving improved catalytic formulations

In situ IR study of the nature and mobility of sorbed species on H-FER during but-1-ene isomerization

The sorption and reaction of but-1-ene over a ferrierite sample was studied by in situ IR spectroscopy to relate the selectivity to isobutene and the sorbed species. The spectra highlighted the presence of various unsaturated species including polyenes, which were readily formed upon contacting H-FER with but-1-ene while a poor selectivity to isobutene was observed. Aromatic compounds were concluded to be subsequently formed via dehydrogenation/cyclization of the polyenic deposits while the isobutene selectivity increased via suppression of by-products. These data support earlier assumptions that the initial high yield of by-products was produced by the reaction of butenes with reactive deposits, which were gradually converted into more inert aromatic deposits. Upon purging the catalyst with an inert, branched C8 hydrocarbons and aromatic molecules such as xylenes desorbed. The rates of desorption showed an initially linear variation with the square root of time and were fitted to a monodimensional Fickian model. Because the slowly diffusing species were rather abundant during the course of the reaction, it is proposed that the skeletal isomerization was effectively limited to the acid sites close the pore mouth of H-FER crystallites

Transition state and diffusion controlled selectivity in skeletal isomerization of olefins

Microkinetic and spectroscopic analysis of skeletal isomerization of linear butenes were performed over H-FER. The effect of butene partial pressure and the rate of formation of isobutene and major byproducts are reported. Results show zero order dependence of isobutene production and first order dependence of byproduct formation with butene pressure. Biphenyl type molecules are proposed as coke species and related to byproduct formation

An investigation of possible mechanisms for the water-gas shift reaction over a ZrO2-supported Pt catalyst

The present work investigates the reactivity of the surface species observable by in situ DRIFTS formed over a Pt/ZrO2 during the water\u2013gas shift (WGS) reaction. A DRIFTS cell/mass spectrometer system was operated at the chemical steady state during isotopic transients to yield information about the true nature (i.e., main reaction intermediate or spectators) of adsorbates. Only carbonyl and formate species were observed by DRIFTS under reaction conditions; the surface coverage of carbonate species was negligible. Isotopic transient kinetic analyses revealed that formates exchanged uniformly according to a first-order law, suggesting that most formates observed by DRIFTS were of the same reactivity. In addition, the time scale of the exchange of the reaction product CO2 was significantly shorter than that of the surface formates. Therefore, a formate route based on the formates as detected by DRIFTS can be ruled out as the main reaction pathway in the present case. The number of precursors of the reaction product CO2 was smaller than the number of surface Pt atoms, suggesting that carbonyl species or some \u201cinfraredinvisible\u201d complex (not excluding formates) at the Pt\u2013zirconia interface was the main reaction intermediate. A simple redox mechanism could also explain the present results