M.A. Peña

Partial oxidation of methanol to produce hydrogen over CuZn-based catalysts

In this work CuZnO and Cu/ZnO/Al2O3 catalysts have been studied for the partial oxidation of methanol with oxygen to produce hydrogen. These CuZn based catalysts showed high activity for the partial oxidation of methanol and it was found that the catalytic activity is directly related to the copper metal surface area. In the series CuZn with copper relative content of 20–70 wt%, the catalyst Cu40Zn60 (Cu 40 wt% and Zn 60 wt%) which showed the highest copper area gave the best results for the partial oxidation of methanol. The activation energies and TOF (turnover frequencies) varied with the CuZn catalyst composition. For catalysts with low copper loading very high Ea and TOF were obtained (for Cu30Zn70 Ea=482 kJ/mol and TOF ca. 200 min−1 at 497–499 K) whereas for higher copper contents the Ea and TOF decreased tending to constant values (for Cu70Zn30 Ea=71 kJ/mol and TOF=160 min−1 at 497–499 K). These results are discussed in terms of a possible effect of the CuZnO interaction which depends on the catalyst composition. Catalytic experiments with Cu40Zn55Al5 (Cu 40 wt%, Zn 55 wt% and Al 5 wt%) showed that the presence of aluminium has an inhibiting effect producing slightly lower methanol conversion. On the other hand, higher selectivities for H2 and CO2 were obtained with only traces of the undesirable carbon monoxide. Moreover, the Al is very important for catalyst stability and life-time experiments showed that Cu40Zn55Al5 is stable during the partial oxidation of methanol with no significant change in activity and selectivity even after 110 h operation at 503 K. The catalyst Cu40Zn60 with no Al, deactivates rapidly after 20 h reaction at 503 K. Experiments using N2O as oxidant showed higher activity to convert methanol but producing large amounts of H2O and CO. The impregnation of catalyst with Na produced similar effect increasing the selectivity for H2O and CO. The results presented seem to indicate that the copper metal is active for partial oxidation of methanol to H2 and CO2 whereas Cu+1 favour the formation of H2O and CO. Cu+2 as CuO shows very low activity for methanol conversion producing only CO2 and H2O.

Selective oxidation of o-xylene over ternary V-Ti-Si catalysts

Abstract The selective oxidation of o -xylene to phthalic anhydride (PA) has been studied on a series of ternary V-Ti-Si oxides. Fresh and used catalysts, containing one monolayer of V 2 O 5 on TiO 2 -SiO 2 supports with 0.5, 1, 2 and 3 monolayers of TiO 2 on SiO 2 , were characterised by Raman spectroscopy, X-ray diffraction (XRD) and temperature-programmed reduction (TPR). The characteristics and catalytic behaviour of these catalysts were compared with those of catalysts containing one monolayer of V 2 O 5 supported on SiO 2 and on TiO 2 . In the ternary V-Ti-Si catalysts, the presence of V 2 O 5 and TiO 2 (anatase) was detected, TiO 2 (rutile) being also found in those with two and three monolayers of TiO 2 . The formation of TiO 2 (rutile) due to exposure to reaction conditions was not observed. A moderate rearrangement of surface vanadium oxide species into crystalline V 2 O 5 is observed. In addition, the formation of V 2 O 4 was detected by XRD in the used catalyst containing one monolayer of V 2 O 5 on the support with one monolayer of TiO 2 on silica. The TPR of used and fresh samples showed that the reduction of vanadium occurs during reaction, such reduction being partially reversible after reoxidation in air flow at high temperature. This partial reduction may favour the aggregation of dispersed vanadia species into crystalline V 2 O 5 . The catalytic behaviour of ternary systems in the selective oxidation of o -xylene depends strongly on the characteristics of the support. An increase in the content of TiO 2 leads to a decrease in the temperature required to achieve total conversion. The use of SiO 2 as support causes formation of large amounts of undesirable C 8 products, whereas the presence of TiO 2 deposited on SiO 2 leads generally to the formation of lower amounts of such products. The formation of both CO 2 and tar is strongly favoured on the support composed of two monolayers of TiO 2 on SiO 2 .

Production of hydrogen by partial oxidation of methanol over carbon-supported copper catalysts

Copper–zinc catalysts deposited by impregnation on different oxidised carbon supports (activated carbon, black carbon and carbon fibres) were tested in the partial oxidation of methanol (POM) (CH3OH + ½ O2 → 2 H2 + CO2). Characterisation of the samples by N2 adsorption, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) revealed differences in the dispersion, location and relative distribution of the active phase on the surface of the carbonaceous supports. These properties have important implications in the catalytic behaviour of the samples in the POM reaction. The location of active phases on the microporous structure implies lower initial activities and better thermal stability. These results may be due to limitations in the access of the reagents to the active phase and the prevention of copper-sintering effects.

