The Role of NiO in Reactive Adsorption Desulfurization Over NiO/ZnO-Al₂O₃-SiO₂ Adsorbent

Round 1

Reviewer 1 Report

The manuscript presents the thiophene desulfurization performance of NiO/ZnO-Al2O3-SiO2 materials treated under different conditions such as reduced, unreduced and N2 treatment. They showed that NiO is also active in the reaction but hydrogen improves the activity. There some comments that need to be addressed in the text before publications.  

1- Please add the index of the identified peaks to the XRD pattern. 

2- the XPS spectra for Zn should be consistent with other. Please change the BE axis from right to left. 

3- Please cite appropriate papers for XPS results and discussion, especially when attributing different binding energies. 

4- I recommend authors add at least one more temperature, i.e. 350 C, to Fig. &, instead of 420 C which is not informative. This is interesting and could provide information at what temperature sulfur removal reaches a constant rate and why. 
5- How do the authors explain the initial reduction in sulfur removal (blue line) in Fig. 7?

6- change supporter to the common term of "support" within the text. 
7- support your conclusion quantitatively with the values of sulfur removal to give a better and clear sense. 

8- How is the performance of these materials for DBT desulfurization after different treatment? 

9- Explain the source of materials and the sulfur analysis instrument. 

Author Response

Dear reviewer: 1- Please add the index of the identified peaks to the XRD pattern. The index of the identified peaks to the XRD pattern has been added in Figure 2. 2- the XPS spectra for Zn should be consistent with other. Please change the BE axis from right to left. The BE axis of the XPS spectra for Zn has been revised to be consistent with other. 3- Please cite appropriate papers for XPS results and discussion, especially when attributing different binding energies. The following publications were cited for XPS results analysis and discussion. [27]Greczynski, G.; Hultman, L. Reliable determination of chemical state in x-ray photoelectron spectroscopy based on sample-work-function referencing to adventitious carbon: Resolving the myth of apparent constant binding energy of the C 1s peak. Appl. Surf. Sci. 2018, 451, 99-103, 10.1016/j.apsusc.2018.04.226. [28]Herron, S.M.; Lawal, Q.O.; Bent, S.F. Polysulfide ligand exchange on zinc sulfide nanocrystal surfaces for improved film formation. Appl. Surf. Sci. 2015, 359, 106-113, 10.1016/j.apsusc.2015.10.059. [29]Smart, R.S.C.; Skinner, W.M.; Gerson, A.R. Xps of sulphide mineral surfaces: metal-deficient, polysulphides, defects and elemental sulphur. Surf. Interface Anal. 1999, 28, 101-105, 10.1002/(sici)1096-9918(199908)28:13.0.co;2-0. 4- I recommend authors add at least one more temperature, i.e. 350 C, to Fig. &, instead of 420 C which is not informative. This is interesting and could provide information at what temperature sulfur removal reaches a constant rate and why. The primary purpose of the experiment at 300 C is to test the adsorbent activity at lower reaction temperature. If the RADS process could operate under 300°C, it means NiO could act as the main component to react with thiophene. The data of the experiment at 350°C has been added. The section 2.6. RADS performance at different temperatures has been revised as follow: Figure 7 is a comparison of the desulfurization effects of unreduced adsorbents at 420 °C, 400 °C, 350 °C and 300 °C. The former two reactions have little difference on sulfur removal rate from the beginning to the end, mainly because a 20 °C temperature