Synthesis of Highly Porous Cu2O Catalysts for Efficient Ozone Decomposition
Round 1
Reviewer 1 Report
The authors describe in their manuscript interesting results in the important issue of ozone decomposition. Some points of the article must be improved before publication.
- 2.2 The characterization of the catalysts is insufficient describe. The authors should describe the experiments in a manner that they are reproducible and understandable. Only the XRD experiments are described in a right way. I am missing any informtion about the sample preparation, the exact measurement conditions, the data reduction.
- Where are Fig. S1 and Fig. S2?
- Figure 4 is not mentioned and explained in the main text.
- Fig. 5 the binding energy should be shown in decreasing order, deconvoluted spectra not decomposed.
Author Response
Please see the attachment.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The authors studied decomposition of ozone passing through nano-sized Cu2O grains. The topic of the manuscript is relevant as ozone pollution causes big recent problems in towns and agglomerations. However, to my opinion, the work lacks some crucial/critical evaluations, and should be reconsidered/published after major revisions will be made.
There are some specific points which should be addressed before a final decision:
Formal: Surface areas of 26.67-58 m2*g-1 are declared as “low”, and 79.5 m2*g-1 as high. In fact, there is no significant (order) difference between these materials, the declaration should not be so strict (abstract+p2,lines54-57).
Porous structure of Na+NH3+P123+But material is not direct result of slowing the reaction, but of small grains (2.7 nm) formed during reaction. Grains aggregate to much larger structures, but with some space between them, as seen in Fig1d. The sentence should be rephrased.
Why characterization data of material obtained with NaOH/butanol are not given in Table 1? If morphology is similar to that of “Na” catalyst, BET and grain size should be accessible by measurement.
According to Fig1a and Fig1b, Cu2O grains are single-crystalline (as supported by cubic shape of material “Na” and measured grain size by XRD). In such case, parameter “pore size” by BJH is somewhat meaningless, as it correspond to packing of individual crystallites, whereas for materials shown in 1c and 1d, this parameters also bring some information on “really” porous structure formed by aggregates of very small single-crystalline grains.
Maybe, (nano)crystal/crystallite-size should be used for size measured by XRD, and grain size for “whole size” obtained by e.g. statistical analysis of TEM images, to better resolve between these parameters.
Discussion of data shown in Fig4 is missing.
Major objection: According to experimental, it seems that 200 pm concentration of ozone was generated in oxygen, which was further mixed with air and concentration of ozone in flowing gas was thus decreased several times (if not, this should be clarified in the experimental section, as readers opinion is such). According to degradation process shown in Fig4, it seems that catalytic efficiency is more-less linearly decreased. If flow-rate of 480 dm3*g-1*h-1 was used, amount of ozone flown through the material is in milimolar scale per hour (maybe sub-milimolar, taken into account dilution of O2/O3 mixture with air, see above) per gram of the Cu2O material. So, the amount of ozone is comparable to molar amount of de-activated Cu2O material (order of hundredths-tenths of gram per hour). So, main question arises – behaves prepared material really as catalyst, or “just” as chemi-absorbent? Some calculation/discussion convincing readers that behavior of the material is really catalytic should be added. Probably, analysis of oxidation states of copper in fresh material and material after O3 exposure should be done.
Author Response
We gratefully thank the editor and all reviewers for their remarks and suggestions, which has significantly rise the quality of the manuscript. Each suggested revision and comment, brought forward by the reviewers was accurately incorporated and considered. Below the comments of reviewers are response point by point and the revisions are indicated.
Reviewer 2
Point 1: Surface areas of 26.67-58 m2*g-1 are declared as “low”, and 79.5 m2*g-1 as high. In fact, there is no significant (order) difference between these materials, the declaration should not be so strict (abstract+p2,lines54-57).
Response: We gratefully appreciate for your valuable comment. Because the reported specific surface area of pure cuprous oxide nanomaterials is generally small (less than 100 m2/g), we describe the size according to this range, which may cause misunderstanding to the readers, so we decided to change “high” to “ relatively high”.
Point 2: Porous structure of Na+NH3+P123+But material is not direct result of slowing the reaction, but of small grains (2.7 nm) formed during reaction. Grains aggregate to much larger structures, but with some space between them, as seen in Fig1d. The sentence should be rephrased.
Response: Thank you for your rigorous consideration, Indeed, we agree that smaller grains sizes are the key factor. The combination of surfactants and the introduction of ammonia is to obtain smaller particles and increase the specific surface area of materials, maybe some of our expressions are not very accurate. So we changed “After the template was washed away, small pores were formed in former position of n-butanol, leading to more abundant pore structure of the material” into “Under the action of combined surfactants, Cu2O crystallized into smaller grains and aggregated into highly porous structure”.
