Study On Preparation Of Silver Catalysts Via Double Solvent Technique And Catalytic Properties For Preferential Oxidation Of CO

In recent years, the fuel cell with its high power density, less environmentalpollution, good reliability and maintainability has attracted more and more attention,and the polymer electrolyte membrane fuel cell （PEMFC） with its many advantages hasbecome the most potential application of fuel cell. However, a problem hinderingPEMFC application is its electrode poisoning to CO. Among the currently availablemethods for solving electrode poisoning, the selective oxidation of CO with molecularoxygen is recognized as the simplest, the most straightforward, economic and effectiveone. Catalysts proposed for this process are mainly noble metals, such as Pt, Rh, Au,and the cost is relatively high. Recently it has been reported that silver catalyst alsoshowed activity for preferential oxidation of CO. Since the cost of silver is relativelylow, it has attracted much attention and has in-depth research value.In this paper, nano-silver catalysts were prepared with mesoporous molecular sieveSBA-15as support and silver nitrate as silver source. The support and catalysts werecharacterized by X-ray diffraction, transmission electron microscopy, N₂adsorption-desorption and inductively coupled plasma-atomic emission spectroscopy. The catalyticactivities for preferential oxidation of CO were studied, and the effects of preparation,silver loading et al. on the activities of catalysts were investigated. The main resultsobtained are as follows.1. Among the nano-silver catalysts with different loadings prepared by doublesolvent technique, all of the silver particles in the sample with low loading were nearlygot into the channels of SBA-15with uniform size and dispersion. With the increase ofloading, agglomeration of the silver particles began to appear and the dispersion beganto deteriorate too, however, most of silver particles can be still kept within the channelsof SBA-15. So it can be concluded that the double solvent technique can effectivelysolve the introduction of metal particles into SBA-15channels and overcome metalparticles aggregation problem.2. Another group of nano-silver catalysts with different loadings were alsoprepared by double solvent technique but using modified SBA-15as support. Theresults showed that the modified SBA-15was still retained a well-orderedmesostructure. Silver particles in the low loading sample were almost dispersed withinthe channels of SBA-15，and the particles were embedded between channels of SBA-15with uniform size and dispersion. The pore wall of the high loading samples began to collapse and the agglomeration of the silver particles along with the deterioration of theparticles dispersion arose, however, most of silver particles can still be kept within thechannels of SBA-15and the catalysts were still retained a well-ordered mesostructure.In a word, the functional modification of SBA-15had played a certain role in catalystpreparation for the particles within the channels, with uniform size and high dispersion.3. Catalytic performances of nano-silver catalysts for preferential oxidation of CO havebeen also investigated, and the differences of catalytic activities for different preparationcatalysts have been studied as well. The results showed that nano-silver catalystssupported on SBA-15had considerable catalytic activities at low temperatures, besides,it can achieve the highest CO conversion at the PEMFC operating temperatures. Thecatalysts also have shown ideal CO selectivity at low temperatures. With the increase ofthe reaction temperature, the competition reaction of H2will increase and selectivity ofCO will decrease. So the catalytic activities for preferential oxidation of CO are relatedto the properties of support and metal loading, and the SBA-15catalysts have shownbetter activities than modified SBA-15catalysts, and proper loading catalysts haveshown better activities than high loading catalysts.