Molecular Structure and Thermal Stability of Oxide-Supported Phosphotungstic Wells-Dawson Heteropolyacid

We present, for the first time in literature, a systematic study of the molecular structure of the Wells-Dawson heteropolyacid H6P2W18O62.24H2O (HPA) dispersed on TiO2, SiO2, ZrO2 and Al2O3. The heteropolyacid-based materials were synthesized through a conventional impregnation method (in aqueous and ethanol media) at a loading that corresponds to the theoretical monolayer coverage (dispersion limit loading). The combination of Raman and infrared studies demonstrates the presence of crystals of HPA (regardless of the nature of the medium used during the synthesis) suggesting that the dispersion limit loading was greatly exceeded. In situ temperature programmed analyses demonstrated that the Raman shift of the distinctive W=O Raman mode of the fosfotungstic Wells-Dawson heteropolyacid is sensitive to the local environment, that is the amount of water molecules associated with the structure. Moreover, the aqueous based species associated with such structures are recognizable through infrared spectroscopy.Fil: Matkovic, Silvana Raquel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico la Plata. Centro de Investigación y Desarrollo En Ciencias Aplicadas; ArgentinaFil: Collins, Sebastián Enrique. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Santa Fe. Instituto de Desarrollo Tecnológico Para la Industria Química (i); ArgentinaFil: Bonivardi, Adrian Lionel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Santa Fe. Instituto de Desarrollo Tecnológico Para la Industria Química (i); ArgentinaFil: Bañares, Miguel A.. CSIC, Consejo Superior de Investigaciones Cientificas, Instituto de Catalisis y Petroquimica.Catalytic Spectroscopy Laboratory; EspañaFil: Briand, Laura Estefania. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico la Plata. Centro de Investigación y Desarrollo En Ciencias Aplicadas; Argentin

CO on Pd(100)/SiO2: a computational study of the effect of CaO on the energetics of adsorption

The semiempirical atom superposition and electron delocalisation molecular orbital (ASED-MO) theory was used to study the CO/Pd-Ca-SiO2 interaction. Addition of a single CaO molecule to simulate a different interaction with both the metal and the CO molecule was also considered. Ca was added as a sublayer between the SiO2 and the metallic cluster. A simple diatomic CaO molecule was positioned on the Pd21 surface. The energetics of CO adsorption was studied in both cases. In our adsorption scheme, CO was considered to be perpendicular to the surface. The results indicate that in general adsorption on twofold sites seems to be more favourable. The Ca sublayer stabilises CO adsorption by less than 0.1 eV for twofold short bridge adsorption site. Fourfold site is de-stabilised by 0.6 eV and is 2 eV less favourable than the twofold sites. The effect of Ca and CaO on the adsorption is of small magnitude. © 2000 Published by Elsevier Science B.V. All rights reserved.status: publishe

Infrared and Raman investigation of supported phosphotungstic Wells- Dawson heteropolyacid

The combination of both infrared and Raman spectroscopy are powerful tools to obtain information of catalytic materials at a molecular level. The present investigation shows, a systematic study in situ about the effects of the temperature in the molecular structure of the supported phosphotungstic Wells-Dawson heteropolyacid (HPA). The infrared and Raman analyses of the samples were performed under in situ conditions from RT to 500 oC range in flowing helium. The oxide supported heteropolyacid was synthesized through a conventional impregnation method (in aqueous) at theoretical monolayer coverage. These studies provided evidences on the dehydration of the HPA upon in situ calcination. Moreover, the in situ investigation allows establishing the surface molecular structure or the thermal stability of the HPA. Additionally, the presence of crystals of HPA over TiO2 suggests that the theoretical monolayer loading should be re-evaluated.Fil: Bonivardi, Adrian Lionel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico - Conicet- Santa Fe. Instituto de Desarrollo Tecnológico Para la Industria Química (i); ArgentinaFil: Matkovic, Silvana Raquel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico la Plata. Centro de Investigación y Desarrollo En Ciencias Aplicadas; ArgentinaFil: Collins, Sebastián Enrique. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico - Conicet- Santa Fe. Instituto de Desarrollo Tecnológico Para la Industria Química (i); ArgentinaFil: Bañares, Miguel A.. Consejo Superior de Investigaciones Científicas (CSIC). Instituto de Catálisis y Petroleoquímica; Españ

Molecular structure and thermal stability of the oxide-supported phosphotungstic Wells-Dawson heteropolyacid

We present, for the first time in the literature, a systematic study of the molecular structure of the Wells-Dawson heteropolyacid H6P2W18O62·24H2O (HPA) dispersed on TiO2, SiO2, ZrO2 and Al2O3. The heteropolyacid-based materials were synthesized through a conventional impregnation method (in aqueous and ethanol media) at a loading that corresponds to the theoretical >monolayer> coverage (dispersion limit loading). The combination of Raman and infrared studies demonstrates the presence of crystals of HPA (regardless of the nature of the medium used during the synthesis) suggesting that the dispersion limit loading was greatly exceeded. In situ temperature programmed spectroscopy analyses demonstrated that the Raman shift of the distinctive WO Raman mode of the phosphotungstic Wells-Dawson heteropolyacid is sensitive to the local environment, that is, the amount of water molecules associated with the structure. Moreover, the aqueous based species associated with such structures are recognizable through infrared spectroscopy.The authors acknowledge the financial support from CONICET (project PIP 0083 and PIP 0278), Universidad Nacional de La Plata (project 11-X626), Argentinian ANPCYT (PICT-2012-1280) and Spanish Ministry project CTQ2011-25517. S. R. Matkovic acknowledges the Spanish Ministerio de Asuntos Exteriores y de Cooperación (MAEC) and Agencia Espan˜ola Cooperacio´n Internacional (AECI).Peer Reviewe

