Patricia Pizarro

Turning TS-1 zeolite into a highly active catalyst for olefin epoxidation with organic hydroperoxides.

A new synthesis route, based on the silanization of zeolitic seeds, has been applied to prepare nanocrystalline TS-1 zeolite with hierarchical porosity, leading to materials exhibiting high catalytic activity in 1-octene epoxidation with organic hydroperoxides as oxidizing agents.

Thermochemical heat storage based on the Mn2O3/Mn3O4 redox couple: influence of the initial particle size on the morphological evolution and cyclability

Thermochemical energy storage (TCS) based on reduction–oxidation cycles of multivalent metal oxides is of great interest for concentrating solar facilities, as it can allow enhancing the global plant efficiency and improving the energy generation dispatchability. However, to guarantee the feasibility of the process, selected materials should present long term durability, which requires the evaluation of the redox couple cyclability. In this work we have demonstrated, for the first time, the suitability of the Mn2O3/Mn3O4 pair for this application during 30 cycles, performed in thermobalance. Nevertheless, an appropriate design of such materials is crucial since it has been found that initial particle size influences the redox behaviour of these oxides. Results showed a 2-fold influence of particle size of the as-prepared materials on the redox reversibility. Firstly, this parameter affects to the thermodynamics and kinetics of the redox reactions. Namely a decrease of the particle size shifted the oxidation temperature to lower values and produced slower reduction and oxidation reactions. Secondly, depending on the particles size, samples followed different sintering mechanisms. This fact influenced dramatically the behaviour of the materials with lower particle size, which suffered from a higher degree of densification that eventually caused a total loss of cyclability.

Development of crystallinity and photocatalytic properties in porous TiO2 by mild acid treatment

Crystallization of amorphous mesoporous TiO2 synthesized by a surfactant-templated sol–gel route has been investigated by different methods in order to generate materials with photocatalytic properties. Crystallization by hydrothermal treatment is not a convenient alternative, as it leads to a strong reduction of the textural properties due to excessive coalescence of the inorganic framework. A new crystallization route, based on the treatment of m-TiO2 xerogels with acid–ethanol mixtures under reflux, has been investigated. Sulfuric and phosphoric acids cause severe structural and porous damage, whereas nitric and hydrochloric acids lead to the development of TiO2 photocatalysts with convenient textural properties. These last two acids produce the selective crystallization into anatase, the most active phase of titania in photocatalysis, with small nanocrystals being present within the pore walls. The materials so obtained are characterized by exhibiting high surface areas (210–260 m2 g−1), with an important contribution of microporosity and good photocatalytic activity for trichloroethylene degradation in the aqueous phase. In particular, the best balance between the textural and photocatalytic properties was achieved when the crystallization was carried out by treatment with a 0.5 wt% HCl–ethanol mixture.

Improving the Thermochemical Energy Storage Performance of the Mn2O3/Mn3O4 Redox Couple by the Incorporation of Iron

Redox cycles of manganese oxides (Mn2 O3 /Mn3 O4 ) are a promising alternative for thermochemical heat storage systems coupled to concentrated solar power plants as manganese oxides are abundant and inexpensive materials. Although their cyclability for such a purpose has been proved, sintering processes, related to the high-temperature conditions at which charge-discharge cycles are performed, generally cause a cycle-to-cycle decrease in the oxidation rate of Mn3 O4 . To guarantee proper operation, both reactions should present stable reaction rates. In this study, it has been demonstrated that the incorporation of Fe, which is also an abundant material, into the manganese oxides improves the redox performance of this system by increasing the heat storage density, narrowing the redox thermal hysteresis, and, above all, stabilizing and enhancing the oxidation rate over long-term operation, which counteracts the negative effects caused by sintering, although its presence is not avoided.

Evaluation of transition metal phosphides supported on ordered mesoporous materials as catalysts for phenol hydrodeoxygenation

A series of transition metal phosphides (Ni2P, Co2P and MoP) have been synthesized by temperature programmed reduction of the corresponding metal phosphate precursors loaded over mesostructured Al-SBA-15, mesoporous γ-Al2O3 (m-Al2O3) and ordered mesoporous carbon (CMK-3). Both the dispersion and metal phosphide phases attained are strongly influenced by the support features, such as their acidic and textural properties. XRD, TEM and H2 chemisorption results revealed that the MoP phase was probably formed but it underwent a fast re-oxidation in air. On the other hand, metal phosphide formation was hindered over m-Al2O3 as only metallic Ni and Co were detected. All the materials prepared have been evaluated as catalysts in hydrodeoxygenation (HDO) using phenol as a bio-oil model compound. The highest phenol conversions were attained with the catalysts based on the acidic supports (Al-SBA-15 and m-Al2O3). Nevertheless, Co2P/Al-SBA-15, Ni2P/m-Al2O3 and Co2P/m-Al2O3 yielded cyclohexanol as the main product denoting very low HDO efficiency. In contrast, Ni2P/Al-SBA-15 showed remarkable catalytic properties, being the only catalyst that provided almost full phenol conversion and extremely high HDO efficiency, with cyclohexane selectivity higher than 90%. This may be due to a synergetic effect between the high electron deficiency, generated by the Niα+ (0 < α < 1) species owing to an electron transfer from Ni to P and the different acidic sites present in the catalyst.

Preparation of bimodal micro-mesoporous TiO2 with tailored crystalline properties.

A new mild crystallization procedure has been applied after a synthesis route in the presence of a non-ionic surfactant, leading to the preparation of bimodal micro-mesoporous TiO2, with remarkable textural properties and pore walls formed by anatase nanocrystals, which exhibit photocatalytic activity.

