Identification of the rate determining step in aging of supported metals

Pulvermacher, B.

doi:10.1016/0021-9517(74)90189-4

Cited by 44 publications

(2 citation statements)

References 25 publications

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“…The TEM image in Figure B shows clearly that the particle size increased significantly up to 5−9 nm. Obviously, larger iron nanoparticles were formed as a result of the coalescence of neighboring particles . The sintering is considered a beneficial effect since iron nanoparticles as small as 1 nm in diameter can hardly catalyze the growth of nanofibers.…”

Section: Resultsmentioning

confidence: 99%

Chemical Vapor Deposition and Synthesis on Carbon Nanofibers: Sintering of Ferrocene-Derived Supported Iron Nanoparticles and the Catalytic Growth of Secondary Carbon Nanofibers

Xia

Birkner

et al. 2005

Chem. Mater.

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The synthesis of homogeneously distributed carbon nanofibers with well-defined morphology on vapor-grown carbon nanofibers was achieved by a sequence of gas-phase steps, thus fully avoiding wet chemistry: first, carbon nanofibers were exposed to oxygen plasma to introduce oxygen-containing functional groups. Then, the chemical vapor deposition of ferrocene was carried out under oxidizing conditions, yielding nanofiber-supported iron oxide nanoparticles.Secondary carbon nanofibers with diameters in the range from 10 to 20 nm were subsequently grown from cyclohexane catalyzed by the sintered metallic iron nanoparticles under reducing conditions. XPS was applied to monitor the chemical changes of the surface composition and the sintering of the metallic iron particles in hydrogen. The morphology and the height distribution of the sintered iron oxide nanoparticles was derived by a unique combined application of scanning electron microscopy and scanning tunneling microscopy. The specific surface area of the nanocomposite was enhanced strongly due to the growth of secondary nanofibers, and it was possible to tune the morphology of the nanofiber-nanofiber composites by the process parameters. Thus, a significant advance in the reproducible synthesis of branched carbon fiber nanocomposites was achieved.

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Section: Resultsmentioning

confidence: 99%

Chemical Vapor Deposition and Synthesis on Carbon Nanofibers: Sintering of Ferrocene-Derived Supported Iron Nanoparticles and the Catalytic Growth of Secondary Carbon Nanofibers

Xia

Birkner

et al. 2005

Chem. Mater.

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“…Several methods are available for the measurement: electron microscopy, X-ray techniques including diffraction and scattering, and gas chemisorption. A review on surface area measurements by Farrauto (1974) gives details on the chemisorption method, and the article by Pulvermacher and Ruckenstein (1974) provides details on the other methods including magnetic measurements. Of these methods, gas chemisorption is perhaps the most accurate and certainly the easiest to implement, e.g., Hz on Pt (Spenadel and Boudart, 1960;Adler and Kearney, 1960) and on Ni (Taylor, Yates and Sinfelt, 1964), CO on Pd (Scholten and Montfoort, 1962), and NO on oxides of Cu, N, and Fe (Gandhi and Shelef, 1973).…”

Section: Introductionmentioning

confidence: 99%

Determination of catalyst surface area from desorption characteristics of physisorbed gases

