Iron oxide overlayers on Au/SiO2/Si(100):: Promoting effect of Au on the catalytic activity of iron oxide in CO oxidation

Iron oxide layers of 5-10 nm thickness were deposited by pulsed laser techniques (PLD) onto either Au films or nano-sized Au particles supported by SiO2/Si(1 0 0). Samples were characterized by atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (TOF SIMS) before and after measurements of the CO oxidation activity. Comparison was made with reference. samples either free of iron oxide and/or free of Au particles/films. The initial activity of iron oxide/Au nano-particles/ SiO2/Si(1 0 0) turned out to be the highest followed by the sample containing a sandwiched Au film. While some reaction-induced changes in the chemical composition of the iron oxide overlayer ("FeOx", which can be regarded as a mixture of Fe2O3, FIron oxide layers of 5-10 nm thickness were deposited by pulsed laser techniques (PLD) onto either Au films or nano-sized Au particles supported by Si02/Si(1 0 0). Samples were characterized by atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (TOF SIMS) before and after measurements of the CO oxidation activity. Comparison was made with reference. samples either free of iron oxide and/or free of Au particles/films. The initial activity of iron oxide/Au nano-panicles/ Si02/Si(I 0 0) turned out to be the highest followed by the sample containing a sandwiched Au film. While some reaction-induced changes in the chemical composition of the iron oxide overlayer ("FeOx", which can be regarded as a mixture of Fe2O3, FeO and FeOOH according to the XPS analysis of the Fe 2p and 0 Is core levels) were seen, no Au segregation at the surface was detected by TOF SIMS. The XPS Au 4f spectra indicated, however, that An atoms might be injected and trapped in this layer. The catalytic activity of the FeO,/Au/SiO2/Si(I 0 0) samples must be attributed to active sites located on the iron oxide overlayer promoted by gold underneath. Since Au nano-particles and Au films caused promotion we infer that an electronic effect is in operation due to the occurrence of an FeO,/Au interface in both cases. Since the promotion is stronger for Au nano-particles the hypothesis of a particle size dependent electronic effect may be advanced. For thicker FeO. of about 40-80 nm no promotion by Au was found.(c) 2005 Elsevier B.V. All rights reserved.eO and FeOOH according to the XPS analysis of the Fe 2p and O Is core levels) were seen, no Au segregation at the surface was detected by TOF SIMS. The XPS Au 4f spectra indicated, however, that An atoms might be injected and trapped in this layer. The catalytic activity of the FeO,/Au/SiO2/Si(1 0 0) samples must be attributed to active sites located on the iron oxide overlayer promoted by gold underneath. Since Au nano-particles and Au films caused promotion we infer that an electronic effect is in operation due to the occurrence of an FeO,/Au interface in both cases. Since the promotion is stronger for Au nano-particles the hypothesis of a particle size dependent electronic effect may be advanced. For thicker FeO. of about 40-80 nm no promotion by Au was found. (c) 2005 Elsevier B.V. All rights reserved.