Henri Charcosset

Platinum-rhenium-alumina catalysts: III. Catalytic properties

Abstract The activity of PtRe catalysts on α- or γ-alumina has been determined for various reactions. In no case is the activity of bimetallic catalysts the sum of the activity of platinum and rhenium of which the catalyst is composed. Curves of activity as a function of the composition of the catalyst exhibit one or two maxima in benzene hydrogenation, benzene-deuterium exchange, cyclopentane, and butane hydrogenolysis, those maxima being more or less pronounced according to the reaction and the support. Conversely, the rate of 1,1,3-trimethyl cyclohexane dehydrogenation decreases when the percentage of rhenium is increased. A pronounced analogy has been established between, on the one hand, the changes of activity with bimetallic composition, and, on the other hand, with the position of the metal in the Periodic Table for the monometallic period Pt, Ir, Os, Re deposited on alumina.

Platinum-rhenium-alumina catalysts: II. Study of the metallic phase after reduction

Abstract Two percent ( Pt + Re ) Al 2 O 3 catalysts prepared by co-impregnation of Al2O3 with H2PtCl6 and Re2O7 and generally reduced by hydrogen at 500 °C were mainly investigated by thermogravimetry (H2O2 cycles at 25 °C), electron microscopy, and infrared spectroscopy (chemisorption of CO at 25 °C). The greater reducibility in the presence of Pt of oxygen chemisorbed by Re, and the vCO frequency shifts of the PtCO and ReCO species against the percentage of Re indicated a strong interaction between both metals. Most probably Pt and Re are alloyed. The H2O2 cycles at 25 °C allowed the determination of the overall dispersion of the (Pt + Re) phase while the optical density of the infrared band of CO adsorbed on platinum gave some information about the surface concentration of Pt in the (Pt + Re) phase.

Platinum-rhenium/alumina catalysts: I. Investigation of reduction by hydrogen

Abstract Pure Re2O7 when heated in a hydrogen atmosphere, sublimes before reduction. However, when Re2O7 is mixed with Pt or Pd or Re powder, instead of volatilization complete reduction to metallic Re occurs below 200 °C. A strong activation by Pt of the reduction of Re2O7 also takes place on Al2O3 samples coimpregnated with H2PtCl6 and Re2O7. With such catalysts, the reduction of Re2O7 to metallic Re under 1 atm pressure of hydrogen seems to be complete at a rather low temperature.

Catalysis of coal hydroliquefaction by synthetic iron catalysts

Abstract The following synthetic iron catalyst precursors were investigated: FeOOH and FeOOH-Al 2 O 3 (90:10 wt%) co-precipitated by ammonia, washed and dried either in an oven or by spray-drying, and Fe 2 O 3 prepared by flame decomposition of FeCl 3 in the gas phase. These catalyst precursors were sulphided in situ by CS 2 during the hydroliquefaction of a highly volatile bituminous coal. An increasing catalytic activity of the iron sulphide was observed as its particle size decreases down to a very low value (0.05 μm), compared to 2–3 μm and to ⪢ 5 μm. Al 2 O 3 did not act as an efficient promoter, even if it gives rise to a decrease of the iron sulphide crystallite size. Diffusional limitations and/or plugging by carbonaceous or mineral solids could result in a low efficiency of the iron sulphide crystallites which lie inside one iron catalyst particle. The above cited flame method, allowing the preparation of pure or doped Fe 2 O 3 with particle size even less than 0.05 μm, is worthwhile for further work in coal hydroliquefaction catalysis, where the catalyst acts as Fe 1−x S.

Study of the formation and removal of carbonaceous deposits on platinum - ruthenium alumina supported bimetallic catalysts

Abstract Alumina supported mono and bimetallic catalysts based on platinum and ruthenium were studied for the conversion of n-heptane at 400 and 500°C. The combustion of the 0.1 to 0.3 wt % of resulting carbonaceous deposits was followed by means of thermal programmed oxidation (TPO), that is by simultaneous quadrupole analysis of the oxygen consumption and the carbon dioxide evolution. No significant variation of the molecular ratio of oxygen consumed to carbon dioxide evolved was observed, either with the temperature during a single TPO experiment or with the ruthenium to platinum atomic ratio in the catalyst. On the other hand, the lowest temperature necessary to complete the combustion of the carbonaceous deposits was found to be much smaller for the ruthenium monometallic catalyst than for the platinum mono or platinum - ruthenium bimetallic catalysts.

Increase of reducibility of NiO by H2, due to pretreatment with salt solutions

The impregnation of a high temperature preparation of pure NiO with various salt solutions, followed by drying and extraction with water, affects the reducibility of NiO differently according to the nature of the cation. Whereas no positive effect is observed with Li, Cs, Zn, Cd, Hg, Al, La, Fe, or Co, the rate of reduction in dry hydrogen becomes about twice as much that of the starting NiO for Na, K, Rb, Mg, Sr, Ba, Au, and Ni. This is ascribed to the formation of some disordered superficial areas on NiO. More pronounced effects occur with Cu, Pt, Pd, Ru, Rh, Os, Ir, and are related to a cationic exchange with surface Ni2+ during impregnation. The effect of most Group VIII a elements changes with the percentage of d character of the metallic bond as does the specific catalytic activity in some particular reactions. A large difference of the thermal stability of artificial nucleation areas formed during the considered treatments is evidenced. From this point of view Pt is much more favorable than Os. Finally, the fraction of platinum which is extractable by water after impregnation of NiO with H2PtCl6 or Pt(NH3)2(NO2)2 gives rise to a metallic phase which increases the rate of reduction only slightly.

