Mn-Ce Catalysts/LDPC Modified by Mo for Improving NH3-SCR Performance and SO2 Resistance at Low Temperature
Round 1
Reviewer 1 Report
This paper provides useful information on the study of new catalysts operating at low temperature for ammonia-SCR applications for NOx removal from flue gas.
The authors investigated the influence of water and sulfur dioxide, which are two of the main poisoners of catalysts that reduce their activity themselves. Especially SO2 is very dangerous due to sulfite formation and ammonium bisulfite clogging the catalyst. This study could be of interest for purification applications that use de-sox scrubber downstream of SCR.
This work is very interesting in the field of NOx emission control from flue-gas. For this reason, it would deserve chances at publication in this Journal but provided that the authors solve some minor and major points, which I list in detail below.
1. Introduction
In the state of the art is rather poor and gets straight to the fact that SCRs are the currently most widely used technologies, without mentioning other emerging technologies that are not yet widely applied, but they are very important for the future.
After the first sentence in the Introduction section, "The industrial revolution brought prosperity and also nitrogen oxides (NOx) that have damaged the biological environment. Nitrogen oxides not only caused acid rain and smog, but also seriously affected human life and health." the authors could report that several emerging technologies could be adopted to NOx emission control, such as Electron Beam Processes EBP (https://doi.org/10.1016/S0969-806X(02)00340-7), Low-Temperature Adsorption LTA (https://doi.org/10.1038/s41598-021-90532-9) or Wet Oxidative Scrubbing WOS (https://doi.org/10.3303/CET2186067), which is also capable of simultaneously control of NOx/SOx emissions (https://doi.org/10.1016/j.cej.2022.137585).
Before the sentence introducing SCR systems "The selective catalytic reduction of ammonia(NH3-SCR) as a mature denitrification technology had been widely used in power generation industry" no reference was made regarding SNCR technologies that are still quite widely used. Probably at this point, the authors should report some comparison between the two applications SCR and SNCR. The authors could use the many references in the literature, e.g., https://doi.org/10.1504/IJEP.2002.000655
but other references are welcome.
2. Experimental apparatus and tests
The authors have added some details about the tubular furnace used but should add more details, such as: the insulator used to keep the temperature constant in the reactor, the specifications of the temperature control system, how the model flue-gas is generated and all the auxiliary equipment for measuring flow rate, temperature etc... a process flow diagram PFD would also be useful.
In addition, other details should be added for gas characterization. Only an NOx concentration analyzer was used? Please add the species it reads (NO, NO2, N2O etc ?), what are the reading ranges of the species, what are the deviations of the instrument?
SO2 is also monitored, if you add gas analyzer details. Also particulate matter is used in model gas, how do the authors detect it?
Finally, how is NH3 added? By spraying into the flue gas before the catalyst?
The text states that NH3 is 500 ppm i.e., in the same stoichiometric proportion as NOx.
More details on this point are needed.
For NH3 was a gas analyzer used to test for ammonia slip?
3. Results
Figures 1a and 1b are as a function of process temperature, but what is the activity time of the catalyst? would need to add this information.
Figures 2 are not very clear. The authors should clearly indicate in which region there is no H2O or SO2 and when they are added instead. The diagram should be broken down into portions with lines and not using arrows.
Have the authors also investigated the reproducibility of their data? Probably plots with error bars should be reported, or at least a discussion should be made in the text if the uncertainties are low and barely visible in the plot.
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Author Response
Please see the attachment.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
This is an interesting investigation on the design and exploration of Mo modified Mn-Ce oxides loaded on low-density porous ceramic (LDPC) support to achieve dust removal and denitrification at the same time. The investigated catalysts were prepared by a simple impregnation method. Interestingly, the addition of Mo to Mn-Ce/LDPC enhanced the catalytic performance and the Mn2Ce1Mo0.2Ox/LDPC combination catalyst exhibited best denitrification activity in wide temperature range with great H2O and SO2 resistance. The quality of data presented is worthy of publication. In view of the significance of the subject addressed, I recommend this article for publication. However, some amendments as pointed out below are necessary before acceptance.
(1) Catalyst Preparation: Brief description about the preparation of Mo impregnated Mn-Ce/LDPC catalyst should be included.
(2) Page 2: Preparation method and catalyst carry were ----------. Catalyst carry is not sounding properly.
(3) Page 2: The specific description of LDPC preparation in our previous articles [30,31]. Incomplete sentence. A brief description about its preparation should be included.
(4) Page 4: Therefore, LDPC with 8% catalyst loading had a high catalytic performance and relatively low filtration resistance……. No specific explanation is found in the article for this observation.
(5) Page 7: With the increase of Mn/Ce, the diffraction peaks became wider and weaker. The overlap of diffraction peaks at 28.68° for MnO2 and at 28.55° for CeO2 resulted in a wider peak in the pattern and the diffraction peaks became weaker because of the poor crystallization of CeO2 and MnO2. Poor crystallization would enhance catalytic performance [30]. Further, as stated in this discussion, The incorporation of Mo6+ into the lattice induced a decrease in the lattice parameters because the ion radius of Mo6+ (0.059 nm) was less than the ion radius of Ce4+ (0.097 nm).
