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Partial oxidation of methane with Cu-Zn-Al catalyst in a dielectric barrier discharge Manuscript id: CEP-D-06-00402 Author response of the reviewer’s comments Reviewer 1: Question: 1) What is the meaning of the following fragment: "The synergy among Cu-Zn-Al was able to absorb..."? [p. 4] Answer: The basis of almost all present-day commercial units for the production of methanol is the ICI process, which converts a high-pressure gas mixture of CO, CO 2, and H2, with greater than 99% efficiency, into the alcohol, using a catalyst containing Copper-Zinc-Alumina (CZA). The existence of CZA with its properties was believed as the key factor for producing methanol from synthesis gas. The concept of synergism of CZA probably can be described below: It is well-known that for the hydrogenation of CO to methanol on Cu-Zn catalysts, Cu acts as the active center for the CO adsorption and activation. Although there are many studies concerning the role of Zn, the ideas that have been proposed are widely diversified, ranging from hydrogen activation center, structural-electronic promoter, to bifunctional Cu-Zn interfacial active center. Kanai et al.1 have ascribed the synergetic effect of Zn to the creation of active sites such as Cu+ by the ZnOx moieties on the Cu surface. Burch et al.2 have thought that the synergy may involve the transport of one adsorbed species such as H+ from the Cu to the Zn which acts as a sink for the hydrogen atoms and transports them back to the Cu to hydrogenate the formate species to methanol. This information has been put in the revised manuscript in more general form to make the readers easily understand. Question: 2) The stability of the catalyst structure is not an adequate proof for the conclusion of not being deactivated. [p. 7] Kinetic measurements should be also considered. Answer: Thank you for the comment from the reviewer regarding the catalyst performance. Yes, it is correct that catalyst structure of before and after experiments can not be used as the only reason for proving the long activity of the catalyst. In the revised manuscript, we added more and clear information about it. During 6-hours operation, the products difference was relatively small or 1 2
Y. Kanai, T. Watanabe, T. Fujitani, T. Uchijima, J. Nakamura, Catal. Lett. 38 (1996)157. R. Burch, R.J. Chappell, S.E. Golunski, Catal Lett. 1 (1988) 439.
similar which is still much higher (~2 times higher) compared to non catalytic process. We can conclude that the catalyst was still active during that period. Secondly, the XRD analysis of the catalyst before and after running showed that those were not so much different. It means there was no structure transformation of the catalyst (e.g.: metal-oxidation or reduced metal form) due to plasma process. The physical observation also show there is no different of the catalyst color before and after experiments. This means that the carbon or coke deposition (one of catalyst deactivation phenomena) was not occurred. It was proved also by SEM analysis of the catalyst surface. Question: 3) How should be understood the sentence: "Instead of methanol selectivity, the stability of the catalyst was also good."? And following: "The deactivation of catalyst, e.g. by carbon decomposition..." (deposition?) [p. 10] Answer: Thank you for the correction. In the revised manuscript we changed some sentences to avoid the ambiguous of the meaning and also correction for the word ‘decomposition’ into ‘deposition’. The answer will be almost similar to the question no. 2. In this part, we want to tell that our process produced a stable process especially we want to mention more on the stability of catalyst in term of the activity. During 6 hours operation, the products were not much changed, so we concluded that the deactivation of the catalyst was not occurred during 6 hours operation. The most possible reason for catalyst deactivation is by carbon deposition or coke formation from the fragmentation of methane CH4 C + 2 H2 However, in the next sentences we informed that the carbon/coke deposition was not occurred due to reaction with oxygen or oxygen radical. Question: 4) The statement: "...the catalyst loading did not affect the conversion of methane." [p. 10] is not in accordance with Table 2 (conversion 25.3% for 0.5 g and 33.9% for 1.5 g). Answer: Thank you for the correction. What the opinion of the reviewer was correct. The conversion of methane increased as the weight of catalyst loading in the reactor was increased. In the revised manuscript, we corrected the previous statement follows the reviewer correction. Actually, we did not focused so much on the conversion of methane due to catalyst loading but the correction is quite important. Thank you. Question: 5) How two sentences should be understood, beginning from : "However" [p.12], ending: "lowest value" [p. 13]?
