Andreas Mattsson

Adsorption of formic acid on rutile TiO2 (110) revisited: An infrared reflection-absorption spectroscopy and density functional theory study

Formic acid (HCOOH) adsorption on rutile TiO2 (110) has been studied by s- and p-polarized infrared reflection-absorption spectroscopy (IRRAS) and spin-polarized density functional theory together with Hubbard U contributions (DFT+U) calculations. To compare with IRRAS spectra, the results from the DFT+U calculations were used to simulate IR spectra by employing a three-layer model, where the adsorbate layer was modelled using Lorentz oscillators with calculated dielectric constants. To account for the experimental observations, four possible formate adsorption geometries were calculated, describing both the perfect (110) surface, and surfaces with defects; either O vacancies or hydroxyls. The majority species seen in IRRAS was confirmed to be the bridging bidentate formate species with associated symmetric and asymmetric frequencies of the ν(OCO) modes measured to be at 1359 cm(-1) and 1534 cm(-1), respectively. The in-plane δ(C-H) wagging mode of this species couples to both the tangential and the normal component of the incident p-polarized light, which results in absorption and emission bands at 1374 cm(-1) and 1388 cm(-1). IRRAS spectra measured on surfaces prepared to be either reduced, stoichiometric, or to contain surplus O adatoms, were found to be very similar. By comparisons with computed spectra, it is proposed that in our experiments, formate binds as a minority species to an in-plane Ti5c atom and a hydroxyl, rather than to O vacancy sites, the latter to a large extent being healed even at our UHV conditions. Excellent agreement between calculated and experimental IRRAS spectra is obtained. The results emphasize the importance of protonation and reactive surface hydroxyls - even under UHV conditions - as reactive sites in e.g., catalytic applications.

Role of bismuth in nano-structured doped TiO2 photocatalyst prepared by environmentally benign soft synthesis

An environmentally benign synthesis method was used to prepare a nearly uniform dispersion of TiO2 nanoparticles modified by bismuth for photocatalytic purposes. The role of bismuth in the catalyst structure was evaluated using numerous methods such as XRPD, HTXRPD, TEM and HRTEM, and XPS, as well as Raman, FTIR, and UV–Vis spectroscopy. The bismuth doping significantly improved the photocatalytic performance of azo dye RB5 discoloration due to the formation of surface Bi3+ species and the abundant hydroxylation of the catalyst surface. The great advantage of this procedure lies in the low temperature preparation under ambient pressure without use of the titanium organometallic precursors. Therefore, this developed synthesis procedure could be easily adapted to the industrial scale.

Surface characteristics and electronic structure of photocatalytic reactions on TiO2 and doped TiO2 nanoparticles

A molecular approach to understand the photocatalytic degradation of small organic molecules adsorbed from the gas phase on anatase, rutile and doped TiO2 nanoparticles is presented. Using in situ Fourier transform infrared (FTIR) spectroscopy and mass spectrometry the rate determining steps for the photocatalytic degradation of formic acid, acetone and propane are unraveled. Key intermediates are identified and correlated to structural properties of the TiO2 nanoparticles. Specifically, stable bridging bidentate carboxylate (R-CO2) and (bi)carbonate species forms preferentially on rutile particles, and are proposed to inhibit the total photodegradation efficiency. In particular, the concentration of R-CO2 is found to decrease with increasing size of the anatase particles, and may at least partly explain why Degussa P25 is a good photocatalyst. Means to avoid R-CO2 site-blocking is discussed. Improved solar light efficiencies are difficulty to achieve in cation doped TiO2 despite higher visible light absorption and stronger adsorbate-surface interactions.

In Situ FTIR Spectroscopy Study of the Photodegradation of Acetaldehyde and azo Dye Photobleaching on Bismuth‐Modified TiO2

The photocatalytic properties of bismuth‐modified titania were studied by photobleaching of two aqueous azo dyes solutions (Reactive Black 5 and Acid Orange 7), and by photoinduced decomposition (PID) of acetaldehyde using in situ FTIR spectroscopy. Low bismuth doping concentrations up to 3 at.% is shown to lead to an increased photobleaching rate of both azo dyes solutions. Too high Bi dopant concentrations lead to less developed crystallite nanoparticles and exhibit weaker adsorption capacity. Bismuth doping altered the adsorption kinetics of acetaldehyde resulting in different surface products, and a modified photocatalytic reaction pathway was inferred.

Infrared spectroscopy study of adsorption and photodecomposition of formic acid on reduced and defective rutile TiO2 (110) surfaces

Adsorption and photodecomposition of formic acid on rutile TiO2 (110) have been investigated with infrared reflection–absorption spectroscopy (IRRAS) employing p- and s-polarized light along the [0 ...

Chemical warfare agent simulant DMMP reactive adsorption on TiO2/graphene oxide composites prepared via titanium peroxo-complex or urea precipitation

Two water-based methods were used to produce TiO2/graphene oxide (GO) nanocomposites with 1 and 2 wt.% GO. Both procedures exclude the use of organometallic precursors, as well as the high-pressure and high-temperature treatments, which facilitate pure and energy efficient synthesis amenable for larger scale synthesis. Nanocomposites with narrow (<10 nm) and long spindle-like (<100 nm) TiO2 nanoparticles supported on GO flakes were obtained (TiO2/GO), and their properties for reactive destruction of the organophosphorus simile chemical warfare agent (CWA) dimethyl methylphosphonate (DMMP) were investigated by in situ DRIFTS spectroscopy. Both synthesis procedures yielded highly reactive nanocomposites with markedly different properties compared to similarly prepared pure TiO2 nanoparticles. GO also induced morphology and texture changes, which were observed to have a significant impact on the adsorption and reactivity of the nanocomposites, and which were strongly related to synthesis procedure. In particular, the reduction state of GO, as measured by Raman spectroscopy, was observed to play a major role for the reactivity of the TiO2/GO nanocomposites.