Alejandro López Ortiz

INFLUENCE OF THE ANATASE/RUTILE RATIO ON THE TIO2 PHOTOCATALYTIC ACTIVITY FOR THE PHOTODEGRADATION OF LIGHT HYDROCARBONS

A set of TiO2 samples with different anatase/rutile ratios was prepared by calcinations at different temperatures from commercial photocatalyst Degussa P25. The effects of the two crystalline phases of titanium (IV) oxide on the photocatalytic activity in gaseous phase through oxidation of light hydrocarbons were studied. Crystalline phase transformation from anatase to rutile occurred at 700°C for P25. Results indicate that samples with higher anatase/rutile ratios presented higher intrinsic activities for the photodegradation of a propane/isobutane/butane (40/35/25 %V) mixture. However, the activity did not totally disappear after complete crystalline transformation from anatase to rutile, indicating that the pure rutile phase also presents photoactivity. During the photocatalytic reaction of TiO2 samples, a linear dependence was found between the inverse of the intrinsic reaction rate constant (k intrinsic) and the water adsorption capacity in the surface (WAPS) of the synthesized TiO2 catalyst. The thermal treatment used to induce the formation of rutile by calcination would presumably reduce water adsorption capacity and surface area, leading to a decrease in photocatalytic activity.

Synthesis method effect of CoFe 2 O 4 on its photocatalytic properties for H 2 production from water and visible light

Currently, the need for more efficient materials that work in the visible light spectrum for hydrogen production has been increasing. Under this criterion, ferrites are ideal because their energetic properties are favorable to photocatalysis as they have a low band gap (1.5 to 3 ev). In this particular research, ferrite is presented as a hydrogen producer. Cobalt ferrites were synthesized by chemical coprecipitation (CP) and ball milling (BM) for comparison of their performance. The characterization of the materials was carried out with X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), BET surface area, UV-VIS spectroscopy, and water adsorption/desorption tests. Evaluation of the photocatalytic activity under visible light was followed by gas chromatography. The results showed that cobalt ferrite by ball milling had a higher photocatalytic activity; this is attributed to the vacancies generated during the milling process at which the sample was exposed.

Redox Stabilization Effect of TiO2 in Co3O4 as Oxygen Carrier for the Production of Hydrogen through POX and Chemical Looping Processes

Novel proposed processes for H2 production and energy generation such as partial oxidation of hydrocarbons (POX-MeO) and chemical looping process (CLP), respectively require the use of solid oxides as oxygen carriers. In POX-MeO the required oxygen for the partial oxidation of methane is provided by a transition metal oxide (MeO). First, H2 is produced through CH4+MeO = CO+H2+Me. Secondly, Me is re-oxidized through Me+O2 = MeO to regenerate the oxygen carrier. In the CL process, CH4 is being completely oxidized through CH2 + MeO = CO2 + H2O + Me producing heat and CO2 ready for sequestration. Finally, Me is re-oxidized using air to regenerate the Me back to MeO. In both processes the regenerated MeO is sent back to the initial step to result in a cyclic operation. Continuous exposure of MeO to Redox cycles frequently produces sinterization and MeO stabilization is needed to avoid loss of activity. The objective of this study is to investigate the stabilization effect of TiO2 in Co3O4 during Redox cycles to be used as an oxygen carrier using CoxTiOy type spinnels. Characterization of the synthesized samples included XRD, TPR, and SEM. Co2TiO4 and CoTiO3 spinnels were synthesized by solid state reaction. TGA and TPR Redox performance cycles of Co3O4 produced sintering, while results using a Co2TiO4 spinnel structure suggest a strong stabilization effect of TiO2 on Co. Ten Redox cycles using H2 and CH4 as reducing agents and a mixture of O2/N2 as oxidizer resulted in fixation of Co to TiO2 avoiding sintering.

Synthesis, Characterization and Kinetic Evaluation of Manganese Oxide Nanoparticles for the H2O2 Catalytic Decomposition

Nanostructured manganese oxides were synthesized towards the catalytic decomposition of hydrogen peroxide. The effect of the type of precipitant (Na2CO3 and NH4OH) over the particle size and crystallographic phase was determined. Also, the influence of particle size and crystal phase over the catalytic activity was established. Characterization of the obtained nanoparticles was performed through XRD analysis, BET surface area and TEM microscopy. Catalytic activity was followed through gasometry of the oxygen evolution [H2O2(l) ? H2O(l) + O2 (g)] at 25◦C. Na2CO3 (Mn-1) and NH4OH (Mn-2) precipitants produced nanoparticles with average sizes of 5-10 nm and 20 nm, respectively. This result was attributed to higher pH stability of precipitant Na2CO3 during the synthesis process. XRD results revealed the presence of ß-MnO2, Mn5O8 and Mn3O4 crystal phase mixture for Mn-1 and Mn5O8 for Mn-2 catalyst. Catalytic activity resulted in intrinsic rate constants (kint = min−1m−2) of 5.2 and 2.3 for Mn-1 and Mn-2, respectively. The greater catalytic activity of Mn-2 catalyst was attributed to the presence of the Mn55O8 phase.

Synthesis, Characterization and Stability Performance of CoWO4 as an Oxygen Carrier under Redox Cycles towards Syngas Production

A redox reaction scheme consisting of two steps as an alternate to the partial oxidation of methane (POX) for synthesis gas production is proposed. With the use of a thermodynamic analysis the CoWO4 spinel was introduced as a new oxygen carrier material, capable to be reduced with methane (15% CH4/Ar) and reoxidized with steam (30% H2O/Ar) to produce syngas and hydrogen in each redox step, respectively. H2 production is feasible during the regeneration of CoWO4 by oxidation of reduced species (Co and W) with steam. CoWO4 synthesis was performed using the solid state reaction. Characterization included: XRD, SEM and particle size determination. Redox-cycle performance was followed by TGA. Thermal stability of CoWO4 was tested by exposing the material to ten consecutive reduction/oxidation (H2/Ar, H2O/Ar) cycles at 850°C and after. Results indicate that CoWO4 showed no significant loss of activity. This stable behavior is associated to its crystalline structure, which allows the fixation of the active metals (Co + W) through the spinel structure, thus inhibiting particle nucleation and migration, which are the main causes of sintering after exposure to steam at high temperatures. The addition of Ni to CoWO4 as a POX catalyst produced an enhancement of the reduction step of 20% in metal oxide conversion with respect to CoWO4.