Jorge Laine

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.

Preparation and characterization of activated carbons from coconut shell impregnated with phosphoric acid

Abstract A series of activated carbons was prepared from coconut shell impregnated with phosphoric acid using a one-step carbonization procedure and varying conditions in order to optimize preparation parameters. The mode and nature of gaseous flow during carbonization were found to affect the surface area and yield, suggesting that oxygen derived from the gas phase may play an important role in activation, and that coking of volatiles could be contributing to the building of the porous structure. The optimum activation temperature for a higher surface area was 450°C. Higher surface area and mesoporosity were favored by increasing the acid concentration of the impregnating solution. Infrared spectra showed bands corresponding to surface oxygen complexes and a band assigned to P-O groups. Temperature-programmed reduction and desorption spectra showed two signals ascribed also to surface oxygen complexes, and other higher-temperature signals corresponding to hydrogen desorption. These were all significantly more intense in the carbons prepared with phosphoric acid as compared with a standard prepared using high-temperature activation.

Effect of the preparation method on the pore size distribution of activated carbon from coconut shell

The pore size distribution of activated carbon prepared from coconut shell employing different activation methods was investigated. “Physical” activation with pure CO2 at 800°C resulted in a bimodal pore size distribution featuring both the widest macropores and the narrowest micropores among the samples studied. The introduction of potassium phosphate as a catalyst for the gasification with CO2 led to a decrease in macropore volume and diameter and to a slight increase in micropore diameter. Wider micropores and significant mesoporosity resulted when employing “chemical” activation with phosphoric acid at a lower temperature (500°C). Surface area was found to increase with increasing median micropore diameter between about 7–15 A. Three simple schemes were proposed depicting the effect of the preparation method on pore size distribution.

Comparative study of alumina-supported CuO and CuCr2O4 as catalysts for CO oxidation

Alumina-supported CuO and CuCr2O4 catalysts of various compositions were prepared and their activity for CO oxidation measured. Fresh, pretreated with CO, and reoxidized catalysts were studied. In general, the activity increased with the CO pretreatment. The extent of the activation depended on the catalyst composition. Thus, copper catalysts were more active than copper chromite at low metal concentrations (<12 wt%), but the opposite was observed at concentrations higher than about 12 wt%. The results on activity behavior, together with TPR and XRD spectra, suggest that the active species are related to copper cations. In the low concentration copper catalysts, the most active surface is generated after CO pretreatment, followed by a fast reoxidation occurring at the first stages of the reaction. In the high concentration copper catalysts, the CO pretreatment produced an induction period as a result of excessive reduction. It is suggested that the role of chromium is to limit the extent of reduction through the formation of the CuCr2O4 phase. The presence of this phase also resulted in catalysts less prone to deactivation, as compared to copper on alumina catalysts.

Temperature-programmed reduction of Ni-Mo oxides

Temperature-programmed reduction (TPR) has been employed to study Ni-Mo mixed oxides which were previously used as model hydrodesulphurization (HDS) catalysts, using compositions ranging from pure MoO3 to pure NiO. An assignment of TPR signals to the different bulk phases was attempted. Good agreement between TPR spectra and structural data obtained previously from X-ray and electron diffraction was observed. TPR traces were consistent with proposed mechanisms of reduction of the bulk oxides MoO3, MoO2, NiO and NiMoO4. The variations in TPR spectra were interpreted in terms of effects such as crystallite size, ageing of the samples, hydrous state and chemical interactions between the different species. The significance of these reducibility results for HDS catalysis is discussed.

Nickel molybdate as precursor of HDS catalysts: Effect of phase composition

Sulfided αand β-NiMoO4 have been employed as model HDS catalysts, aiming to investigate the effect of phase composition of the precursor mixed oxide on the physicochemical characteristics and activity of the sulfides. Both sulfides are highly amorphous or microcrystalline, and could not be differentiated by means of XRD. The differences in BET area were also found to be minimal. However, it was found by means of TPR that the stable α-isomorph is reduced at lower temperatures than the unstable β-phase in both the oxidic and sulfided states. Sulfided β-NiMoO4 was found to be a better catalyst for the HDS of thiophene than the sulfided α-isomorph. This could be related to the higher stability of the former in H2, as decomposition of the active, amorphous Ni-Mo-S structures results in less active and more crystalline phases, as found by XRD.