In situ FTIR and Raman study on the distribution and reactivity of surface vanadia species in V2O5/CeO2 catalysts

The surface structure and reactivity of vanadia supported on ceria catalysts were studied by Raman spectroscopy and methanol adsorption and temperature-programmed surface reaction (TPRS) of the adsorbed species monitored in situ by infrared spectroscopy. Monomeric and polymeric vanadia surface species and CeVO4 were identified on the catalysts. Methanol adsorption reveled that mostly reduced Ce3+ sites are present in the surface of the catalysts, produced as a result of the reductive solid reaction of VOx species with the ceria surface. Adsorbed methanol/methoxy decomposed stepwise to formate and, finally, carbonate/carbon dioxide. The reactivity is linked to interphase sites, Ce–O–V, which could favor the abstraction of hydrogen during the methoxy transformation to formates.The authors acknowledge the financial support of the National Agency for Scientific and Technological Promotion (projects PICT-2012-1280, PICT-2010-0836 and PME 311/06), the Universidad Nacional del Litoral (grants CAI+D 2011 PI 50120110100311 and 50120110100100) and Spanish Ministry (grant CTQ2011- 25517-E).Peer Reviewe

Monoxide carbon and formate species frequency shift on fully oxidized and partially reduced TiO2 and CeO2 surfaces

Trabajo presentado en el 5th Workshop on Novel Methods for Electronic Structure Calculations, celebrado en Montevideo (Uruguay) del 3 al 5 de diciembre de 2013.Understanding the surface chemistry of oxide materials holds great promise for impacting countless technologies that will be critical for our energy and environmental future. Oxide surface chemistry is also crucial for making and using catalysts for the manufacture of chemicals and for pollution cleanup, and for the production and use of fuel cells(1-2), solar fuel photocatalysts(3-4) among others. In this presentation we show the studies of monoxide carbon frequency shift as a tool for the characterization of TiO2 surfaces and the stability of the formate species (HCOO-) in the surface of CeO2 (111) partially hydroxilated. In the first case the adsorption and vibrational frequency of CO on defective and undefective titanium dioxide surfaces is examined applying first-principles molecular dynamics simulations. In particular, the vibrational frequencies are obtained beyond the harmonic approximation, through the time correlation functions of the atomic trajectories. For example, at low CO coverages we find, in good agreement with experimental results an upshift in the vibration frequency with respect to the free CO molecule, of 45 and 35 cm−1 on the stoichiometric rutile (110) and anatase (101) faces, respectively. On the other hand, formate species are considered as potential intermediary molecules in the reaction of water gas shift (WGS) as well as the decomposition of formic acid and methanol, among others. One of the important aspects in the elucidation of these reactions is the identification of the structure of the formate imtermediate: unidentate, bidentate or bridge in the presence of partially hydroxylated surface.Peer Reviewe

Study of formate species in the CH3OH/CeO2 reaction: combining IR spectroscopy and statistical thermodynamics techniques

Trabajo presentado en el Meeting COST Action CM1104-Reducible oxide chemistry, structure and functions, celebrado en Barcelona (España) del 12 al 14 de noviembre de 2014.Formate species (HCOO-) have been suggested as intermediates or spectators in chemical reactions of industrial relevance, such as the water-gas-shift and the methanol steam reforming reaction [1]. Several catalysts, some of them based on ceria, have been proposed to improve the performance of these reactions. One of the main reasons of choosing ceria has been its intrinsic redox properties, but only few works describe the formate formation under the oxidizing/reducing environment, usually modulated by the reaction itself. In this work, we have studied the stability of formate groups in the CH3OH/CeO2 system. In situ transmission infrared spectroscopy was used to study the temperature-programmed surface reaction (TPSR-IR) of adsorbed methanol. Three types of formates were observed based on the OCO stretching frequencies [(OCO)] (Figure 1). Type III formate (as = 1550 cm-1) was the first one obtained over 450 K, where no Ce3+ was detected. However, after increasing the temperature, types I and II (as = 1580 and 1561 cm-1) species emerged accompanied by the appearance of surface Ce3+ species. The phase diagram of formate structures in contact with a gas environment of O2 and H2, to simulate oxidizing/reducing conditions, was calculated using density-functional theory and statistical calculations. The formate binding structure was found to crucially depend on temperature and partial pressures of the reactants in the gas phase. In the absence of methanol the CeO2, CeO2-x, O2/CeO2 and several OH-coverages phases coexist. As methanol pressure increases, we found a bridge formate with two next neighbors hydroxyls adsorbed (NN-OHads) and no Ce3+, named Brg-A (1535 cm-1). At higher methanol chemical potential (Figure 2), a monodentate formate with three NN-OHads and a bridge state with one NN-OHads, named Mono-B and Brg-B, respectively, appeared together with the formation of Ce3+ species (1564 and 1540 cm-1, respectively). We conclude that the experimentally observed formate species correspond to those monodentate and brigde types.Peer Reviewe