The effect of curvature in thawing models

We study the evolution of spatial curvature for thawing class of dark energy models. We examine the evolution of the equation of state parameter, $w_\phi$, as a function of the scale factor $a$, for the case in which the scalar field $\phi$ evolve in nearly flat scalar potential. We show that all such models provide the corresponding approximate analytical expressions for $w_\phi(\Omega_\phi,\Omega_k)$ and $w_\phi(a)$. We present observational constraints on these models.Comment: 14 pages, 6 figures. Accepted for publication in Phys. Lett.

Kinetic Study of Oxidative Dehydrogenation of Ethane over MoVTeNb Mixed-Oxide Catalyst

This document is the Accepted Manuscript version of a Published Work that appeared in final form in Industrial & Engineering Chemistry Research, copyright © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see [insert ACS Articles on Request author-directed link to Published Work, see http://doi.org/10.1021/ie402447h[EN] A MoVTeNb multimetallic mixed oxide was studied for the oxidative dehydrogenation of ethane, a promising alternative for catalytic ethylene production. Lab-scale steady-state experimental reaction data were obtained according to a 3(k) experimental design to investigate the simultaneous effect of temperature (400-480 degrees C) and space time [23-70 g(cat) h (mol of ethane) I]. A fixed-bed reactor at atmospheric pressure was employed, feeding a mixture of ethane, oxygen, and nitrogen. Ethane conversion varied from 17 to 85%, whereas selectivity for ethylene and COx varied from 94 to 76% and from 4.0 to 24%, respectively. These types of analyses are useful for determining the optimum reaction conditions to enhance the catalytic performance of the mixed oxides presented herein.This work was financially supported by the Instituto Mexicano del Petroleo. Technical support from Eng. G. Alonso-Ramirez is gratefully acknowledged.Valente, J.; Quintana-Solorzano, R.; Armendariz-Herrera, H.; Barragan-Rodriguez, G.; López Nieto, JM. (2014). Kinetic Study of Oxidative Dehydrogenation of Ethane over MoVTeNb Mixed-Oxide Catalyst. Industrial and Engineering Chemistry Research. 53(5):1775-1786. doi:10.1021/ie402447hS1775178653

Understanding the kinetic behavior of a Mo-V-Te-Nb mixed oxide in the oxydehydrogenation of ethane

Two kinetic models based on Langmuir-Hinshelwood (LH) and Eley-Rideal (ER) mechanisms were developed to describe the oxydehydrogenation of ethane to yield ethylene over a Mo-V-Te-Nb catalyst. Obtained in a lab-scale fixed-bed reactor, experimental data at the steady-state were used to estimate the kinetic models parameters via a nonisothermal regression. Experiments were performed using an ethane, oxygen and nitrogen mixture as feedstock, spanning temperatures from 673 to 753 K, inlet partial pressures of oxygen and ethane between 5.0 and 22.0 kPa, and space-time from 10 to 70 g(cat) h(molethane)- (1). Ethylene, CO and CO2 were the only detected products, the selectivity for ethylene ranged from 76% to 96% for an ethane conversion interval 4-85%. A series of tests feeding ethylene instead of ethane were also performed at 713 K, varying inlet partial pressures and space-time in the same ranges as was done for ethane. Ethylene conversion was relatively low, 3-14%, the dominant product being CO with CO/CO2 ratios from 0.73 to 0.79. The LH mechanism was found to represent better the experimental data. The oxydehydrogenation of ethane was the reaction with the lowest activation energy, 108-115 kJ mol (1). Except for the conversion of ethane into CO2, deep oxidations were detected as very energetically demanding steps, 156-193 kJ mol (1). Competitive adsorption between reagents and products occurred in the two mechanisms particularly at relatively high reaction severity, water re-adsorption being weaker in comparison with COx re-adsorption. (C) 2014 Elsevier Ltd. All rights reserved.This work was financially supported by the Instituto Mexicano del Petroleo.Quintana-Solorzano, R.; Barragan-Rodriguez, G.; Armendariz-Herrera, H.; López Nieto, JM.; Valente, JS. (2014). Understanding the kinetic behavior of a Mo-V-Te-Nb mixed oxide in the oxydehydrogenation of ethane. Fuel. 138:15-26. doi:10.1016/j.fuel.2014.07.051152613

