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Photoadsorption and photodesorption of oxygen and related photocatalysis on inorganic semiconductors
Classified
abstracts
720-726
energy of the catalytic reaction on the bond strength is discussed. (Germany) F Steinbach, Elektron Yavlen Adsorb Moscow 1969.292-3 16 (in Russian).
Kataliz
Poluprovod,
Mir
16
(USSR) Results of investigations of the effects of radiation on the adsorption and catalytic properties of non-metallic solids are presented and explained. The influence of electron excitations due to irradiation on catalysis is studied in detail. The present state of the electron theory of adsorption and catalysis under the action of irradiation is described. Sorption of Oe, CO, CO, and H, on Al,O, under the action of x-ray and y-radiation is analysed. Photoconductivity of ZnO and its photosorption ability with respect to oxygen are examined. Influence of radiation and illumination on the catalytic oxidation of CO is discussed. (Belgium) R Coekelbergs and A Crucq, Elektron Yavlen Adsorb Kataliz Poluprovod, Mir Moscow 1969,369-398 (in Russian).
720. Adsorption
and catalysis
under irradiation.
16 721. Electron phenomena in chemisorption process of free atoms and radicals on semiconducting adsorbents. (USSR)
The interaction of active particles with asdorbent surfaces and their reactivity in the chemisorbed layer are discussed. Change of electrophysical properties in semiconductors due to chemisorption serves as a sensitive detector for investigation and check of the interaction. It is known that the electrical conductivity and work function of thin polycrystalline layers of oxide-type semiconductors prepared on quartz substrates are considerably changed on chemisorption of a very small number (106-lo* cme3) of free active particles. The electrical conductivity of a zinc oxide polycrystalline layer, about one micron thick, is changed by tens to hundreds of per cent after chemisorption of hydrogen. Increase in the electrical conductivity of the adsorbent is caused by ionization of most of the chemisorbed hydrogen atoms. The kinetics of the electrical conductivity change on hydrogen atom chemisorption on semiconductors is studied. It is found that chemisorption of metal atoms also considerably increases the electrical conductivity of metal oxides with electronic type conductivity. For example, the electrical conductivity of semiconducting films is changed by the presence of small amounts of sodium in the vacuum system even at room temperature, when the sodium vapour pressure is lo-l1 torr. I A Myasnikov, Elektron Yavlen Adsorb Kataliz Poluprovod, Mir Moskva 1969, 1 IQ-1 32 (in Russian). 16 722. Photoadsorption and photodesorption of oxygen and related photocatalysis on inorganic semiconductors. (USSR) Results of experimental investigations of oxygen photoadsorption and photodesorption on ZnO, TiO, and ZrOz are explained on the basis of electron paramagnetic resonance measurements. The method of electron paramagnetic resonance permits investigation of the photodynamic behaviour of adsorbed oxygen on semiconductor surfaces. Special attention was devoted to the protection of samples against oil vapour from the vacuum system. A liquid nitrogen trap was used for this purpose and as a thermostat for the Pirani gauge. All samples were heated in vacuum at 10e5 torr and 500°C. It is found that at high oxygen pressure (20 torr), the surface concentration of oxygen on ZnO is sufficient to trap most of the photoelectrons, and oxygen molecular ions 0, are formed. The photodesorption effect on ZnO was investigated with evacuation of the system down to 10e5 torr. It is found that on oxygen photodesorption, neutral oxygen molecules are formed by interaction of molecular oxygen ions with holes originating from the illumination. The neutral oxygen molecules on the ZnO surface determine the sign of the photosorption effect. On oxidized samples, oxygen photoadsorption takes place and on reduced samples, oxygen phbtodesorption is observed. (Japan) T Kwan. Elektron Yavlen Adsorb 1969,278-291 (in Russian). 723. Mechanisms surface structure.
Kataliz
Polwrovod.
