Photochemical Properties of Precipitated Titania Aerosol Produced by Powdering of Crystal Rutile under Ambient Air

It is shown that the quantum yields of oxygen photoadsorption and carbon dioxide photodesorption on TiO2 are signi�cantly higher under illumination by quanta with energy from the surface absorption region of TiO2 when it is not produced by the traditional way, but from the rutile crystal. e magnitudes and spectral dependencies of the quantum yield of photoadsorption and photodesorption for TiO2 produced from a monocrystal are determined. A mechanism of a sharp increase of the titania photoadsorption activity in the surface absorption region is proposed.


Introduction
e Earth's atmosphere has a capacity for self-cleaning from various gaseous pollutants under photoinduced heterogeneous processes over aerosols particle surfaces.To determine the separation efficiency it is necessary to obtain quantitative data on efficiency of photoinduced processes under atmospheric conditions and to simulate conditions of aerosol particle creation in the troposphere.
e creation of troposphere aerosol particles with �ne crystallised structure was the result of a dispersion of various lithosphere minerals over very long time [1].us, aerosol particles are microcrystals with a sufficiently ideal crystalline structure as opposite to particles of the high-dispersion adsorbents (photoadsorbents) and catalysts (photocatalysts) produced by traditional way, that is, under conditions of preparation TiO 2 with a high speci�c surface in laboratory and industry.e powder-like samples from minerals can be expected to have properties differing from those of the arti�cially produced compounds.In accordance with the published data, photocatalytic reactions over semiconductor metal oxides [2], as well as photosorption processes in oxide insulators [3], include a stage of transfer of electrons and holes formed under illumination to the surface.Consequently, the probability that free carriers will reach the surface and change the reactivity of surface atoms of the lattice or adsorbed atoms and molecules depends on the conditions of the carriers transfer in the bands (valence and conductivity bands).At small sizes of particles (highdispersion oxides insulators -Al 2 O 3 and SiO 2 with a spe-ci�c surface of 150 and 300 m 2 ⋅ g −1 , resp.), these bands fail to be formed and the photosorption activity relative to halogen-containing organic compounds has not been observed [3].
A signi�cant part of the tropospheric continental aerosol mainly consists of well-crystallised silicate and quartz microparticles.In this case, photosorption activity relative to halogen-containing organic compounds is observed.As data [4][5][6] show, efficient photoinduced processes actually proceed on particles of the coastal or desert sand with the participation of organic and halogen organic compounds.To simulate conditions for producing well-crystallised particles in tropospheric aerosol, the powder-like titanium dioxide was made through grinding a titania crystal in rutile modi�cation under ambient air.is paper is devoted to the photochemical properties of the titania dispersed powders obtained from a monocrystal.

Experimental
e titania crystal in rutile modi�cation produced by the Czochralski method was broken and grinded in a corundum mortar.e speci�c surface of prepared powder was equal to 1.6 m 2 ⋅ g −1 .
e X-ray diffraction spectrum of the powder-like sample was recorded with an HZG-4C diffractometer using CuK radiation.According to the results of the X-ray analysis, the titanium dioxide sample matched the titanium dioxide of the rutile crystalline structure.
e titania spectra were subjected to transmission electron microscopy (TEM) analysis on a JEOL model JEM-2010 instrument operated at acceleration at 200 V (Figure 1).
To expose the oxides under UV radiation, an OSL-1 illuminator with a high-pressure mercury lamp (DRSH-250, power 250 �) including a thermal water �lter and UV �lter (transmission band between 270 and 390 nm) was used.e total radiation �ux density reaching the reactor�s surface and recorded by an RTH-20C thermopile was equal to ∼1 milliwatt ⋅ cm −2 for this �lter.To separate monochromatic radiation, interference �lters were used.
e diffuse re�ectance spectra of the powder-like titania were recorded with a SPECORD M-40 spectrophotometer under ambient air.In the studies a powder-like magnesium oxide was used as the reference standard.
e titanium dioxide as a water suspension was applied to the interior wall of the cylindrical quartz reactor and dried in air at room temperature for a week.Aer soldering to a high vacuum setup, the reactor with the sample was pumped out at room temperature for 20 min.Later on the pumping out was performed through a trap with a cooling liquid (ethyl alcohol cooled to 173 K) in order for water in the gas phase of the reactor volume and at the oxide surface to be present continuously.e prolonged evacuation of the reactor volume for 1 hour in high-vacuum installation through a trap resulted in partial removal of carbon dioxide.As a result, quasi-equilibrium �lling of the surface with CO 2 was established.
e amount of O 2 , N 2 O, NO, and CO 2 molecules, halogen-containing organic compounds, and products of their interaction with the titania surface were determined using a Pirani gauge and mass spectrometer by sampling gas from the reaction volume through a dosing valve.
e quantum yield of the photoadsorption (photodesorption) was found as the ratio of the quantity of photoadsorbed (photodesorbed) molecules to the number of quanta passed through the reactor's frontal (transparent) wall.

