Probing Oxide Reduction and Phase Transformations at the Au-TiO2 Interface by Vibrational Spectroscopy

By a combination of FT-NIR Raman spectroscopy, infrared spectroscopy of CO adsorption under ultrahigh vacuum conditions (UHV-IR) and Raman spectroscopy in the line scanning mode the formation of a reduced titania phase in a commercial Au/TiO2 catalyst and in freshly prepared Au/anatase catalysts was detected. The reduced phase, formed at the Au-TiO2 interface, can serve as nucleation point for the formation of stoichiometric rutile. TinO2n−1 Magnéli phases, structurally resembling the rutile phase, might be involved in this process. The formation of the reduced phase and the rutilization process is clearly linked to the presence of gold nanoparticles and it does not proceed under similar conditions with the pure titania sample. Phase transformations might be both thermally or light induced, however, the colloidal deposition synthesis of the Au/TiO2 catalysts is clearly ruled out as cause for the formation of the reduced phase

Molecular Titanate and Titania as photocatalysts in the photocatalytic conversion of CO2_2 and H2_2O

Die Ergebnisse dieser Arbeit sind dem Ziel gewidmet die photokatalytische CO$_2$ Reduktion besser zu verstehen. Alle Katalysatoren, die für die Aktivitätstests benutzt werden, sind basiert auf TiO$_2$ oder molekularem Titanat. Zusätzlich zu der Erläuterung der fundamentalen Aspekte der Ladungsträgerentstehung und deren Dynamiken, bewertet die Einleitung die Trends und Konzepte, die in der Literatur vorhanden sind in Bezug auf TiO$_2$ und Titanaten. Abschließend wird verdeutlicht, dass die photokatalytische CO$_2$ Reduktion nur unzureichend mechanistisch verstanden wird und bis zum heutigen Tage kein Katalysator bekannt ist, der annähernd an eine kommerzielle Nutzung kommt. Diese Herausforderung wird in dieser Arbeit durch eine wissensbasierte Strategie angegangen, welche in Katalysatormodifikation, in situ Charakterisierung, und Aktivitätstest eingeteilt werden kann

A high-purity gas–solid photoreactor for reliable and reproducible photocatalytic CO2 reduction measurements

Reactions between a gas phase and a solid material are of high importance in the study of alternative ways for energy conversion utilizing otherwise useless carbon dioxide (CO2). The photocatalytic CO2 reduction to hydrocarbon fuels like e.g., methane (CH4) is such a potential candidate process converting solar light into molecular bonds. In this work, the design, construction, and operation of a high-purity gas–solid photoreactor is described. The design aims at eliminating any unwanted carbon-containing impurities and leak points, ensuring the collection of reliable and reproducible data in photocatalytic CO2 reduction measurements. Apart from the hardware design, a detailed experimental procedure including gas analysis is presented, allowing newcomers in the field of gas–solid CO2 reduction to learn the essential basics and valuable tricks. By performing extensive blank measurements (with/without sample and/or light) the true performance of photocatalytic materials can be monitored, leading to the identification of trends and the proposal of possible mechanisms in CO2 photoreduction. The reproducibility of measurements between different versions of the here presented reactor on the ppm level is evidenced

Fabrication of Gold/Titania Photocatalyst for CO₂ Reduction Based on Pyrolytic Conversion of the Metal–Organic Framework NH₂‑MIL-125(Ti) Loaded with Gold Nanoparticles

Titania exhibits unique photophysical and -chemical properties and can be used for potential applications in the field of photocatalysis. The control of TiO₂ in terms of phase, shape, morphology, and especially nanoscale synthesis of TiO₂ particles still remains a challenge. Ti-containing metal–organic frameworks (MOFs), such as MIL-125, can be used as sacrificial precursors to obtain TiO₂ materials with diverse phase compositions, morphologies, sizes, and surface areas. MIL-125 is composed of Ti/O clusters as the secondary building units (SBUs) bridged by 1,4-benzenedicarboxylate (bdc). In this study, preformed and surfactant-stabilized gold nanoparticles (GNPs) were deposited onto the surface of amino functionalized NH₂-MIL-125 during solvothermal synthesis. Targeted gold/titania nanocomposites, GNP/TiO₂, were fabricated through the pyrolysis of GNP/NH₂-MIL-125 nanocrystals. The modification of TiO₂ with GNPs significantly increased the photocatalytic activity of the MOF derived TiO₂ material for the reduction of CO₂ to CH₄ as compared to TiO₂ reference samples such as P-25 and AUROlite (Au/TiO₂). The new materials GNP/TiO₂ and TiO₂ derived by the MOF precursor route were thoroughly characterized by PXRD, FTIR and Raman, TEM, and N₂ adsorption studies

