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ХИМИЧЕСКИЕ НАУКИ
THE SIZE EFFECT IN THE ADSOPTION AND RADIOLYSIS PROCESSES OF _THE WATER ON THE SURFACE OF BEO_
Haciyeva S.R., Cafarov Y.D*., Ramazanova N.K., Gadirova E.M., Abbasov. M.Y.
Baku State University Azerbaijan, Baku, Z. Halilova st. -23, AZ1148 * Institute of Radiation Problems Baku, Azerbaijan
Abstract. The influence ofthe catalyst (BeO) dispersion on adsorption and radiolysis processes of the water have been investigated by iR spectroscopy diffuse - scattering method. The size, dependence of adsorption capacity and radiation -catalytic activity of the water on the grain sizes 4<d<106^ has been revealed. The dimensions of the depth of light-diffusing layer have been estimated for these processes. A role of the surface polaritions in the of revealing size effect region has been considered.
Keyword: BeO/ads. H2O system, molecular hydrogen, IR spectroscopy.
Introduction
The efficiency of the heterogeneous catalytic radiolysis on the boundary of division of the phase ( oxide-adsorbed molecule) is defined with formation of exited and ionized surface states in the y- irradiate process [1,2]. The contribution of these surface states are clearly appeared in optimal values of dispersion of oxides. The influence of the dispersion some y- irradiated oxides on the regularities of accumulation and migration of localized charge carriers has been studied by ESR spectroscopy [3,4]. However, in this work role of the surface at that oxides with adsorbed molecules was not sufficiently considered and not investigated the IR spectra. This is connected with unacceptability of transmission IR spectroscopy method at the study of the grain size effects, as in usually pressing of the powder to the pill, a difference of effects expected for large and finely dispersed powders is disappear in consequence of mechanical deformation of the surface. This is received with application of the diffuse-scattering IR spectroscopy method [5]. A high sensitivity of given method and the preservation of the integrity of each particle of the disperse adsorbents in registration of the spectra are open a new possibilities to investigate in the more pure form the effects connected with a grain sizes[6].
The results of investigations of the influence of beryllium oxide dispersion on the regularities of water adsorption and its radiolytic-heterogeneous conversion in the BeO/ads. H2O system received by diffuse-scattering spectroscopy IR method are presented in this work.
EXPERIMENTAL
The BeO powders of the for "luminofors" ( S=60m2/g) with the grain sizes 4^<d<106^ characterized in [7] were used. A particles form has been studied by microphotographyin the electron microscope and was like to spherical. The size distribution of the particles and a main value of their sizes are measured by "SK laser micron sazer" after dispersion of the powder by ultrasound in water.
Fractions of samples with grain sizes d<4, 4-32, 32-53, 53-75, 75-106^ are selected.
The samples were preliminarily thermally treated at T=973K for 8 hours in air. Then for the purpose of full cleaning from organic pollutions and dehydroxylation of the BeO surface, the thermal treatment was carried out at T=1073K in vacuum P=10-3Pa. The inspection check a purity of the surface was realized over the intensities of the IR bands referring to carbohydrogenic pollutions.
The IR spectra were recorded on "Specord 7 IR" spectrophotometer, which was equipmented by the diffusescattering devices and the external radiation sources in quartz cell with CaF2windows [8]. The cell has been jointed with vacuum system, that permit to carry out the treatment of samples in vacuum (P=10-4Pa) and in atmosphere of gases and vapors at temperatures right up to 1073K, also to obtain the spectra of adsorbed H2O and to follow the exchanges in it, taking place after radiolysis of BeO/ads. H2O system.
The IR reflection spectra were received on same spectrophotometer in spectral range 3000-4000 cm-1 at room temperature. The measuring were carried out in angles of incidence near to normal [9].
The water was spectroscopic pure, where the purity to be maked up 99,8%. The water adsorption was studied by manometric method of volume - adsoption installation and by the IR spectroscopy method.
A total amount of the adsorbled water molecules has been determined by the difference of H2O molecules introduced into the measuring cell and remained in equilibrium gas phase after the contact with BeO at T=300K.
The amount of the adsorbed water molecules has corresponded to the occupancy value of BeO surface, 6=0,8 [7,8].
The samples were irradiated with 60Co y-rays at a radiation dose rate dDy/dt=.........
The adsorption intensities (I) were defined for bands of adsorbed H2O, as for bands apperaed in results from radiolysis of the BeO/ads. H2O system. The summary intensity (I) for adsorbed forms of the water has been determined with calculation of intensities ( Vmax = 3280 cm-1 molecular, H-bonded ( Vmax = 3500 cm-1 molecular and Vmax = 3630 cm-1 dissociative chemisorption) adsorbed H2O.
