CO conversions

The reaction between carbon monoxide and hydrogen is exothermic (Ai/gQQp. = —100.5 kJ or 24.0 kcal) and plants must be designed to remove heat efficiently. In order to control the exotherm, CO conversions are typically maintained well below the equiUbrium conversion, 45% at 523 K. This necessitates a substantial recycle of carbon monoxide and hydrogen. 

CO conversion is a function of both temperature and catalyst volume, and increases rapidly beginning at just under 100°C until it reaches a plateau at about 150°C. But, unlike NO catalysts, above 150°C there is Htde benefit to further increasing the temperature (44). Above 150°C, the CO conversion is controUed by the bulk phase gas mass transfer of CO to the honeycomb surface. That is, the catalyst is highly active, and its intrinsic CO removal rate is exceedingly greater than the actual gas transport rate (21). When the activity falls to such an extent that the conversion is no longer controUed by gas mass transfer, a decline of CO conversion occurs, and a suitable regeneration technique is needed (21). 

It has been reported that below about 370°C, sulfur oxides reversibly inhibit CO conversion activity. This inhibition is greater at lower temperatures. CO conversion activity returns to normal shortly after removal of the sulfur from the exhaust (44). Above about 315°C, sulfur oxides react with the high surface area oxides to disperse the precious-metal catalytic agents and irreversibly poison CO conversion activity. 

Catalyst contamination from sources such as turbine lubricant and boiler feed water additives is usuaUy much more severe than deactivation by sulfur compounds in the turbine exhaust. Catalyst formulation can be adjusted to improve poison tolerance, but no catalyst is immune to a contaminant that coats its surface and prevents access of CO to the active sites. Between 1986 and 1990 over 25 commercial CO oxidation catalyst systems operated on gas turbine cogeneration systems, meeting both CO conversion (40 to 90%) and pressure drop requirements. 

CO conversions over Au/Ce02 catalyst were measured in the dry and wet condition as shown in Fig. 1. Similar to other supported gold catalysts, Au/Ce02 catalyst showed higher CO conversions in the presence of water vapor than in the absence of it at the same temperature. Catalytic activities for CO oxidation over Au/Ce02 catalysts prepared at different calcinations temperature were compared in the dry and wet condition as shown in Fig. 2. Au/Ce02 catalyst calcined at 473 K showed the highest initial CO conversion in the absence of water vapor. However, the CO conversion decreased steadily and reached a steady-state value over this catalyst. 

Fig. 1. CO conversions at different reaction temperatures in the dry (open points) and wet condition (filled points) over 0.95wt% Au/CeOj catalyst calcined at 573 K. FAV = 1,000 ml/min/gcat..

Fig. 2. CO conversions at 363 K in the dry condition (open points) and 353 K in the wet condition (filled points) over lOOmg and 50 mg of Aa/CeOi catal) containing 0.95 wt% Au prepared at different calcination temperatures (373 K (circle), 473 K(square), 573 K(triangle up), 673 K (triangle down), 773 K (diamond), 873 K (hexagon)). The reactants of 100 ml/min, 1 vol% CO and 1 vol% O2 in He, were fed to the catalyst.

The Arrhenius plots of the CO conversion rate in Fig. 2 indicate that the activation energy for the Au/Nano-Ti02 catalysts is nearly zero. Haruta et al. [6] also reported similar observations. They suggest that this occurs when the CO adsorbed on gold particles reacts with adsorbed O2 on the support at the interfacial junction between the two surfaces. 

Fig. 4. CO conversions over the catalysts with various compositions of Pt/Ru. GHSV 80,000hr, O2/CO = 1.0 (stoichiometric ratio).

Actually, various efforts have been made to develop the compact and efficient microchannel PrOx reactor for portable PEMFC applications. Goerke et al. [2] reported micro PrOx reactor employing stainless steel microchannel foil and Cu/Ce02 catalyst. They showed more than 99% CO conversion at less than 150 C and residence time of 14ms while CO selectivity was about 20%. Chen et al. [3] also developed microchannel reactor made of... 