difference does not impact RADS greatly. The reaction at 350 °C performs a reduction of sulfur removal at the beginning, and then keep a high sulfur removal rate as the former two reactions. It suggests that the reaction temperature may affect the activation of adsorbent at the beginning of RADS. For the RADS process which happened at 300 °C, the curve decreases first and then increase. This special trend does not accord with the usual decreasing activity found in most of RADS processes. Especially, at 300 °C, the strong rising trend of desulfurization capacity is observed after the volume of feed reaching 22ml-G/g-A. This phenomenon inspires us to design three experiments to pretreat the adsorbents used in the experiments at 300 °C. 5- How do the authors explain the initial reduction in sulfur removal (blue line) in Fig. 7? From the experiments under different temperature, it can be found that NiO played an important role of desulfurization. However, the desulfurization mechanism of NiO need to be further researched. The reason of the initial reduction of sulfur removal under 300°C temperature may be that the sulfur compounds are physically adsorbed on the adsorbent at the beginning, which presents a high sulfur removal. After that, the physical adsorption sites are less and the adsorbent is not activated, so the sulfur removal reduces quickly. Thus, we designed some experiments to research the effect of different pretreatments on the activation of adsorbents. After some different pretreatments, even under 300°C, the adsorbents perform a high desulfurization ability, which means the activation of adsorbent is beneficial to reducing the reaction temperature and energy consumption of RADS. That the adsorbent is not activated is a main reason of the initial reduction in sulfur removal. 6- change supporter to the common term of "support" within the text. The “supporter” has all been revised as “support” in the text. 7- support your conclusion quantitatively with the values of sulfur removal to give a better and clear sense. The conclusion has been revised with the values of sulfur removal. 8- How is the performance of these materials for DBT desulfurization after different treatment? Compared with thiophene, DBT is more difficult to be removed in RADS process (Applied Catalysis B: Environmental 106 (2011) 26-8). In this paper, we focus on the effect of NiO component and activation of adsorbent for the removal of thiophene. As DBT is not a main sulfur compounds in FCC gasoline, the removal of DBT is not discussed. In the future work, we will research the desulfurization performance of FCC diesel, and DBT desulfurization after different treatment. 9- Explain the source of materials and the sulfur analysis instrument. The materials are all of analytical grade and bought from SinoPharm Group. The sulfur analysis instrument is Antek Sulfur detector. This instrument detects the concentration of sulfur by ultraviolet fluorescence method. The sample is oxidized at high temperature and the sulfide in the sample is converted to SO2. Before entering the reaction chamber, the gas is dried by a membrane dryer. SO2 is irradiated by ultraviolet light to produce a specific wavelength spectrum, which is detected and received by photomultiplier tube. The emission fluorescence intensity is directly proportional to the sulfur content in the original sample. After microcurrent amplification and computer data processing, the emission fluorescence intensity can be converted into an electrical signal proportional to the light intensity. The sulfur content of the sample can be calculated by measuring its size.

