Hydrogen adsorption/desorption method

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. 

Platinum was deposited on Nafion membrane by the method described earlier. Ibe cyclic voltammogram is shown in Fig. 2-i6 using 3 M H2SO4 as the electrolyte in the solution side of the membrane. The hydrogen adsorption—desorption features are not very well-defined probably because of some impurities. 

The cyclic voltammograms of RSn/C eletrocatalyts in the absence of ethanol are shown in Fig. 2. The PtSn/C eletrocatalysts prepared by ARP method do not have a well-defined hydrogen adsorption-desorption region (0-... 

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]. 

Electrochemical Active Surface Area (ESCA) of Pt-based catalysts through the hydrogen adsorption/desorption peak area, CO stripping, and underpotential deposition of copper (Cu-UPD) methods. 

Typically, two electrochemical based methods are used to estimate the ECASA. The first method relies on the hydrogen adsorption/desorption (Hads/des) charges [145-152]. In the earlier discussion, we have mentioned that Pozio [145] has used a mean value of Qh-UPD to estimate ECASA. If the amount of Pt used within the catalyst layer (wpt) is known, then ECASA can be calculated by using following equation ... 

The Cr203 content of each catalyst was determined by atomic absorption spectroscopy (Varian/Spectr AA-20 plus) on acid-digested samples. Total surface areas were determined by a single point BET method (nitrogen adsorption-desorption at 77.5 K) using a mixture of 29.7% N2 in helium. Samples were wet-loaded into the flow tube and dried at 423 K in a hydrogen flow for 15 minutes and then for another 30 minutes at 513 K before cooling in helium. 

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. 

Adsorption of acetic acid on Pt(lll) surface was studied the surface concentration data were correlated with voltammetric profiles of the Pt(lll) electrode in perchloric acid electrolyte containing 0.5 mM of CHoCOOH. It is concluded that acetic acid adsorption is associative and occurs without a significant charge transfer across the interface. Instead, the recorded currents are due to adsorption/desorption processes of hydrogen, processes which are much better resolved on Pt(lll) than on polycrystalline platinum. A classification of adsorption processes on catalytic electrodes and atmospheric methods of preparation of single crystal electrodes are discussed. 

In [119], the hydrogen adsorption and desorption reactions in thin palladium electrodes were studied using the potential step method in order to analyze the mechanism of phase transformation. Transient current responses were recorded at the onset of the potential step for 47 pm thick Pd electrodes in 1 mol dm H2SO4 at ambient temperature. A model based on a moving boundary mechanism was proposed to account for the experimental i-t curves. It was found that the hydrogen adsorption reaction shows interfacial kinetic limitations and only numerical solutions can be obtained. Such kinetic limitations were not found for the desorption reaction and a semianalytical solution that satisfactorily fits the experimental data was proposed. 

A related method involves the use of the tip reaction to perturb a reaction at a surface an example of this approach is SECM-induced desorption (SECMID) (22). For example, the adsorption/desorption kinetics of protons on a hydrous metal oxide surface can be studied in an unbuffered solution by bringing the tip near the surface and reducing proton (to hydrogen) at the tip. This causes a local change in pH that results in proton desorption from the surface. The tip current can be used to study the kinetics of proton desorption and diffusion on the surface (Chapter 12). 

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