Characterization of supported MoO3 by temperature-programmed reduction

Abstract Temperature-programmed reduction (TPR) was employed for the characterization of molybdena catalysts. The effect of several variables on reduction of the supported catalysts was determined. The TPR spectra changed significantly with both the amount of MoO3, from 5 to 80 wt %, and the type of support, several silica-alumina carriers, of compositions ranging from pure SiO2 to pure Al2O3. The peak assigned to buildup of multilayers of molybdena on the γ-Al2O3 supports was affected by the calcination temperature and by the amount of promoter (0–5 wt % NiO). Nickel-promoted catalysts activated hydrogen easier than both cobalt-promoted and unpromoted catalysts.

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.

HDS activity of carbon-supported Ni–Mo catalysts derived from thiomolybdate complexes

Abstract Activated-carbon (AC)-supported Ni–Mo sulfide catalysts were prepared employing ammonium thiomolybdate complexes and nickel salts as metal-sulfide precursors and a commercial AC as support. Textural and chemical characterization were carried out by XRD, TPR and BET surface-area measurements. A well-defined Ni(NH3)6MoS4 phase was detected in Ni–Mo/AC co-impregnated samples before pretreatments or catalytic reaction, but it disappeared or became too amorphous to be detected by XRD after prereduction or presulfiding. No relationship between the presence of the mixed phase in the precursor and the HDS activity was obvious. Although both types of pretreatments activated the samples for the hydrodesulfurization (HDS) of thiophene, presulfiding always produced higher activities than prereduction. The nickel salt used (chloride or acetate) did not affect the activity, but the type of thiomolybdate employed had a considerable influence on the HDS activity: The dimeric complex (NH4)2Mo2S12·2H2O was a better precursor than the monomer (NH4)2MoS4, while the trimer (NH4)2Mo3S13·1.8H2O was the poorer. This is probably related to the different decomposition pathways that follow the complexes, as both, the monomer and dimer, decompose to MoS2 through amorphous MoS3 while the trimer directly produces MoS2, presumably more crystalline. The monomeric thiotungstate complex (NH4)2WS4 produced a more active Ni–W/AC catalyst than any of the Ni–Mo/AC samples measured, although for the unpromoted samples the situation was the reverse. Comparison with the alumina-supported catalyst suggests that the thiomolybdate derived samples should be further explored for HDS.

Structure and activity of NiCoMo/SiO2 hydrodesulfurization catalysts

Abstract A series of NiCoMo hydrodesulfurization (HDS) catalysts supported on silica was prepared varying the ratio r = Co /( Co + Ni ) between 0 and 1, maintaining Mo and (Co + Ni) concentrations constant. Calcination temperatures employed were 500 and 600°C. The promoter (Co and/or Ni) was present in the precursor as a hydrated molybdate phase that suppressed both anhydrous MoO 3 crystallite growth and decrease in surface area resulting from sintering of the support. Both phenomena occurred in the nonpromoted catalyst after calcining at 600°C, probably by a dehydroxylation process resulting from dislinking of polymolybdates from the silica support. A correlation between TPR and dispersion given by XRD was established, confirming that the dispersion of molybdenum on silica was lower than that on the aluminas previously studied. According to the proposed correlation, sulfidation caused an increase in dispersion in all the catalysts. The presence of anhydrous MoO 3 was held responsible for the formation of amorphous MoO 2 observed by TPR after sulfidation. The increase in dispersion after sulfidation was accompanied by a remarkable increase in the initial HDS activity of both nonpromoted and promoted catalysts. However, the nonpromoted sample showed a pronounced initial deactivation during use. Compared with another similar, but supported on alumina, series of catalysts, a minimum instead of a maximum steadystate HDS activity was found for an intermediate value of r .