Miller

Lee

1984

AIChE Journal

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A new experimental method for the measurement of catalyst surface area of supported catalysts has been developed using selective physisorption. The desorption characteristics of a gas are studied separately on the catalyst, the support, and the supported catalyst by carrying out thermal desorption experiments in a continuous flow sorptometer. Differences in the coverage vs. temperature curves, obtained from the thermal desorption experiments, are a measure of the selectivity of the physisorbing gas, and allow calculation of the fraction of total surface area occupied by the catalyst.Two systems have been studied utilizing the thermal desorption with carbon dioxide as adsorbate: potassium carbonate/carbon black and silver/alumina. Supported catalyst surface area was determined for each system; the results were confirmed using physical mixtures of the two components (where the actual area of each component is known) and oxygen chemisorption for the silver/alumina system. The experimental technique allows for straightforward calculation of the catalyst area. D. J. MILLER and H. H. LEE Department of Chemical EngineeringUniversity of Florida Gainesville, FL 3261 1The exposed surface area of a catalyst on a support is an important parameter in the characterization of catalytic behavior, particularly in sintering and dispersion studies. Several experimental methods of determining catalyst area have been developed, including chemisorption, electron microscopy, and X-ray techniques. The difficulties involved in applying electron microscopy to actual catalysts and the sophistication of small angle X-ray scattering prevent their practical application in many cases. Chemisorption techniques have met with success in several catalyst systems, but the chemisorption techniques are limited to well-defined metal catalysts and are not in general applicable to nonmetallic catalysts. This paper presents a new method for determining catalyst area using physical adsorption. The method examines the adsorption characteristics of a gas separately on the catalyst (in powder or gauze form), the support, and the supported catalyst using a thermal desorption technique for the determination of the total exposed catalyst area as a fraction of the total catalyst and support area. This method has several advantages over other methods used in catalyst area measurements:1) The physisorption experiments are easy to carry out and the equipment needed is low cost. The physisorption experiments show good reproducibility, and sample preparation is the same as in conventional physisorption.2) The method can be applied to any catalyst system with the appropriate choice of adsorbing gas, since gases physisorb on all surfaces at low temperatures. This is in contrast to chemisorption, where the specific gas-solid interactions limit application to a few systems.3) The total catalyst area is determined directly from the experiments. Unlike chemisorption, there is no need to know the adsorption stoichiometry to determine the total area. CONCLUSIONS AND SIGNIFIC...

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Two kinds of self‐preserving size spectra of a cloud of particles

Pulvermacher¹,

Ruckenstein²

1975

AIChE Journal

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where V, i and A,, are the van der Waals volumes and areas of the molecule given by Bondi ( 1968) and where V, , and A,, are the van der Waals volume and area of a standard segment. The choice of a standard segment is somewhat arbitrary. Here it is defined as a sphere such that for a linear polymethylene molecule of infinite length the identity (z/2)(r -9 ) = r -1 (B3) is satisfied. The coordination number z is set equal to 10. The volume of the standard sphere in terms of its radius Rws is given by V, , = 4/3 x Rws3 (B4) and the area by A,, = 4 n Rws2 (B5) The van der Waals volume and area of an n-mer of polymethylene are n times the volume and area of a methylene group as given by Bondi; that is The population balances describing the time dependence of the size distibution can, under some conditions, be transformed by means of a similarity transformation into an ordinary integro-diff erential equation containing two instead of three variables, If there is compatibility between the transformed equation and the constraints given by the total mass conservation equation and the equation for the total number of particles, a self-preserving spectrum of the first kind can be obtained. There are, however, many situations such as the sintering controlled aging of supported metal catalysts, coagulation of colloidal particles in laminar shear flow, and coagulation of colloidal particles in a turbulent flow when the particles are smaller than the size of the smallest eddy for which, although a similarity transformation is possible, the transformed equation has no solution because of incompatibility with the above mentioned constraints. A second kind of self-preserving spectrum is suggested for these situations. The new variables are induced from a particular case for which an analytical result is available.A detailed presentation of the sintering controlled aging of supported metal catalysts is presented.

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Identification of the rate determining step in aging of supported metals

Cited by 44 publications

References 25 publications

Chemical Vapor Deposition and Synthesis on Carbon Nanofibers: Sintering of Ferrocene-Derived Supported Iron Nanoparticles and the Catalytic Growth of Secondary Carbon Nanofibers

Chemical Vapor Deposition and Synthesis on Carbon Nanofibers: Sintering of Ferrocene-Derived Supported Iron Nanoparticles and the Catalytic Growth of Secondary Carbon Nanofibers

Determination of catalyst surface area from desorption characteristics of physisorbed gases

Two kinds of self‐preserving size spectra of a cloud of particles

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