The yield of molecular hydrogen in a heterogeneous radiolysis BeO/ads. H2O system have been estimated chromatoqraphicly.
Results and discussion.
The adsorbtion centers of the water in preliminary irradiated BeO are arised by molecular and dissociative mechanisms at the room temperature [7]. The dissociative chemisorptions takes place in the result of interaction of adsorbed water with non-equilibriumcharge carries ( ncc ) and exited states ( ex) formed under the action of y- quanta in BeO.
As show kinetic peculiarities of water adsorption on the preliminarily Y-irradiated samples of BeO, the adsorption capacity of water in comparison with unradiated samplesin ~5 times as result of the activation of the surface of BeO. The generation of adsorption active centers and the additional adsorption of molecules H2O on these centers are take place in linear region of the irradiation dose 25<Dy<75 kRad [7]. Therefore at the study of the size dependence of adsoption capacity of BeO with reference to H2O by the IR spectra has been choosed a linear region of kinetic curves of the adsorbtion - desorption equilibrium for preliminarily Y- irradiated BeO ( the activation was carried out in value at adsorbed dose D = 60kRad = const).
The IR spectra of the water adsorbed on preliminarily Y- irradiated surface of BeO in the range of the stretching vibrations of O-H groups ( 3000-3800cm-1) with grain sizes of powder d < 4^ (curvel) and d=100^ (curve1) are shown in figure1. The comparison of the spectra indicate to the decreasing grain size accompained with theincreasing of the intensities of bands molecular and chemical water. This is in a good agreement with concentration values of adsorbed molecules H2O, obtained from manometric measurings.
The size dependence of summary intensities changes of the adsorbed forms or the water on the surface BeO is shown in figure 2 ( curve1). It is seen, that kinetic pecularities are consist of a linear region at values 4^<d<75^ and stationary region of saturation at d> 75^. The observing size effect of radiation -stimulated adsorption of water and increasing its adsorption capacity with decreasing grain sizes may be concerned with:
1) The availabilities of the effective ionized and excited surface states, density of which a strongly dependent on degree of dispersion of beryllium oxide;
2) Increasing of the interaction force inter defects for the small finely dispersion powders.
The taken out of the interaction force defects owing to decreasing of distance inter particles to lead to increasing and accumulation of the energy in defect states with following transference this energy to adsorbed molecules of H2O. The break up of chemical Be-O bond must be accompanied by additional dissociative chemisorption'sof the water. However kinetic curves of formation of dissociative chemisorbed water ( Vmax 3630 cm-1)at different values of the grain sizes are a linear regions practically with same values of inclination
( — = const). Therefore we may be neglect the contribution to force of interaction of inter defects ( figure 2, invention).
Thus, in the size dependence of the adsorption capacity the change of density of effective ionized and exited surface defect states is played a basic part at d<4^. This has been confirmed also with ESR - data of the influence of the surface field on PMC concentrations in the of revealing size effect region d<100^ [3]. According to [3] the role of the surface field( E= 105V/sm) is considerable in the small grain sizes region of BeO; it practical a completely suppress the increase of probability of the recombination processes in the near surface layer. With the increase of grain sizes the velocity of accumulation pmc almost not changes in result of decrease of the actions sphere of the surface field to the volume processes.
It should be noted, that the surface polaritions( SP ) may be played essential part as exited states on the boundary of division of the phase BeO/ads. H2O [10]. It has been shown from the IR reflection spectra of polycristallic samples of BeO, that on the surface of dielectric in the residual rays region, that is in the spectral diapason of TO-LO splitting (700- 1100 cm-1) may be spreaded SP. The spread run of the SP at frequency 950 cm-1 makes up from 50 to 120^ depending on the surface states (on the roughness, connected also with grain sizes) of investigated crystals.
In figure 3 are shown the IR reflection spectra of polycrystal samples of BeO, the surfaces of which are characterized with different graininess. As seen from figure, in depending on the spectrum of irregularities distribution on reflection surface it takes place a transformation of the TO-modes band. This is influence to behavior of failings in the TO-LO splitting region of position its minimum have been observed by the author of [11] in the VTIR spectra of the finely and large grain-iness samples of BeO.
The observed values of the spread run of SR on the surface of BeO [10,11] are in the region of optimal values of grain sizes, at which has been observed the size dependence of the adsorption capacity of the H2O. It is testify to that in grain sizes commensurable with values of the spread runs of SR on the surface of BeO it has been carried out effective transfer and transmission of the energy, absorbed by SP to the molecules of H2O.
