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Ruthenium-Modified Titanate Nanowires for the Photocatalytic Oxidative Removal of Organic Pollutants from Water Beatriz Barrocas, M. Conceição Oliveira, Helena I.S. Nogueira, Sara Fateixa, and Olinda Coelho Monteiro ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b02215 • Publication Date (Web): 07 Feb 2019 Downloaded from http://pubs.acs.org on February 9, 2019
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Ruthenium-Modified Titanate Nanowires for the Photocatalytic Oxidative Removal of Organic Pollutants from Water Beatriz T. Barrocas,a M. Conceição Oliveira,b Helena I.S. Nogueira,c Sara Fateixa,c Olinda C. Monteiroa,* a
Centro de Química Estrutural and Centro de Química e Bioquímica, Faculdade de Ciências, Universidade de Lisboa, 1749-016 Lisboa, Portugal b Centro Química Estrutural, Instituto Superior Técnico, ULisboa, 1049-001 Lisboa, Portugal c Department of Chemistry - CICECO, University of Aveiro, 3810-193 Aveiro, Portugal * Corresponding author:
[email protected] KEYWORDS: Titanate nanowires; ruthenium incorporation; structural rearrangement; Ru-Na exchange; Ru-Ti replacement; charge recombination; caffeine; photocatalytic oxidation. ABSTRACT Titanate elongated nanomaterials have been studied as promising catalysts for photo-assisted oxidation processes, and various methods have been used to tailor their properties. In this context, the synthesis and photocatalytic evaluation of novel ruthenium modified titanate nanowires is described. In this work, pristine (TNW) and modified nanowires (RuTNW) were obtained through the hydrothermal treatment of an amorphous precursor and they were characterized by XRD, Raman, XRF, XPS, TEM, DRS and PL. The results indicate some alterations on the structure and on the optical properties of these semiconductor nanoparticles, owing to ruthenium incorporation. Regarding the structure, several possible Ru positions can be anticipated: in the TiO6 octahedra, substituting Ti4+, or localized in interstitial sites, or in the interlayers, replacing some Na+. Anticipating their potential use for oxidation photocatalysis, namely for pollutants removal, the samples were evaluated for hydroxyl radical production, using the probe molecule terephthalic acid. Both samples were catalytic for this photo-activated process, with RuTNW being the best photocatalyst. Afterwards, the degradation of caffeine, used as a model pollutant, was evaluated under UV-vis and visible radiation. Regardless of the radiation type in use, a clear improvement on TNW photocatalytic performance was observed after Ru incorporation. In fact, RuTNW was the best catalyst for caffeine photodegradation (20 ppm; 0.13 g/L), with a complete pollutant removal after 60
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min, using UV-vis radiation. Through the identification and quantification of the intermediates produced during irradiation, a longer time (more than 120 min) is however required to complete the degradation process. A proposal for the photo-generated charge-transfer mechanism in these photoactivated processes is also proposed and discussed.
1. INTRODUCTION At the moment, millions of pharmaceuticals and personal care products (PPCPs) are extensively used and discharged in the environment after being used. Some of them are extremely difficult to be degraded and their accumulation in nature results in dramatic effects for humans and other living beings. Numerous removal approaches have been evaluated to tackle this issue but improvements on the effectiveness and efficiency of such methods are mandatory. Of all these methodologies, several advanced oxidation processes have been studied for PPCPs removal, and photocatalytic degradation is one of the most promising [1]. Nanocrystalline TiO2 is the most studied semiconductor, regarding pollutants photocatalytic degradation [2,3]. Even knowing that its photocatalytic properties are excellent, there are still some limitations that need to be overpassed before TiO2 applications scale-up, including high photogenerated charge carriers recombination and reduced absorption of Sun energy (Eg = 3.2 eV). Titanate elongated nanostructures have been regarded as an excellent alternative to TiO2, mainly in photocatalytic applications [4]. The crystalline structure of these elongated nanoparticles is formed by TiO6 octahedra layers, very similar to the nanocrystalline TiO2 structure, but with Na+ ions localized in the interlayers. Due to this crystalline arrangement and to the considerable higher surface area, the photocatalytic properties of elongated titanate are even better than the TiO2 ones. Furthermore, the excellent ion exchange ability of these lamellar materials, conferred by the Na+ high mobility, can be seen as an added value. For instance, to produce upgraded photocatalysts, by incorporating foreign active species, such as metallic or other cationic entities [5,6]. Depending on the synthesis
