Differentiating the Reactivity of ZrO2-Bound Formates Formed on Cu/ZrO2 during CO2 Hydrogenation
Libo Niu
Round 1
Reviewer 1 Report
1. english should be improved
2. author used DRIFTS as main tool to clarify the pathway of CO2 hydrogenation over Cu-Zr catalyst, the actual catalytic performance was not included in the manuscript. Is it an efficient catalyst?
3. the morphology of catalyst should be showed in manuscript.
4. author claimed that tow kinds of zirconia-bound formated presented on Cu-Zr and significant reactivity existed between them, but I consider the effects on the actual reaction results are not clear only based on DRIFTS.
Author Response
1. english should be improved
Authors’ reply : the English grammar and style have been improved.
2. author used DRIFTS as main tool to clarify the pathway of CO2 hydrogenation over Cu-Zr catalyst, the actual catalytic performance was not included in the manuscript. Is it an efficient catalyst?
Authors’ reply : the reviewer is correct and we have now added the conversion and productivity of methanol in the following added sentence : « Methanol and CO were the only C-containing products observed at 220 °C and 3 bar when the 20% CO2 + 60% H2 feed was introduced. Steady-state was achieved within one hour on stream in terms of methanol and CO concentrations measured at the exit of the DRIFTS cell and in terms of DRIFTS signals of the various adsorbates observed. Differential conditions were obtained with a CO2 conversion of about 0.8 %. The selectivity to methanol was about 47 % and the productivity was 1.0 µmol(CH3OH) s–1 gCatalyst–1. These values are similar to those reported by Fischer and Bell21 at 250 °C and 6.5 bar for a Cu/ZrO2/SiO2, that were 1.3 µmol(CH3OH) s–1 gCatalyst–1 and a selectivity to methanol of 43 %. »
It can be concluded that our catalyst is as efficient as that reported by Fisher and Bell.
3. the morphology of catalyst should be showed in manuscript.
Authors’ reply : we have carried out TEM analyses (on a JEOL 2010) but the contrast between the zirconia support and the very small Cu nanoparticles was not sufficient to allow determining the shape of the Cu nanoaparticles.
3. author claimed that tow kinds of zirconia-bound formated presented on Cu-Zr and significant reactivity existed between them, but I consider the effects on the actual reaction results are not clear only based on DRIFTS
Authors’ reply : we are not sure to understand the point of the reviewer. Our transient DRIFTS data clealry show that two types of formates, whose exhibit spectra typical of those measured on zirconia, have two very different reactivity.
Reviewer 2 Report
Comment:
This paper reports on an operando spectroscopy study of CO2 hydrogenation on Cu-ZrO2 catalysts. The authors identify two types of formate intermediates on the ZrO2 surface during the hydrogenation reaction and measure their different reaction kinetics. This spectroscopic finding is very insightful and can be further applied to catalyst design for efficient CO2 hydrogenation to methanol. However, there are remaining questions on the catalyst characterization and the structural understanding of two types of surface formate intermediates. Also, more details are need for the data processing part in this manuscript. Thus, I think a minor revision is needed for this manuscript before it published on Catalysts at this stage.
Specific comments:
1. The authors use N2O frontal chromatography to measure the surface area of Cu on support and estimate a structure of Cu spherical particles with an average diameter of 4.9 nm. However, the real structure of Cu size and distribution on ZrO2 need more characterization to unravel, such as TEM or crystallite size analysis from XRD.
2. The authors decompose the total signal into a slow linear contribution and a faster decay. This calculation seems to be arbitrary, and more discussion or reference is needed to explain the data processing method used here.
3. The authors attribute the different kinetics of two types of surface formate intermediates to the hydrogen spillover from Cu. This phenomena was observed and well-studied in previous studies. Base on these, what is the distance limit to differentiate these two types of formate to generate such a kinetics difference?
Author Response
Many thanks for the supporting comments and useful questions. Here are our replies!
1. The authors use N2O frontal chromatography to measure the surface area of Cu on support and estimate a structure of Cu spherical particles with an average diameter of 4.9 nm. However, the real structure of Cu size and distribution on ZrO2 need more characterization to unravel, such as TEM or crystallite size analysis from XRD.
Authors’ reply : as indicated to reviewer one, we have carried out TEM analyses (on a JEOL 2010) but the contrast between the zirconia support and the very small Cu nanoparticles was not sufficient to allow determining the shape of the Cu nanoparticles. The particles are so small that the XRD analysis cannot be more informative, especially ex situ. Since the specific rate of methanol formation is similar to that reported by Fisher and Bell (see new added section), we feel that our catalyst is representative of typical Cu/ZrO2 materials.
2. The authors decompose the total signal into a slow linear contribution and a faster decay. This calculation seems to be arbitrary, and more discussion or reference is needed to explain the data processing method used here.
Authors’ reply : the decomposition could appear at first arbitrary, but in fact it is quite justifiable. The final part of the signal varies very little with time and a linear fit is thus justified. The remaining signal (total minus linear fit, in green in figure 4A) appears to be a single exponential decay, as the corresponding semi-logarithmic plot given in figure 4B is a near-perfect straight line. This indicates that the fast formate species exhibited a uniform reactivity, as is usually observed for most adsorbates. The latter observation justifies fully the signal decomposition that was used, to our opinion.
3. The authors attribute the different kinetics of two types of surface formate intermediates to the hydrogen spillover from Cu. This phenomena was observed and well-studied in previous studies. Base on these, what is the distance limit to differentiate these two types of formate to generate such a kinetics difference?
Authors’ reply : This is a very important and pertinent question, but unfortunately we do not have the answer yet, as we had indicated at the end of the discussion section. We have added the following comment as a basis for this yet unanswered question: “It is possible that the slow formates were located on domains or ZrO2 crystallites on which no Cu nanoparticles were present and thus almost no spillover H would be available. On the contrary, the fast formates could be located on ZrO2 crystals on which Cu nanoparticles would be present and spillover H would be readily available, enabling a uniform reactivity of formates on such domains. ”.
Round 2
Reviewer 1 Report
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The revised manuscript can be accepted.
