Defect-engineered or substoichiometric TiOx is of interest for use in photo- and electrocatalytic processes both as active material and catalyst support. Electrochemical doping of TiO2 via cathodic polarization is an appealing preparation method and frequently employed. Here, we explored the electrochemical preparation of TiOx in an undivided cell using iridium-based (iridium mixed-metal-oxide) and boron doped diamond (BDD) counter electrodes. Cyclic voltammetry and impedance spectroscopy revealed superior charge transfer properties of crystalline TiOx electrodes prepared with BDD (TiOx-BDD). It is shown that the electrochemical properties correlate well with intensities of the H-signals determined using Time of Flight - Secondary Ion Mass Spectrometry (ToF-SIMS). Thus, it is concluded that electrochemical preparation using BDD causes favourable H+ intercalation and/or H diffusion into the sub-surface layers of TiOx. Our extensive analysis using a combination of electrochemical and surface characterization (LEIS and XPS) techniques, additionally suggests that cathodic deposition of Ir, originating from the Ir-based counter electrode, present in sub-ppm concentrations only results in less-efficient doping. Instead in the presence of sub-ppm level Ir contamination hydrogen evolution is favoured during cathodic polarization. The results presented within this study highlight the necessity to use inherently stable counter electrodes for electrochemical preparation and reveal the pronounced influence of trace contamination in electrochemistry in general and the doping mechanism of TiOx electrodes in particular.

One of the major challenges in reusing cooling tower blowdown water (CTBD) utilizing membrane processes is its remaining organic compounds, e.g., humic substances leading to biofouling. Besides, the possible abundance of chloride in CTBD imposes the concern of the formation of chlorinated by-products. To choose a pre-treatment process for the studied CTBD composition, various advanced oxidation processes (AOPs), including electrooxidation (EO), photocatalytic degradation (PCD), heat-activated persulfate oxidation (PS), UVC/vacuum UV (UVC/VUV), and UVC processes, were evaluated and compared based on two main targets: i) highest removal and mineralization of the organics, especially humic substances; and ii) lowest formation of chlorinated byproducts including adsorbable organic halides and oxychlorides. All the processes were conducted in the natural condition of the real CTBD, while solution pH was monitored. Based on results of chemical oxygen demand, total organic carbon, dissolved organic carbon, UV254 absorbance, liquid-chromatography-organic carbon detection (LC-OCD), and fluorescence excitation-emission matrices (FEEM), it is concluded that PS leads to complete removal of organic compounds along with the lowest formation of low molecular weight organic acids and organic neutrals. FEEM and LC-OCD data also indicated that EO, PCD, and UVC/VUV processes brought about substantial removal of organic compounds and broke down the humic substances into low molecular weight building blocks and organics. Besides, EO exhibited the highest AOX and oxychlorides formation, while these were limited when using the other AOPs. Summarizing, PS, PCD, and UVC/VUV were efficient processes for the degradation and mineralization of organics without generating significant amounts of chlorinated byproducts.

Sub-stoichiometric titanium dioxide (TiOx) is known for its corrosion resistance and electrical conductivity.1 Thus, it is recognized as promising material for (photo-)electrochemical applications and used as protective layer in PEC water splitting or electrode material for water treatment.1,2 In general, its properties strongly depend on the synthesis and the conductivity of TiOx can be tuned by its Ti/O ratio.1,3 TiOx is commonly prepared by hydrogenation;3 however, the requirements for high pressure, high temperature and hydrogen atmosphere are cumbersome imposing a potential safety risk. Over the past years, electrochemical preparation of TiOx via cathodic polarization of TiO2 substrates became an interesting alternative.4 Pt is widely used in electrochemistry as counter electrode5 and frequently applied for TiOx preparation;4 however, the suitability of Pt as (anodic) counter electrode (CE) is questionable since the stability of Pt is often overestimated.5 As Pt contamination can severely influence the electrocatalytic properties this study focuses on the preparation of TiOx using three different CE (Pt, Iridium mixed metal oxide, Boron doped diamond) and the influence of the CE will be discussed in detail. It will be shown using state of the art techniques such as SEM, XPS and highly surface sensitive low energy ion scattering (LEIS) spectroscopy that indeed Pt contamination dominates the properties of TiOx (Figure 1a). In contrast, for Iridium mixed metal oxide and Boron doped diamond CEs no related contaminations were observed allowing to determine the intrinsic properties of TiOx electrodes. Using Fe2 +/3+ redox couple experiments and electrochemical impedance spectroscopy we determined the charge transfer properties of the TiOx electrodes prepared with BDD and IrMMO (Figure 1b). Although both BDD and IrMMO appear to be suited for the preparation of TiOx differences in the doping are observed that will be discussed in detail in this contribution. References: F. C. Walsh and R. G. A. Wills, Electrochim. Acta, 55, 6342–6351 (2010), doi: 10.1039/c6ra14507h B. Xu, H. Y. Sohn, Y. Mohassab, and Y. Lan, RSC Adv., 6, 79706–79722 (2016), doi: 10.1039/c6ra14507h. X. Yan, L. Tian, X. Tan, M Zhou, L. Liu and X. Chen, MRS Commun., 6, 192–203 (2016), doi: 10.1557/mrc.2016.33. J. Swaminathan, R. Subbiah, and V. Singaram, ACS Catal., 6, 2222–2229 (2016), doi: 10.1021/acscatal.5b02614. J. G. Chen, C. W. Jones, S. Linic, and V. R. Stamenkovic, ACS Catal., 7, 6392–6393 (2017), doi: 10.1021/acscatal.7b02839. Figure 1

