Study of chemically reactive surfaces of TiO2 and Ag/TiO2 for adsorptive desulfurization of liquid fuels and photocatalysis applications

Abstract

The first part of this study focuses on the mechanisms study of UV photo-induced adsorptive desulfurization of liquid model fuels on anatase TiO2 adsorbents. Previous studies showed UV irradiation during or prior to the desulfurization breakthrough experiment can improve the sulfur removal breakthrough capacity of anatase TiO2 by up to 65%. To further improve the sulfur removal efficiency of the adsorbent, it’s of primary importance to understand the surface alteration mechanism caused by UV irradiation. X-ray photoelectron spectroscopy (XPS) is a surface analytical instrument that can reveal surface chemical properties of solid samples. By comparing the XPS spectral change of Titanium, O, Ag, and C elements before and after UV treatment, XPS can provide us a scheme of the surface chemical composition alteration caused by UV irradiation. In situ XPS technique can further assist us to access the reaction mechanisms by showing the chemical property changes during the reaction procedure. In this study, XPS and in situ XPS spectral results illustrate that UV irradiation converts lattice oxide oxygen into surface hydroxyl groups, and those photo-generated surface hydroxyl groups are essential factors for the improvement of sulfur removal capacities. Qualitative and quantitative analysis showed the ratio between the rate of hydroxyl production and lattice oxide oxygen consumption is 2:1. Thus, the mechanism of UV photo-induced adsorptive desulfurization is UV irradiation converts one oxide oxygen into two surface hydroxyl groups on anatase TiO2 and the additional hydroxyl groups improved the desulfurization capacity of anatase TiO2 adsorbent. To further understand and estimate the mechanisms of UV irradiation on TiO2 adsorbent, two different types of anatase TiO2 adsorbents were applied to XPS characterization. It is proposed that UV photo-catalytic reaction of anatase TiO2 is influenced by adsorbed H2O, so XPS measurements were conducted on UV treated and untreated TiO2 adsorbents that were prepared at 110 °C and 450 °C, respectively. Temperature has a significant effect on the relative amount of H2O can be adsorbed on TiO2 samples as it is indicated by TGA (Thermogravimetric Analysis). XPS spectra showed that UV treatment on 110 °C prepared TiO2 can produce more hydroxyl groups than TiO2 prepared at 450 °C. XPS spectral results also showed that UV irradiation on TiO2 with higher surface area produced more additional hydroxyl groups. The results illustrated that the reactions caused by UV irradiation were that one adsorbed H2O and one lattice oxygen in anatase TiO2 went through photo-catalytic reactions under UV treatment to form two additional surface hydroxyl groups. The second part of this study investigated the reaction mechanism for the UV induced decomposition of carbon contaminants on anatase TiO2. Earlier research efforts showed hydrocarbon contaminants inevitably present on the surface of every catalyst during the preparation and transfer procedure and they could affect the overall catalytic efficiency of the catalyst. Besides, the hydrocarbon layer could be removed by UV irradiation as indicated by their spectral results. The current presents valuable evidence to qualitatively explore the potential reaction mechanism of UV induced carbon contaminant decomposition on anatase TiO2 and quantitatively estimate the reaction stoichiometry and quantum efficiency of UV photons at various irradiation intensities within natural sunlight intensity range by means of in situ Fourier-transform infrared spectroscopy (FTIR) and in situ X-ray photoelectron spectroscopy (XPS) coupled with a 375 nm UV laser diode. In situ FTIR spectral change between untreated TiO2 sample and after 2 hours UV treated TiO2 sample showed decomposition of carbon contaminant with consumption of adsorbed water and formation of oxidant carbon and isolated surface hydroxyls. UV induced decomposition of carbon species associated with consumption of oxygen species was also observed by tracking the XPS spectral change of UV irradiated spot comparing to the dark reference spot. The stoichiometry ratio between the decomposition of carbon species and consumption of oxygen species was 2:1. Which implies a potential reaction pathway where the UV irradiation initiated a photocatalytic reaction on anatase TiO2 surface between one adsorbed oxygen/water and two carbon species and caused decomposition of two carbon contaminant