H.Y. Wang

Carbon dioxide reforming of methane to synthesis gas over supported rhodium catalysts : the effect of support

Abstract The carbon dioxide reforming of methane over the reduced supported Rh (0.5xa0wt.%) catalysts was investigated. Two kinds of oxides, reducible (CeO2, Nb2O5, Ta2O5, TiO2, and ZrO2) and irreducible (γ-Al2O3, La2O3, MgO, SiO2, and Y2O3) were used as supports. Among the irreducible metal oxides, γ-Al2O3, La2O3 and MgO provided stable catalytic activities during the period of study, and the activity increased in the sequence: La2O3

Carbon dioxide reforming of methane to synthesis gas over supported cobalt catalysts

Abstract The CO2 reforming of CH4 over Co supported on an alkaline earth metal oxide (MgO, CaO, SrO, or BaO) as well as on γ-Al2O3 and on SiO2 was investigated. Among these supports, only MgO exhibited a high and stable activity. It provided a CO yield of 93% and a H2 yield of 90% at the high space velocity of 60xa0000xa0mlxa0g−1xa0h−1, which remained unchanged during the period of study of 50xa0h. γ-Al2O3 provided initially a high CO yield, which, however, rapidly decayed. All the other supports exhibited low CO yields and CaO and SiO2 also low stabilities. A solid solution of CoO and MgO was identified by XRD in the calcined MgO supported catalyst, but not in the other supports. Because the oxygens in the solid solution are shared by both Mg and Co and their interactions with Mg are strong, the solid solution is less reducible than the pure CoO and small clusters of metallic Co are generated. Being at least partially embedded in the substrate, these clusters are more stable to sintering than the usual ones; being small they do not favor coke formation. For these reasons, this catalyst exhibits high stability.

CO2 reforming of CH4 over Co/MgO solid solution catalysts — effect of calcination temperature and Co loading

Abstract The effect of calcination temperature on the CO 2 reforming of CH 4 over Co/MgO of various Co loadings was investigated and the results can be summarized as follows: (i) the catalysts with Co loadings between 8 and 36xa0wt.% precalcined at 500 or 800°C provided high and stable activities; (ii) for Co loadings between 4 and 48xa0wt.%, the high calcination temperature of 900°C yielded low activities or very long activation periods; (iii) a high Co loading (48xa0wt.%) combined with a calcination temperature of 500 or 800°C resulted in unstable activities. Depending on the calcination temperature and Co loading, one, two or three of the following Co-containing species, Co 3 O 4 , MgCo 2 O 4 , and (Co,xa0Mg)O (solid solution of CoO and MgO) were identified by combining temperature programmed reduction and X-ray diffraction (XRD) results. Their reducibility decreased in the sequence: Co 3 O 4 >MgCo 2 O 4 >(Co,xa0Mg)O. It is suggested that the high and stable activities observed over most catalysts (except the 48xa0wt.% one) precalcined at 500 or 800°C as well as the low activities or the long activation periods observed over the 900°C calcined catalysts were induced by the formation of a solid solution between CoO and MgO. The activity decay observed over the 48xa0wt.% Co catalyst precalcined at 500 or 800°C was most likely caused by the large metallic particles, formed particularly through the reduction of Co 3 O 4 and MgCo 2 O 4 . These particles sintered and stimulated coke formation.

Formation of filamentous carbon during methane decomposition over Co–MgO catalysts

Abstract The carbon formation during methane decomposition was investigated at 900xa0°C over the 48 wt% Co–MgO catalysts as a function of the calcination temperature T c used in their preparation. Examination of the carbonaceous deposits by transmission electron microscopy revealed three kinds of structures: shapeless tangles, shell-like materials, and carbon filaments. In another set of experiments, the structural characteristics of the calcined catalysts were investigated using temperature-programmed reduction (TPR) and X-ray diffraction (XRD). Co 3 O 4 , Co 2 MgO 4 , and (Co, Mg)O (solid solution of CoO and MgO) were identified for T c ≤700xa0°C, Co 3 O 4 and (Co, Mg)O for T c =800xa0°C and only (Co, Mg)O for T c =900xa0°C. It was found that the metal particles originated from the reduction of the solid solution favored the formation of filamentous carbon. A possible explanation is proposed.

Partial oxidation of methane to synthesis gas over MgO-supported Rh catalysts: the effect of precursor of MgO

Abstract Five magnesium-containing precursors were used to prepare magnesium oxides of different surface areas. With these oxides as supports, catalysts (1xa0wt.% Rh loading) with different Rh dispersions after reduction were prepared. At a Tfurnace of 750°C and 1xa0atm, all these catalysts provided a conversion >80% and selectivities of 95–96% to CO and 96–98% to H2, at the high space velocity of 7.2xa0×xa0105xa0mlxa0g−1xa0h−1, with very high stability. No significant deactivation of the catalyst was observed for up to 96xa0h of reaction. However, no notable effect of the precursor of MgO was noted and possible explanations are provided. Temperature-programmed reduction (TPR) experiments indicated the presence of up to three kinds of rhodium-containing species, Rh2O3, Rh2O3-interacting with the support, and MgRh2O4, in the oxidized MgO-supported Rh catalysts. The strong interactions between rhodium and magnesium oxides are suggested to be responsible for the high stability of the catalyst. It was found that the methane conversion increased with the amount of catalyst when it was below 5.0xa0mg, but remained almost unchanged when it was greater than 5.0xa0mg. The catalytic assays also provided some information about the reaction mechanism.