For the practical use of this CO removal reactor, the microchannel reactor should be operated carefully to maintain operating temperature ranges because the reaction temperature is critical for the microchannel reactor performance such as CO conversion, selectivity and methanation as disclosed in the above results. It also seems that the present microchannel reactor is promising as a compact and high efficient CO remover for PEMFC systems. 

Fig. 3. CO conversion and selectivity with respect to reaction temperature in a microchannel reactor...

A microchannel reactor for CO preferential oxidation was developed. The reactor was consisted of microchannel patterned stainless steel plates which were coated by R11/AI2O3 catalyst. The reactor completely removed 1% CO contained in the Ha-rich reformed gas and controlled CO outlet concentration less than Ippm at 130 200°C and 50,000h. However, CH4 was produced from 180"C and CO selectivity was about 50%. For high performance of present PrOx reactor, reaction temperature should be carefully and uniformly controlled to reach high CO conversion and selectivity, and low CH4 production. It seems that the present microchaimel reactor is promising as a CO removal reactor for PEMFC systems. 

In this study, Pt/AliOj having high activity for CO oxidation and different affinities for fee adsorption of CO and Hi was selected as a catalyst/adsorbent In a conventional packed bed reactor (PBR), fee surface of fee catalyst is dominantly covered by COads with small amotmt of Oads fee CO conversion is therefore low. Several investigations on periodic operation have illustrated feat fee reaction front wife comparable amount of fee two adsorbed species leads to enhancement of fee CO conversion. Conceptually, this type of the reaction front should be generated by application of a CMBR, as well. Figure 1 illustrates an image of... 

The surface properties of three types of methanation catalysts obtained by oxidation of selected Intermetallics were examined In relation to their CO conversion activity. The first type (Ni Si, N1 A1 ) which corresponds to active phase-supporl iX the coXventionally prepared catalyst Is little affected by the oxidation treatment. The surface Nl is oxidized and relatively more abundant In the active solids. The second type (active phase-promoter ex Ni Th ) is extensively decomposed on oxidation. The transformation of these alloys Is accompanied by a surface enrichment in Nl. 

The overall behavior of the third type [mixed active phase-promoter ex (Nl Fej )Th] Is similar to the second. Preferential segregation of Fe, as compared to Nl, occurs on oxidation. A close correlation between CO conversion rate and surface Nl content has been observed. ... 

CO conversion data relative to (N1 SI ) and (ThNl Fe, series were taken from ref. ( ) and (,9), respectively. Catalytic measurements were obtained for oxygen treated N1 Th Intermetallics. Prior to each run, a sample mixture (50 mg cata ys + 50 mg ground quartz) was reduced In H. at 275 C for 16 hours. CO hydrogenation was carried out at 275 C using H /C0 ratio 9. More experimental details are given elsewhere (10). 

Correlation Between Surface Structure and CO Conversion Activity... 

Before any attempt to establish a correlation between the surface structure of the oxidized alloys and their CO conversion activity one must stress that the surface composition of the samples under reaction conditions may not necessarily be Identical to that determined from ESCA data. Moreover, surface nickel content estimates from ESCA relative Intensity measurements are at best seml-quantlta-tlve. This can be readily rationalized If one takes Into consideration ESCA finite escape depth, the dependence of ESCA Intensity ratio... 

Figure 5 Correlation between surface composition and CO conversion activity of oxidized Nij Sl alloys...

Variation of CO conversion activity ( ) and CO sorption capacities ( ) as a function of bulk nickel content. 

Ternary Alloys. The variations of CO conversion rate as a function of N1 content in (ThNi Fe, catalysts are compared in Figure 7 to N1 surface concentration as determlned from ESCA data. It Is evident... 

Variation of CO conversion activity as a function of bulk nickel content. 

Figure 6. Direct evidence for bridged-to-1inear CO conversion isotopic CO experiments.

CO and 5 CO2 at 613 K and atmospheric pressure. Flowrates were maintained such that the CO conversion was kept to less than 7 percent. 

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