Structure-Activity Relationship Studies And Anodic Activation For Nano PtRu Catalysts

The direct methanol fuel cell (DMFC) powered by a liquid fuel has been attracting much attention in recent decades. With the system-related advantages and simpler miniaturization, DMFC is regarded as a promising power source for transport applications and portable electric appliances as well. Up to now one of the main technological obstacles for DMFC to commercial application is still the lack of high-efficiency anode catalysts for methanol electrooxidation. PtRu catalyst is the anode catalyst most widely used today in DMFC. Although numerous works have been reported concerning the different aspects of the PtRu catalyst, this system remains not fully understood and therefore, there is still a space for improvement. In this thesis, the activation effect of anodic treatment for PtRu/C catalysts was discovered. Such finding is not only a practical method for catalyst activity promotion and also provides new clues for the study on the structure-activity relationship of PtRu catalyst. On such basis, this thesis is devoted to some basic studies on the structure-activity relationship. The main contents and conclusions are as follows:1. Pattern recognition for the structure-activity relationship of nano PtRu catalystsIt is well known that the activity of PtRu/C catalysts is affected by several factors. However, the studies reported in the literature have been mainly confined to the investigation on a single factor without controlling other influencing factors in experiments. As a result, the conclusions are not fully convincing. In this thesis, a database was built up consisting of the particle size and alloy degree data from XRD, the amounts of hydrous ruthenium oxides found by thermal analysis and the catalytic activity measured by electrochemical experiments. The combined effects of several factors on the activity were studied by pattern recognition methods. It was found that within the range of parameter variations of the database the decrease of PtRu particle size and increase of the amount of ruthenium oxides are beneficial to the catalytic activity while the alloy degree is much less influencing. This work is valuable and castsa new light on the structure-activity relationship research of nano PtRu catalysts2. Analyses of the cyclic voltammetry (CV) curves for nano PtRu/C catalystsThe CV curves contain rich information about the PtRu/C catalyst. However, it is difficult to assign the curve features since the composition of PtRu/C catalyst is complicated. In this thesis, the CV features of PtRu/C catalyst are analyzed by comparing with the CVs of metallic Ru and ruthenium oxide, and by observing the systematic changes of CV in the process of anodic treatment of the catalyst. It was indicated that the CV of nano-PtRu/C are mainly determined by the alloy and ruthenium oxides. The CV of PtRu/C is Ru-like but not Pt-like even if the ruthenium alloy degree is as low as 3%. The CVs of ruthenium oxides can be either electrochemically reversible or irreversible depending on the details of their composition and structure. These two kinds of ruthenium oxides are gradually lost in the process of anodic treatment; at the same time, the Ru in the alloy is oxidized to reversible oxides and the alloy degree decreases accordingly. It was also shown by multiple regression that the CV charges can be reasonably separated into the three components contributed from the carbon support, alloy particles and ruthenium oxides, respectively.3. The finding and the mechanistic study of the anodic activation effect for nano PtRu/C catalystsWe discovered that the activity of PtRu/C can be considerably promoted by polarization at 1.3 V (vs. RHE) for about half an hour. All the catalysts tested, including Johnson-Matthey commercial catalyst, showed marked promotion effects without exception. The catalytic activity gains ranged from tens percent to 5 times depending on individual catalysts. COadS stripping, cathodic treatment, chlorine ion effect and CV characterization experiments were conducted to seek for the promotion mechanism. The results indicated that the anodic activation correlates strongly with the increase of the reversible ruthenium oxides and the decrease of the irreversible oxides.4. Preliminary solid-state H-NMR study on the hydrous ruthenium oxides in nano PtRu/C catalystSolid-state 'H-NMR studies were attempted to gain a new insight into the status and dynamics of protons in hydrous ruthenium oxides in PtRu/C. The experiments illustrated that three kinds of protons exist in the catalyst. With reference to the thermal analysis data, the three protons were tentatively assigned to physically adsorbed water and two kinds of structural waters bonded differently to RuC^. The temperature dependence of the spin-lattice relaxation time (Ti) of protons in the catalyst was measured by variable-temperature H-NMR experiments and was used to deduce the activation energy of thermal diffusion for the protons. The activation energy thus found was 8.16 kJ/mol, which is markedly higher than the lowest value 2.74 kJ/mol reported in the literature for hydrous ruthenium oxides, implying that the activity of PtRu/C catalysts could hopefully be further improved by optimizing the proton diffusivity in the hydrous ruthenium oxide.