Catalyst Coating And Micro-reactor For Preferential CO Oxidation

As CO is very harmful to the Pt-based electrocatalysts in proton exchange membrane fuel cells, CO concentrations in the feed stream to the anode must be decreased to a fairly low level of less than 10 ppm. Preferential oxidation of carbon monoxide (PROX) is one of the most effective and promising methods. PROX is often conducted in a packed bed reactor; however, traditional PROX reactors with randomly packed catalytic beds display slow heat and mass transfer and are not able to fulfill the dynamic demands of fuel cell systems. In addition, the highly exothermic oxidation reaction may cause hot spots in the catalyst bed. To address these issues, several reactors have been developed, including straight-channel monolith reactors, microchannel reactors, and metal foam reactors. However, for most of these reactors, an outlet CO concentration of less than 10 ppm could only be obtained at high temperatures or with the use of large amount of precious metals. Thus, further investigations should be conducted to improve the catalytic activity and economy of PROX catalyst coatings and reactors. Based on the above discussion, the present work is focused on the design of a effective and stable catalyst coating and that of a new catalyst-coated channel plate reactor (CCPR). The main results are summarized as follows:(1) Efficient Pt-Co/?-Al2O3 catalyst coatingThis work has investigated the catalytic properties of Pt-Co/?-Al2O3 catalyst coatings. The most active catalyst coating, designated 1Pt2Co (1 wt% Pt and 2 wt% Co), could decrease CO concentration from 1% to a value of less than 10 ppm for GHSV values ranging from 40000 to 120000 ml g-1h-1. This catalyst coating can work at a wide window of operation in terms of temperature (120-150?corresponding to 10 ppm level outlet CO concentration), and they are very stable for PROX reaction. Combination property of the catalyst coating is better than that of the same kind published recently. The addition of Co improves catalytic activity significantly. This improvement is partly related to the formation of Pt3Co. The co-existence of Co metal and its oxides on the catalyst surface were revealed to be a consequence of the gradual oxidation of Co by the gas phase O2 during the initial stages of the PROX reaction. This synergetic effect of Co0 and Cox+ may be yet another reason for the superior activities of Pt-Co catalysts. The high accessibility of the reactant to Pt3Co species is favorable and crucial for PROX.(2) PtCoK/y-Al2O3 catalyst coatings In order to further improve the activity and selectivity of Pt-Co/?-Al2O3 catalyst coatings, the effect of K addition has been investigated. The activity test shows that an appropriate amount of K addition (K/Pt=1.5) enhances the performance of Pt-Co catalyst coatings greatly. Moreover, this catalyst coating has good resistance to H2O and CO2, and is very stable. Characteristic results show that, Pt3Co is clearly present on the surface of the catalyst coating, and the average size of metallic particles increased with an increase of K addition. Small amount addition of K (K/Pt=1.5,1.5K-PtCo) increases the Pt-Co bimetallic interface. As a result,1.5K-PtCo promotes the formation of the Pt3Co intermetallic compound in the catalyst coating preparation. The multiplication of Pt3Co intermetallic compound formation on the catalyst coating surface may be the reason why the K promoted catalyst coatings exhibits high PROX activity. However, when the K/Pt molar ratio increased to 3, CO conversion decreased. There seems to be two reasons for the decreased PROX activity. First, hydrogen spillover occurs. As is well known, hydrogen spillover can cause more hydrogen oxidation, leaving insufficient oxygen with CO. Second, strong interaction between Pt3Co and alumina support might reduce the PROX active site. When he K/Pt molar ratio increased to 5, Pt atoms and Co atoms are covered by K phase, and in consequence, CO and O2 can hardly be adsorbed on metallic particles. As a result, CO conversion decreased greatly.(3) Design and manufacture of the new type of CCPRA CCPR with a rectangular channel of 120mm×10mm×0.5mm (length×width×depth) was produced via conventional mechanical milling, and a 1Pt2Co catalyst coating was coated on the upper and lower walls of the reactor. The proposed reactor can operate at a wide range of temperatures (CO concentrations of 1-10 ppm at 140-170?) and can be used under realistic conditions (both CO2 and H2O can be present in the inlet). In addition, the reactor exhibited excellent tolerance to undesirable conditions, such as reaction temperature runaway and feeding stream control failures. The capacity per channel of the proposed CCPR was approximately 50-100 times greater than those of microchannel reactors; thus, problems associated with excessive reactors are significantly reduced. The proposed CCPR has great potential in small-scale hydrogen production for fuel cells.