Metal solution precursors: their role during the synthesis of MoVTeNb mixed oxide catalysts

[EN] Synthesized via the slurry method and activated at high temperature (873 K), MoVTeNb multimetallic mixed oxides are applied to catalyze the oxidative dehydrogenation of ethane to ethylene (ODHE). Mixed oxides typically contain M1 and M2 crystalline phases, the relative contribution of these phases and the respective catalytic behaviour being notably influenced by the preparation conditions of the metallic aqueous solution precursor, given the complexity of the chemical interactions of metal species in solution. Thus, detailed in situ UV-vis and Raman studies of the chemical species formed in solution during each step of the synthetic procedure are presented herein. The main role of vanadium is to form decavanadate ions, which interact with Mo species to generate an Anderson-type structure. When niobium oxalate solution is added into the MoVTe solution, a yellow-coloured gel is immediately formed due to a common ion effect. When liquid and gel phases are separated, the M1 crystalline phase is produced solely from the gel phase. Attention is also devoted to the influence and role of each metal cation (Mo, V, Te and Nb) on the formation of the active M1 crystalline phase and the catalytic behaviour in the ODHE. The catalyst constituted mostly of M1 crystalline phase is able to convert 45% of the fed ethane, with a selectivity to ethylene of around 90%.This work was financially supported by the Instituto Mexicano del Petroleo (IMP) Project D.61010. EMF thanks CONACyT Mexico and IMP. JMLN thanks DGICYT in Spain (Project CTQ2015-68951-C3-1-R).Sánchez-Valente, J.; Maya-Flores, E.; Armendariz-Herrera, H.; Quintana-Solorzano, R.; López Nieto, JM. (2018). Metal solution precursors: their role during the synthesis of MoVTeNb mixed oxide catalysts. Catalysis Science & Technology. 8(12):3123-3132. https://doi.org/10.1039/c8cy00750kS31233132812Ushikubo, T., Oshima, K., Kayou, A., Vaarkamp, M., & Hatano, M. (1997). Ammoxidation of Propane over Catalysts Comprising Mixed Oxides of Mo and V. Journal of Catalysis, 169(1), 394-396. doi:10.1006/jcat.1997.1692Ushikubo, T., Oshima, K., Kayou, A., & Hatano, M. (1997). Ammoxidation of propane over Mo-V-Nb-Te mixed oxide catalysts. Spillover and Migration of Surface Species on Catalysts, Proceedings of the 4th International Conference on Spillover, 473-480. doi:10.1016/s0167-2991(97)80871-3Ushikubo, T. (2000). Recent topics of research and development of catalysis by niobium and tantalum oxides. Catalysis Today, 57(3-4), 331-338. doi:10.1016/s0920-5861(99)00344-2Ueda, W., & Oshihara, K. (2000). Selective oxidation of light alkanes over hydrothermally synthesized Mo-V-M-O (M=Al, Ga, Bi, Sb, and Te) oxide catalysts. Applied Catalysis A: General, 200(1-2), 135-143. doi:10.1016/s0926-860x(00)00627-xWatanabe, H., & Koyasu, Y. (2000). New synthesis route for Mo–V–Nb–Te mixed oxides catalyst for propane ammoxidation. Applied Catalysis A: General, 194-195, 479-485. doi:10.1016/s0926-860x(99)00394-4Botella, P., Solsona, B., Martinez-Arias, A., & López Nieto, J. M. (2001). Catalysis Letters, 74(3/4), 149-154. doi:10.1023/a:1016614132694Oshihara, K., Hisano, T., & Ueda, W. (2001). Topics in