Results and Discussion
Aer pumping out the reactor through the trap cooled down to 173 K, mainly NO gas evolved from the titania surface (fourfold greater than CO 2 accumulated inside the reactor).Our earlier measurements of the amount and composition of gases evolved from the oxide surfaces in the dark (with other metal oxides produced in laboratory or industrial conditions) were characterised mainly by CO 2 desorption.us, the amount of carbon dioxide evolved from the MgO surface is equal approximately to 10% of the monolayer, and nitrogen oxides (predominantly N 2 O) are equal approximately to 1% [7].
It can be supposed that nitrogen oxide (II) is produced in a sufficient amount when grinding the TiO 2 crystal in air due to molecular nitrogen oxidation at centres formed at the break of the Ti-O bonds of the titania lattice.e NO generation is also possible through the well-known reaction of N 2 O decomposition at the electron donor centre of the metal oxide surface [8,9].In this case, N 2 O is adsorbed from air like the carbon dioxide.
e dark adsorption of Freon 134a (CF 3 -CH 2 F) or Freon 22 (CHF 2 Cl), insigni�cant in magnitude (0.03% of the oxide surface monolayer at a pressure of ∼10 −2 Torr inside the reactor volume), is observed for the powder-like titania produced from a crystal of the rutile crystalline modi�cation.e adsorption kinetics of these gases are presented in Figure 2. 3.1.Photodesorption.When illuminating the titania surface through a �� �lter in the presence of Freon 134a or Freon 22 (aer saturation of the dark adsorption of these gases [7,8]), the carbon dioxide photodesorption is observed, while the photoadsorption of the Freons is low.
e transmission spectrum of the �� �lter is given, for example, in [10].e CO 2 photodesorption kinetics in the presence of Freon 134a or Freon 22 is shown in Figure 3.
e kinetics of photodesorption is characterised by the fast desorption of carbon dioxide in the initial time of illumination (up to 3-5 min), followed by slow desorption with a constant rate.In the �rst case, probably, desorption is connected with classic CO 2 photodesorption under illumination of the surface under quanta with energy from the fundamental absorption band of TiO 2 [11].In the second case, desorption is attributed to the photocatalytic reaction of the oxidation oxygen of an oxide of the by adsorbed carbon-containing compounds (e.g., carbon oxide from air) followed by reducing the titanium dioxide surface [12].During this process, as it follows from mass spectrometric data, the magnitude of mass peak 44 increases by several times (Figure 4, curve 1).However, photocatalytic oxidation of the considered Freons by surface oxygen does not occur, because peaks 33 (Freon 134a) and 51 (Freon 22) do not decrease under illumination both through the �� �lter and without a �lter (Figure 4, curve 2).
e titanium dioxide is characterised by the absence of photocatalytic activity in the oxidation of methane, ethane, and their halogen derivatives like Freon 134a (CF 3 -CH 2 F) and Freon 22 (CHF 2 Cl) [13].However, this activity of TiO 2 relative to carbon oxide, unsaturated hydrocarbons, carbon acids, alcohols, and other organic compounds is very considerable [14].
Measurements of the CO 2 photodesorption rate under illumination by monochromatic light at different wavelengths (via the use of interference Filters) and light �ux intensities allowed to calculate quantum yield of the CO 2 photodesorption.e calculation results are given in Figure 5 (curve 1).Curve 1 shows the diffusion re�ection spectrum of the powder-like TiO 2 measured relative to magnesium oxide as the reference standard.

