Catalysts hydrogen adsorptivity

Figure 2. Hydrogen adsorption and desorption Isotherms for rhodium catalysts. Solid lines denote total adsorption and dashed lines denote reversible adsorption. The meaning of symbols Is as follows ...

Figures 3 and 5 give examples of hydrogen TPD profiles observed on both catalysts reduced at 473K. The more intense patterns concern the desorption of the hydrogen fixed on the solids during the reduction step (curve a) while the weaker peaks (curve b) are related to the hydrogen retained after readsorption at room temperature. Hydrogen adsorption is thus an activated process.

The amoimt and strength of hydrogen adsorption changed with the adsorption temperature for all catalysts examined. As the adsorption temperature was raised, the amount and strength of adsorption increased. 

Another factor producing an apparent decrease in the activity of disperse catalysts is steric hindrance. In reactions involving relatively large molecules, not aU of the inner surface area of the catalyst may by accessible for these molecules, so that the true working surface area is smaller than that measured by BET or hydrogen adsorption. 

Diemant T, Hager T, Hosier HE, Rauscher H, Behm RJ. 2003. Hydrogen adsorption and coadsorption with CO on well-defined himetallic PtRu surfaces—A model study on the CO tolerance of himetallic PtRu anode catalysts in low temperature polymer electrolyte fuel cells. Surf Sci 541 137. 

Shimizu H, Christmann K, Ertl G. 1980. Model studies on bimetallic Cu/Ru catalysts II. Adsorption of hydrogen. J Catal 61 412. 

Evidence for a marked difference between the surface and bulk compositions of dilute copper-nickel alloys has been reported recently by a number of investigators (82, 87-90). Much of the experimental evidence comes from hydrogen adsorption data (74, 82, 87, 90). The conclusions of van der Plank and Sachtler were based on the premise that nickel chemisorbs hydrogen while copper does not (82, 87). The total adsorption of hydrogen at room temperature was taken as a measure of the amount of nickel in the surface. However, in hydrogen adsorption studies on the catalysts used to obtain the catalytic results in Fig. 6, the amount of adsorption on the copper catalyst, while small compared to the adsorption on nickel, is not negligible (74) However, the amount of strongly adsorbed... 

Hou et al. [73] considered small "carbon islands" as the main hydrogen-adsorption sites in an MWNT. The hydrogen-storage capacity of a CNT varies widely and the reason for such a variation is not clear, possibly caused by the impurity such as metal catalysts or amorphous carbon. It is not clear yet how the metallic catalyst particles, which are used during the preparation of nanotube samples, affect the hydrogen-storage capacity of nanotubes. 

Lee, Y.-W., R. Deshpande, A.C. Dillon, M.J. Heben, H. Dai, B.M. Clemens, The role of metal catalyst in near ambient hydrogen adsorption on multi-walled nanotubes. Mat. Res. Soc. Proc. 2004. 

Hydrogen uptake of reduced catalysts (X) was measured by volumetric method with an AUTOSORB-l-C analyzer (Quantachrome Instruments). Hydrogen adsorption was carried out at 373 K after in situ H2 reduction at 773 K for 6 h in the adsorption cell. The dispersion and particle size of metallic Co were calculated by the following equations, assuming that the stoichiometry for hydrogen adsorption on the metallic site is unity ... 

---