Analysis of Unregulated VOCs Downstream a Three-Way Catalyst in a Simulated Gasoline Engine Exhaust under Non-Optimum Conditions

Round 1

Reviewer 1 Report

Dear authors, 

thank you for submitting this work for consideration. I personally do not have many technical comments: I think that the experiments have been carried out in a good way and the results are well presented. 

My main problem with this manuscript is that is unclear what is the novelty of the work, TWC has been extensively studied in the literature in a range of configuration ranging from model ones (like in this case) to realistic ones. It is essential to clarify this and motivate in details why this manuscript is new compared to previous literature and worth of publication. This is very important.

The overall manuscript is ok, with some points that will benefit from additional clarification but it is not enough: the novelty, if any, must be clearly shown and discussed against the existing literature. 

In addition, it is not very clear what is the overall objective of this work: it is simply to analyse different compounds produced by a TWC in model conditions or the authors want to propose reaction mechanisms? this has to be clarified as well.

Also, please be mindful of the writing: there are few typos (NOx is written in different ways across the manuscript and at the beginning N2O is written with an extra space, this just in the first pages)

ABSTRACT 

NOx: this normally indicates nitrogen oxides and not nitric oxides

INTRODUCTION

Reactions reported are global ones and do not cover the complexity of the chemistry involved, this should be clarified

The term efficiency is used in an appropriate way, normally when talking about catalysts conversion, selectivity, yield or in general activity are used.

"Therefore, most of the pollution emitted by gasoline vehicles occurs during the cold-start period, before the catalyst is functional." While this is true, it is only partially true. Gasoline vehicles can emit significant amount of pollutants during dynamic driving on the highway for example, additional literature can be referenced here and will help justify the experiments done in the following.

"The control of rich and lean conditions along light-off experiments is very tricky to achieve with a real engine, justifying the implementation of a synthetic gas bench. " This statemnt is approximative. The authors need to discuss this choice in details, depending on what they want to achieve in this manuscript which, at the end of the introduction, is not very clear. Indeed, while using model rectors/mixtures/... has some advantages, it has also disadvantages. For example in dynamic accelerations the space velocity changes, influencing conversion. How the impact of this has been taken into account?

MATERIALS

"All the lines placed downstream of the CEM were heated to avoid condensation problems." At which temperature?

"This small volume allows to reach a space velocity of around 50 000 h– 1 for an overall flow of 20 NL min– 1, representative of real gasoline exhausts." Can you provide references? A quick look in the literature (works at ORNL, Houston) resulted in a bit higher SV (75k - 100k)

Conversion rate X is this expression correct? I do not think this is a rate

The time resolution of the instruments used is very low. The authors maybe can dedicate additional effort in explaining how they have handled this in dynamic experiments with temperature ramps (and why they have used these instead of steady state experiments for example). I understand that there is an interpolation process to align the signals but how the authors excluded the possibility of side reactions between normally very reactive HC fragments during the long analysis? 

RESULTS

3.2

Please note that N2O is not only a GHG but also a pollutant (e.g. https://www.science.org/doi/full/10.1126/science.1176985?casa_token=yZ1zQI3-2ekAAAAA%3AxymJRNDycVr2fZFdyjfIjInoENvoUMOdsG0Z-cfgbLg7Bd6hA2KRS3XflBrCveDK1KErIFVSJhLc8Q)

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

The manuscript by Louarn et al. describes the experimental investigation of the chemical treatment of model gasoline exhaust mixtures by a commercial three-way catalyst. Using a synthetic gas bench, the authors report the conversion of the exhaust gaseous species into CO, NOx, hydrocarbons, and VOCs. The experimental study offers a controlled way to characterize the catalyst efficiency under several fuel conditions, which is a good alternative to direct engine sampling. The experiments are appropriately designed with series of analytical tools. The data are worth publishing because they reveal important limitations in the efficiency of the commercial catalyst as well as emission of unregulated and toxic compounds. However, the manuscript remains very limited in its scope and scientific merit. Below are the two main concerns I would like to see addressed before publication.

-The study is limited to a single commercial catalyst. To broaden the scope of the paper it would have been interesting to study the VOC formation as a function of catalyst load and catalyst composition. This is particularly important when discussing the possible reaction mechanisms for the VOC formation. Different catalyst composition could provide better information about the chemistry. In addition, if the main contribution of the manuscript is to reveal the formation of unregulated compounds by commercial catalyst, this should not be limited to only one catalyst. One could argue that the VOC are only relevant to this specific catalyst. These findings are likely to be relevant for a wide range of engine exhaust catalysts.

-The manuscript does not report any quantitative information about the VOC formation. All the data for the VOC formation are in arbitrary units or relative signal. Quantitative information is required and needs to be compared to toxicity thresholds in order to really understand the impact of the present findings on the catalytic treatment.

If the authors are unable to address these limitations by running new experiments, I would suggest adding a more exhaustive discussion of the possible impact of these findings. As presented here they seem relatively limited.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Thank you for the review