E-mail: Solonin@materials.kiev.ua
Yu.M.Solonin
Institute for Problems of Materials Science, National Academy of Sciences of Ukraine, 3 Krzhyzhanovsky str., UA-03142 Kyiv, Ukraine
The structural aspects of hydrogen interaction with activated textured molybdenum trioxide layer
ABSTRACT
The textured palladium activated MoO3 thick films have been prepared and the phase transformations during its interaction with hydrogen at temperatures 20 - 400OC studied. The TEM and SAED of the individual MoO3 crystals, covered by palladium and undergone the similar treatment, were performed to investigate the mechanism of a reaction. The exposure to hydrogen already at room temperature tinges the layer in light blue color as a result of the penetration of hydrogen atoms into MoO3 lattice by "hydrogen spillover" mechanism. At temperature 200OC the dark blue cubic phase H163MoO3 with lattice parameter a = 0,380 nm forms. The further heating of the film in hydrogen at temperatures 300 and 400OC leads to a gradual transformation of H 63MoO3 to a non-stoichiometric molybdenum suboxide Mo1 XO also with cubic structure. During the formation Mo1XO the strong orientation correlation preserved:
(010)MoO3 || (010)H163MoO3 || (010)Moi XO [001]MoO3 || [001]Hw.3MoO3 || [001)]Moi-XO The external shape of the crystal during these transformations also preserved. But at the second stage owing to the loss of oxygen the suboxide pseudocrystal possesses a very fine porous inner structure. The size of the pores is less then 10 nm.
1. INTRODUCTION
To realize the hydrogen widespread use as a clean energy carrier and fuel not only the cost-efficient technologies of hydrogen production, storage and transportation but also reliable methods of hydrogen detection for its safe handling must be developed. The use of gas sensors for detecting chemical species is important for numerous industrial and consumer processes. But conventional gas sensors, based on coarsegrained materials, are limited by several characteristics: poor long-term stability, response reproducibility and others.
Current major technological trends for gas sensors preparation are thin films and nanocrystalline materials [1]. Na-nocrystalline materials offer many advantages over their conventional counterparts. In particular drastic increases in sensitivity are expected as particle sizes are reduced. Nanocrystalline materials with particle size smaller than 100 nm show a lot of special properties not found in bulk materials, because of their small particle size and large surface areas.
The activated oxides are the traditional materials for gas sensor application. Gas sensors based on SnO2 are used since the beginning 70's. Some other metal oxides such as MoO3, WO3 in form of thin films are considered as the sensing materials of gaseous species [2,3]. Many of metal oxides are capable of easy exchange of oxygen between the solid body and the gas phase, many of the oxides also easy form the compounds with hydrogen type HXMeOY. Employing nanostruc-tured materials with large specific surface area can increase the rate of such heterogeneous reactions. But the kinetics and especially mechanisms of the nanoparticles interaction with gases are generally different from reactions of bulk materials.
It is known that hydrogen is fairly easily implanted into the oxide matrix, forming hydrogen bronzes HXWO3, HXMoO3, etc. [4,5]. The valence of the transition metal is not changed in the process, as the hydrogen is "dissolved" in the oxide matrix in a non-stoichiometric ratio.
In 1949 it had already been shown that at ordinary temperatures WO3 reacts with atomic hydrogen obtained in a gas [6] or aqueous [7] phase, or through the "hydrogen spillover" process, forming tungsten bronze, HXWO3, which changes the yellow color of WO3 to blue. This color change is used as a test for determining the presence of atomic hydrogen. The "hydrogen spillover" effect is the result of the reaction of the highly dispersed palladium and platinum on oxide materials with molecular hydrogen yielding atomic spilt-over hydrogen, which spills from the surface of the metal and is absorbed by the oxide. The "hydrogen spillover" and formation of the hydrogen bronzes on the base of MoO3 and WO3 were investigated by Sermon and Bond [8,9] and by author in [10,11].
In this work attention was focused on the creation the textured thick MoO3 film, consisting of very thin (nano-sized in one dimension) activated by Pd plate-like oxide crystals, and on investigation the structural transformations during interaction of this film with hydrogen at temperatures from 20 to 450OC.
