Hydrogen partial pressure gradient

Palladium and palladium-silver alloy membranes on porous alumina tubes were prepared by means of simultaneous and sequential electroless plating techniques [234], The membrane reactor was used for the direct formation of hydrogen peroxide by catalytic reaction of H2 and 02 at 293 K. The concentration of H202 increased with increases in the transmembrane partial pressure gradient of H2. A high concentration of H202 was obtained with a membrane that consisted of a palladium layer on the outer surface, porous alumina in the middle, and a palladium-silver alloy layer on the inside. 

The n-butane aromatization was performed, using an automatic flow unit, in a quartz microreactor (the charge was 0.5g). The reaction conditions were varied in a wide range temperatures of 573-773 K, space velocities of 300-10000 h, total pressure of 0.1-0.5 MPa, butane partial pressure in He stream of 0.5-100 kPa. Some runs were made in hydrogen atmosphere, the hydrogen partial pressure was varied from 5 to 400 kPa. To avoid the flow gradient caused by butane condensation the feed was injected as a liquid at 1.5 MPa into a mixer heated to 673 K. The use of separate flow lines for catalyst activation and for the feedstock make it possible to measure the conversion and product distribution immediately after feedstock injection. The probes for analysis were taken automatically every 3-5 min. Normally, stable activity was reached after 5-10 min and remained practically constant over 4 hours. 

For materials with mixed electronic and ionic conduction e.m.f. measurements on a concentration cell have often been used to separate the different contributions. In this method a concentration gradient is established across the sample, which is equipped with electrodes reversible to only one ion, e.g. platinum black electrodes and different hydrogen partial pressures. The potential difference is measured, preferably at several partial pressure differences. The transference number of the ion can then be deduced from the deviation from the Nemst law behaviour by use of the equation " ... 

Hydrogen permeation through palladium membranes occurs through a solution-diffusion mechanism. The main steps involved in hydrogen transport across the membrane, under a partial pressure gradient, are as follows (Uemiya,1999) ... 

At W/F = 0, which is equal to the initial rate (r0), the above derivative reduces to CD. The best values of C and D were obtained by a gradient search using a digital PDP-9 computer. Mezaki and Kittrell (6) have demonstrated that this expression is a convenient and relatively nondiscriminating way to extrapolate to zero conversion. The range of partial pressures (torr) of reactants used were hydrogen sulfide, 4-22, and sulfur dioxide, 2-22. 

The minus sign in front of the flux equations arises from the flux going downhill, that is, in the direction of negative gradient. The flux is here defined as going in the direction from Side I towards Side II and thus becomes positive if the partial pressure of hydrogen is larger at Side I than at Side II. 

Meanwhile, the Vqc can indicate the presence of electronic conduction and provide the ionic transfer number. By establishing a concentration cell, in this case hydrogen, the transportation of protons due to the gradient in partial pressure of hydrogen between the anode and the cathode causes a measured Vqc. Compared to the theoretical Voc calculated from Nemst equation, the proton transfer number wUl be revealed (Sutija, Norby, Bjombom, 1995). 

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