Advances in the design of ordered mesoporous materials for low-carbon catalytic hydrogen production

It is believed that hydrogen will play a relevant role as an energy vector in the near future, but in order to fulfil these expectations the development of economically feasible routes with limited CO2 emissions is required. In this respect, catalysis is crucial in most of the possible processes for hydrogen generation and, accordingly, improvement in the catalyst properties should have a significant impact on the efficiency of the production. In particular, ordered mesoporous materials of different chemical composition (oxides and non-oxides) possess a high potential for improving a variety of catalytic routes for the production of low-carbon hydrogen. The activity of this type of catalyst is frequently enhanced due to a combination of high surface area, low diffusion restrictions, and high dispersion of the supported active phases. In this work, we review the latest advances in the use of these mesostructured catalysts in the most relevant routes for hydrogen generation with reduced greenhouse emissions: steam reforming of biomass derived feedstocks (biogas, ethanol, and glycerol), methane and ammonia decomposition and photocatalytic reduction using sacrificial electron donors.

Assessing biomass catalytic pyrolysis in terms of deoxygenation pathways and energy yields for the efficient production of advanced biofuels

The present work focuses on the pathways through which catalytic pyrolysis of biomass into bio-oil proceeds and the effect of the operation conditions on parameters like bio-oil oxygen composition and mass yield, and also additional indicators, such as the distribution of both the oxygen and the chemical energy contained in the initial biomass among the different products. Acid washed wheat straw was used as biomass feedstock. The pyrolysis tests were performed in a lab-scale downdraft fixed-bed reactor working at atmospheric pressure, employing a nanocrystalline H-ZSM-5 zeolite as catalyst. A systematic study was carried out by decoupling both the thermal and catalytic reactions in order to evaluate the influence of three key variables: temperature of the thermal zone; temperature of the subsequent catalytic step and catalyst/biomass ratio. Increasing the pyrolysis temperature in the thermal zone resulted in more bio-oil* (bio-oil in water-free basis) production to the detriment of char and water fractions. In contrast, a significant reduction in bio-oil* fraction, due to decarbonylation and decarboxylation, occurred when increasing the catalytic bed temperature from 400 up to 500 °C. A similar effect was observed by varying the catalyst/biomass ratio since it increased the production of CO, CO2, light olefins and coke at the expense of a decline in the bio-oil* yield. Nevertheless, this bio-oil contains oxygen amounting to as low as 10 wt%, while retaining about 38% of the energy yield. Char, coke and gaseous hydrocarbons contain a great part of the biomass chemical energy, hence their formation should be suppressed or minimized to further improve the bio-oil* energy yield. At high catalyst/biomass ratios, the bio-oil becomes rich in aromatic compounds, both oxygenated and hydrocarbons, while the amount of sugars, furans, carboxylic acids, and other oxygenated products is strongly reduced.

Mixed NaNbxTa1−xO3 perovskites as photocatalysts for H2 production

Mixed perovskites NaNbxTa1−xO3 were prepared by solid state reaction (SSR) as well as by hydrothermal (Hyd) methods, and their photocatalytic activity for hydrogen production was studied using the water–methanol system. The assessment of the NaNbxTa1−xO3 materials obtained by the SSR method reveals that the activity of the individual NaTaO3 and NaNbO3 perovskite semiconductors is largely improved in their combined form. Among several compositions employed, the 1 : 1 molar ratio (NaNb0.5Ta0.5O3 sample) shows the best performance for H2 production. On the other hand, using the Hyd method, which implies lower synthesis temperature, the photocatalytic activity of NaNb0.5Ta0.5O3 is enhanced compared to the material obtained by the high temperature SSR method. The characterization of the materials reveals that catalyst properties like high surface area, a larger proportion of the monoclinic crystalline phase and lower crystal defects for the NaNb0.5Ta0.5O3 photocatalyst synthesized by the hydrothermal route may be responsible for its superior activity. Further significant improvement in the activity of the NaNb0.5Ta0.5O3 semiconductor is achieved by the addition of Pt as the co-catalyst, showing that the loading amount has a great influence on the activity. The highest H2 production rate (37.8 μmol g−1 min−1) is obtained for the catalyst prepared by the hydrothermal method (Hyd-NaNb0.5Ta0.5O3) with 0.125 wt% of Pt loading. Moreover, the developed Hyd-NaNb0.5Ta0.5O3 sample shows a stable H2 evolution activity for several reuse cycles.

Synthesis of Nickel Phosphide Nanorods as Catalyst for the Hydrotreating of Methyl Oleate

Arrays of nickel phosphide nanorods were successfully synthesized by nanocasting using mesostructured silica SBA-15 as a hard template (HT-Ni2P). After temperature-programmed reduction of the phosphate precursor infiltrated within the pore walls of SBA-15, the unsupported material was obtained by removing the silica matrix with diluted HF. The pore channel of the SBA-15 template stabilizes the Ni2P particles, preventing sintering after the high reduction temperature and shaping their elongated morphology. Moreover, HT-Ni2P catalyst shows an improvement in the textural properties with a significantly higher surface area than the reference sample synthesized in the absence of template. X-ray diffraction revealed that the only crystalline phase present in this material was Ni2P. On the other hand, transmission electron microscopy shows that the catalyst is mainly constituted by agglomerates of nanorods. Through EDX microanalysis the efficient removal of silicon was confirmed. Under hydrotreating conditions, nanorods of Ni2P show a fourfold enhancement in the conversion of methyl oleate with respect to conventional Ni2P synthesized in absence of hard template. Nevertheless, when these data are normalized to surface area, the specific activity of HT-Ni2P nanorods is significantly lower than that of the conventionally prepared sample. Differences in selectivity were also observed as Ni2P nanorods favored the decarboxylation reaction leading to a higher yield of n-heptadecane.