Catalytic zeolite membranes for CO clean-up

Hydrogen purification is a critical technology for power generation by PEM-FCs (proton exchange membrane fuel cells). In order to avoid cell performance degradation, carbon monoxide concentration must be kept below 10 ppm in the hydrogen feed streams [1]. CO selective oxidation (CO Selox) is the primary method used for hydrogen deep purification [2]. A catalyst active for CO oxidation in the presence of high concentrations of H2, CO2, steam, is required to avoid H2 consumption. Interesting new engineering devices such as catalytic MRs were investigated in a few papers for CO Selox [3, 4]. The catalytic membrane provides active and nano-sized particles in a thin zeolite layer (a few microns thick), reducing by-passing and misdistribution generally shown in a packed bed. The aim of this research is to investigate the role of the catalytic membrane (Pt-loaded zeolite) in a MR as an effective device for CO Selox. The CO removal performance of catalytic membranes having different permeation properties and activated at two different temperatures has been studied and their stability has been also investigated. Experimental Tubular zeolite membranes having different permeation properties were prepared by using the innovative secondary growth method [5,6] on ceramic supports. Catalytic active Pt/Na-Y membranes were obtained from Na-Y membranes by means of ion-exchange followed by calcination and reduction carried out at two temperatures. The MR was operated in a continuous flow-through mode (Fig. 1) with only one entrance (the feed) and only one exit on the other membrane side (the permeate). The retentate was closed and no sweep gas was employed. This system is not designed for a separation, but to operate as a catalytic "contactor". Fig. 1. MR continuous flow-through mode. The H2 (50-60 %) and CO (~ 1 %) concentrations in the Feed mixtures are in the range of a typical reformate gas stream after its upgrading by means of high and low-temperature water gas shift stages. The reaction tests have been carried out at ca. 200°C (usual value for the low-temperature water gas shift reactor). A low O2/CO feed molar ratio (1) has been used. Results The catalytic MRs succeeded in reducing the outlet CO concentration from 10,000 ppm (1%) down to 10-50 ppm. Among the different prepared membranes, the best CO removal was obtained using a low-permeance catalytic membrane also at low pressure [7]. For the more permeable membranes the increase in operating pressure (up to 550 kPa) was effective in improving the MR CO conversion and CO2 yield. Also an increase in the oxygen excess (?=3) had a positive impact on the CO conversion but, at the same time, the selectivity towards CO2 decreased. Comparing the different membranes, it was observed that the reduction temperature has significant effects on the catalytic activities: the activity was found to improve when a lower reduction temperature was used producing smaller Pt catalytic particles. Finally, the CO2 presence in the feed (15%) slightly reduced the MR performance. Conclusions CO Selox in the presence of large quantities of hydrogen was successfully carried out in continuous flow-through MRs using different Pt-loaded catalytic zeolite membranes. The catalytic MRs succeeded in reducing the outlet CO concentration from 10,000 ppm (1%) down to 10-50 ppm. The results here presented confirm the good potentiality of catalytic membranes for a deep purification of H2-rich streams, allowing the hydrogen final use, e.g., in fuel cell applications.