The utilization of red mud waste as industrial honeycomb catalyst for selective catalytic reduction of NO

As a new way for the high-value utilization of red mud (RM) waste, we proposed an improved approach to prepare the RM-based sludge/powder via the sulfuric acid hydrothermal dissolution and NH3 aqueous precipitation route and then the RM-based industrial-sized honeycomb (150 × 150 × 600 mm) was successfully produced by the extrusion moulding method in pilot scale. The synthesized RM-based powdery/honeycomb catalyst exhibited more than 80% deNOx activity and good durability of H2O and SO2 above 350°C. But the decline of NO conversion was also observed above 350°C, which was confirmed to result from the increased oxygenation of NH3 at high temperature. To improve the NO conversion at high temperature, NH3 was shunted and injected into the catalyst bed at two different places (entrance and centre) to facilitate its uniform distribution, which relieved the oxidation of NH3 and increased deNOx efficiency with 98% NO conversion at 400°C. This work explored the industrial application feasibility for the RM-based honeycomb catalyst as well as the possible solution to decrease the oxygenation of NH3 at high temperature, which presented a valuable reference for the further pilot tests of RM catalyst in industry.


Introduction
You wrote that the improving catalytic property is based on the sulfuric acid hydrothermal dissolution of red mud and then NH3 aqueous precipitation. I suggest to discuss chemistry of this approach.
Experimental -I think that Table 1 (Chemical composition of red mud and catalysts) should be moved to this section. Table 1. Is this weight percent?
Results and Discussion -Can chemical composition of red mud affect the properties of catalyst? -According to Table 1 Title: The utilization of red mud waste as industrial honeycomb catalyst for selective catalytic reduction of NO Manuscript ID: RSOS-191183 Thank you for your submission to Royal Society Open Science. The chemistry content of Royal Society Open Science is published in collaboration with the Royal Society of Chemistry.
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When submitting your revised manuscript, you must respond to the comments made by the referees and upload a file "Response to Referees" in "Section 6 -File Upload". Please use this to document how you have responded to the comments, and the adjustments you have made. In order to expedite the processing of the revised manuscript, please be as specific as possible in your response. Comments to the Author(s) This work aims at the evaluation of the performance in the selective catalytic reduction of NO of a catalyst prepared from red mud waste. It is a very interesting approach since this catalytic process is still attracting the interest of many research groups and the valorization of industrial wastes is a priority in order to minimize the negative environmental impact of many industrial chemical processes. However, there are several points requiring to be addressed in order to clarify the information provided in this manuscript: 1. (Introduction, p. 4., l. 45) What does it mean that the low reaction efficiency between HNO3 and RM may be not convenient for the industrial amplification (implementation?)? 2. It would be interesting to include a picture of the RM honeycomb catalyst prepared in the present study. 3. (Experimental, p. 8, 19) They refer to the determination of BET and BJH, but this is meaningless, because these are two methods for the determination of specific surface area (BET) and pore size distribution (BJH), respectively. 4. How do the authors explain that, by comparing the chemical composition of original RM and fresh RM catalyst (after the acid-hydrothermal and neutralization steps), the weight (is it true? or is it molar?) percentages of Fe2O3 and TiO2 remain almost unchanged, Al2O3 decreases and SiO2 increases)? 5. (p. 10, l. 11) The BET is not a test, as previously indicated, and this method is usually used for the calculation of specific surface area (BET surface area), excepting for microporous materials. This method does not provide information about the pore size distribution, as is indicated. 6. It would be very useful to study catalysts by X-ray photoelectron spectroscopy (XPS) in order to get insights into the surface composition of fresh and used RM catalysts. 7. The scale of the x-axis of Figure 3D does not allow to distinguish clearly the evolution of pore size distribution curves. The representation until 100 nm, or less, could be enough. 8. Have they detected the formation of a particular metal sulfate, since the percentage of SO3 is high. By considering the formation enthalpy of different potential metal sulfates, which is the most probable to be formed in the presence of Fe(III), Al(III) and Ti(IV)? 9. What is the reason of the slight increase in NO conversion in the presence of H2O and SO2 at 450ºC ( Figure 5A)? 10. The total acidity can be evaluated from NH3-TPD data. An explanation, taking into account the chemical composition of original RM and fresh RM catalyst, must be given to justify the drastic difference between the corresponding desorption curves ( Figure 7D). The existence of strong Brönsted acid sites and Lewis acid sites is put forward by the authors, but nothing is said about the nature of chemical species responsible of these acid sites. 11. It is necessary to include in situ DRIFTS experiments of reactants in order to elucidate the mechanisms involved in SCR activity and ammoxidation. In this sense, is it actually an ammoxidation process leading to nitriles by reaction of ammonia and oxygen, using alkenes as substrate?. If this is not the case, the nature of oxidation products must be explained. 12. A paragraph comparing the present results with data already reported in the literature should be very useful for readers. 13. Minor errors: (Figure 1) Molding addictive?, Grind and sieve instead of Grind and seive; (p. 7, l. 22) N2O formation, not information. Therefore, it is an interesting methodological approach for the design of a new catalyst for NH3-SCR, but these issues must be clarified before recommending the manuscript for publication.

