ISSN 0036-0244, Russian Journal of Physical Chemistry, 2006, Vol. 80, No. 2, pp. 294–295. © Pleiades Publishing, Inc., 2006. Original Russian Text © A.N. Emel’yanov, N.V. Shcherbakov, M.V. Vishnetskaya, D.P. Shashkin, Yu.N. Rufov, 2006, published in Zhurnal Fizicheskoi Khimii, 2006, Vol. 80, No. 2, pp. 363–364.
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Emission of Active Oxygen Forms from a Deposited Bismuth Oxide A. N. Emel’yanov*, N. V. Shcherbakov*, M. V. Vishnetskaya*, D. P. Shashkin**, and Yu. N. Rufov** * Gubkin State Academy of Oil and Gas, Leninskii pr. 65, Moscow, 117917 Russia ** Semenov Institute of Chemical Physics, Russian Academy of Sciences, ul. Kosygina 4, Moscow, 117977 Russia E-mail:
[email protected] Received December 23, 2004
Abstract—The specific features of the emission of singlet oxygen from deposited bismuth oxide were examined. It was established how the rate of the emission of singlet oxygen depends on the concentration of deposited bismuth oxide. The largest amount of singlet oxygen was emitted from the sample with a crystalline structure. It was shown that only singlet molecular oxygen is emitted from deposited bismuth oxide. DOI: 10.1134/S0036024406020324
Bismuth oxide is a component of selective catalysts for the partial oxidation of unsaturated and aromatic hydrocarbons [1–3]; therefore, the generation of active oxygen forms by bismuth oxide is of considerable interest. It was discovered [4, 5] that, at 470–520°ë, bulk bismuth oxide emits atomic oxygen, which recombines in the gas phase to yield ozone. We performed a series of experiments to study these processes on deposited bismuth oxide. The concentration of singlet molecular oxygen in the gas phase was determined as described in [6]. The experiments were performed with bismuth oxide deposited on silica gel (Ssp = 120 m2/g) in amounts of 6.0, 3.0, and 1.5 at % (the ratio of the number of bismuth atoms to the total number of atoms in the Bi2O3/SiO2 composition). To remove water, the silica gel was preliminary calcined at 500°ë for 2 h. The samples were prepared by depositing bismuth nitrate from a solution on silica and calcining the composite obtained (the procedure was described in detail in [3]). The concentration of ozone in the gas phase was measured as in [7]. This procedure is similar to that described in [6] except that eosin was used in [7] as a detector, a compound the reaction of which with ozone is characterized by a high quantum yield. A joint use of two selective methods [6, 7] for determining active oxygen forms made it possible to establish that the samples under study emitted only singlet molecular oxygen—no other active forms of oxygen were observed. The crystallinity of samples was analyzed by the XRD method on a DRON-3 diffractometer with a CuKα radiation source. X-band EPR spectra were recorded on an EPR-V spectrometer, using CuSO4 · 5ç2O and chloride Mn2+–MgO as references. All the samples exhibited two types of singlet oxygen emission. Nonequilibrium amounts of singlet oxygen were first detected at 350–375°ë, with the amount of 1∆gO2 depending on the concentration of deposited of bismuth oxide. The first superequilibrium-emission of oxygen from the sample occurred in the form of a
single peak, while the second nonequilibrium emission exhibited several peaks. The second nonequilibrium emission from all the samples was observed at a fixed temperature (480°ë). It was established that the second emission of 1∆gO2 starts at 480°ë. The data on the specific amount (per gram of sample) of singlet oxygen released as a function of the concentration of deposited bismuth oxide are given in the table. XRD analysis showed that the samples containing 1.5 and 3.0 at % deposited bismuth oxide were amorphous. By contrast, the sample containing 6.0 at % deposited bismuth oxide yielded a well-resolved diffractogram (figure). This means that, in the sample containing 6 at % of deposited bismuth oxide, a SiBi2O5 surface compound with a tetragonal or rhombohedral structure is formed, as follows from the diffractogram displayed in the figure. The XRD data obtained provided no evidence of αBi2O3 transition; therefore, the formathe βBi2O3 tion of atomic oxygen associated with the βBi2O3 αBi2O3 phase transition can be excluded [4, 5]. Emission of singlet oxygen from bismuth oxide deposited on silica gel in various concentrations
294
[Bi2O3], % t(I), °C 1.5 3 6
373 359 351
1∆ O (I) g 2
× 10–15, 1∆gO2(II) × 10–15, molecules/g molecules/g 1.3 1.83 5.73
1.71 3.17 5.85
Note: t(I) is the temperature at which the rate of the first emission attains its maximum value; 1∆gO2 (I) and 1∆gO2 (II) are the concentrations of singlet oxygen after the first and the second emissions, respectively; the temperature of onset of the second emission was 480°C.
295
3.064
EMISSION OF ACTIVE OXYGEN FORMS FROM A DEPOSITED BISMUTH OXIDE I 1.0
8
24
40
1.544
1.913 1.851
2.215
3.211
2.752 2.542 2.388
0.2
3.726
7.748
0.6
56
γ, deg
Diffractogram of bismuth oxide deposited on silica gel (6 at %); I is the relative intensity, γ is the diffraction angle.
REFERENCES
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Attempts to detect O 2 in EPR spectra of sample subjected to various treatments also failed, and, there– fore, the possibility of the formation of 1∆gO2 from O 2 , can also be excluded. We believe that the formation of singlet molecular oxygen on a silica gel surface containing deposited bismuth ions can be explained by structural transformations of the surface bismuth compounds near ~480°ë, the temperature at which surface ions become mobile. The formation of singlet oxygen within 350–370°ë has a different nature, probably the 3 – conversion of Σg O2 into 1∆gO2 under the action of the electric field of a bismuth ion. This work was supported by the Russian Foundation for Basic Research, project no. 04-03-32513.
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