This opinion is in contrary to the reported observations in the literature. As reported earlier [Applied Catalysis B: Environmental 162 (2015) 122–132] there is a clear evidence on the incorporation of Mn ions into the ceria lattice and a decrease in the lattice parameter due to the smaller size of manganese ions in comparison to Ce4+ ions (Ce4+ = 0.097 nm; Mn2+= 0.083 nm; Mn3+= 0.065 nm; Mn4+= 0.053 nm).
Why there is no incorporation Mn ions into ceria lattice in this study? What is the driving force to incorporate Mo into the ceria lattice? A proper justification with reference to the above publication is necessary in the revision.
Some improvement is necessary.
Author Response
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Author Response File: Author Response.pdf
Reviewer 3 Report
After carefully reviewing the paper “Mn-Ce catalysts/LDPC modified by Mo for improving NH3-SCR performance and SO2 resistance at low temperature” I recommend that it should be accepted after major revision. Here are some recommendations:
- The reference list should be completed with relevant papers from the literature. None of the first 20 papers from a Scopus search of “denitrification and dust removal” (sorted by relevance) is cited;
- rows 47-51: “In the flue gas purification process, the SCR reactor was installed in the back end of the desulfurization device and electrostatic precipitator. The dust removal and denitrification processes caused a series of issues such as large occupation space, high cost and difficult in maintenance.” – needs citation;
2. Materials and Methods. 2.1. Preparation of catalysts
- row 79: “The specific description of LDPC preparation in our previous articles” – at least a very brief description should be offered to the reader about the composition and the preparation method;
- rows 82-83: “The impregnation was operated at a vacuum of approximately 0.8-1.0 MPa …” – these are not vacuum values, please correct;
2.2. Catalytic-activity test
- rows 87-88: a short description of the catalytic tests is needed: type of reactor, amount of catalyst …
- rows 94-95: “The resistance of the catalytic LDPC was measured according to the Chinese standard (GB/T 6165–2008).” – please provide a short explanation of this standard for the readers;
- row 97: “The self-cleaning performance …” – please provide a short explanation of the concept;
2.3. Characterization of catalysts
- rows 115-116: “H2-TPR was carried out in a mixed gas (50 mL/min) with 5% H2 and 5% Ar” – what is the rest 90 % of gas?
3. Results and discussion. 3.1. Catalytic performance
- rows 135-136: “Mn2Ce1Mo0.2Ox/LDPC has best catalytic performance compared with other Mn-Ce catalysts/LDPC.” – the results in Figure 1(a) show that Mn2Ce1Mo0.3Ox/LDPC has also very good activity, even slightly higher than Mn2Ce1Mo0.2Ox/LDPC. Why did the authors consider the one with Mo0.2 to be better? Why the Mo0.3 sample was never mentioned, tested or characterized again? A short explanation should be included in the text. Also, for the readers that are not familiar with the topic, a short explanation should be provided about the cause of the conversion decrease at high temperatures;
3.4. Dust removal efficiency and filtration resistance
- row 213: why did the authors choose to work with 5 g/m3 of dust in the gas?
3.5. Characterization. 3.5.1. Crystal phase analysis
- rows 252-253: “The 3D interconnected porous structure ensured that the dust in the gas does not direct contact with catalysts.” – if the dust can pass through the structure (and the authors measure 0.5 g of dust/m3 after it) how can the authors be sure that the dust does not get in direct contact with the catalyst? (see also the lack of verb from the sentence)
- rows 254-255: “While, for the sample with catalyst loading of 12 wt.%, the channels was blocked by catalyst.” – the SEM Figure 6(d) show channels with small apertures, but the structure is not completely blocked;
3.5.3. Atomic species and valence analysis
- rows 281-282: “… more Mn4+ could produce more oxygen vacancy (Vo) which was helpful to the catalytic process as shown in equation 2” – please provide an explanation for this claim: why oxygen vacancies are helpful?
- Table 2: “Mn speciesa”, “Ce speciesa”, “O speciesa” – please correct. Also, the 0.47 value for the Oγ+Oß/Oα ratio for the Mn2Ce1Mo0.2 sample is not correct;
3.5.4. Reducibility and acidity
- rows 321-323: “It is well known that the surface acidity of the catalyst played an essential role in the NH3-SCR reaction and the overall area of the desorption peaks represents the numbers of acid sites and the temperature of desorption peak is related to the acid strength” – citation needed;
English language:
- rows 180-181: “When SO2 and H2O were introduced in the reaction gas, the change of NOx conversion showed in Figure 2c.” – check the sentence;
- rows 203, 241: “was benefit”;
Minor improvements needed (see the comments above)
Author Response
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Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
I would like to thank the authors for considering my suggestions.
In my opinion, the manuscript is now ready for publication.
I have only one last suggestion for the authors: emerging technologies allow very high and often even higher performance than conventional SCRs (unlike as written in the updated versione).
The limitations of these technologies for industrial applications are mainly due to energy consumption or the production of solid or liquid wastes to be disposed of and the costs related to them.
Further efforts need to be made to make these processes more efficient and to recover waste raw materials and reduce wastewater management costs.
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
Reviewer 3 Report
I recommend that the paper should be accepted in the present form, with one minor observation: in Figure 1(b), the concentration in the graph varies between 4 and 12, while in the legend is 5-12 %; it should be corrected.
Author Response
Point： In Figure 1(b), the concentration in the graph varies between 4 and 12, while in the legend is 5-12 %; it should be corrected.
Response: Thank you for your reminding. We have corrected the error you pointed out.