Catalysis, 15(2/4), 153-160. doi:10.1023/a:1016630307377Botella, P., López Nieto, J. M., Solsona, B., Mifsud, A., & Márquez, F. (2002). The Preparation, Characterization, and Catalytic Behavior of MoVTeNbO Catalysts Prepared by Hydrothermal Synthesis. Journal of Catalysis, 209(2), 445-455. doi:10.1006/jcat.2002.3648Millet, J. M. M., Roussel, H., Pigamo, A., Dubois, J. L., & Jumas, J. C. (2002). Characterization of tellurium in MoVTeNbO catalysts for propane oxidation or ammoxidation. Applied Catalysis A: General, 232(1-2), 77-92. doi:10.1016/s0926-860x(02)00078-9DeSanto Jr., P., Buttrey, D. J., Grasselli, R. K., Lugmair, C. G., Volpe, A. F., Toby, B. H., & Vogt, T. (2003). Topics in Catalysis, 23(1/4), 23-38. doi:10.1023/a:1024812101856Millet, J. M. ., Baca, M., Pigamo, A., Vitry, D., Ueda, W., & Dubois, J. . (2003). Study of the valence state and coordination of antimony in MoVSbO catalysts determined by XANES and EXAFS. Applied Catalysis A: General, 244(2), 359-370. doi:10.1016/s0926-860x(02)00614-2BOTELLA, P. (2004). Selective oxidative dehydrogenation of ethane on MoVTeNbO mixed metal oxide catalysts. Journal of Catalysis, 225(2), 428-438. doi:10.1016/j.jcat.2004.04.024Holmberg, J., Grasselli, R. K., & Andersson, A. (2004). Catalytic behaviour of M1, M2, and M1/M2 physical mixtures of the Mo–V–Nb–Te–oxide system in propane and propene ammoxidation. Applied Catalysis A: General, 270(1-2), 121-134. doi:10.1016/j.apcata.2004.04.029Grasselli, R. K., Buttrey, D. J., DeSanto, P., Burrington, J. D., Lugmair, C. G., Volpe, A. F., & Weingand, T. (2004). Active centers in Mo–V–Nb–Te–O (amm)oxidation catalysts. 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Active centers, catalytic behavior, symbiosis and redox properties of MoV(Nb,Ta)TeO ammoxidation catalysts. Topics in Catalysis, 38(1-3), 7-16. doi:10.1007/s11244-006-0066-xSafonova, O. V., Deniau, B., & Millet, J.-M. M. (2006). Mechanism of the Oxidation−Reduction of the MoVSbNbO Catalyst:  In Operando X-ray Absorption Spectroscopy and Electrical Conductivity Measurements. The Journal of Physical Chemistry B, 110(47), 23962-23967. doi:10.1021/jp064347lWagner, J. B., Timpe, O., Hamid, F. A., Trunschke, A., Wild, U., Su, D. S., … Schlögl, R. (2006). Surface texturing of Mo–V–Te–Nb–O x selective oxidation catalysts. Topics in Catalysis, 38(1-3), 51-58. doi:10.1007/s11244-006-0070-1Kolen’ko, Y. V., Zhang, W., d’ Alnoncourt, R. N., Girgsdies, F., Hansen, T. W., Wolfram, T., … Trunschke, A. (2011). Synthesis of MoVTeNb Oxide Catalysts with Tunable Particle Dimensions. ChemCatChem, 3(10), 1597-1606. doi:10.1002/cctc.201100089Hävecker, M., Wrabetz, S., Kröhnert, J., Csepei, L.-I., Naumann d’Alnoncourt, R., Kolen’ko, Y. V., … Trunschke, A. (2012). Surface chemistry of phase-pure M1 MoVTeNb oxide during operation in selective oxidation of propane to acrylic acid. Journal of Catalysis, 285(1), 48-60. doi:10.1016/j.jcat.2011.09.012Ishikawa, S., Tashiro, M., Murayama, T., & Ueda, W. (2014). Seed-Assisted Synthesis of Crystalline Mo3VOx Oxides and Their Crystal Formation Mechanism. Crystal Growth & Design, 14(9), 4553-4561. doi:10.1021/cg500661pNieto, J. M. L., Botella, P., Vázquez, M. I., & Dejoz, A. (2002). The selective oxidative dehydrogenation of ethane