We evaluated electrochemical degradation (ECD) and photocatalytic degradation (PCD) technologies for saline water purification, with a focus on rate comparison and formation and degradation of chlorinated aromatic intermediates using the same non-chlorinated parent compound, 4-ethylphenol (4EP). At 15 mA·cm-2, and in the absence of chloride (0.6 mol·L-1 NaNO3 was used as supporting electrolyte), ECD resulted in an apparent zero-order rate of 30 μmol L-1·h-1, whereas rates of ∼300 μmol L-1·h-1 and ∼3750 μmol L-1·h-1 were computed for low (0.03 mol·L-1) and high (0.6 mol·L-1) NaCl concentration, respectively. For PCD, initial rates of ∼330 μmol L-1·h-1 and 205 μmol L-1·h-1 were found for low and high NaCl concentrations, at a photocatalyst (TiO2) concentration of 0.5 g·L-1, and illumination at λmax ≈ 375 nm, with an intensity ∼0.32 mW·cm-2. In the chlorine mediated ECD approach, significant quantities of free chlorine (hypochlorite, Cl2) and chlorinated hydrocarbons were formed in solution, while photocatalytic degradation did not show the formation of free chlorine, nor chlorine-containing intermediates, and resulted in better removal of non-purgeable hydrocarbons than ECD. The origin of the minimal formation of free chlorine and chlorinated compounds in photocatalytic degradation is discussed based on photoelectrochemical results and existing literature, and explained by a chloride-mediated surface-charge recombination mechanism.

TiO2 is one of the most attractive photocatalysts, with applications in water splitting, wastewater treatment, and air purification. Understanding the fundamentals of the functioning of TiO2 requires knowledge of the nature and dynamics of photo-induced excitons and charge carriers. Although photoluminescence (PL) spectroscopy can provide important fundamental insights, photophysical mechanisms are still under debate. To address this problem, the aim of the present work is to investigate the evolution of the PL spectrum in time of nanostructured anatase TiO2 thin films and the nature of associated processes, at room temperature and in aqueous media closely resembling photocatalytic conditions. We show that the PL spectrum of commonly used nanostructured anatase TiO2 thin films in aqueous media is time-dependent, with pH-dependent broadening at the low energy side of the spectrum in time. By global analysis of the spectrotemporal PL behavior and the effect of addition of NaCl at neutral and mildly acidic conditions, we show that this spectral development is due to an increasing contribution of processes sensitive to surface termination relative to bulk processes to the PL in time. The time-dependent PL spectrum and dynamics can be assigned to the recombination of mobile electrons populating the conduction band or shallow traps with immobile hole polarons in deep traps and motion of electrons from the nanoparticle bulk toward the depletion layer/surface in ca. 1 ns. This directionality likely plays an important role in the photocatalytic performance of nanostructured anatase TiO2 and effects of ions such as chloride in aqueous media. Control of the directional motion of electrons and suppression of surface charge recombination via surface engineering show promise to further increase the photocatalytic activity.