Nanocomposites and nanomaterials

Effect of heat treatment on surface acid sites of iron-doped titanium dioxide

Manuilov E. V.1, Shymanovska V.V.1, Khalyavka T.A.2, 
Melnyk A.K.2, Shcherban N.D.3

1 Institute of Physics, NAS of Ukraine, Prospect Nauki, 46, Kyiv-03039, Ukraine, E-mail: evm.sem18@gmail.com
2 Institute for Sorption and Problems of Endoecology, NAS of Ukraine, General Naumov str., 13, Kyiv-03164, Ukraine 
3 L.V. Pisarzhevskii Institute of Physical Chemistry, NAS of Ukraine, Prospekt Nauky, 31, Kyiv-03028, Ukraine 

Doped titanium dioxide is widely used in photocatalysis because doping causes the change of physico-chemical properties of TiO2 often increasing its photocatalytic activity [1, 2]. The aim of this work was to obtain the samples of titanium oxide modified by iron and investigate the effect of heat treatment on their surface. 
The polydisperse nanocrystalline pure samples of anatase (A) and rutile (R) were modified via adsorption of Fe3+ ions (A/Fe, R/Fe) from FeCl3 aqueous solutions with subsequent drying at 150 °C, washing with bi-distilled water and calcination at 300, 600 and 900 °C in air. 
From EPR spectra, the signals with a hyperfine structure for TiO2/Fe (anatase and rutile) samples calcined at 300 °C, 600 °C and 900 °C were observed. Such spectrum indicates surface bound single Fe ions on rutile or anatase TiO2. EPR spectra of anatase A/Fe-300 °C, A/Fe-600 °C and rutile R/Fe-900 °C show only one paramagnetic intense signal at g = 1.995 and g = 1.998, respectively. This indicates the presence of oxygen vacancies as a dominant defect in the samples. For A/Fe-900, EPR resonance transitions at g =8.09, 5.57, 3.47 and 2.65 are observed. Transition at g = 4.22 (4.23) can be ascribed to the Fe (III) ions in orthorhombic sites at the surface of rutile R/Fe-300. The broad peaks detected at g = 2.005 are ascribed to photogenerated holes trapped by lattice oxygen and rich concentration of hole centers such as Ti vacancies in the system.
According to the data of temperature-programmed desorption of ammonia (TPDA) and pyridine ad(de)sorption with IR-spectral control it can be concluded that anatase surface contains active both Lewis and Brønsted acid sites (LAS and BAS) while rutile samples have only LAS regardless of the thermal treatment [3]. The revealed acid sites can be related to weak (the maximum of ammonia desorption < 300 ºC) and strong (Tmax of desorption > 400 ºC) ones. The anatase Fe-doped TiO2 possesses significantly higher concentration of acid sites compared to Fe-doped rutile (Fig. 2, a). An increase in the calcination temperature from 300 to 900 ºC leads to a significant decrease of the concentration of weak acid sites for Fe-doped anatase samples (Fig. 2, b). 

a	b	a	b	
Fig. 1. EPR spectra of Fe/TiO2 rutile (a) and anatase (b) samples calcined at different temperatures (1- 300, 2- 600, 3- 900 °C).	Fig. 2. NH3-TPD profiles for the prepared samples: a: 1 – pure R, 2 – pure A, b: 1 – A/Fe (300 ºC), 2 – A/Fe (600 ºC), 3 – A/Fe (900 ºC). 

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