Cobalt-Based Fischer–Tropsch Synthesis: A Kinetic Evaluation of Metal–Support Interactions Using an Inverse Model System

Round 1

Reviewer 1 Report

The authors present a model system to evaluate the metal (cobalt)- support (alumina) interaction in a catalyst used in Fischer Tropsch synthesis, a process of paramount importance in chemistry. The paper is clear and I detected no particular issues. Minor changes should be addressed by the authors 

1) acronyms should be expanded to facilitate reading

2) equations and chemical reactions should be numbered and cited in the text

3) some references are missing, please check that the reference manager is working properly

4)page 9 lines 319-327 are redundant. No need of these statements most of which are also general.

 

Author Response

The authors present a model system to evaluate the metal (cobalt)- support (alumina) interaction in a catalyst used in Fischer Tropsch synthesis, a process of paramount importance in chemistry. The paper is clear and I detected no particular issues.

 

Minor changes should be addressed by the authors 

acronyms should be expanded to facilitate reading

the acronyms are defined at the first point in the text

 

equations and chemical reactions should be numbered and cited in the text

Equations and chemical reactions are numbered

 

some references are missing, please check that the reference manager is working properly

Missing were references to the Figures (Figure 4 and Figure 5). They have been inserted.

 

4)page 9 lines 319-327 are redundant. No need of these statements most of which are also general.

The first paragraph of the discussion section has been reformulated as:

Figure 6 shows schematically the formation of a reverse model system to investigate metal support interactions by impregnation of Co₃O₄ with a well-defined crystallite size with a solution of aluminum sec-butoxide in n-hexane [18] (see Figure 6) Upon drying and calcination, sub-nanometer-sized alumina islands on cobalt oxide are formed [18]. Lewis acid sites are being introduced in the catalyst as evidenced by pyridine uptake and pyridine TPD due to the presence of aluminum possibly at the interface between metallic cobalt and the alumina islands, which will affect the performance of cobalt in the Fischer-Tropsch synthesis.

Author Response File: Author Response.pdf

Reviewer 2 Report

In this work, the interaction between cobalt and alumina in the Fischer-Tropsch synthesis were investigated by using an inverse model systems prepared by depositing different amounts of alumina on cobalt oxide. The introduction of Al significantly affected the activity, selectivity, and stability. The H₂-chemisorption, CO-TPR and pyridine TPD were used to further studied the reaction kinetics. The aluminum was found to result in the formation of strong acid sites and enhanced CO-dissociation. This is an interesting and well written work.

J. Am. Chem. Soc. 2009, 131, 20, 7197-7203 and J. Catal. 2009, 266, 129-144 reported how cobalt particle size affect on TOF in Fischer–Tropsch synthesis. Their reported results are different with the result in this work. Is this due to the interaction between cobalt and alumina in this work?

Author Response

In this work, the interaction between cobalt and alumina in the Fischer-Tropsch synthesis were investigated by using an inverse model systems prepared by depositing different amounts of alumina on cobalt oxide. The introduction of Al significantly affected the activity, selectivity, and stability. The H₂-chemisorption, CO-TPR and pyridine TPD were used to further studied the reaction kinetics. The aluminum was found to result in the formation of strong acid sites and enhanced CO-dissociation. This is an interesting and well written work.

Am. Chem. Soc. 2009, 131, 20, 7197-7203 and J. Catal. 2009, 266, 129-144 reported how cobalt particle size affect on TOF in Fischer–Tropsch synthesis. Their reported results are different with the result in this work. Is this due to the interaction between cobalt and alumina in this work?

 

We used much larger cobalt crystallites to ensure that particle size will not be a contributing factor to the difference in the performance of our model systems. Hence, we ascribe the obtained changes in the activity and selectivity to the presence of alumina islands on/near the catalytically active metal.

Author Response File: Author Response.pdf

Reviewer 3 Report

In this work, Cobalt oxide/ alumina composites have been synthesized using a reverse modelling approach, whereby the support materials (alumina) are deposited on cobalt oxide (Co being the active site for the Fischer Tropsch reaction). The authors have studied the effect of alumina content in the composites and how it affects their particle sintering, selectivity toward methane and higher hydrocarbon formation. I recommend publication after the authors address the following:

