Juan Matos

Synergy effect in the photocatalytic degradation of phenol on a suspended mixture of titania and activated carbon

The photocatalytic degradation of phenol, chosen as an aromatic model molecule, has been performed at room temperature (20°C) in contact with a suspended mixture of titania and of activated carbon (AC). Non-additive adsorption capacities were observed when the solids were mixed, and this was ascribed to a strong interaction, involving half of the surface of titania and ca. 14% of that of AC. A synergy effect was observed with an increase of the first order rate constant by a factor of 2.5. As for neat titania, the same main intermediate products (hydroquinone and benzoquinone) were found but in much smaller quantities and during a much smaller lifetime, suggesting that the same reaction mechanism occurred in the presence of photoinactive AC. The synergy effect was ascribed to an extended adsorption of phenol on AC followed by a transfer to titania where it is photocatalytically degraded. The synergy effect could not be improved by previous physical treatments of the solid mixture such as grinding and sonication. Some phenol remained adsorbed on AC when no traces of organic compounds were detected in the purified water. This adsorbed phenol could be destroyed by illuminated titania while maintaining UV-irradiation. This combined photocatalytic system may appear as a new performing one, more efficient with a shorter time necessary for decontaminating diluted used waters.

Activated carbon supported NiMo: effects of pretreatment and composition on catalyst reducibility and on ethylene conversion

Activated carbon (AC) supported Mo, Ni and NiMo catalysts have been studied by means of TPR and the catalytic transformation of ethylene as a probe reaction. The initial state of the catalysts varied with different reducing or sulfiding pre-treatments. TPR of the fresh samples suggested that Ni and Mo are well dispersed on the support. Pre-reduction at 500°C produced Mo surface sites with average oxidation number close to two and Ni metallic species. For the sulfided catalysts low peak temperatures were observed, which seem to relate with easily removable sulfur. Activity results correlated well with the TPR spectra. Contrary to what is normally found in Al2O3 or SiO2, both the TPR and activity/selectivity characteristics of Ni seem to dominate over those of Mo in unsulfided samples, which is likely to be due to the lower interaction of Ni with the carbon support. Coke was an important product for most of the samples, but the NiMo combination in general lowered its formation. However, previous reports suggesting that carbon-supported HDS catalysts are less prone to coke deposition than the Al2O3 supported ones could not be substantiated. Regardless of the type of pre-treatment, AC and MoC showed low activity with 100% selectivity to coke, except the sulfided MoC samples, where ethane production was observed. Hydrogenation sites of sulfided MoC were very fragile, as mild post-reduction produced complete deactivation at steady state conditions. However, in sulfided but not post-reduced samples, the feed can protect active sites from the deleterious effect of the H2 atmosphere. In the case of unsulfided samples hydrogenation and cracking were associated to Ni, but sulfiding inhibited these functions of Ni sites in NiC. High selectivity to ethane was observed in sulfided NiMoC, showing that this catalyst is more resistant to sulfur than the single metal ones. From the dependence of selectivity of sulfided catalysts on the type of pre-treatment it is concluded that the hydrogenation sites in NiMoC are of the same type as those of MoC, i.e., they are associated mainly to Mo sulfides.

Influence of surface properties of activated carbon on photocatalytic activity of TiO2 in 4-chlorophenol degradation.

This work shows the influence of specific surface area (BET) and surface pH of different activated carbons on the photocatalytic activity of UV-irradiated TiO2 in the 4-chlorophenol (4CP) photodegradation. Carbons were prepared from sawdust of Tabebuia Pentaphyla wood by physical activation with CO2 and by pyrolysis under N2 flow at tempera- tures from 450 up to 1000oC during 1h Chemical activation was also performed by the impregnation of precursor with ZnCl2, H3PO4 and KOH at different concentrations, followed by activation under N2 flow at 450oC by 1h. Comparison of apparent first-order rate constants between TiO2 against TiO2-AC indicate that TiO2 photoactivity is enhanced or inhibited as a function of AC properties and correlations with surface area and pHPZC were found suggesting that surface pH of car- bons is the most important parameter that influences TiO2 activity in the 4CP photodegradation.

Photocatalytic Activity of TiO2 on Activated Carbon Under Visible Light in the Photodegradation of Phenol

The photodegradation of phenol under visible light by using TiO2 and activated carbon prepared by physical activation was performed. Activated carbon was prepared from Tabebuia Pentaphyla wood by physical activation under CO2 flow at 800oC. UV-Vis lamp with a high proportion of visible light was used. Kinetic results of phenol photodegradation indicated a beneficial effect between TiO2 and AC. An increase of apparent first-order rate constant (kapp) by a factor of 1.74 in presence of TiO2-AC was obtained in comparison than TiO2 alone. This cooperative effect between both solids was ascribed to the high surface area and basic surface pH of this type of AC.

Changes on Texture and Crystalline Phase of Activated Carbon-Supported Ni-Ca Catalyst During Dry Methane Reforming

Changes on texture and crystalline phase of AC-supported Ni-Ca catalyst during dry methane reforming were verified. Characterization by N2 adsorption, XPS, XRD, TEM and HR-TEM was performed. XPS and XRD analysis suggest that the initial activity can be attributed to a nickel reduced phase formed during helium pre-treatment. TEM showed that a homogeneous distribution of Ni nanoparticles between 10-20nm at the tip of multi-walled carbon nanotubes is formed during reaction. HR-TEM showed diffusion of Ni nanoparticles and the formation of carbon-like nano onions with an interlayer separation of 0.5nm suggesting the formation of a ultra-microporous carbon structure. An increase in surface area from 231m 2 .g -1 to 2405m 2 .g -1 after 90min reaction was found and attributed to an in situ activation of carbon deposits by CO2 gasification. The present results suggest that introducing CO2 pulses during reaction carbon-supported Ni-Ca could be employed as potential catalysts for methane and other hydrocarbon reforming reactions at mild experimental conditions.

Texture Properties and Kinetic Parameters Associated to Carbon Materials Obtained from Sawdust of Algarroba Wood. 1. Application in Phenol Photodetoxification

Carbon materials were obtained by pyrolysis of the sawdust of Algarroba wood under CO2 or N2 atmospheres at temperatures from 200oC up to 900oC. Carbon materials were characterized by surface and micropore areas, volume pore and mean pore diameter. An approach of the kinetic parameters (apparent constant-rates and activation energies) as- sociated with the synthesis of these carbon materials is also presented. From the kinetic data of thermal degradation of Al- garroba wood, three different pyrolysis zones were verified. First, an incipient carbonization between 200 up to 350oC, second an extensive carbonization between 350 up to 600oC, and a third step involving the pore formation at temperatures higher than 600oC. As a target application of carbon materials, results of phenol adsorption and phenol photodegradation under UV-irradiation indicated the potential of carbon materials in presence of TiO2 for treatment of polluted waters.