K. Ravindranathan Thampi

A fourier transform infrared spectroscopic study of C02 methanation on supported ruthenium

Diffuse-reflectance infrared Fourier transform (DRIFT) spectroscopy has been used to study in situ, the low-temperature (T < 200°C) methanation of CO2 over Ru on TiO2 supports and on Al2O3. For 3.8% Ru/TiO2, the reaction exhibits an activation energy (Ea) of 19 kcal/mol, is 0.43 ± 0.05 (approximately one-halt) order in H2 concentration, and essentially independent of C02 concentration. At 110°C, 40% of the available metal sites are occupied by CO (Qco = 0.4), a known methanation intermediate. In contrast to Ru/TiO2, Ru/Al2O3, despite having the same Ea and Qco = 0.2, is 15 times less active. Batch catalyst screening experiments showed no dependence of methanation activity on adsorbed CO (COa) formation rate (as modeled by HCOOH dehydration) or on Qco. In view of this, and the fact that CO dissociation is known to be structure-sensitive, heterogeneity in the active sites is invoked to reconcile the data. The high Ru dispersion on TiO2 is believed to contribute to the enhanced activity over this support. Adsorbed CO2 and H2 react, possibly at the metal-support interface, to form COa via rapid equilibration of the reverse water-gas shift reaction, in which HCOOH (and/or HCOO- ion) play a major role. According to this view, the COa and HCOO-a intermediates seen by FTIR represent accumulated reservoirs en route to CH4, in which the COa hydrogenation step is rate-controlling. An interesting synergy occurs for mixtures of Ru/anatase and Ru/rutile, the former being a better catalyst for CO. supply while the latter is more effective in COa hydrogenation.

A study on the La1-xSrxMnO3 oxygen cathode

The impedance responses and current-overpotential characteristics were studied for oxygen reducing cathodes, stoichiometric La1 − xSrxMnO3 (x = 0.16 − 0.20) in solid oxide fuel cells, under varying conditions of temperature (700–900 °C) and oxygen partial pressure (1 − 10−4atm) for two distinctly different electrode morphologies (porous and dense structure). An electrode densified at the interface with the yttria-stabilized zirconia solid electrolyte was more efficient in this study. The reaction at the finely porous LSM proceeds via the triple phase boundary and ineffectively uses the mixed conductivity property of the material. Reaction mechanisms for both cases (porous and dense structure) are discussed and compared with the literature. A new, two-layered cathode structure is proposed.

Fourier transform infrared spectroscopic study of carbon dioxide methanation on supported ruthenium

Diffuse-reflectance infrared Fourier transform (DRIFT) spectroscopy has been used to study in situ, the low-temperature (T < 200°C) methanation of CO2 over Ru on TiO2 supports and on Al2O3. For 3.8% Ru/TiO2, the reaction exhibits an activation energy (Ea) of 19 kcal/mol, is 0.43 ± 0.05 (approximately one-halt) order in H2 concentration, and essentially independent of C02 concentration. At 110°C, 40% of the available metal sites are occupied by CO (Qco = 0.4), a known methanation intermediate. In contrast to Ru/TiO2, Ru/Al2O3, despite having the same Ea and Qco = 0.2, is 15 times less active. Batch catalyst screening experiments showed no dependence of methanation activity on adsorbed CO (COa) formation rate (as modeled by HCOOH dehydration) or on Qco. In view of this, and the fact that CO dissociation is known to be structure-sensitive, heterogeneity in the active sites is invoked to reconcile the data. The high Ru dispersion on TiO2 is believed to contribute to the enhanced activity over this support. Adsorbed CO2 and H2 react, possibly at the metal-support interface, to form COa via rapid equilibration of the reverse water-gas shift reaction, in which HCOOH (and/or HCOO- ion) play a major role. According to this view, the COa and HCOO-a intermediates seen by FTIR represent accumulated reservoirs en route to CH4, in which the COa hydrogenation step is rate-controlling. An interesting synergy occurs for mixtures of Ru/anatase and Ru/rutile, the former being a better catalyst for CO. supply while the latter is more effective in COa hydrogenation.

FTIR spectroscopic study of the interaction of CO2 and CO2 + H2 over partially oxidized Ru/TiO2 catalyst

At least three distinct linearly bound carbonyl species are identified in the adsorption of CO2 or CO2 + H2 over RuRuOxTiO2 catalyst. The relative concentration and the growth of these species depend on metal oxidation state, presence of hydrogen, reaction temperature, and duration of exposure. The presence of preadsorbed or coadsorbed hydrogen promotes formation of x173-1 type species, the RuOx-(CO)ad species develop only on prolonged exposure to a dose of CO2 or CO2 + H2. The oxygen or the hydrogen ligand bonded to ruthenium facilitates CO bond scission. The widely reported lower temperature requirement for the CO2 methanation reaction as compared to that of CO is attributed to the high reactivity of nascent carbonyl species which give methane directly via “active” carbon formation. As shown earlier (Gupta et al., J. Catal. 137, 437 (1992)), the CO methanation requires multistep transformations, making the process energy intensive, particularly in the 300–450 K temperature range. The studies using 2H and 13C labeled adsorbates helped in the identification of oxygenated surface species having vibrational bands in the 1000–1800 cm−1 region. These species are regarded as inactive side products formed on the support and/or at the Ru-support interfaces.

