Interaction Between Electrocatalytic Active Elements And Their Carriers

Electrocatalytic materials are mainly composed of electrocatalytic active sites and their carriers. Their structures play an important role of in the electrocatalytic properties. So, they have a direct influence on the applications in electrochemical industries. Noble metal oxide RuO₂ is the most common electrocatalytic active material, and rutile-type TiO₂ is the common electrocatalytic carrier material. Looking back the last thirty years, almost all the researches focused on two subjects: one was to increase the electrocatalytic activities and the other was to improve the distribution of the electrocatalytic active sites. Comparatively, the researchers laid less stress on the stability of carrier materials. In this work, one of the aims is to study the microstructure of the TiO₂ carrier and the same time to consider replacing rutile-type TiO₂ with non-crystalline oxide to improve the stability of the frame materials. The other is to try to replace RuO₂ with inexpensive component to improve the electrocatalytic activity. The study has both practical and theoretical significancies. Nano-scale RuO₂,TiO₂,RuO₂-TiO₂,RuO₂-SiO₂,RuO₂-SnO₂ and RuO₂-TiO₂-SiO₂ were prepared by combining a sol-gel technique using the precursors of (C4H9O)4Ti and TEOS (tetraethyl orthosilicate) with a Pechini method using commercial SnCl₂·2H₂O,SnCl₄·5H₂O and RuCl₃·3H₂O as source materials. The phase structures, crystal sizes, microstructures and morphologies of the derived oxides were analyzed by DSC-TG-DTA, XRD, TEM and FT-IR techniques. The electrochemical properties of Cl₂ and O₂ evolutions and accelerated lifetime were obtained. The results show that: (1) Nano-sized TiO₂ can be prepared by sol-gel technique. Heated at below 250℃for 1h, amorphous phase is obtained. Heated at between 250℃and 600℃for 1h, Nano-sized anatase TiO₂ is present. The 70%-vol amorphous phase can be transformed to rutile at 750℃for 1h. Irradiated with an intensive electron beam, rutile can be detected from powders heated at 250℃and 360℃. The effect of electron beam on the phase transformation is not so strong for powders with larger size. The values of kinetic parameters such as Avrami exponent n, the crystallization active energy E, the dimensionality m and the active energy Q of crystal growth were investigated on the bases of the principle of non-isothermal crystallization using DSA method. It shows that during the course of crystallization, the special structure of nano-scale crystals requires the special kinetic features. (2) Nano-scale rutile-type TiO2 can be prepared using TiCl3 as the source and DBS as anionic surfactant after the product was dried at 60℃. The derived rutile TiO2 has super thermal stability. The morphological feature ia formed by bundles of parallel branches. The form of the crystals and the preferred orientation structure might have something to do with the addition of the little surfactant. The preferred orientation is <110>. (3) The mixture of RuO2 and Ru were observed from Pechini method by neutral or strong acidic reaction solutions. The forming of Ru is attributed to the disproportionation reaction in solutions and the following redox reaction of RuO2 in solid condition. Upon heating, RuO2 can be easily reduced to metal Ru if the retained carbon atoms possess which come from the incomplete combustion of organic compound. RuO2 can be entirely formed at above approximately 600℃. (4) when SnCl2·2H2O or SnCl4·5H2O was added with the precursor of RuCl3·3H2O, the resultant powders might be greatly different in structures, morphologies, sizes and dispersing state. Rutile-type SnO2,RuO2 and (Ru,Sn)O2 with sizes of about 10 nm are formed. Their structures are little affected by the combustion condition of the organic compound when the ratio of Ru/Sn is in propriety. The preparation at below 800℃, no metal Ru can be detected. The ultrafine SnO2 effectively destroys the preferred orientation structure of RuO2, which improves the dispersing abilities and sizes of powders and would controll the growth tendency of RuO2. (5) xRuO2-(1-x)TiO2 mixed oxides were prepared by Pechini method using RuCl3·3H2O as precursor and also by sol-gel method using (C4H9O)4Ti as precursor. With the increase of Ru content, the crystallization temperatures are lowered. When the Ru content is below to 50% at mol ratio and the preparing temperature at below 600℃, the mixed oxide system provides a formation of a solid solution of RuO2 and TiO2 with rutile-type TiO2-based structure. While the Ru content is over 50%, the preferential disproportionation reaction takes place in solutions. Thus Ru and RuO2 mixture is formed. During the following heating process, some RuO2 is reduced into Ru. On the other hand, some Ru is oxidized into RuO2. A thermal dynamic condition is given. The mixed oxide system forms a solid solution of RuO2 and TiO2 with rutile-type unstoichiometric RuO2-based structure. Curving grain boundaries and some disordered structures in nanocrystalline materials are observed. (6) xRuO2-(1-x)SiO2 mixed oxides can be prepared by Pechini method using RuCl3·3H2O as precursor and by the sol-gel method using TEOS as precursor. The resultant powders sizes are below 20nm. Within our heating temperature range, non-crystalline SiO2 is the only phase. The amorphous SiO2 do not form a solid solution with RuO2. As the Ru content is increasing,the tendence for Ru3+ to RuO2 and Ru is enhanced. The organic compound added cannot be combusted thoroughly during the succedent heat treatment, therefore, the combusting products are influenced by the redox of RuO2 and Ru. The d-spacing values of RuO2 are smaller than that of standard in PDF card. When the temperature heightens, the oxidization of Ru gradually becomes the main process, thereby creates more oxygen vacancies. The amorphous SiO2 also benefits for controlling the grains growth. Numerous defects exist in the crystalline powders. The atom ratio of Si/O in or around the crystals is basically in accordance with the structural formula SiO2. The atom ratio of Si/O in the non-crystalline areas cannot satisfy the formula, and the ratio is not unanimous in the whole non-crystalline areas. (7) RuO2-TiO2-SiO2 were prepared by combining sol-gel technique using the precursors of (C4H9O)4Ti and TEOS with a Pechini method using commercial RuCl3·3H2O as source material. No SiO2 crystalline can be observed below 800℃. With the Ru content increasing, especially above 50%, the disproportionation reaction is drastically accelerated. With the temperature increasing, the non-crystalline area is gradually reduced and metal Ru is gradually oxidated. The smaller the Ru grains, the stronger the oxidation processes. The more the metal Ru exists, the bigger the d-spacing values of the resultant oxides depart from that of standard. SiO2 can strengthen the stability of RuO2 lattice structure and improve the structural uniformity. The possible structural arrangements of metal citrate structure are most in two-tooth complexing and little in bridge complexing. (8) With the same Ru content, the Ru-Si oxide coatings have lower potentials of chlorine and oxygen evolution than those of Ru-Ti oxide coatings. The accelerated lifetimes of the Ru-Si oxide coatings are longer than those of Ru-Ti oxide coatings when the Ru content is below 40%, while shorter when the Ru content is above 40%. The electrochemical properties of the Ru-Ti-Si oxide coatings can be improved when some SiO2 is added into the Ru-Ti binary oxide coatings.