Ar ion bombardment modification of Pd–Au/MWCNTs catalyst surfaces studied by electron spectroscopy

Abstract

AbstractThe Pd–Au multiwall carbon nanotubes (MWCNTs) supported catalyst prepared by polyol method was found to be catalytically active material in direct formic acid fuel cell electrooxidation reaction and during cyclic voltammetry (CV) measurements. The surface of the catalyst after calcination and reduction treatments was investigated previously. The effect of Ar ion bombardment modification on the calcinated catalyst was studied by electron spectroscopy. The chemical and structural changes were deduced from the X‐ray photoelectron spectroscopy (XPS) and X‐ray excited Auger electron spectroscopy (XAES) spectra. The XPS quantitative and qualitative analysis of different chemical forms of atoms indicated that the sputtering causes an increase of Pd surface content, accompanied by decreasing content of N, O, C sp2, and Au (due to preferential sputtering of Au) and significant increase of C sp3 amount. A decrease was also observed in the surface content of (i) carboxyl groups, (ii) water, and (iii) Pd oxide (mainly less stable PdO2). Analysis of XPS inelastic background shape by QUASES indicates that Pd crystallites phase is covered by PdOx + Camorphous overlayer of decreasing thickness after sputtering. Namely, thickness of PdOx overlayer decreased from 8.2 to 3.5 nm and the thickness of amorphous carbon overlayer located on the top increased from 2.3 to 3.5 nm. Significant increase of C sp3 content results from carbon nanotube (CNT) π bonds damaging, whereas decrease of PdO2 content from lower stability of PdO2. Larger thickness of Camorphous overlayer, in contrary to PdOx, may result from higher preferential sputtering of oxygen than carbon atoms and cracking of the MWCNTs structure by Ar+ ions.