Structure and reactivity of undoped and sodium- doped PbO/α-Al2O3 catalysts for oxidative coupling of methane

Abstract Undoped and Na-doped lead oxides supported on α-Al2O3 were used in the oxidative coupling of methane (OCM) by cofeeding methane and oxygen at atmospheric pressure. In general, the methane conversion increased with increasing Pb-loading but the C2+ selectivity decreased slightly. Another major effect observed on incorporation of PbO into α-Al2O3 was the dramatic increase of carbon dioxide with lead content. The combined use of X-ray diffraction, temperature-programmed reduction and photoelectron spectroscopy revealed that a certain proportion of Pb4+ species is present in the undoped catalysts, and this is greater for the catalysts with higher lead oxide content. These techniques also revealed that lead oxide is partially evaporated under severe conditions imposed by the OCM, although the evaporation can be diminished by doping with sodium. The beneficial effect of sodium on the catalyst stability is also accompanied by a better catalyst performance due to the ability of sodium oxide to activate methane.

Characterization of Ni-honeycomb catalysts for high pressure methane partial oxidation

Nickel-honeycomb catalysts prepared by different procedures have been characterized by temperature-programmed reduction and X-ray photoelectron spectroscopy. Prereduced catalysts were tested in the partial oxidation of methane under high pressure (20 bar). The influence of the method of incorporation of the catalytic component, the metal loading and the activation temperature on the catalytic performance were investigated. It appeared that the formation of a NiAl 2 O 4 phase between the Ni precursor and the Al 2 O 3 phase of the carrier, while maintaining high dispersion is a key parameter to maintain high activity and good stability during on-stream operation.

Catalytic partial oxidation of methanol: H2 production for fuel cells

In this work Cu/ZnO and Cu/ZnO/Al2O3 catalysts have been studied for the partial oxidation of methanol with O2 to produce H2. These Cu-Zn based catalysts showed high activity for the partial oxidation of methanol and with activity directly related to the Cu metal area. In the series Cu-Zn with copper relative content of 20–70 wt%, the catalyst Cu40Zn60 (Cu 40 wt% and Zn 60 wt%), which showed the highest Cu area, gave the best results for the partial oxidation of methanol. The activation energies and TOF (turnover frequencies) varied with the Cu-Zn catalyst composition. For catalysts with low Cu loading very high Ea and TOF were obtained (for Cu30Zn70 Ea = 482 kJ/mol and TOF ca. 200 min−1 at 497–499 K) whereas for higher Cu contents the Ea and TOF decreased tending to constant values (for Cu70Zn30 Ea = 71 kJ/mol and TOF = 160 min−1 at 497–499 K). These results are discussed in terms of a possible effect of the Cu-ZnO interaction which depends on the catalyst composition. Catalytic experiments with Cu40Zn55Al5 showed that the presence of Al has an inhibiting effect producing slightly lower methanol conversion. On the other hand, higher selectivities for H2 and CO2 were obtained with only traces of the undesirable CO. Moreover, the Al is very important for catalyst stability and life-time experiments showed that Cu40Zn55Al5 is stable during the partial oxidation of methanol with no significant change in activity and selectivity even after 110 h of operation at 503 K. Overall, the results seem to indicate that Cu0 is active for partial oxidation of methanol to H2 and CO2 whereas Cu+ favors the formation of H2O and CO.

Promotion by Dichloromethane of the Oxidative Coupling of Methane on LiMn/MgO Catalysts

It has been observed that by introduction of halomethane into the gas stream during the testing of catalysts for the methane coupling reaction, the selectivity for the formation of C2 products is greatly enhanced1–4. The presence of gaseous HC1 in the feed has also been reported to promote the OCM reaction5. However, it is interesting to note that the highest C2 selectivity was achieved when the corresponding chloride salts were used to introduce alkali metals in the catalyst1.3, 7–9. Burch et al.1,2 have shown that addition of alkali halides to manganese dioxide can convert its activity from total oxidation to coupling, and that exposure to chlorine can also modify the behavior of manganese catalysts. Despite the studies related to catalytic systems containing chloride ions, important factors such as the nature of surface phase, its stability and the enhancement of C2 yields still remain unclear.