1742
Ind. Eng. Chem. Res. 1999, 38, 1742
Rebuttal to Comments on “Catalyst Temperature Oscillations during Partial Oxidation of Methane” Yun Hang Hu and Eli Ruckenstein* Department of Chemical Engineering, State University of New York at Buffalo, Buffalo, New York 14260
Sir: In a recent paper1 we reported the observation that, during the partial oxidation of methane over Nisupported catalysts, oscillations of the catalyst temperature take place over NiO/SiO2 but not over NiO/Al2O3 or NiO/MgO. Hot layers were observed over all three catalysts, but only over NiO/SiO2 were they moving down and up. The oscillations were attributed to this movement, and the relatively facile reduction and reoxidation of the NiO/SiO2 catalyst, due to the weak interactions between NiO and SiO2, was suggested as the probable cause of the motion. In the preceding paper, Cohn brings in essence two objections to our paper: (1) The interactions between NiO and SiO2 are strong, because they react and generate a nickel-silicate, and not weak as considered by us. Consequently, they cannot be the cause of the oscillations; a coking/decoking process might cause the oscillations. (2) Similar experiments (!) carried out by Choudhary et al.2 for NiO + MgO on a support containing 4.1% Al2O3 and 95% SiO2 have not generated oscillations. While we appreciate his interest in our work, we disagree with his objections for the following reasons: 1. When the amount of NiO is sufficiently large (and in the present case we have 13.6 wt % Ni), only a small amount of NiO reacts with SiO2 to form a very thin film of silicate over the surface of SiO2. Most remains as NiO, and there are no strong interactions between the remaining NiO and SiO2. This issue was discussed by us in a previous paper.3 In addition, temperatureprogrammed reduction (TPR) experiments (unpublished) with 4% H2 in argon indicated that the initial reduction temperature was about 330 °C for NiO/SiO2 (13.6 wt % Ni), which is near that of pure NiO (about 300 °C). In contrast, for NiO/Al2O3 (13.6 wt % Ni) the initial reduction temperature was high (670 °C) and no reduction peak could be detected even at 800 °C for NiO/ MgO (13.6 wt % Ni). These results clearly indicate that there are weak interactions between NiO and SiO2 and much stronger interactions between NiO and Al2O3 and between NiO and MgO. Because of the relatively weak interactions, NiO of the surface layer can be easily reduced to Ni and also easily reoxidized to NiO, when NiO is supported on SiO2. The freshly reduced NiO at the top of the catalytic bed becomes very active, and a hot layer is generated. Reoxidized, this layer loses its activity, but the layer beneath, being already reduced,
becomes active. After a certain time, the oxidized layer near the entrance is again reduced and becomes a hot layer, while the layer beneath is reoxidized. As a result, the hot layer moves down and up. When the interactions are strong, some active sites are generated slowly through reduction and some are slowly reoxidized. An almost steady number of sites is achieved and, as a result, the hot layer is fixed. Cohn suggests that a coking/decoking process might be responsible for the oscillations and attributes the oscillations over NiO/SiO2 catalyst to the higher capability for coking of the SiO2 support. We have, however, excluded this mechanism because (i) coking/decoking should be associated with fluctuating concentrations at the exit, and this was not observed experimentally, but particularly because (ii) experiments have revealed that, during the CO2 reforming of methane, the amount of carbon deposited on Ni/ Al2O3 is higher than that on Ni/SiO2.3 2. Cohn made the comment that, in the experiments of Choudhary et al.,2 the precoating of the SiO2-Al2O3 support with MgO prevents the interactions between NiO and SiO2. This fact is not, however, critical for the issue under discussion. More relevant information is provided by the TPR experiments of Choudhary et al.,2 which revealed that the initial reduction temperature is large, about 550 °C, indicating that there are strong interactions between NiO and the support. These interactions may be similar to those encountered with the NiO/MgO catalyst but can also be more complex because of the presence of SiO2 and Al2O3. The mechanism suggested by us indicates that no oscillations should occur in this case, and this indeed was observed. Consequently, contrary to Cohn’s comment, the experiments of ref 2 are compatible with our mechanism. Literature Cited (1) Hu, Y. H.; Ruckenstein, E. Catalyst Temperature Oscillations during Partial Oxidation of Methane. Ind. Eng. Chem. Res. 1998, 37, 2333. In Table 1, column H2O selectivity, third line from the bottom (100), should be replaced by (0). (2) Choudhary, V. R.; Uphade, B. S.; Mamman, A. S. Oxidative conversion of methane to syngas over nickel supported on commercial low surface area porous catalyst carriers precoated with alkaline and rare earth oxides. J. Catal. 1997, 172, 281. (3) Ruckenstein, E.; Hu, Y. H. Role of support in CO2 reforming of CH4 to syngas over Ni catalysts. J. Catal. 1996, 162, 230 and references cited therein.
IE980827Q
10.1021/ie980827q CCC: $18.00 © 1999 American Chemical Society Published on Web 02/11/1999