Combined catalytic partial oxidation and CO2 reforming of methane over supported cobalt catalysts

The combined partial oxidation and CO2 reforming of methane to synthesis gas was investigated over the reduced Co/MgO, Co/CaO, and Co/SiO2 catalysts. Only Co/MgO has proved to be a highly efficient and stable catalyst. It provided about 94–95% yields to H2 and CO at the high space velocity of 105u2009000 mlu2009g−1u2009h−1 and for feed ratios CH4/CO2/O2=4/2/1, without any deactivation for a period of study of 110 h. In contrast, the reduced Co/CaO and Co/SiO2 provided no activity for the formation of H2 and CO. The structure and reducibility of the calcined catalysts were examined using X-ray diffraction and temperature-programmed reduction, respectively. A solid solution of CoO and MgO, which was difficult to reduce, was identified in the 800u2009°C calcined MgO-supported catalyst. The strong interactions induced by the formation of the solid solution are responsible for its superior activity in the combined reaction. The effects of reaction temperature, space velocity, and O2/CO2 ratio in the feed gases (while keeping the C/O ratio constant at 1/1) were investigated over the Co/MgO catalyst. The H2/CO ratio in the product of the combined reaction increased with increasing O2/CO2 ratio in the feed.

Conversions of Methane to Synthesis Gas over Co/γ-Al2O3 by CO2 and/or O2

The goal of the paper was to investigate the effect of the catalyst precursor on the catalytic activity. For this reason, the structure, the reducibility and the reaction behavior of γ-Al2O3-supported Co (24 wt%) catalysts as a function of calcination temperature (Tc) were investigated using X-ray diffraction, temperature-programmed reduction, CO chemisorption, pulse reaction with pure CH4, and the catalytic reactions of methane conversion to synthesis gas. Depending on Tc, one, two, or three of the following Co-containing compounds, Co3O4, Co2AlO4, and CoAl2O4, were identified. Their reducibility decreased in the sequence: Co3O4>Co2AlO4>CoAl2O4. Co3O4 was generated as a major phase at a Tc of 500°C and Co2AlO4 and CoAl2O4 at a Tc of 1000°C. The reduced Co/γ-Al2O3 catalysts, obtained via the reduction of the 500 and 1000°C calcined catalysts, provided high and stable activities for the partial oxidation of methane and the combined partial oxidation and CO2 reforming of methane. They deactivated, however, rapidly in the CO2 reforming of methane. Possible explanations for the stability are provided.

Catalytic partial oxidation of methane to synthesis gas over γ-Al2O2-supported rhodium catalysts

The partial oxidation of methane was studied over γ-Al2O3-supported catalysts for Rh loadings between 0.01 and 5.0 wt%. It was found that the activity and selectivities for loadings between 0.5 and 5.0 wt% are almost the same. As an example, detailed information is presented for the 1.0 wt% Rh/γ-Al2O3, which provides at 750°C (furnace temperature) an activity of 82% and selectivities of 96% to CO and 98% to H2, at a gas hourly space velocity (GHSV) of 720000 ml g−1 h−1. Its activity remained stable during our experiment which lasted 120 h. Possible explanations for this high stability are proposed based on TPR and XRD experiments. Pulse reactions with small pulses of CH4 and CH4/O2 (2/1) were performed over the reduced and unreduced Rh catalysts to probe the mechanistic aspects of the reaction. The partial oxidation of methane to syngas was found to be initiated by metallic rhodium sites, since the CO selectivity increased with increasing number of such sites.

Effect of calcination conditions on the species formed and the reduction behavior of the cobalt–magnesia catalysts

The structural characteristics and the reduction behavior of the Co/MgO catalysts were investigated using temperature‐programmed reduction (TPR) and X‐ray diffraction (XRD). The variables investigated included the preparation method and the heat treatment conditions (calcination temperature and time). Depending on these factors, one, two or three of the following Co‐containing species, Co3O4, MgCo2O4 and (Co, Mg)O (solid solution of CoO and MgO) were identified. The extent of solid solution formation increased as the calcination temperature and calcination time increased. A much lower calcination temperature was needed to form a solid solution in the impregnated catalysts than in the physically mixed ones. The formation of a solid solution rendered the catalyst less reducible. Finally, the decomposition of CH4, as a probe reaction, was performed and it was found that the amount of carbon deposited decreased with increasing extent of solid solution formation.