Mir Moscow
16 of doping of nickel oxide and their relation to solid
(USSR) Doping mechanisms of nickel oxide are investigated. It is found that by vacuum treatment at 25o”C, a limited number of ions with different valency, as compared to the basic metallic component, can be introduced into the surface layer of nickel oxide. Lithium ions are placed in lattice nodes and anion vacancies are formed. Introduction of gallium
ions in the lattice nodes leads to the reduction of Ni2+ ions. Metallic atoms are formed which may migrate leaving cation vacancies. In both cases surface defects are formed which change the structure of surface oxide. Thus a low-temperature doping influences adsorption ability and catalytic activity. The structural changes result in variation of the number of Ni3+ ions which is always connected with oxygen absorption. Magnetic and electric properties of pure and doped nickel oxide are investigated. Oxygen chemisorption on clean and doped nickel oxide is evaluated. The reaction ability of oxygen sorbed at 250°C on clean and doped nickel oxide is determined. (France) P Gravelle et al, Elektron Yavlen Adsorb Moscow, 1969, 192-210 (in Russian). 724. The role of electronic
Kataliz
and ionic defects
Poluprovod,
in oxidation
Mir
16 catalysis.
(USSR) The influence of impurities on catalytic and electronic properties of semiconductors is analyzed. A model of reacting solid surface is presented. The reactions on solid surface are considered as processes of formation and disappearance of surface layers due to chemical composition changes. If one process predominates, then, depending on the penetration depth, adsorption, desorption or reaction in the volume of solid will take place and when the processes proceed simultaneously, then catalytic reaction takes place. On formation or disappearance of the surface layers, defects in the solid play an important role, the reaction will be extended to the depth, where defects of lattice nodes exist. Generally, this region is deeper than an atomic layer and this effect enables difficulties connected with the determination of the adsorbed layer from the gaseous phase to be overcome. Thermodynamic treatment and kinetics of oxidation catalysis by metal oxides are presented. Experimental data are discussed and it is concluded that the electronic defects play a principal role in the reactivity of NiO at low temperature, COO, TiO,, Fe0 and probably Cr,O,. Adsorbed cations influence the reactivity of CuO, ZnO and NiO at high temperature. (USA) G Parravano, Elektron Yavlen Adsorb Moscow, 1969,175-191 (in Russian). 725. Different
forms
of chemisorption
Kataliz
Poluprovod,
on semiconductors.
Mir
16 (USSR)
A theoretical study of chemisorption on semiconductors is presented. The treatment of chemisorbed particles as surface defects forming localized surface levels leads to differentiation of neutral and charged, valence-saturated and radical, weak and strong, reversible and irreversible forms of chemisorption. It is shown that it is important to consider a great variety of chemisorption bonds. Abandoning the supposition that chemisorption always has an ionic character, considerably broadens the possibilities of the theory. The problem of the neutral form of chemisorption is still‘insufficiently studied but it is clear that this form of chemisorption, the differences in chemical nature of the adsorbent and the adsorbate are manifested. To distinguish the form of chemisorption, it is necessary to determine the interaction energy of the structure of the adsorbed particle with the lattice. Determination of this energy, even qualitatively, enables investigation of the chemisorption mechanisms. It is shown that the theory of chemisorption on semiconductors is not applicable to compounds of transition metals. (Bulgaria) 0 Peshev, Elektron Yavlen Adsorb Kataliz Poluprovod, Mir Moscow, 1969, 94-109
(in Russian). 16
726. Electronic
processes
on solid surfaces
and reactivity
of chemi-
sorbed molecules. (USSR) Catalysis mechanisms based on electronic processes on solid surfaces and the reactivity of chemisorbed molecules are studied. The electron theory of chemisorption discerns two forms of chemisorption. The neutral form when the molecule bond with surface is formed without the participation of free charge carriers from the lattice, leads to the production of empty levels. The charged form, in which localization of charge carriers occurs on an adsorbed particle, results in formation of filled levels. A new mechanism of adsorption is presented which explains charging of the surface without the existence of the radical form of chemisorption. The real surface of solids is characterized by the presence of different surface states of non-adsorption origin. With adsorption on a defect, it is possible that the local level of this defect may dissapear or its energy position may be changed. In the case where direct interaction between the adsorbed molecule and the defect does not occur, the adsorbed molecule may significantly 353
abstracts
720-726
energy of the catalytic reaction on the bond strength is discussed. (Germany) F Steinbach, Elektron Yavlen Adsorb Moscow 1969.292-3 16 (in Russian).