Answer: Thank you. We corrected the sentences to make it clear. In this part, we want to explain the reason why the methanol production was the highest based on the mol-balance of CO and CO 2. The reason is quite simple. By knowing and calculating the CO and CO2 concentration, we will know the oxidation rate of methane by oxygen. Instead of direct methane oxidation into methanol, the most visible pathways of reaction are via synthesis gas. Methane will be oxidized into CO (first oxidation-stage) and continued to CO2 (2nd oxidation-stage). When the total of CO and CO2 is less, the probability of methane oxidation to methanol will be higher. Similar analogy is also applicable in case of CO and CO2. When the produced CO2 is less, higher chance of CO conversion to methanol will be obtained as CO to CO2 and CO to CH3OH are parallel reactions. Our calculation of the CO and CO2 and the ratio of CO2 to CO in the product stream for yttrium/CZA catalyst are the lowest value compared to other catalysts. The existence of yttrium inhibits the second-stage oxidation of carbon from CO to CO2. However, this will create a chance for CO to be converted into methanol. The detail phenomena of partial oxidation of methane with yttrium stabilized zirconia were observed thoroughly in our previous research3. This information was added to the revised manuscript. Question: 6) The last question concerns the idea of a 2-stage process proceeding through the syngas production [p. 9]. Taking into account the equilibrium of the H 2 reactions with CO and CO2, is it really possible to obtain so much methanol as was found at the experiment conditions? Answer: Thank you for the question of reviewer regarding the possibility of high amounts of methanol by this process. It is relative to say whether the production of methanol is very high or very low. Reviewer can compare some papers related to the production of methanol via direct conversion of methane. For example, in Aoki et al. paper 4, they showed very high selectivity of methanol (~73%) but the conversion of methane was less than 1%. In our opinion, it is very possible to produce methanol in high production rate based on the availability of H2 and CO or CO2 produced by plasma processes. The most important actually is not in the case of ‘selectivity’ but in term of ‘yield’. We think that our yield is still low, but what we have presented here is the highest yield of direct methane conversion to methanol as we compared with other papers. Reviewer 2: 3
A. Indarto, J.W. Choi, L. Hwaung, H.K. Song and J. Palgunadi, Partial oxidation of methane with sol-gel Fe/Hf/YSZ catalyst in dielectric barrier discharge: catalyst activation by plasma, J. Rare Earths 24 (2006) 513-518. 4 K. Aokia, M. Ohmae, T. Nanba, K. Takeishi, N. Azuma, A. Ueno, H. Ohfune, H. Hayashi, Y. Udagawa, Direct conversion of methane into methanol over MoO3/SiO2 catalyst in an excess amount of water vapor, Catalysis Today 45 (1998) 29-33
Questi on : 1. Was the electrical power supplied to the discharge kept constant for all the
experiments? Answer: Thank you for the question. Yes, in this experiment we kept the supplied power at constant value which was around 50W. For the power increment experiments, we increased the supplied power from 50W to 120W. Question: 2. Solid carbon production whether it was produced or not. Answer: Thank you. Regarding the production of solid carbon, we explain here that the carbon or coke was not produced or it was produced by the quantity was very small. The solid carbon was also not found in the surface of the catalyst. The reason has been clearly explained on page 10. We thank for the comment and question from the reviewers. We appreciate it so much. We are welcomed for any comment, question, or information in order to make this manuscript better and perfect. Thank you once more again.
Y. Kanai, T. Watanabe, T. Fujitani, T. Uchijima, J. Nakamura, Catal. Lett. 38 (1996)157. R. Burch, R.J. Chappell, S.E. Golunski, Catal Lett. 1 (1988) 439.
