Molten Salts Electrolysis Preparation Of Unusual Valence Transition Metal Oxides And Their Studies As Catalytic Supports For Methanol Electro-oxidation

Direct methanol fuel cell (DMFC) represents an energy conversion technology which has attracted widespread attention by academia and industry due to its high efficiency for energy conversion, environmental friendliness, quick start-up at low-temperatures etc. The problem of Pt catalyst poisoned by CO is one of the most serious bottlenecks for the development of DMFC. Compared to the massive research concerned on Pt and PtRu catalysts at present, the in-depth study on the catalyst support is fewer. In depth research or understanding and provision of more material choices are very important because the catalyst support is not only a structural material but also can exert a great influence on the utilization, stabilisation and even enhancement of the efficiency of the precious metal catalytst. High-surface-area carbon black is widely employed as the catalyst support in the modern fuel cell technology. Due to the relatively weak interaction between the carbon support and the metal catalysts, and the low oxidation resistance of carbon, carbon supports have been one of the main factors that lead to the decay of the catalyst’s activity. In this research, electro-reduction of transition metal oxides to partially metalized metal oxide of the nano- or sub-micro-sizes was carried out in the molten mixture of CaCl2+NaCl at 600-700℃. The partially metalized metal oxides were utilized as the support for Pt or PtRu to prepare the Pt/MOx or PtRu/MOx catalysts. According to the electrochemical test, the Pt/MOx or PtRu/MOx catalysts show higher activity toward the oxidation of CO and methanol than Pt/C or PtRu/C. The main progresses of this study are summarized as follows.1 Several procedures were designed and undertaken to prepare the suitable supports for Pt catalyst toward CO and methanol oxidation. A proper procedure was identified to produce low valence metal oxides with nano or sub-micrometer structures which could meet the need of catalyst support. (1). The product from electro-reduction of TiO2 powder in the form of porous pellets in molten CaCl2 at 850℃was micro-meter-sized agglomerates, which were however found to be challenging to use because they could not be stably dispersed in water as the catalyst support. (2). Smaller particles were obtained by high-energy ball milling the electrolysis product, but the process introduced impurities to the products. (3). Smaller particles could also be obtained by electro-reduction of the TiO2/carbon black mixture in molten CaCl2 at 850℃, but TiC was found to form as a by-product. (4). The product from electro-reduction of nano-TiO2 in the molten mixture of CaCl2 and NaCl (0.92 in mole ratio, eutectic) at 600℃could be ultrasonicated into a highly stable aqueous suspension, proving insignificant sintering between the particles of ca.100-200nm in size. It has confirmed that the proper sized catalyst support could be fabricated by electro-reduction of nano- TiO2 in the molten mixture of CaCl2 and NaCl at 600℃. The electro-reduction mechanism of TiO2 in this relatively low temperature is also discussed.2. Compared with the Pt/C catalyst, the negative potential shift of the CO striping peak on Pt/TiOx indicates higher activity toward CO oxidation on Pt/TiOx. The CO striping peak on different catalysts were found to be as follow:Pt/C-0.733V; Pt/10%CaδTiOx(2.0V)-0.726V; Pt/30%CaδTiOx(2.0V)-0.702V; Pt/TCT-0.625 V. Pt/TCT showed the highest catalytic activity toward both CO oxidation and CH3OH oxidation reaction among those catalysts. The cyclic voltammograms (CVs) of Pt/TCT in the solution of methanol and H2SO4, exhibited a surface area normalized oxidation peak current that was 5.6-fold greater than that on Pt/C. Herein, CaδTiOx(2.0V) represents the product from electro-reduction of TiO2/carbon black(0.22 in mole ratio) mixture in molten CaCl2 at 850℃and 2.0V for 10 hrs. TCT represents the product from electro-reduction of nano-TiO2 in the molten mixture of CaCl2 and NaCl (0.92 in mole ratio, eutectic) at 600℃and 2.8V for 5 hrs.3. Electro-reduction of solid oxides in molten salts to prepare Nb or Zr sub-oxide with the valence of the metal close to zero was investigated. The electrolysis products were used as the support for Pt and then investigated for their capability of anti-poisoning by CO. The Pt nanoparticles appeared uniformly on both Nb and Zr sub-oxide particles, which were confirmed by TEM inspection. The chronoamperometric results obtained on Pt/Nb sub-oxide for methanol oxidation at 0.6V (vs. R.H.E.) showed that the electro-catalytic activity followed the order of Pt/NCN-2.2V> Pt/NCN-2.0V> Pt/NCN-1.8V> Pt/Nb2O5> Pt/Nb> Pt/NCN-2.4V> Pt/C(normalized against the electrochemical surface area of Pt); Pt/NCN-2.2V> Pt/Nb>Pt/NCN-1.8V>Pt/C>Pt/NCN-2.0V>Pt/Nb2O5>Pt/NCN-2.4V(normalized against the mass of Pt). The chronoamperometric results from Pt/Nb sub-oxide for methanol oxidation at 0.6V (vs. R.H.E.) showed that the electro-catalytic activity followed the order of Pt/ZrO0.35-CaZrO3>Pt/ZrO2>Pt/Zr> Pt/ZrO0.27-CaZrO3>Pt/C (normalized against the electrochemical surface area of Pt); and Pt/ZrO0.35-CaZr03>Pt/ZrOo.27-CaZrO3>Pt/C>Pt/Zr>(2.0V)>Pt/ZrO2(normalized against the mass of Pt). Herein, NCN represents Nb sub-oxides.4. At the PtRu loading of 20wt%, compared with the PtRu/C catalyst, the negative potential shift of the CO striping peak on PtRu/ZrO2, PtRu/ZrOo.35, and PtRu/ZrO0.27 indicate much higher anti-poisoning capability for CO on these catalysts than on PtRu/C. PtRu/C showed the highest electro-catalytic activity per electrochemical surface area toward CH3OH oxidation among all the catalysts; but the PtRu/ZrOo.35 showed the highest electro-catalytic activity per milligram Pt toward CH3OH oxidation among all the catalysts.5. Partially metalized solid oxides were used to support PtRu catalysts and investigated for their catalytic-assistance toward CO and methanol oxidation in this work. The CO stripping peak on the PtRu/Nb catalyst showed 25 mV negative shift than on the PtRu/C catalyst. At the PtRu loading of 20wt%, the PtRu/Nb catalyst indicated much higher anti-poisoning capability for CO. For the PtRu catalysts, it was observed that the reduction peak on the backward sweep became higher with continuous cycling the potential. This phenomenon may be due to the dissolution of Ru oxide during the CV scanning. The dissolution of Ru oxide resulted in more Pt exposing on the surface of the catalyst. The i-t curves at 0.6V(vs. R.H.E.) show that the PtRu/Nb catalyst get the highest catalytic activity as normalized against the mass of the metal catalyst among all the catalysts.