Ultrasonic Tuning of TiO2 Based Mixed Oxides Structural Properties and Catalytic Activity

Ultrasound (US) has a potential to impact every stage of the preparation of a material in liquid phase. US can produce nanomaterials in amorphous state (due to the very high cooling rates), which is relevant to fields such as catalysis, magnetism and coating processes [1]. US generates shock waves that facilitate coating and insertion/intercalation processes and improves the distribution of the active phase on a support. We pursue ultrasound powder synthesis as a greener method to lower solvent amount and catalyst precursor, as well as to accelerate lengthy preparation method such as sol-gel and impregnation. US-assisted synthesis produces catalysts with better morphology and activity. Simple tuning of operating conditions increases the range of obtainable features for different purposes [2,3].

We report the ultrasound assisted sol-gel and impregnation synthesis of sulphated ZrO₂-TiO₂ and Mn oxides-TiO₂ systems. We characterized the catalysts with N₂ adsoption, XRD, SEM-EDX and TEM. We tested the sulphated ZrO₂-TiO₂ in the free fatty acids (FFA) esterification reaction and the sulphated ZrO₂-TiO₂ and the Mn_xO_y-TiO₂ systems in the ultrasound-assisted photocatalytic degradation of recalcitrant water pollutants (pharmaceutical products). We adopted a ultrasound probe with a tip of 13 or 20 mm diameter. The ultrasound operating frequency was fixed at 20 KHz. The nominal power was 500 W and we varied the amplitude from 20 to 60 % adopting continuous ultrasound or ultrasound pulses (0.5 on and 0.5 off). In the sol-gel synthesis of sulphated ZrO₂-TiO₂ systems, ultrasound pulses increased the surface area up to 220 m²g^-1 for the lowest solvent: precursors ratio of 15:1 mol, corresponding to the highest power density. Solvent: precursors molar ratios of 30:1 and 60:1 gave a SSA of 150 m²g^-1 and 30 m²g^-1, respectively. The samples obtained with pulses were also the ones exhibiting the highest Brønsted surface acidity. The catalysts with the highest acidity/SSA ratio were the most active in the free fatty acids esterification reaction.

We deposit Mn_xO_y on TiO₂ in one step with the ultrasound-assisted sol-gel synthesis or via ultrasonic impregnation in a second step. We prepared Mn/TiO₂ catalyst in one step with a classical sol–gel method, starting from butyl-titanate, deionized water, nitric acid, anhydrous ethanol, and manganese nitrate [4]. After mixing at room temperature to yield a yellowish transparent sol, we dried the precursor at 80°C for 24h, converting it into a xerogel. The calcination to obtain the Mn/TiO₂ catalyst was standard at 500°C for 5h. We prepared the same catalyst applying US, either maintaining the same calcination parameters or decreasing the calcination temperature to 400°C for 2 hours to trigger the formation of anatase instead of rutile as an active phase.

Mn/TiO₂ two steps synthesis consisted of the US-assisted surface deposition of Mn-oxides on TiO₂ (both nanometric or micrometric), synthesized in a preceding stage [5]. Generally, US promote homogeneous nucleation in a first step, and enhance the crystals growth in a second stage. Acoustic cavitation, in particular for the two-steps Mn deposition, generates high-speed micro jets which impact the TiO₂ support with a cleaning and de-agglomerating effect. Ultrasound increases the dispersion of the Mn species over both supports and has a role in stabilizing Mn_xO_y species that samples synthesized with traditional sol-gel don’t normally exhibit (preliminary XRD results) [6]. The modulation of the main sonication operating parameters produces a mixed-size distribution of Mn and Mn-Ox species on TiO₂ rather than a narrow distribution of small crystallites, which often turns out to be better for the photocatalytic activity. US also improves the distribution of Mn_xO_y over micro-metric TiO₂ (SEM-EDX analysis), which is meaningful from a safety point of view since body barriers may not “recognize” nano-powders. The photocatalytic activity of Mn_xO_y over nano and micro-size TiO₂ is comparable. Considering instead the comparison between Mn and other metals, Mn is cheaper and one of a kind, considering the variety of oxides that can form. By means of US we can also modulate and enhance the formation of specific oxidation states [6,7].

References

[1] T.J. Mason, Ultrasound in synthetic organic chemistry, Chem. Soc. Rev., 26 (1997), pp. 443–451.

[2] S.J. Musevi, A. Aslani, H. Motahari, H. Salimi, Offer a novel method for size appraise of NiO Nanoparticles by PL analysis: synthesis by sonochemical method, J. Saudi Chem. Soc., 20 (2016), pp. 245–252.

[3] C. Pholnak, C. Sirisathitkul, D.J. Harding, Characterizations of octahedral zinc oxide synthesized by sonochemical method, J. Phys. Chem. Solids, 72 (2011), pp. 817–823.

[4] Qi-lin Chen, Rui-tang Guo, Qing-shan Wang, Wei-guo Pan, Ning-zhi Yang, Chen-zi Lu, Shu-xian Wang, J. Taiwan Inst. Chem. Eng., 64 (2016) 116–123.

[5] M. Stucchi, C.L. Bianchi, C. Pirola, G. Cerrato, S. Morandi, C. Argirusis, G. Sourkouni, A. Naldoni, V. Capucci, Ultrason. Sonochem., 31 (2016) 295–301.

[6] M.I. Díez-García, V. Manzi-Orezzoli, M. Jankulovska, S. Anandan, P. Bonete, R. Gómez, T. Lana-Villarreal, Effects of ultrasound irradiation on the synthesis of metal oxide nanostructures, Phys. Procedia, 63 (2015), pp. 85–90.

[7] K.D. Bhatte, D.N. Sawant, D.V. Pinjari, A.B. Pandit, B.M. Bhanage, One pot green synthesis of nano sized zinc oxide by sonochemical method, Mater. Lett., 77 (2012), pp. 93–95.