The coexistence of another energy transmission canals has been confirmed in works [12, 13]. By the author of work [13] has been considered a role of exactions formed under the action of Y-quanta in oxide phase (BeO) at the energy transmission. However, the calculation of the exactionmechanism of the transmission also didn't completely provide the balance of energetic yields of H2G(H2) and the total G0=G0(n.c.c.)+G0(ex) (G0 - initial yields of n.c.c. and exited states). This is once more directly confirms about participation of SP in energy transmission adsorbed by oxide phase through the near-surface layer to the adsorbed molecules.
To study the regularities of radiation-heterogeneous processes of H2O transformation in contact with BeOin different values of the grain sizes and to reveal
the size dependence of radiation-catalytic activity the IR spectra of heterogeneous BeO/ads. H2O system were obtained after y-irradiation. The kinetic regularities of the adsorbed water decomposition (curve 2) and of the total accumulation products of the heterogeneous transformation surface hydroxyl O-H and hydrid Be-H (curve 3), also a curve of molecular H2 accumulation (4) are shown in figure 2.
The comparison of the regularities of the changes of adsorption capacity and of the radiation-catalytic of H2O showthat the observed size dependences are beared analogous character. The difference consists only in velocities the values of the adsorptions and decompositions of the H2O in the adsorption and radioly-sis processes.
To take into consideration that the particle form in investigated powders of BeO was near to spherical, to observed empirical size dependence of intensities of the adsorption bands can be describen by expression
I1/3~ 1-2L/d (1)
where d - is parameter, characterized a linear size of the grain; L - to follow considered as the effective depth of the light - diffusing layer in the middle of IR region, which can be corresponded to the depth of the active near-surface layer finding inter the surface and volume of the dielectric.
The intensities of the bands of the adsorbed molecules on diffuse scattering spectra are determined as
I=R.-R [14], (2)
of these dependences in the coordinates I1/3~f(d-1) permit to defined a values L of the adsorbed H2O (figure 4). A mean values of L are estimated by way of extrapolation (R^-R)1/3 in point R^=R, which are equal to L=4p. ( for case of the adsorption) and L=2p. (for case of the radiolysis).
A reduced values of L in the case of radiolysis in comparative with case of the adsorption probably is connected with screening of the surface of BeO by the final products of decomposition such as O-H and Be-H groups.
The comparison of the finded experimental values of L for adsorption and radiolysis processes of the H2O on the surface of BeO and y-AhO3 [15] show, that BeO is characterized almost with the in ~ 2 times lesser values of this layer. This is explained with: (1)- the differ-ency of the thickness of the surface irregularities, (2)-the differency of the emptiness in these surface-irregularity layers also (3) - H2O on dependence of the charges of cations Be2+ and Al3+.
Thus, a comparative study of the kinetic regularities of the radiation - stimulated adsorption of water on the surface of beryllium oxide and of the radiolysis in the BeO/ads. H2O system processes, also the influence of the oxide dispersity on these regularities by diffuse scattering IR-spectra permit to choose the surface with given properties in which has been carried out the effective energy transfer to the adsorbed molecules. A role of the SP as a possible the energy transmission canals has been evidently in the of revealing size effect region at d<100p..
where RM and R are the reflection coefficienters (albedo) of infinite thick samples to and after adsorption in it's the observed light molecules. The building
4000 3500 3000 V, CM
Fiqure 1. IR spectra of molecular H2O, adsorbed on the preliminarily y-irradiated surface BeO in different grain
sizes: 1-d=4jU; 2-d=106^
Fiqure 2. (a) The size dependence of the summary intensity (Is) of adsorption bands of the adsorbed forms of the water on the preliminarily y-irradiated BeO with a subsequent adsorption H2O (curve 1) and heterogeneous irradiated BeO/ads. H2O system (curve 2) invertion - the kinetic curves of the formation of the dissociative chemisorbed of the H2O in different grain sizes: 1-d=4f; 2-d=106f (b) The size dependence of the summary intensity (Is) of the adsorption bands of the products of decomposition after radiolysis in BeO/ads. H2O system ( curve 3) and of the accumulation of molecular H2 (curve4).
900 700
FREQUENCY, cm"1
Fiqure 3. Reflection IR spectra of the samples BeO with different grain sizes: 1-d=4f; 2-d=106f
Fiqure 4. The summary intensity change I1/3 of the adsorbed H2O depending on inversely value ofgrain size for
the cases of adsorption (1) and radiolysis (2) processes.
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