Noble Metal Promoted TiO₂ from Silver-Waste Valorisation: Synergism between Ag and Au

Round 1

Reviewer 1 Report

The work submitted by Stucchi et al. is a valuable study of the possible promotion of monometallic Ag species, obtained from a silver-rich wastewater and supported on TiO₂, with small amount 84 (0.5 wt.% - 2 wt.%) of Au NPs. It should be mentioned that the manuscript is well-written, the introduction clearly lays out reasons for studying of such catalyst. The surface morphology, structure and electronic properties of the prepared bimetallic catalyst are well characterized by a fruitful combination of physical methods. The work has been carefully done and the results sound. Basically, I have only minor comments listed below as they came upon reading, which have to be addressed before the manuscript can be accepted.

Comments to authors:

- Concerning the descriptions of XPS data – P.10, line 247 – There is the sentence «This shift of binding energies supports the hypothesis of a direct contact between gold and silver nanoparticles, in which an electronic and charge transfer took place». It is not clear which kind of shift authors means. If it is about the formation of metallic specie in case of Ag3d and changing of the Au0/Au+ ratios then from my opinion it is namely the formation of different species, thus it is not correct to discuss it in terms of BE shifts. Usually when the electronic and charge transfer discussed for the bimetallic systems it is namely the shift of BE, but not the appearance of new state in the spectra (see for example [1] - https://doi.org/10.1016/j.apsusc.2016.01.173 or [2] - https://doi.org/10.1021/jp053515r)

 

Author Response

Answers to reviewer 1

Reviewer 1:

The work submitted by Stucchi et al. is a valuable study of the possible promotion of monometallic Ag species, obtained from a silver-rich wastewater and supported on TiO2, with small amount (0.5 wt.% - 2 wt.%) of Au NPs. It should be mentioned that the manuscript is well-written, the introduction clearly lays out reasons for studying of such catalyst. The surface morphology, structure and electronic properties of the prepared bimetallic catalyst are well characterized by a fruitful combination of physical methods. The work has been carefully done and the results sound. Basically, I have only minor comments listed below as they came upon reading, which have to be addressed before the manuscript can be accepted.

 

Comments to authors:

- Concerning the descriptions of XPS data – P.10, line 247 – There is the sentence «This shift of binding energies supports the hypothesis of a direct contact between gold and silver nanoparticles, in which an electronic and charge transfer took place». It is not clear which kind of shift authors means. If it is about the formation of metallic specie in case of Ag3d and changing of the Au0/Au+ ratios then from my opinion it is namely the formation of different species, thus it is not correct to discuss it in terms of BE shifts. Usually when the electronic and charge transfer discussed for the bimetallic systems it is namely the shift of BE, but not the appearance of new state in the spectra (see for example [1] - https://doi.org/10.1016/j.apsusc.2016.01.173 or [2] - https://doi.org/10.1021/jp053515r)

We thank the Reviewer for the suggestion. We agree that the sentence was not very clear using the term “shift” in that context. What we wanted to underline was that the different composition of the metal species in terms of oxidation state in the bimetallic sample could be due to charge transfer between the species.

Therefore, the previous sentence:

“This shift of binding energies supports the hypothesis of a direct contact between gold and silver nanoparticles, in which an electronic and charge transfer took place”

has been replaced by

“The appearance of different metal species in terms of oxidation state supports the hypothesis of a charge transfer between Ag and Au, or even of an interaction between metals and the support [28]”, citing the paper (Applied Surface Science 367 (2016) 214–221) suggested by the reviewer.

(The added reference is n. 28 and all the changes are highlighted in yellow in the main text).

We also corrected the number of the table (as table 3, page 9 line 237, highlighted in yellow).

Please see also the attachement.

Best regards,

Marta Stucchi

Author Response File: Author Response.pdf

Reviewer 2 Report

Manuscript Number: catalysts-1581704

Title: Noble metal promoted TiO2 from silver-waste valorisation: synergism between Ag and Au.

Recovery of noble metals through metal-enriched wastewaters is a very interesting work. Here the authors propose an innovative approach for an efficient recovery of Ag nanoparticles through Ag and Cu metal-enriched wastewaters. Furthermore, they fabricate the 0.5%Au/3%Ag-TiO₂ and found that ethanol was completely converted  after 1 h of UV irradiation, acetaldehyde was formed as the main oxidation product and fully degraded in less than 180 min. However, there are some issues should be explained before the paper can be acceptable.