Acid versus base peptization of mesoporous nanocrystalline TiO2 films: functional studies in dye sensitized solar cells

We report an analysis of the influence of acid/base conditions employed in the synthesis of TiO2 nanoparticles upon the performance of dye sensitised photoelectrochemical solar cells fabricated from these particles. The functional properties of the TiO2 nanoparticles in these devices are investigated by potential step chronoamperometry, transient laser spectroscopy, and photovoltaic device characterisation. We find that base peptization conditions employed in the sol–gel fabrication of the TiO2 nanoparticles result in a reduction in film electron density under negative applied bias, correlated with slower interfacial recombination losses and a higher device open circuit voltage.

Metal chalcogenides as counter electrode materials in quantum dot sensitized solar cells: a perspective

The quantum dot sensitized solar cell (QDSSC), which has an analogous structure and working principle to the dye sensitized solar cell, has drawn much attention due to its characteristic advantages like ease of fabrication, robustness and the potential for multiple electron generation. Much effort to optimize various components of QDSSCs has been taken to boost the overall device performance. It is well known that the counter electrode (CE) plays a vital role in sensitized solar cells and could profoundly impact the device performance. Recently, metal chalcogenides have been explored as superb counter electrode materials for QDSSCs. This review gives a panorama of both conventional noble metals and carbon CEs and newly emerged metal chalcogenide CE materials for QDSSCs, while the influence of CE materials on the overall device performance is stressed in detail. The Conclusions and prospects emphasizes the importance of studies on metal chalcogenide CE materials and puts forward the remaining challenges that need to be addressed.

Highly active meso-microporous TaON photocatalyst driven by visible light

TaON was found to be ca. 20 times more active as a visible light activated photocatalyst for the oxidation of methanol, when compared to the well studied UV-visible activated TiO2 (P25) catalyst.

Conductivity measurements of various yttria-stabilized zirconia samples

Samples of yttria-stabilized zirconia manufactured by the following fabrication procedures, were obtained from commercial sources: (i) hot isostatic pressing; (ii) tape casting; (iii) vacuum plasma spraying, and (iv) calendering. The ionic conductivities of these samples were measured by (a) impedance spectroscopy; (b) the four-point probe method; (c) the current-interruption technique, and (d) the van der Pauw technique. The tape-cast and hot pressed samples showed good and very reproducible conductivity values. The vacuum plasma sprayed samples showed an anisotropy in their conductivity, with the cross-plane value being several times lower than the in-plane value. A simple model based on the porous microstructure of these samples can explain this observation. Sintering of the plasma sprayed samples minimized the anisotropy and significantly improved their conductivity values. The calendered samples also showed a similar anisotropy in their conductivity data when they were inadequately sintered.

Rare earth cuprates as electrocatalysts for methanol oxidation

A series of rare earth cuprates with overall composition Ln2−xMxCu1−yMy′O4−δ (where Ln=La and Nd; M=Sr, Ca and Ba; M′=Ru and Sb: 0.0≤x≤0.4 and y=0.1) have been tested as anode electrocatalysts for methanol oxidation. The evaluation of electrode kinetic parameters was made galvanostatically. The catalyst characterization was carried out by specific conductivity measurements, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Iodometry. These materials exhibit significant activity for methanol oxidation at higher potentials. The linear correlation between Cu(3+) content and methanol oxidation activity suggests that the active sites for adsorption of methanol is Cu(3+). The methanol oxidation onset potential depends on the ease of Cu(2+)→Cu(3+) oxidation reaction. These materials show better tolerance towards the poisoning by the intermediates of methanol oxidation compared to that of conventional noble metal electrocatalysts (supported and bulk). The lattice oxygen in these oxides could be considered as active oxygen to remove CO intermediates of methanol oxidation reaction.

Materials for polymer electrolyte fuel cells

The commercial success of the polymer electrolyte fuel cell (PEFC) will to a large extent be determined by the nature, properties, functionality, and cost of the electrochemical sub-components used in the membrane electrode assembly (MEA). Materials research activities in Switzerland for the PEFC are being pursued at the Paul Scherrer Institut (Villigen AG) and the Swiss Federal Institute of Technology in Lausanne with different objectives. The radiation grafted proton exchange membrane developed at the Paul Scherrer Institut (PSI) has been brought to a near-product-like quality level with encouraging performance close to state-of-the-art materials and a life-time of several thousand hours. Furthermore, the membrane shows low methanol crossover in the direct methanol fuel cell. In addition, polyarylene block copolymer membranes have been investigated as an option for fluorine-free membranes. The electrocatalysis of Pt in acidic solution and in contact with a solid electrolyte, the development of new methanol oxidation and oxygen reduction catalysts, and co-sputtering of Pt and carbon as an alternative method for catalyst preparation are areas of fundamental research. More applied research is performed in the characterization of commercial electrodes in single cells, using standard as well as advanced diagnostic tools developed in-house. This article gives an overview over the research and development projects in Switzerland related to materials and components for the PEFC.