Kataliz
Poluprovod,
Mir
16
(USSR) Results of investigations of the effects of radiation on the adsorption and catalytic properties of non-metallic solids are presented and explained. The influence of electron excitations due to irradiation on catalysis is studied in detail. The present state of the electron theory of adsorption and catalysis under the action of irradiation is described. Sorption of Oe, CO, CO, and H, on Al,O, under the action of x-ray and y-radiation is analysed. Photoconductivity of ZnO and its photosorption ability with respect to oxygen are examined. Influence of radiation and illumination on the catalytic oxidation of CO is discussed. (Belgium) R Coekelbergs and A Crucq, Elektron Yavlen Adsorb Kataliz Poluprovod, Mir Moscow 1969,369-398 (in Russian).
720. Adsorption
and catalysis
under irradiation.
16 721. Electron phenomena in chemisorption process of free atoms and radicals on semiconducting adsorbents. (USSR)
The interaction of active particles with asdorbent surfaces and their reactivity in the chemisorbed layer are discussed. Change of electrophysical properties in semiconductors due to chemisorption serves as a sensitive detector for investigation and check of the interaction. It is known that the electrical conductivity and work function of thin polycrystalline layers of oxide-type semiconductors prepared on quartz substrates are considerably changed on chemisorption of a very small number (106-lo* cme3) of free active particles. The electrical conductivity of a zinc oxide polycrystalline layer, about one micron thick, is changed by tens to hundreds of per cent after chemisorption of hydrogen. Increase in the electrical conductivity of the adsorbent is caused by ionization of most of the chemisorbed hydrogen atoms. The kinetics of the electrical conductivity change on hydrogen atom chemisorption on semiconductors is studied. It is found that chemisorption of metal atoms also considerably increases the electrical conductivity of metal oxides with electronic type conductivity. For example, the electrical conductivity of semiconducting films is changed by the presence of small amounts of sodium in the vacuum system even at room temperature, when the sodium vapour pressure is lo-l1 torr. I A Myasnikov, Elektron Yavlen Adsorb Kataliz Poluprovod, Mir Moskva 1969, 1 IQ-1 32 (in Russian). 16 722. Photoadsorption and photodesorption of oxygen and related photocatalysis on inorganic semiconductors. (USSR) Results of experimental investigations of oxygen photoadsorption and photodesorption on ZnO, TiO, and ZrOz are explained on the basis of electron paramagnetic resonance measurements. The method of electron paramagnetic resonance permits investigation of the photodynamic behaviour of adsorbed oxygen on semiconductor surfaces. Special attention was devoted to the protection of samples against oil vapour from the vacuum system. A liquid nitrogen trap was used for this purpose and as a thermostat for the Pirani gauge. All samples were heated in vacuum at 10e5 torr and 500°C. It is found that at high oxygen pressure (20 torr), the surface concentration of oxygen on ZnO is sufficient to trap most of the photoelectrons, and oxygen molecular ions 0, are formed. The photodesorption effect on ZnO was investigated with evacuation of the system down to 10e5 torr. It is found that on oxygen photodesorption, neutral oxygen molecules are formed by interaction of molecular oxygen ions with holes originating from the illumination. The neutral oxygen molecules on the ZnO surface determine the sign of the photosorption effect. On oxidized samples, oxygen photoadsorption takes place and on reduced samples, oxygen phbtodesorption is observed. (Japan) T Kwan. Elektron Yavlen Adsorb 1969,278-291 (in Russian). 723. Mechanisms surface structure.
Kataliz
Polwrovod.