• Is this method also suitable for otherwastewater? Please explain the role of the ions in the wastewater, and why only Ag was deposited on the TiO₂?
• The conversion of ethanolis high, and do you have checked the published papers with similar catalysts? Please make a comparison other published work and find an reference for your samples.
• Figure 9 b,c) HAADF and elemental mapping of individual metal nanoparticles. The Ag nano-particle is very large according to the pictures while the Au is very small. And the a, b, c pictures are not consistent with each other.

The aforementioned issues needed to be improved, and I would recommend it publish after a major revision.

Author Response

Answers to reviewer 2

Reviewer 2:

Recovery of noble metals through metal-enriched wastewaters is a very interesting work. Here the authors propose an innovative approach for an efficient recovery of Ag nanoparticles through Ag and Cu metal-enriched wastewaters. Furthermore, they fabricate the 0.5%Au/3%Ag-TiO₂ and found that ethanol was completely converted after 1 h of UV irradiation, acetaldehyde was formed as the main oxidation product and fully degraded in less than 180 min. However, there are some issues should be explained before the paper can be acceptable.

• Is this method also suitable for otherwastewater? Please explain the role of the ions in the wastewater, and why only Ag was deposited on the TiO₂?

Considering the very good results obtained using this silver enriched wastewater, we can suppose that the methodology may be suitable also in the case of other metal species, such as gold.

However, it should be noted that concentration of metal ions in the precursor solution is crucial to have the formation of the metal species of interest.

To be note that, as reported in the materials and methods section (see page 14, table 5), the amount (mg/L) of silver cations (Ag⁺) in the wastewater solution is 5 orders of magnitude higher than other metal ions, which can be considered as traces and negligible. Having such enrichment of Ag ions, we were able to calculate the proper reduction potential through a preliminary cyclic voltammetry, and this allows the reduction of Ag⁺ to Ag⁰ by chronoamperometry.

A commenting sentence was added to the Conclusions: “The present procedure for the electrochemical deposition of noble metal NPs from wastewaters by precious metal industries could be extended to other species, such as gold, by tuning the chronoamperometry conditions using wastewaters with a suitable enrichment of the chosen metal ion.”

 

• The conversion of ethanol is high, and do you have checked the published papers with similar catalysts? Please make a comparison other published work and find an reference for your samples.

As reported in the manuscript, the bare Kronos 1077 has been already used for ethanol degradation under UV (see ref. 23 – page 3 line 100), showing a slower acetaldehyde removal. It should be noted that, in addition to the complete degradation of the aldehyde, only the bimetallic sample formed ethylene as product.

In ref. 23 it is also reported the activity of P25, which is a nanometric TiO2 widely used in photocatalysis; P25 is active toward ethanol degradation under UV irradiation and it is able to degrade also acetaldehyde. However, in this case the different specific surface area and morphology does not allow a consistent comparison with our samples. The choice of Kronos 1077 was guided also by considerations about its cost, considering that the price of P25 is 170$/100g while Kronos 1077 costs 150$/tonn.

Considering the recommendation by the Reviewer, we stressed this point adding the sentence:

“This low cost, pigmentary TiO₂ (Kronos 1077) was active under UV light. However, while it showed ethanol degradation and acetaldehyde formation, the degradation of latter required a longer irradiation compared to the metal-modified sample here reported”,

Highlighted in yellow in the text.

• Figure 9 b,c) HAADF and elemental mapping of individual metal nanoparticles. The Ag nano-particle is very large according to the pictures while the Au is very small. And the a, b, c pictures are not consistent with each other.

The fig. 9a reports the HAADF and EDS elemental maps of the 0.5%Au/3%Ag-TiO2 sample, where it is possible to see Au and Ag dispersed on the TiO2 surface; as already discussed describing the TEM images, Fig 9 b and c present the elemental maps of two large encircled metal particles visible on the HAADF image below.

(please see the attachement)

As discussed for the Ag/TiO2 sample, the deposition procedure followed by a high temperature calcination step resulted in a broad distribution of particle size (see also fig. 8). Considering that Au nanoparticles have a controlled dimension (2-3 nm) and they are added in a second step, some of the Au NPs grew on the surface of the biggest Ag particles as shown in the fig. 9 c. We added a sentence to clarify this issue:

“This picture is in agreement with the polydispersity of Ag particles deposited in the monometallic sample (Fig. 8).”

The aforementioned issues needed to be improved, and I would recommend it publish after a major revision.

We thank the reviewer for his comments, and we hope that our explanation is adequate to clarify the raised issues.