Recommendation?
Accept as is

Review form: Reviewer 2
Is the manuscript scientifically sound in its present form? Yes

Recommendation?
Accept as is

Comments to the Author(s)
The authors have made an important effort to adequately answer most of questions raised in the revision process. This has allowed to improve and clarify the information provided in the manuscript, and I could recommend its publication.

Introduction
Issue 2: You wrote that the improving catalytic property is based on the sulfuric acid hydrothermal dissolution of red mud and then NH 3 aqueous precipitation. I suggest to discuss chemistry of this approach. Discussion: This is a good suggestion for better understanding the preparation process of RM catalyst. As shown in Table 1, the main composition of original RM is Fe 2 O 3 , Al 2 O 3 , SiO 2 , TiO 2 , and alkaline/alkaline earth metal. The 50 wt.% H 2 SO 4 was use to remove alkaline/alkaline earth metal through acid base neutralization. At the same time, the bulk oxides such as Fe 2 O 3 and Al 2 O 3 may also partly dissolved by H 2 SO 4 to form soluble ferric sulfate and aluminum sulfate. After the treatment by H 2 SO 4 , ammonia aqueous was added to generate Fe(OH) 3 and Al(OH) 3 precipitate. Then the Fe(OH) 3 and Al(OH) 3 transformed to active Fe 2 O 3 and inert Al 2 O 3 after calcined at 500 °C for 3 h. The Fe 2 O 3 can act as the main active component, and the other oxides (e.g., Al 2 O 3 , TiO 2 , and SiO 2 ) will be as the support components in RM based catalyst.  Discussion: This is a good suggestion. The utilization of red mud waste as SCR catalyst has been studied before. Mohapatro et al. [15] reported the red mud catalyst with 31% deNO x efficiency at 400 °C for CO-SCR, and it increased to 92% when plasma was cascaded with red mud catalyst. The red mud SCR catalyst prepared by Bhattacharyya et al.
[16] obtained 40% NO conversion. Although the deNO x efficiency for red mud catalyst is still unsatisfactory, it demonstrates the feasibility to use red mud waste as high-value SCR catalyst. The main reason for the dissatisfied deNO x performance is the poor dispersity of active Fe species and the adverse effect of alkalis in RM. p.12, Line 14: rephrase: The NO conversion of RM powdery catalyst was investigated and illustrated in Fig. 5A Comments to the Author(s) This work aims at the evaluation of the performance in the selective catalytic reduction of NO of a catalyst prepared from red mud waste. It is a very interesting approach since this catalytic process is still attracting the interest of many research groups and the valorization of industrial wastes is a priority in order to minimize the negative environmental impact of many industrial chemical processes. However, there are several points requiring to be addressed in order to clarify the information provided in this manuscript: 1. (Introduction, p. 4., l. 45) What does it mean that the low reaction efficiency between HNO 3 and RM may be not convenient for the industrial amplification (implementation?)? Discussion: Due to the strong volatility and instability of HNO 3 , only low concentration of HNO 3 solution (20 wt.%) can be used, which may decrease the reaction efficiency between HNO 3 and RM. Besides, the toxic volatile is harmful to the environment, limiting the production of large-scale RM deNO x catalyst via the nitric acid-ball milling and neutralization-washing method. p.3, Line 17: rephrase: However, the strong volatility of HNO 3 results in the low reaction efficiency between HNO 3 and RM, which may be not convenient for the industrial implementation to produce the RM based honeycomb catalyst, and the easy oxidation of NH 3 at the temperature above 350 °C may also prevent the further improvement of deNO x efficiency in industry.
