Improved Co Oxidation Activity in the Presence and Absence of Hydrogen over Cluster-Derived Ptfe/Sio2 Catalysts
Attilio Siani1, Burjor Captain2, Oleg S. Alexeev1, Eirini Stafyla1, Ana B. Hungria3, Paul A. Midgley3, John M. Thomas3, Richard D. Adams2, and Michael D. Amiridis1. (1) Chemical Engineering, University of South Carolina, 301 Main St., Columbia, SC 29208, (2) Department of Chemistry and Biochemistry, University of South Carolina, 631 Sumter St., Columbia, SC 29208, (3) Material Science and Metallurgy, University of Cambridge, Cambridge
The oxidation of CO in the absence and presence of H2 has attracted significant attention recently, due to its potential application in indoor/cabin air clean-up and in the purification of hydrogen streams used in proton exchange membrane (PEM) fuel cells. Hydrogen is currently produced by catalytic steam reforming, partial oxidation, and auto-thermal reforming of hydrocarbons. However, CO is formed as a by-product in all of these processes and must be subsequently removed due to the high sensitivity of the Pt-based PEM electrocatalysts to poisoning by CO. The preferential oxidation (PROX) of CO in the presence of hydrogen is currently used commercially for this application due to its efficiency and relative simplicity. Supported metal catalysts incorporating Pt and other noble metals have been extensively investigated for PROX and have exhibited substantial activity for this reaction. Further attempts have been made to improve their performance by the addition of promoters to the existing catalytic formulations. In this regard, supported bimetallic catalysts, incorporating more than one active component, may provide possibilities of synergism that lead to superior performance which largely depends on the structure and composition of the bimetallic particles, and the nature of the support. Conventional methods used for the preparation of supported catalysts (e.g., incipient wetness, wet impregnation or deposition-precipitation) often provide limited control over the structure of the resulting materials. Better uniformity of the supported metal particles has been achieved through the use of bimetallic molecular clusters as precursors. The catalytic performance of such cluster-derived PtFe/SiO2 bimetallic catalysts for the oxidation of CO has been examined in the absence and presence of H2 and compared to that of Pt/SiO2. PtFe2/SiO2 and Pt5Fe2/SiO2 samples were prepared from PtFe2(COD)(CO)8 and Pt5Fe2(COD)2(CO)12 organometallic cluster precursors, respectively. FTIR data indicate that both clusters can be deposited intact on the SiO2 surface. The clusters remained weakly bonded to the SiO2 surface and could be removed by extraction with CH2Cl2 without any changes in their structure. Subsequent heating in H2 flow led to complete decarbonylation of the supported clusters at approximately 350°C and the formation of bimetallic particles that were more active for CO oxidation than either Pt/SiO2 or a conventionally-prepared PtFe/SiO2 sample. However, substantial deactivation with time on stream was also observed, suggesting that the properties of the PtFe bimetallic sites in the cluster-derived samples can be altered by exposure to the reactants. The Pt5Fe2/SiO2 sample was also more active than Pt/SiO2 for PROX with a selectivity of approximately 92 % at temperatures up to 60°C. In this case the deactivation with time on stream was substantially lower, suggesting that the highly reducing environment under the PROX conditions helps to maintain the properties of the active PtFe bimetallic sites.