over hydrothermally synthesised MoVTeNb catalysts. Chem. Commun., (17), 1906-1907. doi:10.1039/b204037aLópez Nieto, J. ., Botella, P., Concepción, P., Dejoz, A., & Vázquez, M. . (2004). Oxidative dehydrogenation of ethane on Te-containing MoVNbO catalysts. Catalysis Today, 91-92, 241-245. doi:10.1016/j.cattod.2004.03.040Ivars, F., Botella, P., Dejoz, A., Nieto, J. M. L., Concepción, P., & Vázquez, M. I. (2006). Selective oxidation of short-chain alkanes over hydrothermally prepared MoVTeNbO catalysts. Topics in Catalysis, 38(1-3), 59-67. doi:10.1007/s11244-006-0071-0Botella, P., Dejoz, A., Abello, M. C., Vázquez, M. I., Arrúa, L., & López Nieto, J. M. (2009). Selective oxidation of ethane: Developing an orthorhombic phase in Mo–V–X (X=Nb, Sb, Te) mixed oxides. Catalysis Today, 142(3-4), 272-277. doi:10.1016/j.cattod.2008.09.016Deniau, B., Millet, J. M. M., Loridant, S., Christin, N., & Dubois, J. L. (2008). Effect of several cationic substitutions in the M1 active phase of the MoVTeNbO catalysts used for the oxidation of propane to acrylic acid. Journal of Catalysis, 260(1), 30-36. doi:10.1016/j.jcat.2008.08.020SOLSONA, B., VAZQUEZ, M., IVARS, F., DEJOZ, A., CONCEPCION, P., & LOPEZNIETO, J. (2007). Selective oxidation of propane and ethane on diluted Mo–V–Nb–Te mixed-oxide catalysts. Journal of Catalysis, 252(2), 271-280. doi:10.1016/j.jcat.2007.09.019Nguyen, T. T., Burel, L., Nguyen, D. L., Pham-Huu, C., & Millet, J. M. M. (2012). Catalytic performance of MoVTeNbO catalyst supported on SiC foam in oxidative dehydrogenation of ethane and ammoxidation of propane. Applied Catalysis A: General, 433-434, 41-48. doi:10.1016/j.apcata.2012.04.038Nguyen, T. T., Aouine, M., & Millet, J. M. M. (2012). Optimizing the efficiency of MoVTeNbO catalysts for ethane oxidative dehydrogenation to ethylene. Catalysis Communications, 21, 22-26. doi:10.1016/j.catcom.2012.01.026Valente, J. S., Armendáriz-Herrera, H., Quintana-Solórzano, R., del Ángel, P., Nava, N., Massó, A., & López Nieto, J. M. (2014). Chemical, Structural, and Morphological Changes of a MoVTeNb Catalyst during Oxidative Dehydrogenation of Ethane. ACS Catalysis, 4(5), 1292-1301. doi:10.1021/cs500143jTHORSTEINSON, E. (1978). The oxidative dehydrogenation of ethane over catalysts containing mixed oxides of molybdenum and vanadium. Journal of Catalysis, 52(1), 116-132. doi:10.1016/0021-9517(78)90128-8Ishikawa, S., Yi, X., Murayama, T., & Ueda, W. (2014). Heptagonal channel micropore of orthorhombic Mo3VOx as catalysis field for the selective oxidation of ethane. Applied Catalysis A: General, 474, 10-17. doi:10.1016/j.apcata.2013.07.050Ishikawa, S., Yi, X., Murayama, T., & Ueda, W. (2014). Catalysis field in orthorhombic Mo3VOx oxide catalyst for the selective oxidation of ethane, propane and acrolein. Catalysis Today, 238, 35-40. doi:10.1016/j.cattod.2013.12.054Grasselli, R. K., Burrington, J. D., Buttrey, D. J., DeSanto Jr., P., Lugmair, C. G., Volpe Jr., A. F., & Weingand, T. (2003). Topics in Catalysis, 23(1/4), 5-22. doi:10.1023/a:1024859917786Grasselli, R. K., Lugmair, C. G., Volpe Jr., A. F., Andersson, A., & Burrington, J. D. (2010). 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Interaction between Tachyon and Hessence (or Hantom) dark energies

