In-situ infrared spectroscopy studies of surface reactions over irradiated photocatalysts
Date of Issue2013
School of Materials Science and Engineering
A*STAR Institute of Chemical and Engineering Sciences
Interest in photocatalysis driven by solar energy has risen greatly over the past four decades due to the fact that it is one of the most promising approaches to help solve the energy crisis and effectively combat environmental contamination. TiO2-based nanomaterials remain attractive candidates as photocatalysts due to their high photoefficiency, strong oxidizing power, good stability, and non-toxicity. Great attention has been given to improving its conventional (UV-driven) performance by exploiting, inter alia, the synergy of mixed (anatase/rutile) phase junctions, or making more perfect crystals (defects act as charge recombination centers) without sacrificing texture (surface area) etc. In the last two decades, intensive studies have been devoted to overcoming arguably the greatest limitation of TiO2, viz., its insensitivity to visible light. Early attention was given to bulk doping with transition metal ions but many act as recombination centers. This led to a related strategy with anions, where some success has been achieved with nitrogen as dopant. However, no major breakthrough is forthcoming as yet and other approaches are attracting serious interest, e.g., sensitization by narrow-band gap semiconductors, decoration with nanoparticulate metals having plasmon resonance absorption, or organic polymers less susceptible to photo-bleaching than conventional dyes. Furthermore, while major efforts have been expended in materials synthesis, characterization and testing, spectroscopic investigations at the reactant/catalyst interfaces are sparse. In particular, in situ studies at the photocatalyst surface are vital to unravel the mechanistics detail, yielding key information integral to the design and development of new materials with superior performance. In the present work, two complementary techniques, Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) and Attenuated Total Reflection (ATR) Fourier Transform Infrared (FTIR) Spectroscopy have been used for in situ investigations of some typical photocatalytic reactions at the gas/solid and liquid/solid interfaces, respectively. Commercial material, Degussa P25 TiO2, was used mainly for system verification and fundamental studies on the state of material under bandgap excitation, especially ATR-FTIR, and as a benchmark (in pristine and metalized form) for novel visible-sensitized photocatalysts prepared in this thesis. DRIFTS studies were made initially on P25 TiO2 and its platinized form to establish any changes in state under illumination and its relation with the ambient gas environment, water vapour and O2 in particular. Earlier reports of long-lived and reversible photochromism due to free carriers (electrons) were confirmed, and a detailed study made of the conditions that optimize the effect. The photo-generated charge carriers recombine quickly in dry inert atmosphere (N2), but are stabilized in the presence of H2O vapor, which is believed to act principally on the surface-trapped holes. The electron spectrum shows a featureless continuum absorption of increasing intensity towards lower frequency characteristic of free carriers in the bulk. However, deviations from the power law are attributed to partial trapping at surface Ti4+ sites. Under inert gas, and depending on the humidity level, photo-adsorption of water is promoted by surface-trapped holes and repelled (or substantially weakened) by surface-trapped electrons. Oxygen acts as a charge-recombination center and rapidly quenches the electron spectrum in a reversible manner. Addition of a more powerful hole-scavenger, e.g., ethanol, results in the accumulation of excess electrons due to irreversible oxidative photo-abstraction of H. These electrons migrate to the surface and effect photo-metallization, viz., reduction of adsorbed metal ions, such as Pt4+, to deposit metallic Pt nanoparticles. In an O2-rich atmosphere, ethanol itself is seen to be mineralized over P25 TiO2, i.e., photo-oxidized to CO2 and H2O via acetaldehyde and acetic acid (acetate) intermediates. Due to the strong absorption of IR radiation by solvent (H2O in this case), the investigation of photo-reactions at the liquid/solid interface has been severely hampered until recently. ATR-FTIR is applied to liquid-phase photo-oxidation of ethanol over TiO2, both in the pristine and metalized form. Intermediates in both the adsorbed and bulk forms were identified and tracked to elucidate the reaction pathway and assess the rate-limiting step(s). Just as in DRIFTS (vide ultra), development of charge carrier absorption in the pristine photocatalyst was also seen in ATR-FTIR and served as a marker of O2 depletion during photo-degradation experiments. Pt deposits act as efficient electron sinks (as judged by quenching of the continuum absorption) and activate the reductive component of the photocatalytic cycle (through ionosorption of dioxygen and/or reduction of protons), further enhancing the overall efficiency. Knowledge learnt from Degussa P25, a de facto standard, was then applied to investigate two groups of in-house prepared visible light active photocatalysts. These comprised (1) melon-modified titanate/TiO2 prepared from urea and (2) Ag modified titanate/TiO2. The abundance of surface OH groups on titanate promotes the efficient formation of melon, as compared to pre-formed anatase TiO2. In-situ DRIFTS confirmed the presence of melon, an organic polymeric semi-conductor, and further showed that it is a photo-stable visible light sensitizer. Melon-modified titanate/TiO2 nanomaterials were active for degradation of organic dye Methyl Orange (MO), and oxidation of ethanol under visible light irradiation. Decoration of TiO2 with Ag nanoparticles extends the absorption range into the visible due to the surface plasmon resonance (SPR) effect. These materials were also active in degradation of MO dye and ethanol photo-oxidation under visible light irradiation. The best performance for the Ag-decorated material in visible light and the pristine TiO2 under UV excitation were obtained after identical thermal treatment (450 C) thus showing the importance of the semi-conductor. A model is proposed for visible photo-activity via SPRpromoted electron transfer from Ag metal to TiO2. It is hoped that the in-situ spectroscopic investigations at the reactant/catalyst interface expounded in this Thesis give major impetus to subsequent research in the field. More specifically, the real-time inter-relationship between changes in the optical state of the catalyst, adsorbed intermediates, and their correlation with performance, provides valuable clues towards the rational design of better photocatalysts.