The Influence of Surface Structure on H4SiO4 Oligomerization on Rutile and Amorphous TiO2 Surfaces: An ATR-IR and Synchrotron XPS Study

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• 作者：Yantao Song ; Peter J. Swedlund ; Grant J. McIntosh ; Bruce C. C. Cowie ; Geoffrey I. N. Waterhouse ; James B. Metson
• 刊名：Langmuir
• 出版年：2012
• 出版时间：December 11, 2012
• 年：2012
• 卷：28
• 期：49
• 页码：16890-16899
• 全文大小：554K
• 年卷期：v.28,no.49(December 11, 2012)
• ISSN：1520-5827

文摘

Silicic acid (Hb>4b>SiOb>4b>) is ubiquitous in natural aquatic systems. Applications of TiOb>2b> in these systems will be influenced by Hb>4b>SiOb>4b> sorption and oligomerization reactions on the TiOb>2b> surface, and this can affect many aspects of TiOb>2b> reactivity. The spatial arrangement of sorption sites on a metal oxide surface can promote specific lateral interactions, such as oligomerization, between sorbed species. In this work we explore the relationship between surface structure and interfacial Hb>4b>SiOb>4b> oligomerization by quantifying the extent of Hb>4b>SiOb>4b> sorption and oligomerization on three TiOb>2b> phases; a rutile phase having well-developed (110) faces (R180), a rutile phase with poorly developed (110) faces (R60), and an amorphous TiOb>2b> (TiOb>2(am)b>). The in situ ATR-IR spectra measured over time as 0.2 mM Hb>4b>SiOb>4b> reacted with TiOb>2b> were quite different on the three TiOb>2b> phases. The percentage of the surface Hb>4b>SiOb>4b> that was present as oligomers increased over time on all phases, but after 20 h almost all Hb>4b>SiOb>4b> on the R180 surface was oligomeric, while the Hb>4b>SiOb>4b> on TiOb>2(am)b> was predominantly monomeric. The extent of Hb>4b>SiOb>4b> oligomerization on R60 was intermediate. When the TiOb>2b> phases reacted with 1.5 mM Hb>4b>SiOb>4b> the ATR-IR spectra showed oligomeric silicates dominating the surface of all three TiOb>2b> phases; however, after 20 h the percentage of the surface Hb>4b>SiOb>4b> present as three-dimensional polymers was 30, 10, and 0% on R180, R60, and TiOb>2(am)b> respectively. The Si 2s photoelectron peak binding energy (BE) and the Hb>4b>SiOb>4b> surface coverage (螕b>Sib>) were measured by XPS over a range of 螕b>Sib>. For any given 螕b>Sib> the Si 2s BE鈥檚 were in the order R180 > R60 > TiOb>2(am)b>. A higher Si 2s BE indicates a greater degree of silicate polymerization. The ATR-IR and XPS results support the existing model for interfacial Hb>4b>SiOb>4b> oligomerization where linear trimeric silicates are formed by insertion of a solution Hb>4b>SiOb>4b> between suitably orientated adjacent bidentate sorbed monomers. The TiOb>2(am)b> has previously been shown to consist of 2 nm diameter particles with a highly disordered surface. When compared to the TiOb>2(am)b> surface, the regular arrangement of TiOb>6b> octahedra on the rutile (110) face means that sorbed Hb>4b>SiOb>4b> monomers on adjacent rows of singly coordinated oxygen atoms are oriented so as to favor linear trimer formation. Higher silicate polymers can form between adjacent trimers, and this is favored on the rutile (110) surfaces compared to the TiOb>2(am)b>. This is also expected on the basis of the arrangement of surface sites on the rutile (110) surface and because the high surface curvature inherent in a 2 nm spherical TiOb>2(am)b> particle would increase the spatial separation of adjacent trimers.