species. Eight different reaction cases were examined using eight TiO2 specimens treated by UV at four different intensities within natural sunlight radiation intensity range repeated twice each. The measured stoichiometry for the above noted conversion varied from 2.09 to 1.72 with an average value of 1.92 and a standard deviation of 0.11. The photon efficiency was estimated based on the number of carbon species decomposed by one UV photon, and the results showed the highest photon efficiency was obtained as 0.181 ‰ from 5-10 minutes UV exposure at 1.5 mW/cm2. All the reaction cases showed a significant efficiency drop after 30 minutes of UV irradiation due to the consumption of adsorbed reactant species. UV irradiation showed a high selectivity of hydrocarbon decomposition, which was 75%±5% for all the reaction cases. Earlier research on Ag/TiO2 adsorptive desulfurization showed 450 °C calcined 4wt% Ag/TiO2 adsorbent had a supreme sulfur removal capacity. It is proposed the capacity enhancement was due to the additional surface hydroxyl groups created by Ag2O on TiO2. However, upon UV irradiation treatment, the desulfurization capacity was significantly decreased. In the current investigation, in-situ time-resolved XPS characterization was carried out to understand the mechanisms of the photo-induced chemical reactions on 4wt% Ag/TiO2 adsorbent. When compared with the XPS spectral results from pure TiO2 and 4wt% HNO3/TiO2 samples, a pair of anomalous Ti 2p1/2 and Ti 2p3/2 peaks were observed by XPS characterization of 4wt% Ag/TiO2, which can be identified as Titanium hole species. It is also envisioned from high-resolution O 1s spectra that the Titanium hole species were stabilized by hydroxyls. High-resolution Ag 3d spectra showed Silver Oxide was reduce to Silver metal accompanied by the color of the white sample was turned into black. Time-resolved XPS characterization showed a conversion of Titanium hole, hydroxyls, and Silver metal into Ti4+, lattice Oxygen, and Silver oxide (sample color was less dark by this procedure). UV irradiation accelerated the conversion of Titanium hole species into Ti4+. After all the Titanium hole species was converted into Ti4+, further XPS/UV irradiation would not cause any spectral change to the sample. To observe again the spectral changes, the sample has to be re-calcined at 450 °C in flowing air. By quantitatively analyzing the deconvolution details of the photoelectron peaks, the reaction mechanism is proposed according to the following stoichiometry: Sample excitation: TiO2 + AgOH + hv → TiO2(OH) + Ag; Sample relaxation: 2TiO2(OH) + Ag + hv → 2TiO2 + AgO + H2O. Previous XPS studies on the state of dispersion and growth of Ag on TiO2 have shown that Ag/Ti intensity ratios increased linearly with Ag content up to 4 wt% and increased less significantly thereafter from 8 wt% to 20 wt% indicating nucleation and growth of Ag crystallites. Based on the earlier observations, the peak intensity of Titanium hole species at the first XPS characterization (exposed to X-ray irradiation for 2 minutes) and its relaxation period should also be dependent on the weight percentage loading of Ag on TiO2. Silver loading from 1wt% to 20wt% on 169 m2/g Titania was investigated. The Titanium hole species peak intensities at 2 minutes of X-ray exposure increased linearly with silver content up to 4wt% and increased less significantly thereafter. The relaxation period for all the Titanium hole species to be converted into Ti4+ was increased linearly from 1wt% to 4wt% but increased less significantly thereafter. All the above observations indicated that the dispersion of Ag on Titania support determines the Titanium hole species peak intensity at 2 minutes X-ray exposure. Therefore, it is proposed that the sample preparation procedure (incipient wetness impregnation and 450 °C calcination in flowing air) created Oxygen vacancies on Titania supports which are occupied by Silver and hydroxyls. With Silver and hydroxyls on Oxygen vacancies, photo irradiation created holes and electrons can be transferred to the Titanium bonded Silver and Titanium holes were stabilized by hydroxyls. Once Silver received electron to be reduced into isolated Ag metal, holes and electrons created by further photo irradiation would recombine and no more Titanium hole peak intensity would be observed. Under ultrahigh vacuum condition, photo irradiation would cause desorption of hydroxyls (in form of adsorbed H2O) and caused a decrease of Titanium hole species and hydroxyl peak intensity. Thus, the relaxation reaction occurred.