In this paper, we have considered that the universe is filled with tachyon, hessence (or hantom) dark energies. Subsequently we have investigated the interactions between tachyon and hessence (hantom) dark energies and calculated the potentials considering the power law form of the scale factor. It has been revealed that the tachyonic potential always decreases and hessence (or hantom) potential increases with corresponding fields. Furthermore, we have considered a correspondence between the hessence (or hantom) dark energy density and new variable modified Chaplygin gas energy density. From this, we have found the expressions of the arbitrary positive constants B0 and C of new variable modified Chaplygin gas

Palatini formulation of the R−1R^{-1}modified gravity with an additionally squared scalar curvature term

In this paper by deriving the Modified Friedmann equation in the Palatini formulation of $R^2$ gravity, first we discuss the problem of whether in Palatini formulation an additional $R^2$ term in Einstein's General Relativity action can drive an inflation. We show that the Palatini formulation of $R^2$ gravity cannot lead to the gravity-driven inflation as in the metric formalism. If considering no zero radiation and matter energy densities, we obtain that only under rather restrictive assumption about the radiation and matter energy densities there will be a mild power-law inflation $a(t)\sim t^2$, which is obviously different from the original vacuum energy-like driven inflation. Then we demonstrate that in the Palatini formulation of a more generally modified gravity, i.e., the $1/R+R^2$ model that intends to explain both the current cosmic acceleration and early time inflation, accelerating cosmic expansion achieved at late Universe evolution times under the model parameters satisfying $\alpha\ll\beta$.Comment: 14 pages, accepted for publication by CQ

Biological synthesis of nanosized sulfide semiconductors: current status and future prospects

There have been extensive and comprehensive reviews in the field of metal sulfide precipitation in the context of environmental remediation. However, these works have focused mainly on the removal of metals from aqueous solutions-usually, metal-contaminated effluents-with less emphasis on the precipitation process and on the end-products, frequently centering on metal removal efficiencies. Recently, there has been an increasing interest not only in the possible beneficial effects of these bioremediation strategies for metal-rich effluents but also on the formed precipitates. These metal sulfide materials are of special relevance in industry, due to their optical, electronic, and mechanical properties. Hence, identifying new routes for synthesizing these materials, as well as developing methodologies allowing for the control of the shape and size of particulates, is of environmental, economic, and practical importance. Multiple studies have shown proof-of-concept for the biological synthesis of inorganic metallic sulfide nanoparticles (NPs), resorting to varied organisms or cell components, though this information has scarcely been structured and compiled in a systematic manner. In this review, we overview the biological synthesis methodologies of nanosized metal sulfides and the advantages of these strategies when compared to more conventional chemical routes. Furthermore, we highlight the possibility of the use of numerous organisms for the synthesis of different metal sulfide NPs, with emphasis on sulfate-reducing bacteria (SRB). Finally, we put in perspective the potential of these methodologies in the emerging research areas of biohydrometallurgy and nanobiotechnology for the uptake of metals in the form of metal sulfide nanoparticles. A more complete understanding of the principles underlying the (bio)chemistry of formation of solids in these conditions may lead to the large-scale production of such metal sulfides, while simultaneously allowing an enhanced control over the size and shape of these biogenic nanomaterials