The loading of alumina jumped from 0.5% to 2.5%. Is there any explanation on why higher alumina loading was not tested? It is shown in Table 1 how the presence of Al prevents particle sintering, but the size of only Co is reported, not the entire Co/alumina composite. To have a clearer idea, the particle size of the entire composite should be reported.  If more alumina is present (by weight percentage), the wt % of Co reduces automatically. This should also lead to lower extent of Co sintering. The authors should address this in the manuscript.  Have there been any control experiment performed where Cobalt is deposited on alumina? Such a material would be ideal to establish the importance and effect of the inverse model system. In line 116, pg 3/14, the authors explain the decrease of TOF in the catalysts with higher Al loading on the basis of increased metal loading. This is unclear, since the total metal content (Al+Co) should be same irrespective of the relative weight percentage of Al and Co. The authors should explain it more clearly. Could this be a result of lower Co loading with increased Al loading, since Co is the actual active site? The references in certain places appear just as ERROR- Reference not found. Please fix this (see line 290. pg 8, line 269, pg 7 etc). Minor typos, such as "ac" instead of "act" in line 345, pg 10.

Author Response

In this work, Cobalt oxide/ alumina composites have been synthesized using a reverse modelling approach, whereby the support materials (alumina) are deposited on cobalt oxide (Co being the active site for the Fischer Tropsch reaction). The authors have studied the effect of alumina content in the composites and how it affects their particle sintering, selectivity toward methane and higher hydrocarbon formation. I recommend publication after the authors address the following:

 

The loading of alumina jumped from 0.5% to 2.5%. Is there any explanation on why higher alumina loading was not tested?

We originally did not anticipate the formation of alumina islands (nor the strong sintering upon reduction). Our calculations showed that a 2.5 wt.-% of aluminum loading on cobalt particles with a dispersion of 10% should give a surface ratio of Co/Al = 1: 1.3. Hence, we did not venture into higher aluminum loadings. The formation of alumina islands (although their 3D structure is still unknown) would allow for higher aluminum loadings, which we will try in the future.

It is shown in Table 1 how the presence of Al prevents particle sintering, but the size of only Co is reported, not the entire Co/alumina composite. To have a clearer idea, the particle size of the entire composite should be reported. 

We imaged the particles as well using TEM, but it is difficult to obtain statistically relevant data from these images seeing the particles are large (in particular for the sample Al0).

 

If more alumina is present (by weight percentage), the wt % of Co reduces automatically. This should also lead to lower extent of Co sintering. The authors should address this in the manuscript. 

We are not sure what the reviewer implies here. The alumina loading is very low (less than 2.5 wt.-%) and the catalyst is in essence a bulk cobalt catalyst. We would rather follow the line of thinking (as stated in the manuscript) that alumina is preventing direct contact between cobalt particles (acting as a space) and thus reducing the extent of sintering via coalescence.

Have there been any control experiment performed where Cobalt is deposited on alumina? Such a material would be ideal to establish the importance and effect of the inverse model system.

We have in the past performed a number of experiments with Co/Al2O3. We have tested impregnated Co/Al₂O₃ (now included as reference [22]) and model systems of Co/Al₂O₃ with minimal metal support interactions [6]. In order for the reader to compare the performance with the more common catalyst systems we have include:

The obtained turnover frequencies for the sample without aluminum (Al0) resembles the turnover frequency obtained over model catalyst supported on alumina with minimal metal support interactions [6], whereas the obtained turnover frequency for the sample Al2.5 resembles the turnover frequency obtained with a normal impregnated cobalt on alumina catalyst [22].

 

In line 116, pg 3/14, the authors explain the decrease of TOF in the catalysts with higher Al loading on the basis of increased metal loading. This is unclear, since the total metal content (Al+Co) should be same irrespective of the relative weight percentage of Al and Co. The authors should explain it more clearly. Could this be a result of lower Co loading with increased Al loading, since Co is the actual active site?

We apologize for the lack of clarity here. We meant to state that the turnover frequency decreases, whilst at the same time the rate per unit mass increases upon increasing the aluminum loading, because of the change in the metal dispersion. Reformulated:

The decrease in the turnover frequency with increasing aluminum content despite the increase in the rate of reaction per unit mass is a consequence of the strong effect of the increasing aluminum content on the cobalt dispersion (see Table 1)

 

 

The references in certain places appear just as ERROR- Reference not found. Please fix this (see line 290. pg 8, line 269, pg 7 etc). Minor typos, such as "ac" instead of "act" in line 345, pg 10.

All corrected

Author Response File: Author Response.pdf

Round 2

Reviewer 3 Report

The authors have addressed the comments that were given in their previous version of the manuscript. The revised manuscript is certainly a significant improvement, and the experimental facts and results are  more clearly explained. I would suggest publication in the present form.