similar which is still much higher (~2 times higher) compared to non catalytic process. We can conclude that the catalyst was still active during that period. Secondly, the XRD analysis of the catalyst before and after running showed that those were not so much different. It means there was no structure transformation of the catalyst (e.g.: metal-oxidation or reduced metal form) due to plasma process. The physical observation also show there is no different of the catalyst color before and after experiments. This means that the carbon or coke deposition (one of catalyst deactivation phenomena) was not occurred. It was proved also by SEM analysis of the catalyst surface. Question: 3) How should be understood the sentence: "Instead of methanol selectivity, the stability of the catalyst was also good."? And following: "The deactivation of catalyst, e.g. by carbon decomposition..." (deposition?) [p. 10] Answer: Thank you for the correction. In the revised manuscript we changed some sentences to avoid the ambiguous of the meaning and also correction for the word ‘decomposition’ into ‘deposition’. The answer will be almost similar to the question no. 2. In this part, we want to tell that our process produced a stable process especially we want to mention more on the stability of catalyst in term of the activity. During 6 hours operation, the products were not much changed, so we concluded that the deactivation of the catalyst was not occurred during 6 hours operation. The most possible reason for catalyst deactivation is by carbon deposition or coke formation from the fragmentation of methane CH4 C + 2 H2 However, in the next sentences we informed that the carbon/coke deposition was not occurred due to reaction with oxygen or oxygen radical. Question: 4) The statement: "...the catalyst loading did not affect the conversion of methane." [p. 10] is not in accordance with Table 2 (conversion 25.3% for 0.5 g and 33.9% for 1.5 g). Answer: Thank you for the correction. What the opinion of the reviewer was correct. The conversion of methane increased as the weight of catalyst loading in the reactor was increased. In the revised manuscript, we corrected the previous statement follows the reviewer correction. Actually, we did not focused so much on the conversion of methane due to catalyst loading but the correction is quite important. Thank you. Question: 5) How two sentences should be understood, beginning from : "However" [p.12], ending: "lowest value" [p. 13]?
Answer: Thank you. We corrected the sentences to make it clear. In this part, we want to explain the reason why the methanol production was the highest based on the mol-balance of CO and CO 2. The reason is quite simple. By knowing and calculating the CO and CO2 concentration, we will know the oxidation rate of methane by oxygen. Instead of direct methane oxidation into methanol, the most visible pathways of reaction are via synthesis gas. Methane will be oxidized into CO (first oxidation-stage) and continued to CO2 (2nd oxidation-stage). When the total of CO and CO2 is less, the probability of methane oxidation to methanol will be higher. Similar analogy is also applicable in case of CO and CO2. When the produced CO2 is less, higher chance of CO conversion to methanol will be obtained as CO to CO2 and CO to CH3OH are parallel reactions. Our calculation of the CO and CO2 and the ratio of CO2 to CO in the product stream for yttrium/CZA catalyst are the lowest value compared to other catalysts. The existence of yttrium inhibits the second-stage oxidation of carbon from CO to CO2. However, this will create a chance for CO to be converted into methanol. The detail phenomena of partial oxidation of methane with yttrium stabilized zirconia were observed thoroughly in our previous research3. This information was added to the revised manuscript. Question: 6) The last question concerns the idea of a 2-stage process proceeding through the syngas production [p. 9]. Taking into account the equilibrium of the H 2 reactions with CO and CO2, is it really possible to obtain so much methanol as was found at the experiment conditions? Answer: Thank you for the question of reviewer regarding the possibility of high amounts of methanol by this process. It is relative to say whether the production of methanol is very high or very low. Reviewer can compare some papers related to the production of methanol via direct conversion of methane. For example, in Aoki et al. paper 4, they showed very high selectivity of methanol (~73%) but the conversion of methane was less than 1%. In our opinion, it is very possible to produce methanol in high production rate based on the availability of H2 and CO or CO2 produced by plasma processes. The most important actually is not in the case of ‘selectivity’ but in term of ‘yield’. We think that our yield is still low, but what we have presented here is the highest yield of direct methane conversion to methanol as we compared with other papers. Reviewer 2: 3
A. Indarto, J.W. Choi, L. Hwaung, H.K. Song and J. Palgunadi, Partial oxidation of methane with sol-gel Fe/Hf/YSZ catalyst in dielectric barrier discharge: catalyst activation by plasma, J. Rare Earths 24 (2006) 513-518. 4 K. Aokia, M. Ohmae, T. Nanba, K. Takeishi, N. Azuma, A. Ueno, H. Ohfune, H. Hayashi, Y. Udagawa, Direct conversion of methane into methanol over MoO3/SiO2 catalyst in an excess amount of water vapor, Catalysis Today 45 (1998) 29-33
Questi on : 1. Was the electrical power supplied to the discharge kept constant for all the
experiments? Answer: Thank you for the question. Yes, in this experiment we kept the supplied power at constant value which was around 50W. For the power increment experiments, we increased the supplied power from 50W to 120W. Question: 2. Solid carbon production whether it was produced or not. Answer: Thank you. Regarding the production of solid carbon, we explain here that the carbon or coke was not produced or it was produced by the quantity was very small. The solid carbon was also not found in the surface of the catalyst. The reason has been clearly explained on page 10. We thank for the comment and question from the reviewers. We appreciate it so much. We are welcomed for any comment, question, or information in order to make this manuscript better and perfect. Thank you once more again.