Photoadsorption. e photoadsorption activity of titania
produced from a monocrystal of the rutile crystalline modi�cation (TiO 2 -I) was compared with that of the titania sample produced from powder-like TiO 2 of the anatase modi�cation (TiO 2 -II).e anatase was heated at 1273 K for two hours in air in order to obtain from the anatase phase (basically) the rutile phase (basically).Further, the surface of TiO 2 -II obtained in such a way was cleaned from the adsorbed layer by long-time, high-temperature, oxygen vacuum treatment.Manometric and mass spectrometric data have shown the absence of the dark adsorption of oxygen on TiO 2 -II, which probably is connected with the oxygen preadsorption during the sample cooling in oxygen.e dark adsorption was also absent for the TiO 2 -I sample, because the surface at its formation through grinding the crystal was saturated with oxygen from air.Moreover, the surface reconstruction by means of high-temperature treatment in a vacuum or in some reducing atmosphere was not conducted in the experiment.
e spectral dependence of the quantum yield of oxygen photoadsorption for the TiO 2 -sample is presented in Figure 6 (curve 1).Of interest are high-photoadsorption quantum yields at    nm (corresponding to the TiO 2 surface absorption band), which are close in magnitude to quantum yields in the range of the TiO 2 absorption.At a wavelength of about 465 nm, the quantum yield is 0.7%; at  = 340 nm, it is 0.75% (Figure 6, curve 1).e spectral dependence of the photoadsorption quantum yield for the TiO 2 -II sample is shown by curve 2. Comparing, the di�usion re�ection spectrum for TiO 2 -II is also given.e quantum yield at a wavelength of 465 nm is 0.03% and at 340 nm is 0.15% for this sample.Close quantum yields in the surface absorption band were observed also for titanium dioxide produced through the burning in air of a pyrotechnic mixture including titanium microparticles [15].e absorption in the range of energies of quanta less than the TiO 2 forbidden band can be related to electron transition from deep surface levels of the oxide to the conduction band [16].At the same time, the oxygen photoadsorption proceeds on the electron localised in the surface trap of the semiconductor oxide [9,17].us, it can be supposed that the sharp increase of the quantum yield in the range of TiO 2 surface absorption relative to oxygen is connected with the increase of deep surface levels formed at a break of the Ti-O bonds when producing powder-like oxide from a monocrystal.

Conclusion
It is discovered that in the band of solar tropospheric radiation (   nm), the photochemical activity of titanium dioxide produced through grinding the monocrystal under ambient air is signi�cantly higher than that of the compound produced by traditionally way, that is, TiO 2 prepared with a high speci�c surface.
It can be expected that the photochemical properties of other oxides produced from minerals (MgO from periclase, SiO 2 from quartz, Al 2 O 3 from corundum, etc.� will signi�cantly differ from the properties of compounds produced in laboratory or industrial conditions.e formation of oxide particles from mineral crystals under natural conditions is accompanied by the formation of photochemically active solid tropospheric aerosol.

F 1 :
TEM picture of the titania powder produced by grinding of crystal rutile.

F 4 :F 5 :
peak 44 (rel.u.) Magnitude of mass peak 51 (rel.u.) Time (s) Kinetics of the variation of peaks 44 (1) and 51(2) of masses in the mass spectra of CO 2 and Freon 22, respectively, under illumination of the TiO 2 surface through the �� �lter in the presence of Freon.Spectral dependencies for the titanium dioxide sample produced from a monocrystal: (1) the quantum yield of CO 2 photodesorption in the presence of Freon 134a and (2) the optical density relative to MgO.