Estimation of Average Crystallites Size of Active Phase in Ceria-Supported Cobalt-Based Catalysts by Hydrogen Chemisorption vs TEM and XRD Methods

Abstract

Cobalt catalysts with CeO 2 support, unpromoted and promoted with potassium were prepared by an impregnation method. Reduced catalysts were subjected to hydrogen chemisorption at different temperatures in the range of 313-453 K. Studies have shown that calculated Co crystallites size depends on the temperature of chemisorption. The average of size crystallites of the Co active phase obtained from the total and strong chemisorption data were compared with those from measurements by other techniques, TEM and XRD. The results of comparison allowed us to indicate the most suitable chemisorption temperature, different for unpromoted (383 K) and potassium-promoted (413 K) catalysts to determine the proper Co crystallites size of active phase, compatible with the crystallites size determined by the most objective method, i.e. by TEM measurements. In the case of small metal crystallites of active phase (4-5 nm) the divergence of their average size determined by hydrogen chemisorption and TEM methods and particularly by the XRD method is definitely higher than that in the case of larger crystallites (~12 nm).can significantly influence on obtained results. More than 40 years ago Bartholomew et al. [23][24][25] show that the chemisorption of hydrogen over cobalt catalysts with various supports (SiO 2 , Al 2 O 3 , C, MgO, ZSM-5, TiO 2 ) and over unsupported cobalt is activated and highly reversible. The activation was a function of the interaction of cobalt with a support or promoter. Frequently used in catalysis, potassium promoter significantly increased the adsorption activation energy for hydrogen [26]. It is also proved that some methods for determining the hydrogen uptake, i.e. flow adsorption methods such as thermal desorption and pulse methods, measure only irreversible (strong) chemisorption [23,27]. No hydrogen desorption was detected for MgOand ZSM-5-supported and low-loaded (1-3 wt.%) aluminasupported cobalt catalysts using TPD method, even though those catalysts adsorbed hydrogen when the static technique was applied [23,24]. The special case is the Co/TiO 2 system, where due to the strong metal-support interactions the stoichiometry of hydrogen chemisorption is lower than one hydrogen atom per surface cobalt atom, even in static measurements. With exception of the cobalt systems in which the strong metal-support interactions take place, the static technique was recommended for estimation of the average size of cobalt crystallites on the basis of maximal hydrogen chemisorption uptake, usually at 373 K. A good correlation with TEM measurements was found for the Co/SiO 2 , Co/γ-Al 2 O 3 and Co/C catalysts. The paper [9] also shows that the volume of chemisorbed hydrogen (including strong and weak chemisorption) on the CoRe/γ-Al 2 O 3 catalyst varies with the temperature of chemisorption, and the use of different data leads to the determination of different average size of crystallites, even though the actual crystallites size remains unchanged. They obtained the best results when the total chemisorption...

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Effect of metal precursor and pretreatment conditions on the catalytic activity of Ni/C in the aqueous phase hydrodechlorination of 1,1,2-trichloroethene

“…However, the general trend has been preserved. Based on the literature data [23], we could explain that the average size of nickel-based nanoparticles estimated from XRD data is congruous with that measured by the TEM method only in the case when there are no very small ones among the crystallites.…”

Surface State and Catalytic Performance of Ceria‐Supported Cobalt Catalysts in the Steam Reforming of Ethanol

“…The active metal surface area, dispersion and mean Co particle size of catalysts pre‐reduced in situ in H 2 at 420 °C (1 h) were calculated from H 2 chemisorption data obtained by using an ASAP 2020 (Micromeritics) analyser at 130 °C . A H/Co stoichiometry of 1:1 and a spherical shape of the Co particles were used to calculate the mean size.…”

“…A H/Co stoichiometry of 1:1 and a spherical shape of the Co particles were used to calculate the mean size. A detailed description of the calculation method has been given elsewhere …”