2. EXPERIMENTAL
First step of the MoO3 thick film preparation consisted in creation on the substrate of Al2O3 or other the loose layer of arbitrary oriented very thin plate-like MoO3 crystals. Then the special treatment was used for densification of this layer and reorientation of the crystals to obtain the texture with some predominant crystal orientation. The activation was performed by an impregnation of the layer with solution con-
Солонин Ю. М. Особенности взаимодействия водорода с активированным структурированным слоем триоксида молибдена.
taining same palladium salt and following heating in air at 300OC. The activated MoO3 layer was exposed to hydrogen at different temperatures from 20OC to 450OC and structural changes after such treatments were studied by X-ray diffrac-tometer method. The more detailed study of a phase and structural transformations was carried out with use of the individual MoO3 crystals, similar to crystals involved to layer. These crystals were placed to thin carbon substrate. The individual MoO3 crystals were undergone to same treatment as layer. The structural changes were investigated by transmission electron microscopy (TEM) and selected area electron diffraction (SAED). We used two methods of palladium activation of such individual crystals. The first one consisted in thermal vacuum evaporation of palladium on the carbon substrate, already covered by individual MoO3 crystals. The second one consisted in evaporation of palladium on a clean substrate, on which the MoO3 crystals were deposited later.
3. RESULTS AND DISCUSSION
The molybdenum trioxide possesses the orthorhombic lattice with parameters a = 0,3963 nm, b = 1,3855 nm and c = 0,3696 nm [12,13]. It can form the very thin plate-like crystals with extremely developed (010) type crystal face and elongated in [001] direction. The thickness of such crystals may be 10-100 nm and area about 2-4 mm2. In Fig.1,1 the X-ray diffractogram of the loose layer obtained at the first stage of textured MoO3 film preparation is shown.
The phase transformations during exposure of palladium activated MoO3 layer to hydrogen at different temperature were studied by X-ray diffractometer method. The results are presented in Table 1. The exposure already at room temperature tinges the layer in light blue color as a result of the penetration of hydrogen atoms into MoO3 lattice by "hydrogen spillover" mechanism. At temperature 200OC the dark blue cubic phase with lattice parameter a = 0,380 nm forms. The cubic phases type HXMoO3 were obtained and studied by Sermon and Bond [8]. Comparison of the lattice parameters, obtained in our work and in [8], allows us to consider, that we observed the similar phase with x=1,63. The cubic phases HXMoO3 differ from classic Glemser's phases [14], which are orthorhombic and based on a no distorted MoO3 lattice.
The further heating of the film in hydrogen at temperatures 300 and 400OC leads to a gradual transformation of H163MoO3 to a non-stoichiometric molybdenum suboxide Mo1-XO also with cubic structure. Such phase never forms during conventional reduction of coarse and not activated MoO3 powders. In our case this phenomenon is a result of specific mechanism of activated nano-sized MoO3 crystals reduction.
Table 1.
Type of sample Phase content
20UC 200UC 300UC 400UC
Activated thick film A B B+C C
Individual crystals Activation I - B' B'+C C
Activation II MoU3 MoQ3 MoQ3 Moö2
30 35 20
Fig.1. The X-ray diffractograms of MoO3 thick film before (1) and after (2,3) special treatment.
The all X-ray lines with correct intensity correlation are observed. On the diffractograms of the layer additionally treated (Fig.1,2,3) the intensities of the reflexes (040) and (060) sharply increased. Such diffractograms confirm the existence of a texture with (010)MoO3 parallel to the substrate surface. All the diffractograms of Fig.1. were obtained from the substrate surface, which was parallel to axis of diffractome-ter. At the same time the diffractograms in Fig.1,2 and Fig.1,3, obtained at two layer position, which differed by rotation on angle of 90 degrees in layer plane, were similar. This indicate that preferable orientation of the crystals in layer plane is absent. But the method can be developed to provide the preparation of a textured layer also with predominant orientation of the [001]MoO3 along fixed direction. As we will see below it may be very important for the creation of the layers with special deformative properties.
A - H^MoO? light blue; B - H1 63MoO, cubic, a = 0,380 nm, dark blue;
B'-H1 55172MoOy monoclinic, a=0,380 nm, b=1,398 +1,374 nm, c=0,4 0,406 nm;
C - Mo1XO, cubic, a = 0,408 nm., black.
For more detailed study of structure transformations the TEM and SAED of individual palladium activated MoO3 crystals on carbon substrate were performed. We used two methods of activation: vacuum deposition of palladium immediately on crystal surface (Activation I); deposition of palladium on carbon substrate before exposure to MoO3 "smoke" (Activation II). For the second method not so close contact of the Pd particles with MoO3 surface can be obtained. The application of these two methods allows to estimate the influence of a contact quality of catalyst with crystal on reaction kinetic and mechanism.
In Fig.2 the TEM image of the thin MoO3 cryst
Для дальнейшего прочтения статьи необходимо приобрести полный текст. Статьи высылаются в формате PDF на указанную при оплате почту. Время доставки составляет менее 10 минут. Стоимость одной статьи — 150 рублей.