Mir Moscow
16 of doping of nickel oxide and their relation to solid
(USSR) Doping mechanisms of nickel oxide are investigated. It is found that by vacuum treatment at 25o”C, a limited number of ions with different valency, as compared to the basic metallic component, can be introduced into the surface layer of nickel oxide. Lithium ions are placed in lattice nodes and anion vacancies are formed. Introduction of gallium
ions in the lattice nodes leads to the reduction of Ni2+ ions. Metallic atoms are formed which may migrate leaving cation vacancies. In both cases surface defects are formed which change the structure of surface oxide. Thus a low-temperature doping influences adsorption ability and catalytic activity. The structural changes result in variation of the number of Ni3+ ions which is always connected with oxygen absorption. Magnetic and electric properties of pure and doped nickel oxide are investigated. Oxygen chemisorption on clean and doped nickel oxide is evaluated. The reaction ability of oxygen sorbed at 250°C on clean and doped nickel oxide is determined. (France) P Gravelle et al, Elektron Yavlen Adsorb Moscow, 1969, 192-210 (in Russian). 724. The role of electronic
Kataliz
and ionic defects
Poluprovod,
in oxidation
Mir
16 catalysis.
(USSR) The influence of impurities on catalytic and electronic properties of semiconductors is analyzed. A model of reacting solid surface is presented. The reactions on solid surface are considered as processes of formation and disappearance of surface layers due to chemical composition changes. If one process predominates, then, depending on the penetration depth, adsorption, desorption or reaction in the volume of solid will take place and when the processes proceed simultaneously, then catalytic reaction takes place. On formation or disappearance of the surface layers, defects in the solid play an important role, the reaction will be extended to the depth, where defects of lattice nodes exist. Generally, this region is deeper than an atomic layer and this effect enables difficulties connected with the determination of the adsorbed layer from the gaseous phase to be overcome. Thermodynamic treatment and kinetics of oxidation catalysis by metal oxides are presented. Experimental data are discussed and it is concluded that the electronic defects play a principal role in the reactivity of NiO at low temperature, COO, TiO,, Fe0 and probably Cr,O,. Adsorbed cations influence the reactivity of CuO, ZnO and NiO at high temperature. (USA) G Parravano, Elektron Yavlen Adsorb Moscow, 1969,175-191 (in Russian). 725. Different
forms
of chemisorption
Kataliz
Poluprovod,
on semiconductors.
Mir
16 (USSR)
A theoretical study of chemisorption on semiconductors is presented. The treatment of chemisorbed particles as surface defects forming localized surface levels leads to differentiation of neutral and charged, valence-saturated and radical, weak and strong, reversible and irreversible forms of chemisorption. It is shown that it is important to consider a great variety of chemisorption bonds. Abandoning the supposition that chemisorption always has an ionic character, considerably broadens the possibilities of the theory. The problem of the neutral form of chemisorption is still‘insufficiently studied but it is clear that this form of chemisorption, the differences in chemical nature of the adsorbent and the adsorbate are manifested. To distinguish the form of chemisorption, it is necessary to determine the interaction energy of the structure of the adsorbed particle with the lattice. Determination of this energy, even qualitatively, enables investigation of the chemisorption mechanisms. It is shown that the theory of chemisorption on semiconductors is not applicable to compounds of transition metals. (Bulgaria) 0 Peshev, Elektron Yavlen Adsorb Kataliz Poluprovod, Mir Moscow, 1969, 94-109
(in Russian). 16
726. Electronic
processes
on solid surfaces
and reactivity
of chemi-
sorbed molecules. (USSR) Catalysis mechanisms based on electronic processes on solid surfaces and the reactivity of chemisorbed molecules are studied. The electron theory of chemisorption discerns two forms of chemisorption. The neutral form when the molecule bond with surface is formed without the participation of free charge carriers from the lattice, leads to the production of empty levels. The charged form, in which localization of charge carriers occurs on an adsorbed particle, results in formation of filled levels. A new mechanism of adsorption is presented which explains charging of the surface without the existence of the radical form of chemisorption. The real surface of solids is characterized by the presence of different surface states of non-adsorption origin. With adsorption on a defect, it is possible that the local level of this defect may dissapear or its energy position may be changed. In the case where direct interaction between the adsorbed molecule and the defect does not occur, the adsorbed molecule may significantly 353