2. It would be interesting to include a picture of the RM honeycomb catalyst prepared in the present study. Discussion: The picture of the RM honeycomb catalyst was included in Fig. 1 8,19) They refer to the determination of BET and BJH, but this is meaningless, because these are two methods for the determination of specific surface area (BET) and pore size distribution (BJH), respectively. Discussion: The corresponding part has been revised as suggested. p.8, Line 5: rephrase: A nitrogen adsorption-desorption apparatus (ASAP 2020, Micromeritics Instrument Corp, USA) was used to determine the surface area and pore size distribution of samples at 77 K. 4. How do the authors explain that, by comparing the chemical composition of original RM and fresh RM catalyst (after the acid-hydrothermal and neutralization steps), the weight (is it true? or is it molar?) percentages of Fe 2 O 3 and TiO 2 remain almost unchanged, Al 2 O 3 decreases and SiO 2 increases)? Discussion: After acid-hydrothermal treatment, the alkaline/alkaline earth metal together with part Al, Fe and Ti oxides were dissolved, but the SiO 2 is nearly insoluble. The subsequent neutralization and washing process will remove the alkaline/alkaline earth metal element and recover of the Fe 2 O 3 , Al 2 O 3 and TiO 2 . However, the different solubility (Al>Fe>Ti>Si) [16] resulted in the different recover rate of these Al, Fe and Ti species during the neutralization process. More Al was lost during the process, but SiO 2 was almost completely retained. Ultimately, we observed the increased percentage of SiO 2 , almost unchanged percentage of Fe 2 O 3 and TiO 2 , and decreased percentage of Al 2 O 3 . p.4, Line 9: rephrase: The preparation process of RM powdery/honeycomb catalyst was illustrated in Fig. 1. Original RM (Table 1a) and 50 wt.% H 2 SO 4 were mixed at a molar ratio of 1/1.2. Then the whole mixture was transferred into steel autoclave and maintained at 150 °C for 10 h, during which the alkaline/alkaline earth metal was leached, the bulk oxides such as Al 2 O 3 , Fe 2 O 3 and TiO 2 may also partly dissolved by H 2 SO 4 to form soluble aluminum sulfate, ferric sulfate and titanyl sulfate, but the SiO 2 was nearly insoluble [16]. After the treatment by H 2 SO 4 , the composite was washed for several times and neutralized to pH value of 8 with NH 3 aqueous to remove the alkaline/alkaline earth metal. The product of Al(OH) 3 , Fe(OH) 3 , Ti(OH) 2 and SiO 2 was obtained in the neutralization process. 5. (p. 10, l. 11) The BET is not a test, as previously indicated, and this method is usually used for the calculation of specific surface area (BET surface area), excepting for microporous materials. This method does not provide information about the pore size distribution, as is indicated. Discussion: The corresponding part has been revised as suggested. p.9, Line 12: rephrase: The BET surface area measurement is also performed to have a deep insight into the change of surface area and pore-size distribution. Fig. 3 illustrates the N 2 adsorption-desorption isotherms curves (Fig. 3C) and the corresponding pore size distribution (Fig. 3D) of catalysts, whose isotherms are close to type IV with a H 3 -type hysteresis loop, indicating the typical mesoporous characteristics. Obviously, the pore size distribution of fresh RM catalyst (Fig. 3C-b) is broader than original RM (Fig. 3C-a). The BET surface area, pore volumes, average pore diameter are listed in Table 2. 6. It would be very useful to study catalysts by X-ray photoelectron spectroscopy (XPS) in order to get insights into the surface composition of fresh and used RM catalysts. Discussion: The main focus of this paper is providing an available method to the disposal of RM solid waste as deNO x catalyst. And the RM based catalyst showed good activity and stability as demonstrated in Fig. 5 and Fig. 6. Thus, the difference in surface composition for the fresh and used RM catalysts was not discussed and the XPS was not performed in the manuscript.
7. The scale of the x-axis of Figure 3D does not allow to distinguish clearly the evolution of pore size distribution curves. The representation until 100 nm, or less, could be enough. Discussion: Fig. 3D has been checked and corrected in the revised manuscript as suggested. 8. Have they detected the formation of a particular metal sulfate, since the percentage of SO 3 is high. By considering the formation enthalpy of different potential metal sulfates, which is the most probable to be formed in the presence of Fe(III), Al(III) and Ti(IV)? Discussion: This is a good suggestion. Fe 2 (SO 4 ) 3 is the most possible metal sulfates by considering the formation enthalpy. However, the peak of Fe 2 (SO 4 ) 3 or other metal sulfates was not found after carefully compared XRD pattern with the standard card. It may exist in the form of adsorptive sulfate considering its comparatively low content.
