Hydrogen adsorption measurement methods

The electrochemical active surface area (EASA) of fuel cell Pt-based catalysts could be measured by the electrochemical hydrogen adsorption/desorption method. For carbon supported Pt, Pt alloy, and other noble metals catalysts, the real surface area can be measured by the cyclic voltammetry method [55-59], which is based on the formation of a hydrogen monolayer electrochemically adsorbed on the catalyst s surface. Generally, the electrode for measurement is prepared by dropping catalyst ink on the surface of smooth platinum or glassy carbon substrate (e.g, a glassy carbon disk electrode or platinum disk electrode), followed by drying to form a catalyst film on the substrate. The catalyst ink is composed of catalyst powder, adhesive material (e.g., Nafion solution), and solvent. 

Studies on the adsorption of hydrogen from the gas phase had provided strong evidence for the existenee of two forms of adsorbed hydrogen and the AC impedance studies were supported by the results of the new LSV and CV techniques. The early measurements using the voltammetry methods were hampered by the use of impure electrolytes which resulted in ill-defined hydrogen adsorption and desorption peaks but the realisation of the need for a clean electrochemical system soon resulted in the routine observation of the now familiar twin Hads peaks. 

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 ... 

B.E.T. method using nitrogen, since nitrogen is chemisorbed at — 196°C. The hydrogen adsorption at this temperature measures the surface more accurately and is in close agreement with the chemisorption of carbon monoxide at both liquid nitrogen and room temperature and with the van der Waal s adsorption of krypton. 

The signals in Figure 2 are assigned to Pd (I) ions located at two different sites, A and B. Their respective g values are site A, g = 2.41, gL = 2.11 and site B, g = 2.28, g = 2.10. ESR quantitative measurements showed that about 8% of the loaded palladium was in the form of Pd(I) after hydrogen adsorption. Subsequent to the ESR experiments, the hydrogen uptake by sample (A) was measured by a volumetric method. 

The best approach is normally an in situ determination based on voltammetry or charging curves, usually within the hydrogen adsorption region [96]. It is of course necessary to know the actual value of 0H for absolute determinations, but the method is practicable on a relative basis. The method becomes absolute only in a few cases, in particular for Pt electrodes [97] for which the catalytic activity per metal atom, which is the parameter really needed to evaluate electrocatalytic effects, can be calculated [98]. Sometimes, results are reported relative to the surface area measured on the basis of the limiting current for a redox reaction [99], but what is obtained is only the macroscopic surface in which asperities of a height higher than the diffusion layer thickness can only be accounted for. 

To measure hydrogen adsorption using the flow method a sample previously reduced and flushed free of hydrogen by an inert gas stream (usually Ar) is exposed to a constant flow of, for example, a H2/Ar mixture (typically containing 2% H2) and the quantity of hy-... 

Supports and Catalysts. The catalyst supports used in this work are described in Table I. The surface areas, except for the sillcalite, were measured by the multi-point BET method. The surface area for the sillcalite was obtained from the manufacturer. Silicalite is an essentially aluminum-free pentasil zeolite (14) manufactured by Union Carbide. The chlorine contents of the supports were determined by neutron activation analysis, and sulfur contents were obtained with a Leco sulfur analyser. Sulfur and chlorine contents were measured since these elements may influence subsequent hydrogen adsorption on the supported platinum catalysts (15). 

Alumina supported nickel, cobalt and iron catalysts containing 10, 30 and 50 wt % of metal were prepared by co-precipitation method at constant pH conditions (pH = 8 for Ni and Co and pH = 7 for Fe) as described earlier [7], Hydrogen and oxygen adsorption measurements were carried out over the catalysts using a conventional high vacuum system according to the procedure described elsewhere [7]. BET surface area was measured... 

Direct current plasma technique (DCP) was used to determine the metal content of the catalysts. The metal dispersions was measured by hydrogen adsorption at 298 K (363 K for Pd-catalyst). Extrapolation of the adsorption isotherms to zero pressure was applied for the determination of adsorbed hydrogen. The amount of reversibly adsorbed hydrogen was determined by back-sorption method. Dissociative adsorption of hydrogen was considered and the metal particle sizes were calculated assuming a spherical geometry. The mean metallic particle sizes were also investigated by transmission electron microscopy (TEM) and X-ray diffraction (XRD) techniques. 

In technical applications zeolite molecular sieves and catalysts are generally used under conditions of multicomponent diffusion. Selective diffusion measurements of the individual components are therefore of immediate practical relevance. In the conventional adsorption/desorption method such measurements are complicated, however, by the requirement of maintaining well-defined initial and boundary conditions for any of the components involved. Being applied at equilibrium, such difficulties do not exist for PFG NMR. The traditional way to perform such experiments is to use deuterated compounds or compounds without hydrogen, thereby leaving only one proton-containing component, which then yields the H NMR signal [163-165]. 

A Spanish group from the University of Alicante performed adsorption measurements on activated carbon materials at room temperature using a mixed gravimetric and volumetric method. The highest value of hydrogen adsorption they reported was close to 1 wt% at 10 MPa for an activated carbon obtained from anthracite and with a specific surface area of 1058 m g [29]. 

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