Transient Kinetic Experiments within the High Conversion Domain: The Case of Ammonia Decomposition

Round 1

Reviewer 1 Report

The report is uploaded as a pdf file.

Comments for author File: Comments.pdf

Author Response

Response to Reviewer 1

We are grateful to Reviewer 1 for their time in assessing our submission.  Several points are raised which will help us to clarify general misconceptions about the TAP technique.

Reviewer Comment: “The authors have studied the kinetics ammonia decomposition on Fe, CoFe and Co by using the so-called pure-diffusion Temporal Analysis of Products (TAP) reactor. Such reactors are rarely employed, and as far as I understand one of the authors' goals is to attract attention to this area.”

Response: We agree with the reviewer that the TAP technique may be qualified as ‘rare’ or ‘niche’, there are only 20 such devices in the world.  However, it is a goal of all authors to attract attention to this area by submission of this manuscript; in addition to adding to the general body of scientific knowledge of course.  Though rare, there is in fact a large body of work from multiple independent groups using the technique for numerous applications in catalysis; a recent review article that collects this work should be considered:

Morgan, K., et al., Forty years of temporal analysis of products. Catalysis Science & Technology, 2017. 7(12): p. 2416-2439,

and the following which is independent of the authors:

Perez-Ramirez, J. and E.V. Kondratenko, Evolution, achievements, and perspectives of the TAP technique. Catal. Today, 2007. 121(3-4): p. 160-169.

In addition, the TAP technique is included in texts on advanced kinetics, catalyst characterization and catalysis:

Murzin, D. and T. Salmi, Catalytic kinetics. 2005: Elsevier Amsterdam.

Che, M. and J.C. Védrine, Characterization of solid materials and heterogeneous catalysts: From structure to surface reactivity. 2012: John Wiley & Sons.

Thomas, J.M. and W.J. Thomas, Principles and practice of heterogeneous catalysis. 2014: John Wiley & Sons.

Nonetheless the TAP technique is still not widely understood and it is the hope of the authors that this manuscript will increase comprehension while attracting attention.

Reviewer Comment: “The results presented might, however, discredit the use of such reactors.” … “The interpretation of the results implies the applicability of the simplest first-order reaction kinetics.  For example, the Arrhenius plot [Fig. 5(A)] implies that the reaction rate constant is expressed via conversion as

k = X/(1 - X).

                                                                                                                                                (1)

This expression corresponds to the first-order reaction kinetics in CSTR with

X = kt/(1+kt),                                                                                                                       (2)

where t is the residence time. Earlier (Ref. [29]; Eq. (12)), one of the authors postulated that the latter expression is applicable to the first-order reaction kinetics in a TAP reactor, but it was not proved because the corresponding reaction-diffusion equation was not exactly solved.”

Response:  The expression does indeed correspond to the case of the first-order CSTR.  The corresponding first order reaction-diffusion equation has been solved explicitly in the other work we referred to concerning TAP and uniformity, reference 30:

Phanawadee, P.; Shekhtman, S.O.; Jarungmanorom, C.; Yablonsky, G.S.; Gleaves, J.T. Uniformity in a thin-zone multi-pulse TAP experiment: numerical analysis. Chem. Eng. Sci. 2003, 58, 2215-2227, doi:10.1016/S0009-2509(03)00080-0.

The derivation can be found therein, and we have made explicit reference to the derivation in our revised manuscript so that the reader can be assured that k is proportional to X/(1-X).

Reviewer Comment: “For real reactions, the first-order reaction kinetics are usually not applicable.”

Response: Certainly in ‘real’ reactions first-order kinetics are not the rule.  The rate expressions encountered in industrial catalysis are often complex functions of multiple component concentration with nonlinear exponential dependences.  This arises in an advecting device where multiple reactants and products interact with variable surface coverages. 

The TAP technique is distinguished from the steady-state advecting device in that one reactant interacts with a well-defined surface coverage under conditions far from equilibrium such that backward reactions can be considered negligible (it is analogous to molecular beam scattering experiments).  TAP is rooted in the concept of insignificant perturbation and the simplicity of the measurement brings it closer to that of an elementary process.  Elementary gas/solid interactions are almost always first order with respect to the gas phase molecule.  The TAP approach is analogous to molecular beam scattering where gas/solid interactions are not influenced by gas/gas collisions.  This should clarify the conceptual basis for the use of a linear rate expression and we have added such explanation to the manuscript.

We assumed a first order reaction and the Arrhenius dependence in Figure 5 supports this hypothesis, even at high conversion.  Of course, one can only be certain of a negative result but the model assumptions and data work together nicely.

Moreover, it is quite common to use a linear approximation to estimate an apparent rate constant even in steady-state kinetics.  Generally, the whole kinetic dependence will be nonlinear but the apparent rates constant in the linear domain is always applicable, to illustrate:

  (figure in attached word document)

Reviewer Comment: “For the introduction and interpretation of the results, the authors use Eq. (1) which was also borrowed from Ref. [29]. The author's presentation implies that Eq. (1) is applicable at arbitrary conversion. Looking through Ref. [29], one can find, however, that the derivation of Eq. (1) is based on the assumption that the concentration in the center of the reaction zone is equal one half of the sum of the concentrations on the boundaries. This approximation is obviously not applicable at hight (sic.) conversions (to describe this limit, one should exactly integrate the reaction-diffusion equation, and as I have already noticed it was not done). Practically, this means that at hight (sic.) conversions the authors fail even at the level of the first-order reaction kinetics.”

Response: TAP is distinguished from a conventional reactor in that diffusion is the only mode of transport (there are no gas/gas collisions).  The differential catalyst zone combined with the diffusion transport acting as an efficient impeller is what creates uniformity.  The same is not possible using a thin-zone approach in an advecting device at high conversion.  Furthermore, the data in Figure S3 of the supplementary information demonstrates that over long pulse series the shape of the response does not change.  The kinetic response does not change hence the assumption of uniformity is valid even at high conversions.  Integration of equation (1) is not necessary and a linear approximation may be used.

Reviewer Comment: “The reaction mechanism is not discussed in detail”.

Response: The intent of the work is to demonstrate kinetic assessment at high conversion and the additional information that comes from observing the time dependence of the reaction rate.  Discussion of the reaction mechanism is however a topic of a detailed manuscript in preparation based on the same data. 

Reviewer Comment: “The presentation might be better. (i) The English is far from academic.”

Response: Alas, we humbly agree but no attempts have been made to improve the writing style.  This reminder invokes thoughts of the deleted stanzas from In Memory of W.B. Yeats by great English-American poet W.H. Auden:

Time that is intolerant

Of the brave and the innocent,

And indifferent in a week

To a beautiful physique,

Worships language and forgives

Everyone by whom it lives;

Pardons cowardice, conceit,

Lays its honours at their feet.

Reviewer Comment: “(ii) The figure captions are not informative.”

Response: We thank the reviewer for this observation and have amended the figure captions to be more descriptive.

Reviewer Comment: “There are misprints.  In Fig. 5(A and B), for example, ln((X/1-X)) should be read as ln((X/(1-X))”

Response: We thank the reviewer for catching this misprint but have amended the figure to read ln(X/(1-X) instead.

Author Response File: Author Response.pdf

Reviewer 2 Report

The article in its present form proves to be a good scientific research. Therefore, I recommend the publication.

Author Response

Response to Reviewer 2

Reviewer Comment: The article in its present form proves to be a good scientific research. Therefore, I recommend the publication.

Response: We are grateful to Reviewer 2 for their time in assessing our submission. 

Round 2

Reviewer 1 Report

The revision is adequate.