Point 3: Why characterization data of material obtained with NaOH/butanol are not given in Table 1? If morphology is similar to that of “Na” catalyst, BET and grain size should be accessible by measurement.
Response: We feel sorry for the inconvenience brought to the reviewer. We did characterize this sample, but at first we thought it was of little significance, so we intend to put it in the supplementary part. Now we filled the data in Table 1, and TEM characterization was put in SI.
Please see the attachment.
Point 4: According to Fig1a and Fig1b, Cu2O grains are single-crystalline (as supported by cubic shape of material “Na” and measured grain size by XRD). In such case, parameter “pore size” by BJH is somewhat meaningless, as it correspond to packing of individual crystallites, whereas for materials shown in 1c and 1d, this parameters also bring some information on “really” porous structure formed by aggregates of very small single-crystalline grains.
Response: Thank you for your rigorous thinking. Strictly speaking, the first two samples are not porous structure. Therefore, the BJH pore size describes the space formed by the accumulation of small particles. But because of their small crystal particles, the “pore” may contribute to the specific surface area. In addition, these pore size data may help readers to better compare different samples.
Point 5: Maybe, (nano)crystal/crystallite-size should be used for size measured by XRD, and grain size for “whole size” obtained by e.g. statistical analysis of TEM images, to better resolve between these parameters.
Response: The reviewer put forward a constructive opinion. What we really want to express is (nano)crystal/crystallite-size. For possible ambiguity, we have revised the corresponding parts of the manuscript.
Point 6: Discussion of data shown in Fig4 is missing.
Response: Thank you for attention to this problem. Lack of explanation is really inappropriate, we conducted the analysis of figure 4 as follow:
In general, the catalytic performance corresponded to the specific surface area. As illustrated in Figure 4 a, in dry air, Cu2O synthesized by Na+NH3+P123+But. demonstrated the highest activity among these catalysts, with ozone conversion over 90% at 960000 cm3 g-1 h-1. The humidity resistance of the catalyst is also an important index to measure its practical application ability. As shown in Figure 4 b, performance of different samples at 90% relative humidity was basically consistent with the drying condition. Their activities were all decreased due to competitive adsorption of aqueous vapour, yet larger specific surface area can provide more active sites to reduce the impact. In addition, the durability test results of the catalyst Cu2O by using Na+NH3+P123+But. (in Figure 4 c) displayed that Cu2O catalyst still has high catalytic activity after 18 hours of continuous use.
Point 7: Major objection: According to experimental, it seems that 200 pm concentration of ozone was generated in oxygen, which was further mixed with air and concentration of ozone in flowing gas was thus decreased several times (if not, this should be clarified in the experimental section, as readers opinion is such). According to degradation process shown in Fig4, it seems that catalytic efficiency is more-less linearly decreased. If flow-rate of 480 dm3*g-1*h-1 was used, amount of ozone flown through the material is in milimolar scale per hour (maybe sub-milimolar, taken into account dilution of O2/O3 mixture with air, see above) per gram of the Cu2O material. So, the amount of ozone is comparable to molar amount of de-activated Cu2O material (order of hundredths-tenths of gram per hour). So, main question arises – behaves prepared material really as catalyst, or “just” as chemi-absorbent? Some calculation/discussion convincing readers that behavior of the material is really catalytic should be added. Probably, analysis of oxidation states of copper in fresh material and material after O3 exposure should be done.
Response: We totally understand the reviewer’s concern. In fact, the ozone concentration is really 200ppm. We calculate the ozone inlet concentration after mixing with air. In order to avoid misunderstanding, we decided to draw a schematic diagram of the equipment. The details are as follows:
For the doubt of whether the catalyst depends on its reducibility to treat ozone, we have made the following calculation and added it to the manuscript.
According to its durability test data: 480000 cm3 g-1 h-1 with 200 ppm ozone for 18h, the ozone treatment capacity for 1g Cu2O is:
Please see the attachment.
1g Cu2O = 1g/144g mol-1 = 0.007mol
In other words, the amount of ozone treated by Cu2O is more than 10 times that of its own substance, according to the following equation:
O3 + Cu2O = 2CuO + O2
Cu2O is obviously not enough to reduce so much ozone. So its catalytic effect is confirmed.
Author Response File: Author Response.pdf
Round 2
Reviewer 2 Report
All objections were solved, and I support publication of the manuscript in its present form.