Dibenzothiophene hydrodesulfurization by noble metal supported catalysts

Abstract The hydrodesulfurization of dibenzothiophene (DBT) is investigated at 240°C, under one atmosphere pressure of H 2 + DBT, in a DBT conversion range giving rise to a H 2 S concentration between 30 and 200 ppm. The turnover number (TN) of platinum slightly decreases as the dispersion of platinum (Ptins) to Pt ratio = D (Pt)) increases from 0.01 to 0.7, whatever support (α-Al 2 0 3 , SiO 2 , chlorinated γ-Al 2 O 3 , SiO 2 -Al 2 O 3 , montmorillonite) was used. Furthermore, the TN is much lower for highly dispersed Pt/γ-Al 2 O 3 -C1 (D(Pt) = 0.9; o (Pt) = 1.0 nm). Pt/TiO 2 (D(Pt) = 0.06 or D(Pt) = 0.34) reduced or re-reduced at 240°C has a higher TN than the preceding catalysts. Reduction or re-reduction of Pt/TiO 2 , at 500°C results in only a relatively small decrease in the catalytic activity, in spite of the appearance of a strong metal-support interaction observed by several workers. For different supported metals on Al 2 O 3 , the TN follows the sequence: Pt > Ir > Ru > Re. Alloying Ir or Ru (25 to 30 at %) with Platinum results in a relatively small decrease in the TN (per one (Pt s + Ir s or Ru s ) atom). The activity, per unit weight of catalyst, of an industrial Co-Mo/γ-Al 2 O 3 sulfided catalyst (HR 306) is equivalent to that of 1 % Pt/γ-Al 3 0 3 (D(Pt) = 0.7). The present platinum supported catalysts have a very low selectivity in hydrogenation (production of phenylcyclohexane, PCH 2 -Al 2 O 3 or montmorillonite; the Co-Mo/γ-Al 2 0 3 catalyst has a selectivity which is intermediate between Pt/γ-Al 2 O 3 and Pt/SiO 2 -Al 2 O 3 or montmorillonite. The deactivation (decrease in activity versus the time en-stream) is lower for Co-Moγ-Al 3 0 3 than for Pt/γ-Al 3 O 3 , but the addition of Ge to Pt/γ-Al 2 O 3 gives rise to a more stable catalyst.

Hydrogen titration, scanning electron microscopy, and associated X-ray emission studies of PtRuAl2O3 catalysts

Abstract Two series of platinum-ruthenium catalysts deposited on γ-Al 2 O 3 pellets and equivalent to 2 wt% Pt in number of (Pt + Ru) atoms were investigated. The first series (I) was prepared by successive impregnation of H 2 RuCl 6 and of H 2 PtCl 6 in water, and the second (II) by coimpregnation of H 2 PtCl 6 and of H 2 RuCl 6 in ethanol (75%) plus water. The chemisorption (O 2 , H 2 ) and titration (mainly hydrogen temperature-programmed titration of chemisorbed oxygen) data showed catalysts (I) to be composed of mixtures of Pt and Ru particles, and catalysts (II) to be composed of alloy particles. This conclusion was on the whole corroborated by single particle X-ray emission analysis using X-ray emission in a scanning transmission electron microscope. This last technique further showed that the platinum particles in catalysts (I) contained a small amount of Ru. The reasons for the difference of state of the metallic particles in catalysts (I) and (II) are not totally elucidated but a tentative interpretation is given.

Increase of the rate of reduction of NiO by H2, due to pretreatment with CO or NH3

Abstract When the reduction of pure NiO is initiated with CO or NH 3 at a sufficiently high temperature, an increase of the subsequent rate of reduction by H 2 in standard conditions is observed. This effect is larger than the increase of reducibility due to the addition of platinum. It arises from the formation of an increased number of nickel nuclei when NiO is treated with CO or NH 3 . The pretreatment with NH 3 is preferably performed below 360 °C, the Curie point of nickel. The rate of growth of the nickel nuclei in H 2 at 200 °C is unchanged after a pretreatment with NH 3 or CO unless the latter is performed at 300–320 °C (formation of Ni 3 C) or above 390 °C (formation of a rather large amount of free C).

Conversion of tetralin over sulphided iron catalysts, in connection with coal liquefaction

Abstract The gas phase dehydrogenation of tetralin under atmospheric pressure was studied at 660 K using reduced, reduced and sulphided or directly sulphided iron oxide catalysts. It was found that, in tetralin conversion, metallic iron - possesses activity for dehydrogenation and hydrogenolysis simultaneously, - is strongly poisoned, even at very low S/Fe ratios, - is sulphided, more slowly than the initial oxide, to a S/Fe ratio near unity, where it shows good intrinsic activity and its selectivity is oriented to the dehydrogenation reaction. Directly sulphided iron oxides have a higher specific catalytic activity with the same (100%) selectivity for dehydrogenation. The tetralin dehydrogenation activity of the catalyst depends strongly on the stoichiometry of the Fe1-xS sulphide. These results are in agreement with the fact that the iron-based catalysts used in the liquefaction of coal are more active if they are not pre-reduced and used with the addition of a sulphiding agent, particularly in the case of sulphur poor coals.