(please see also the attachement)

 

best regards,

Marta Stucchi

Author Response File: Author Response.pdf

Reviewer 3 Report

The authors presents noble metal promoted TiO2 from silver-waste valorisation: synergism between Ag and Au. An innovative approach for an efficient recovery of noble metals contained in these metal-enriched wastewaters as precursors for the synthesis of noble metal nanoparticles (NPs) and supported metal catalysts has been proposed. My comments are the following:

 

1. In section 2.1, the EtOH degradation and CH3CHO production has been presented. However, the figures of carbon balance and CO2 selectivity (if CO2 was produced) for the using the four catalysts are required.
2. In Fig.1, the sampling time (30min) for photo catalyst test is little bit long. It is suggested to reduce the sampling time, especially at the beginning of the reaction.
3. The scale of Fig.1 is not clear. The y-axe is only for EtOH or shared by EtOH and CH3CHO. Moreover, according to Fig.1, it seems the production of CH3CH with Ag-TiO2 is more important than others. Could the authors present their calculation?
4. In fig.9, please add the Ti elemental map.
5. In Fig.8, the particle size distribution seems large. In these articles (10.1016/j.cej.2018.04.106 and /10.1021/jp303356y ) a narrow size distribution and homogenous nanoparticle has been obtained by the reduction of ag/au ions at UV irradiation. Could these methods be applied to achieve better photocatalytic activity?

Author Response

Answers to reviewer 3

Reviewer 3:

The authors present noble metal promoted TiO2 from silver-waste valorisation: synergism between Ag and Au. An innovative approach for an efficient recovery of noble metals contained in these metal-enriched wastewaters as precursors for the synthesis of noble metal nanoparticles (NPs) and supported metal catalysts has been proposed. My comments are the following:

1. In section 2.1, the EtOH degradation and CH3CHO production has been presented. However, the figures of carbon balance and CO2 selectivity (if CO2 was produced) for the using the four catalysts are required.

We thank the reviewer for this comment. We did not specify that acetaldehyde converted to CO2. Indeed, the carbon balance is largely negative on the product side in case of all catalysts except Ag/TiO2 as estimated from the conversion curves and this indicates the formation of gaseous products not detected by GC with FID, likely CO₂.

The sentence “No other intermediates were detected: on this grounds and based on previous literature reports, acetaldehyde can be assumed to be further converted to CO₂, as reported in fig. 2.” was added in Section 2.1, highlighted in yellow.

1. In Fig.1, the sampling time (30min) for photo catalyst test is little bit long. It is suggested to reduce the sampling time, especially at the beginning of the reaction.

Unfortunately, this sampling time is required by the instrument we used. Closer analyses would decrease the reproducibility and increase the error. We appreciate this suggestion, and we take it into account for a possible improvement of our instrumentation.

1. The scale of Fig.1 is not clear. The y-axe is only for EtOH or shared by EtOH and CH3CHO. Moreover, according to Fig.1, it seems the production of CH3CH with Ag-TiO2 is more important than others. Could the authors present their calculation?

We apologize for the scale reported in the y-axes of fig. 1. EtOH(ppm) was replaced by “ppm”, as it indicates the concentration of each species. The formation of CH3CHO on the monometallic Ag-TiO2 is higher than in other cases because that sample is less efficient in converting acetaldehyde, leading to its accumulation in higher concentrations.

1. In fig.9, please add the Ti elemental map.

As requested, we replaced Fig. 9c with this new images, including Ti elemental mapping.

(please see the attachement)

1. In Fig.8, the particle size distribution seems large. In these articles (10.1016/j.cej.2018.04.106 and /10.1021/jp303356y) a narrow size distribution and homogenous nanoparticle has been obtained by the reduction of ag/au ions at UV irradiation. Could these methods be applied to achieve better photocatalytic activity?

The Reviewer is correct in pointing out a large particle size distribution of Ag NPs. In the present study, this seeming drawback can be advantageous to create junctions between Ag and Au NPs, considering that Au nanoparticles can grow on Ag. However, a narrow size distribution could be obtained by means of other synthesis methodologies, such as those suggested by the Reviewer, and it would be interesting to investigate their behaviour in future studies.

We thus added the following sentence in the Conclusions, citing the papers suggested by the Reviewer:

“A possible enhancement of these materials could be obtained improving the metals particle size distribution, for example using other promising synthesis methodologies already reported [38].”

Please see also the attachement.

Best regards,

Marta Stucchi

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

accept

Reviewer 3 Report

All the comments have been carefully responded  by the authors and the manuscript  could be accepted