First Principles Analysis of H2O Adsorption on the (110) Surfaces of SnO2, TiO2 and Their Solid Solutions

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• 作者：Konstanze R. Hahn ; Antonio Tricoli ; Gianluca Santarossa ; Angelo Vargas ; Alfons Baiker
• 刊名：Langmuir
• 出版年：2012
• 出版时间：January 17, 2012
• 年：2012
• 卷：28
• 期：2
• 页码：1646-1656
• 全文大小：670K
• 年卷期：v.28,no.2(January 17, 2012)
• ISSN：1520-5827

文摘

Both associative and dissociative H₂O adsorption on SnO₂(110), TiO₂(110), and Ti-enriched Sn_1鈥?i>xTi_xO₂(110) surfaces have been investigated at low (¹/₁₂ monolayer (ML)) and high coverage (1 ML) by density functional theory calculations using the Gaussian and plane waves formalism. The use of a large supercell allowed the simulation at low symmetry levels. On SnO₂(110), dissociative adsorption was favored at all coverages and was accompanied by stable associative H₂O configurations. Increasing the coverage from ¹/₁₂ to 1 ML stabilized the (associatively or dissociatively) adsorbed H₂O on SnO₂(110) because of the formation of intermolecular H bonds. In contrast, on TiO₂(110), the adsorption of isolated H₂O groups (¹/₁₂ ML) was more stable than at high coverage, and the favored adsorption changed from dissociative to associative with increasing coverage. For dissociative H₂O adsorption on Ti-enriched Sn_1鈥?i>xTi_xO₂(110) surfaces with Ti atoms preferably located on 6-fold-coordinated surface sites, the analysis of the Wannier centers showed a polarization of electrons surrounding bridging O atoms that were bound simultaneously to 6-fold-coordinated Sn and Ti surface atoms. This polarization suggested the formation of an additional bond between the 6-fold-coordinated Ti_6c and bridging O atoms that had to be broken upon H₂O adsorption. As a result, the H₂O adsorption energy initially decreased, with increasing surface Ti content reaching a minimum at 25% Ti for ¹/₁₂ ML. This behavior was even more accentuated at high H₂O coverage (1 ML) with the adsorption energy decreasing rapidly from 145.2 to 101.6 kJ/mol with the surface Ti content increasing from 0 to 33%. A global minimum of binding energies at both low and high coverage was found between 25 and 33% surface Ti content, which may explain the minimal cross-sensitivity to humidity previously reported for Sn_1鈥?i>xTi_xO₂ gas sensors. Above 12.5% surface Ti content, the binding energy decreased with increasing coverage, suggesting that the partial desorption of H₂O is facilitated at a high fractional coverage.