دانلود رایگان مقاله مطالعه جذب اشعه X از نانوذرات پالادیم و مس سریا پشتیبانی شده

ABSTRACT

 The detailed structures of bimetallic Pd-Cu on various CeO2 and Al2O3 supports were investigated by X-ray absorption technique in-situ in hydrogen and WGS conditions. No indication of the neighboring Pd atoms in both Pd-K edge and Cu-K edge EXAFS fittings showed that Pd is highly dispersed in the lattice of metallic Cu. The Cu-Cu bond distance was markedly shortened by alloying with Pd and correlated to decrease of coordination number, which reflects particle size. A higher coordination number for Pd-Cu than that for Cu-Cu on Al2O3 support suggested that Pd is in the interior of the nanoparticles on Al2O3 while these coordination numbers were close on CeO2 support indicating a uniform distribution of Pd and Cu atoms. The CO shift activity was not simply correlated to the Cu-Cu bond distance or particle size, but the high activity of Pd-Cu/CeO2 was attributed to surface Pd interacting with Cu on CeO2 surface.

4. Conclusions

Using the X-ray absorption technique, the local structures of Pd and Cu in Pd-Cu catalysts were investigated in comparison with monometallic catalysts. From Cu K-edge spectra, only Cu neighbors were recognized and Pd neighbor was invisible. The particle size estimated from the Cu-Cu coordination number was much smaller for Pd-Cu system than monometallic Cu. The Cu-Cu bond distance in the Pd-Cu system was much shorter than that of monometallic Cu, which was correlated to the small particle size. The Pd K-edge XANES looks like an alloy, but in the EXAFS spectra, only Cu neighbors were identified. Thus, Pd is atomically dispersed in the Cu lattice. The Fourier transform of Pd K-edge spectra showed that Pd is geometrically identicalto Cu, i.e.the Pd-Cu bond distance is much shorter than that of monometallic Pd (2.75Å) and is close to Cu-Cu bond distance of monometallic Cu (2.55Å). Although such structural feature (small particle size and lattice shrinkage) of Pd-Cu is not a sole factor for the enhanced CO shift activity, the interactions among these two metal components and CeO2 support were suggested to control the CO shift activity.