Segregation of K and its effects on the growth, decoration, and adsorption properties of Rh nanoparticles on TiO₂(1 1 0)

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• 作者：Andrá ; s Berkó ; ^a ; ^b ; Ná ; ndor Balá ; zs^a ; Gyö ; rgy Kassab^b ; Lá ; szló ; Ó ; vá ; ri^a ; ^{ovari@chem.u-szeged.hu}
• 关键词：Alkali promoter ; Potassium segregation ; Rhodium ; Strong metal support interaction ; TiO2(1  ; 1  ; 0) ; XPS ; STM ; LEIS
• 刊名：Journal of Catalysis
• 出版年：2012
• 期刊代码：97_00219517
• 类别：ce
• 出版时间：May, 2012
• 卷：289
• 期：Complete
• 页码：179-189
• 文件大小：1967 K

摘要

In order to understand the promoting mechanism of alkali additives, we have studied potassium overlayers on TiO₂(1 1 0). The surface was prepared by thermal segregation of K from the bulk to the surface above 750-800 K. The bulk diffusion of the Ti/O ions is required for the migration of large K ions inside the lattice. STM and LEIS revealed that segregated potassium forms small clusters of 1-2 nm on the surface, containing also oxygen. These clusters are located preferentially on the one-dimensional defect sites (Ti₂O₃ strings) of the (1 脳 1) rutile surface and on the Ti₂O₃ rows of the (1 脳 n) reconstructed surfaces. According to XPS, the potassium on the surface after segregation at 1000 K is only partially ionized and the Ti 2p region is dominated by the Ti⁴⁺ component. XPS and LEIS provided evidence of a very clear preference for the Rh clusters to grow near 300 K not on the potassium structures but on the potassium-free parts of titania surface. This finding may imply the absence of a direct contact between Rh and K at low Rh coverages. Nevertheless, evaporation of Rh on K/TiO₂(1 1 0) results in more cationic K sites, due to an electron transfer from K to Rh through titania. The decoration and encapsulation of Rh nanoparticles by TiO_x proceed also in the presence of potassium. The capping layer does not contain potassium. At large Rh cluster sizes, the wheel-like structure of the cover layer could be identified with a structure found on top of Rh crystallites formed on the K-free titania surface. The presence of potassium stabilized CO on Rh nanoparticles, which is attributed to the indirect charge transfer from potassium structures to rhodium (long range effect).