PROX catalysts

Present catalysts are developed for process plant service where transient conditions are not a concern. Typical shift catalysts, such as copper-zinc oxide, are reduced in place and must be isolated from air. There is a need for smaller, high surface area catalyst beads on low-density monolith substrate to be developed without reducing activity. This need applies to all fuel processor catalyst, not just the shift catalysts. There is also a need to demonstrate that the low-temperature, PROX catalysts have high selectivity toward CO and long term stability under operating conditions. 

Tanaka et al. have recently reported low-temperature active PROX catalysts consisting of FeOx/Pt/Ti02 [53]. They studied the mechanism of selective CO... 

The Pd/Al203 system and Pd/PrOx systems were studied in more detail than the others and kinetic and thermodynamic results suggest that the Pd/PrOx catalysts have a much higher density of specific sites (about 1.6 times) for aromatization than on the alumina support. 

Figure 7.5. Schematic illustration of CO and H2 light-off on Prox catalyst.

A patent was granted to Shore et al.42 for a process utilizing a Cu/Ce02 catalyst that also contained Pt. It was found that the precious metal lowered the temperature necessary for the reduction of the base metal from its inactive oxide to the active metal form. The operating temperature is 100°C, compared with the minimum of 140°C required for the reduction of Pt-free copper/ceria in the reformate. One limitation of the copper Prox catalyst is that it is CO inhibited CO shifts the reduction temperature of the catalyst. 

The development of catalyst formulations for Prox is continuing. The discussion just presented should serve as a very good starting point when researching Prox catalyst formulation development, but is not an end. Especially with the rapid development of nanocatalysts, there will likely be future developments for some time. 

Shore, L. and Farrauto, R.J. Preferential oxidation of CO in H2 streams. In PROX Catalysts, Handbook of Fuel Cells Fundamental Technology and Applications (ed. Vielstich, W., Lamm, A., and Gasteiger, H.A.), Vol. 3, Part 2. New York John Wiley and Sons, p. 211, 2003. 

However, Pt catalysts are more stable than An catalysts. In general, nano-Au catalysts are known to undergo rapid deactivation during CO oxidation [18,188,196,283,284,305,309,315,349]. Extensive efforts have been undertaken to study the deactivation mechanism [18,283,349-353] STM/STS studies on model Au catalysts have shown that the deactivation is induced by oxygen. So, in order to employ Au as PROX catalysts, it is essential to S3mthesise nano-Au catalysts with high stability. 

Catalyst development and catalyst/ microreactor integration. Several ATR, WGS, and PrOx catalysts have been prepared. The ATR catalysts include beta-alumina supported nickel. These catalysts are being evaluated. Transition metal carbide and oxide supported gold catalysts were demonstrated to be highly active for the WGS... 

Decrease of 20% to 90% observed in autothermal reforming (ATR) and preferential oxidation (PrOx) catalysts, including loss in activity and CO conversion in PrOx... 

After extended operation, the catalysts present in the fuel processor were characterized to observe any potential degradation of the catalysts. The relative catalyst surface areas of the ATR, HTS, LTS and PrOx catalysts all decreased after extended operation. The initial portion of the ATR catalyst, where the fuel oxidation occurs, showed a large decrease in surface area, over an order of magnitude decrease from about 3 m /g to <0.2 m /g (note that the surface area is low because the support material is included in the measurement). Other portions of the ATR catalyst did not show as big a decrease in surface area. The LTS catalyst surface area decreased about 50%, which appears to be independent of the catalyst location in the LTS section. The measured PrOx catalyst surface area, shown in Figure 5, decreased as a function of the axial location in the reactor catalyst volume. The measured PrOx catalyst surface area shows a high decrease in the upstream section, while in downstream sections of the PrOx, approximately 75% of the original surface area is maintained. 

Figure 5. PrOx Catalyst Surface Area after Total On-Stream Time of -1200 hrs of Operation...

Shore L. and Farrauto R.J. 2003. PROX catalysts. In, Handbook of Fuel Cells-Fundamentals, Technology and Applications, Vielstich W., Lamm A. and Gasteiger A. (Eds.), Vol. 3, Wiley, Chichester, pp. 211-218. 

PrOx Study with Grooved Stainless-steel Foils and Au-based Catalysts [38] Another investigation to study PrOx catalyst activity and to examine heat transfer properties was performed by the Institute for Micro Process Engineering (IMVT) [38] using microreactors with grooved stainless-steel foils (Figures 27.8 and 27.9). The... 

Preferential oxidation (PROX) catalysts were prepared by washcoating of alumina suspensions with PVA and acetic acid [61]. The stainless-steel reactor contained heat exchangers at the inlet and outlet and the stacks were sealed with graphite seals. Temperature was measured with a thermocouple at the reactor outlet. Noble metal-based catalysts supported on alumina were tested. The influence of the temperature, space velocity and O2/CO ratio and the effect of water were studied. The CO content could be reduced from ca 1% to 10 ppm. The authors recommended a dual-stage PROX reactor in order to minimize the hydrogen loss. 

In this chapter, we highlight the development of gold catalysts for WGS and PROX. There are many relevant publications dealing with the development of WGS and PROX catalysts for these reactions, optimization of catalysts, nature of active sites, reaction mechanisms, and deactivation mechanisms [15-19]. The chapter is not intended to be comprehensive. Rather, examples on the development of new gold catalysts for WGS and PROX will be highlighted and some thoughts on the insufficiencies of the current research as well as some perspectives on future research will be furnished. 

The additives for PROX catalysts mentioned earlier are all minority species. Sometimes, mixed oxides with the same content of two oxides were used as supports for making gold catalysts. Examples include Au/MnO -CeO, [129-131], Au/FeO -CeO ... 

There is also a need to demonstrate that the low-temperature, PROX catalysts have high selectivity toward CO and long term stability. 

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