9. What is the reason of the slight increase in NO conversion in the presence of H 2 O and SO 2 at 450 ºC ( Figure 5A)? Discussion: As shown in Fig. 5   10. The total acidity can be evaluated from NH 3 -TPD data. An explanation, taking into account the chemical composition of original RM and fresh RM catalyst, must be given to justify the drastic difference between the corresponding desorption curves ( Figure 7D). The existence of strong Brönsted acid sites and Lewis acid sites is put forward by the authors, but nothing is said about the nature of chemical species responsible of these acid sites. Discussion: As shown in Table 1, original RM mainly consists of Fe 2 O 3 , Al 2 O 3 , SiO 2 , TiO 2 , Na 2 O, CaO and K 2 O etc. The high Na 2 O content and other alkaline-earth metals in the original RM will interact with the major active components during catalytic reaction, resulting in the decrease of surface area and activity. After the acid-hydrothermal and neutralization method, the majority of alkali/alkaline-earth metals were removed, which may increase the active acid sites as well as the NH 3 adsorption capacity [17]. As for the used RM catalyst, the residual sulfate after SCR reaction in the presence of SO 2 /H 2 O may further increase the NH 3 adsorption. NH 3 was mainly absorbed on the Brönsted acid sites at relative low temperature to form NH 4 + , and then the reaction between NH 4 + and NO occurred to generate N 2 and H 2 O. While the coordinated NH 3 bound to Lewis acid sites usually response for the SCR activity at high temperature. Both of the Brönsted acid sites and Lewis acid sites are important in the SCR reaction with NH 3 . p.17, Line 12: rephrase: The oxidizability of NH 3 is related to the acid sites of RM catalysts, thus NH 3 -TPD test was carried out as profiled in Fig. 7D. It can be seen that the adsorption capacity of NH 3 increased significantly for the fresh RM sample compared with the original one due to the elimination of alkali/alkaline-earth metals. The desorption peak centered at about 100 °C can be assigned to physically adsorbed NH 3 . Another obvious desorption peak can be observed from 300 °C to 500 °C, which may be due to the desorption of ionic NH 4 + bound to strong Brønsted acid sites and coordinated NH 3 bound to Lewis acid sites. It was reported [17,27] that NH 3 was mainly absorbed on the Brönsted acid sites at relative low temperature to form NH 4 + and then react with NO to generate N 2 and H 2 O. While the coordinated NH 3 bound to Lewis acid sites usually response for the SCR activity at high temperature. Both of the Brönsted acid sites and Lewis acid sites are important in the SCR/SCO reaction with NH 3 . As for the used RM catalyst, the residual sulfate may further increase the NH 3 adsorption. These strong acid sites between 300 °C and 500 °C will facilitate the strong adsorption of ammonia and will be also beneficial for the NH 3 -SCO reaction, resulting in the decreased SCR selectivity. All of these results indicate that the severe oxidation of NH 3 resulted in the low actual ratio of NH 3 /NO and thus low reaction efficiency of SCR. 11. It is necessary to include in situ DRIFTS experiments of reactants in order to elucidate the mechanisms involved in SCR activity and ammoxidation. In this sense, is it actually an ammoxidation process leading to nitriles by reaction of ammonia and oxygen, using alkenes as substrate?. If this is not the case, the nature of oxidation products must be explained. Discussion: The in situ DRIFTS study of RM based catalyst have been done in our previous work, which showed that the NH 3 -SCR reaction over RM catalyst follows the Eley-Rideal mechanism at high temperature [17]. Besides, the detailed mechanism of NH 3 oxidation was well elaborated by other reports for Fe based catalyst [8,23], and the main purpose of this manuscript is to find new way to lower down the ammoxidation rate at high temperature.