Reviewer 2 Report

Dear Authors,

the paper deals with an interesting topic. However it need a revision to highlight its novelty with respect to your previous work (ref 24). 

I suggest you to rewrite the abstract making some changing with respect to your previous work.

In addition, you should also revise the English language. As following I want to catch your attention on some sentences:

Lines 17-20

Lines 89-90

Lines 154-158

Lines 166-167

Lines 203-205

Lines 230-238

Try to revise these parts of the paper and make them more understandable for the readers.

Please, revise the chemical formula and use the subscript properly.

I am really sorry, but I think you need to revise the paper following the above suggestion.

Best regards

Author Response

Dear reviewer: the paper deals with an interesting topic. However it need a revision to highlight its novelty with respect to your previous work (ref 24). In our previous work, we researched the effect of hydrogen pretreatment on RADS process and found that for NiO/ZnO-Al2O3-SiO2 adsorbent, under 420°C reaction temperature, NiO and Ni0 could be assumed as the main active components. In this work, under 300°C reaction temperature, NiO may be the main active component. To avoid the misunderstanding, the highlights has been revised as follow: 1. The results of various characterizations indicate a probable strong metal-support interaction between NiO and the support. 2. Under low reaction temperature (300°C ), NiO is assumed as an active component. 3. NiSx formed during the process of RADS may promote the desulfurization. 4. The result of activation performances of different pretreatment on RADS process shows that with the participation of hydrogen and hexane, the RADS process can be well performed at a lower temperature than which used to be. I suggest you to rewrite the abstract making some changing with respect to your previous work. In this paper, we focus on the role of NiO component and activation of adsorbent. Metal Ni is an intermediate reduction product of NiSx, which is not conflicted with the mechanism of NiO and metallic Ni mentioned in our previous work. Under lower reaction temperature, NiO is difficult to be reduced. Also, the desulfurization mechanism of NiO is quite different from that of Ni. As the suggestions, the abstract has been revised as follow: The reactive adsorption desulfurization (RADS) of a model gasoline n-hexane containing thiophene was carried out with a NiO/ZnO-Al2O3-SiO2 adsorbent in N2 and H2, respectively. A declining RADS trend has been observed in N2, without the presence of H2, indicating that NiO is sulfurized and exhibits activity for RADS. TPR and XPS results presented NiO in the adsorbent is hard to be reduced because of the powerful interaction between NiO and the support. The sulfurization of NiO into NiSx is a primary condition for RADS process the same as the presulfurization of hydrotreating catalyst, while metallic Ni is an intermediate reduction product of NiSx. Results of a low RADS temperature at 300 °C, much lower than the reduction temperature of NiO, suggest that NiO plays an important role. Based on assumption of NiO as the main active component, the RADS could reduce the reaction temperature and energy consumption significantly. The participation of hydrogen and n-hexane in pretreatment conducted at 420 °C contributes to the activation of adsorbent. Also, these methods of pretreatment improved the desulfurization performance under the reaction temperature of 300 °C. In addition, you should also revise the English language. As following I want to catch your attention on some sentences: Lines 17-20 Lines 17-20 are revised as ‘The participation of hydrogen and n-hexane in pretreatment conducted at 420 °C contributes to the activation of adsorbent. Also, these methods of pretreatment improved the desulfurization performance under the reaction temperature of 300 °C.’ Lines 89-90 Lines 89-90 are revised as ‘ Figure 1 shows that the sulfur removal rate still keeps at 25% in nitrogen atmosphere after treating model gasoline about 120 ml-G/g-A.’ Lines 154-158 Lines 154-158 are revised as ‘indicating that there exists a strong interaction between NiO and the support. This probable interaction may make NiO much harder to be reduced. It can be inferred from the profiles that NiO contained in the fresh adsorbent seems unlikely to be reduced to Ni0 at 420 °C. Considering the excellent performance of desulfurization in our study, NiO may play the role of an active component in place of nickel.’ Lines 166-167 Lines 166-167 are revised as ‘It indicates that S elements are adsorbed well and carbon deposits are formed on the surface of the adsorbents. For the adsorbents which have been used in nitrogen, the weak peak of sulfur shows the poor desulfurization performance in N2.’ Lines 203-205 Lines 203-205 are revised as ‘However, the curve of the RADS process which happened at 300 °C decreased first and then increased. This special trend did not accord with the usual decreasing activity found in most of RADS processes.’ Lines 230-238 Lines 230-238 are revised as ‘However, during the process before the feed reaching 10ml-G/g-A, the sulfur removal rate indicated by the curve of case A is slightly lower compared with which of case D. Interestingly, with the participation of n-hexane during the process of pretreatment, the drop of sulfur removal at the beginning of RADS that occurs in case A disappears in case B and C. The function of n-hexane is considered while discussing the various results of different types of pretreatments in case A, B and C at the beginning of the process. It is assumed that n-hexane reacts with the adsorbent, during which some carbon-metal bond is formed. This type of bond may promote the electric charge transfer on the surface of the adsorbent. As a result, the desulfurization capacity of the adsorbent is enhanced regardless of the much lower reaction temperature.’ Try to revise these parts of the paper and make them more understandable for the readers. Please, revise the chemical formula and use the subscript properly. The chemical formula has been revised carefully. Best regards