(274e) Contaminant Tolerant Catalysts for High Temperature Membrane Wgs Reaction
The water?gas shift (WGS) reaction using membrane reactors are attractive largely because hydrogen can be selectively permeated through a membrane, making complete conversion possible. In this manner, the broad consequences of complete conversion can be explored. The present investigation was aimed at developing new contaminant tolerant (H2S) high temperature catalysts for the WGS reaction that can operate in extreme conditions as posed in the membrane reactor regime. Of particular interest for a H2 selective membrane reactor is the effect of CO2, since the continuous H2 removal through the membrane will create a CO2-rich condition over the shift catalyst. Therefore, it is important to evaluate the influence CO2 on the catalyst activity and stability.
In this work, metal (M)-doped ferrite catalysts using either the first-row transition atoms (M = Cr, Co, Ni, Cu), the non-transition atom (M = Zn), or the inner-transition atom (M = Ce) series were considered. These catalysts were synthesized and their performance for the WGS reaction evaluated in the presence of 100 ppm H2S. Metal dopant additives (M) influence the physicochemical, the structural properties and catalytic performance. The experiments were performed in a temperature range of 400?500 °C. A relatively high space velocity of 60,000 h-1 was maintained in the WGS reactor. The feed gas comprised of a regulated stream of steam and CO (Steam-to-CO = 3.5) along with 100 ppm H2S. The shift activity was found to increase with increasing temperature. Interestingly, at high temperatures, the activity approached the equilibrium conversion value over Fe/Cr and Fe/Ce catalysts. Among the various catalysts studied, Fe/Ce is found to be an ideal catalyst for operating in H2S-rich conditions at high temperatures. Iron?ceria-based WGS catalysts are promising, because the oxygen storage capacity (OSC) of ceria and the cooperative effect of Ce?Fe leads to active sites. Interestingly, both iron and ceria undergo facile charge-transfer reactions between FeIII ↔ FeII and CeIV ↔ CeIII redox couples, respectively; the synergism between the two couples could be responsible for the improved WGS activity. In addition, the increased WGS activity with increase in temperature could be due to the improved OSC of ceria. The information obtained from various techniques, including X-ray diffraction (XRD), temperature programmed reduction (TPR), pore size distribution (PSD), Raman, X-ray Photoelectron, Mössbauer spectroscopy, and transmission electron microscopy (TEM), is used to fully characterize the crystalline structure and morphology of the catalysts and their WGS activity. In another set of WGS experiments, iron-chrome and iron-cerium oxide catalysts were evaluated in CO2 rich stream conditions. All these interesting findings will be presented in detail during the course of presentation.