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methodology, these foreign metallic ions can replace Na+, and/or can substitute some Ti4+ in lattice positions, or can be localized in interstitial sites. In this context, improvements on photocatalytic performance were recently obtained, using Co modified titanate nanoparticles, with the dopant localized in two distinct positions: partial substitution of Ti4+ and replacement of Na+ in the interlayers [7]. Ruthenium, a 4d transition metal, has been described as a dopant element that can significantly promote photogenerated electron–hole pairs separation and can extends the material light absorption to the visible range, by inducing the creation of intermediate bands (IB) within the forbidden zone. This will promote a redshift in the absorption band edge, with the IB acting not only as intermediate energetic levels, during photo-activation, but also as recombination center, extending the lifetime of the charge carriers [8,9]. For a Ru doped TiO2 material, with a partial substitution of Ti with Ru, the probability of success for both excitation and recombination is conditioned by the bandgap energy and on the IB energetic position [9]. If a forbidden zone is split into 2 sub-gaps regions owing to an IB creation, it is desired that the lower gap will be the narrower one. With this, the probability for driving an electron from the valence band (BV) up to the IB would be higher than the possibility to have an electron, from the conduction band (CB), combining with a hole located at the same IB. Thus, the IB can behave as an efficient step to promote the relay of the electrons into the CB [9]. The Ru oxidation state is critical in the photocatalyst performance. For example, if Ti4+ is substituted by Ru3+ and/or Ru2+, a new electron donor level is formed, whereas if Ti4+ is substituted by Ru4+ and/or Ru5+ species, an acceptor level in the bandgap is formed, to preserve the total charge balance. Either way, the charge transport and transfer pathway will be altered. As an outcome, the substitution of Ti4+ in TiO2 based systems by both Ru4+ and Ru3+ has been proposed [10].
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Several methodologies have been described to produce Ru modified TiO2 based materials, with some of them producing RuO2-TiO2, or TiO2 combined with Ru metallic nanoparticles. Ru-doped TiO2 nanofibers have been produced using electrospun Ru-TiO2/poly(vinyl acetate) fibers, [11] and monodispersed sea urchin-like Ru-doped rutile TiO2 architectures have been successfully synthesized using an acid-hydrothermal method and low temperatures [8]. TiO2-RuO2 nanocomposites have been described as photoactive for acetone decomposition under visible light. In this configuration, RuO2 is essential for composite material bandgap decreases [12]. The synthesis of TiO2 nanotubes combined with metallic Ru nanoparticles, via impregnation methods, has also been reported. These nanocomposite particles are catalytic for vinyl acetate and cyclohexene hydroformylation, [13] and can also be used for the Fischer-Tropsch synthesis [14]. The absorption edge of nanocrystalline TiO2 can be extended via Ru doping, through the formation of new energetic levels in the bandgap, [12] leading to modifications on the electron-hole recombination process. This can be further improved by using iron (III) ions as electron acceptors [15]. Based on these findings, this work describes the incorporation of Ru in titanate nanowires (TNW) and the evaluation of their photocatalytic performance on pollutants removal. Due to its environmental impact and resistance to degradation, caffeine, an emergent pollutant, was chosen as model. Here, Ru-modified titanate nanowires (RuTNW) were produced through the hydrothermal treatment of an amorphous doped precursor containing Ru. After characterization, the prepared samples were used as photocatalyst for hydroxyl radical production and, afterwards, for caffeine photo-assisted oxidation. The obtained results indicate that the incorporation of Ru in the TNW structure induces structural and optical modifications and clearly contributes to the improvement of these materials photocatalytic performance.
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2. EXPERIMENTAL All used reagents were from Aldrich and Across (analytical or chemical grade) and were used as received. To prepare the solutions, distilled water was used as solvent. 2.1. Synthesis of the Undoped and Ru-Doped Amorphous Precursor The TNW amorphous precursor was prepared based on a published procedure [16]. The obtained solid was used for TNW production. A similar procedure was followed to produce the Ru-containing precursor, by adding the required molar amount (1%, nominal) of RuCl3 to the titanium containing solution. 2.2. TNW and RuTNW Synthesis The pristine and Ru-modified TNW particles were produced using a reported procedure using 160 °C and 24 hours of reaction [6]. The two powders obtained through the hydrothermal treatment of the amorphous and the Ru-containing precursors were labeled as TNW and RuTNW respectively. 2.3. Photodegradation Studies The photocatalytic degradation experiments were performed using a photo-reactor (250 mL) with refrigeration previously described [17]. For the visible experiments, a Pyrex filter was used to remove the UV range, cutting off the wavelengths below 400 nm. Before being irradiated, the suspensions were stirred during 1 hour in the dark, to allow adsorption equilibrium. The photo-assisted photodegradation runs were performed using a 20 ppm caffeine aqueous solution (150 mL, 0.13 g cat/L). Three runs were performed for each photodegradation experiment, (error