Evidence for Metal–Support Interactions in Au Modified TiO_<i>x</i>/SBA-15 Materials Prepared by Photodeposition

Gold nanoparticles have been efficiently photodeposited onto titanate-loaded SBA-15 (Ti­(<i>x</i>)/SBA-15) with different titania coordination. Transmission electron microscopy shows that relatively large Au nanoparticles are photodeposited on the outer surface of the Ti­(<i>x</i>)/SBA-15 materials and that TiO_<i>x</i> tends to form agglomerates in close proximity to the Au nanoparticles, often forming core–shell Au/TiO_<i>x</i> structures. This behavior resembles typical processes observed due to strong-metal support interactions. In the presence of gold, the formation of hydrogen on Ti­(<i>x</i>)/SBA-15 during the photodeposition process and the performance in the hydroxylation of terephthalic acid is greatly enhanced. The activity of the Au/Ti­(<i>x</i>)/SBA-15 materials is found to depend on the TiO_<i>x</i> loading, increasing with a larger amount of initially isolated TiO₄ tetrahedra. Samples with initially clustered TiO_<i>x</i> species show lower photocatalytic activities. When isolated zinc oxide (ZnO_<i>x</i>) species are present on Ti­(<i>x</i>)/SBA-15, gold nanoparticles are smaller and well dispersed within the pores. Agglomeration of TiO_<i>x</i> species and the formation of Au/TiO_<i>x</i> structures is negligible. The dispersion of gold and the formation of Au/TiO_<i>x</i> in the SBA-15 matrix seem to depend on the mobility of the TiO_<i>x</i> species. The mobility is determined by the initial degree of agglomeration of TiO_<i>x</i>. Effective hydrogen evolution requires Au/TiO_<i>x</i> core–shell composites as in Au/Ti­(<i>x</i>)/SBA-15, whereas hydroxylation of terephthalic acid can also be performed with Au/ZnO_<i>x</i>/TiO_<i>x</i>/SBA-15 materials. However, isolated TiO_<i>x</i> species have to be grafted onto the support prior to the zinc oxide species, providing strong evidence for the necessity of Ti–O–Si bridges for high photocatalytic activity in terephthalic acid hydroxylation

Surface Termination and CO₂ Adsorption onto Bismuth Pyrochlore Oxides

The catalytic activity and gas-sensing properties of a solid are dominated by the chemistry of the surface atomic layer. This study is concerned with the characterization of the outer atomic surfaces of a series of cubic ternary oxides containing Bi­(III): Bi₂M₂O₇ (M = Ti, Zr, Hf), using low-energy ion scattering spectroscopy. A preferential termination in Bi and O is observed in pyrochlore Bi₂Ti₂O₇ and related cubic compounds Bi₂Zr₂O₇ and Bi₂Hf₂O₇, whereas all three components of the ternary oxide are present on the surface of a Bi-free pyrochlore oxide, Y₂Ti₂O₇. This observation can be explained based on the revised lone-pair model for post-transition-metal oxides. We propose that the stereochemically active lone pair resulting from O 2p-assisted Bi 6s–6p hybridization is more energetically favored at the surface than within a distorted bulk site. This leads to reduction of the surface energy of the Bi₂M₂O₇ compounds and, therefore, offers a thermodynamic driving force for the preferential termination in BiO_<i>x</i>-like structures. CO₂ adsorption experiments <i>in situ</i> monitored by diffuse reflectance IR spectroscopy show a high CO₂ chemisorption capacity for this series of cubic bismuth ternary oxides, indicating a high surface basicity. This can be associated with O 2p–Bi 6s–6p hybridized electronic states, which are more able to donate electronic density to adsorbed species than surface lattice oxygen ions, normally considered as the basic sites in metal oxides. The enhanced CO₂ adsorption of these types of oxides is particularly relevant to the current growing interest in the development of technologies for CO₂ reduction