Lucien Bonnetain

Morphology and crystalline order in catalytic carbons

Abstract The influence of temperature and of catalyst composition on the morphology and crystalline order has been studied for a series of catalytic carbons prepared by carbon monoxide disproportionation (2CO→CO2+C) over an iron-cobalt, an iron-nickel, and a nickel catalyst, and by methane decomposition (CH4→2H2+C) over a nickel catalyst. The morphology was studied by TEM in a bright field mode. The products are filamentous when prepared at low temperature and granular when prepared at high temperature. The ranges of temperature corresponding to each morphology depend both on the alloy composition and on the reacting gas. The structure of the catalytic carbons, as studied by TEM in SAD mode and by X-ray diffraction, is turbostratic (2D, i.e. biperiodic) when the carbons are prepared at low temperature and crystalline (3D, i.e. triperiodic) when prepared at high temperature. When the particle size is large, the temperature of transition is about 500°C, whatever the composition of the alloy and of the reacting gas are. However, small particles are always 2D. The carbon layers, as observed by TEM in lattice fringe mode, are short and curved for turbostratic, and large and stiff for crystalline phase. Biperiodic products are not graphitizable.

Formation and characterization of catalytic carbons obtained from CO disproportionation over an iron nickel catalyst—I: Fragmentation and rates of carbon deposition

Abstract A series of catalytic carbons has been prepared by CO disproportionation (2CO→CO2 + C) over an iron nickel catalyst. The catalyst is under the form of filings (particle sizes ranging from 10 to 160 μm) of an iron nickel alloy (75 wt % nickel, 25 wt % iron). The dependence of the rate of carbon deposition on temperature, between 400 and 650°C, and on the CO partial pressure in the reacting CO CO 2 mixtures has been investigated. Since the specific area of the catalyst is initially very small, its fragmentation by carbon deposition is necessary to obtain an appreciable rate of reaction. Experimental evidence suggest that the existence of a high density of dislocations in the catalyst is a necessary condition for its fragmentation.