Improved CO oxidation activity in the presence and absence of hydrogen over cluster-derived PtFe/SiO2 catalysts

摘要

The catalytic performance of cluster-derived PtFe/SiO2 bimetallic catalysts for the oxidation of CO has been examined in the absence and presence of H-2 (PROX) 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 support. The clusters remained weakly bonded to the SiO2 surface and could be extracted with CH2Cl2 without any significant changes in their structure. Subsequent heating in H-2 led to complete decarbonylation of the supported clusters at approximately 350 degrees C and the formation of Pt- Fe nanoparticles with sizes in the 1-2 nm range, as indicated by HRTEM imaging. A few larger nanoparticles enriched in Pt were also observed, indicating that a small fraction of the deposited clusters were segregated to the individual components following the hydrogen treatment. A higher degree of metal dispersion and more homogeneous mixing of the two metals were observed during HRTEM/XEDS analysis with the cluster-derived samples, as compared to a PtFe/siO(2) catalyst prepared through a conventional impregnation route. Furthermore, the cluster-derived PtFe2/SiO2 and Pt5Fe2/ SiO2 samples were more active than Pt/SiO2 and the conventionally prepared PtFe/SiO2 sample for the oxidation of CO in air. However, substantial deactivation was also observed, indicating that the properties of the Pt-Fe bimetallic sites in the cluster-derived samples were 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 50 degrees C. In this case, the deactivation with time on stream was substantially slower, indicating that the highly reducing environment under the PROX conditions helps maintain the properties of the active Pt- Fe bimetallic sites.