Peer review of: Metal 3D printing technology for functional integration of catalytic system (2020)

Reviewers' comments: 

Reviewer #1 (Remarks to the Author): 

The authors describe an interesting approach to fabricate self-catalytic reactors (SCR) made by selective 
laser sintering of catalytically active alloy powders. As demonstrated by experiments the 3D printing 
approach provides control over Fischer-Tropsch (FT) conversion rate and selectivity by changing the 
design of the 3D printed SCR. While this concept is very interesting and will certainly be of interest to 
others in the community and the wider field, the manuscript has two main shortcomings: 1) although 
the authors printed and tested different reactor designs to demonstrate that the FT product distribution 
changes with the reactor design, there is no discussion of the particular reactor designs and why the 
product selectivity changed (other than a statement that the design may change the contact time); and 
2) many of the claims made by the authors are simply not supported by the material and discussions 
provided in the manuscript and the supplementary information. The specific comments below regarding 
additional information, characterization and discussion of the results need to be addressed to support 
the authors’ claims and to allow other researchers to reproduce the results. After this major revision, 
the manuscript may be considered for publication. 

Specific comments: 

1)To highlight their approach, the authors state that the T316 reaction tube was produced by 
subtractive manufacturing. While I could not check this statement (no product number was provided) 
tubes are typically manufactured by extrusion or drawing. 

2)What is the porosity and the specific surface area of the laser sintered material, before and after the 
calcination pretreatment? This information is critical for comparing catalytic properties of the SCR with 
the precursor alloy powders. 

3)What is the surface area and volume of the reactor as defined by the channel design, as printed as 
well as after the calcination/hydrogen treatment? 

4)What is the particle size and the specific surface area of the metal powders used for catalytic 
experiments (after the pretreatment), and how much powder was used in these tests? The SEM image 
shown in Figure S9b and the TEM image shown in S11a/b suggest a dramatic morphology change during 
the pretreatment. 

5)What is the elemental surface composition of the SCRs and the metal powders after the described 
pretreatment method? It is not clear (but assumed by the authors) that the same pretreatment method 
for SCRs and metal powders leads to the same surface composition as new alloy phases may be formed 
during the laser sintering process which respond differently to the pretreatment. This requires the use 
of surface sensitive characterization techniques like photoelectron spectroscopy as it is the surface 
composition and not the bulk phase that controls the catalytic performance. 
6)What is the measured conversion of the reactants CO (FT), CO2 (CO2 hydrogenation) and CO2/CH4 
(CO2 reforming of CH4) into hydrocarbons? The authors report only how much of the reactants is 
missing in the outlet, but not to which amount the missing reactants were converted into hydrocarbons. 
The Moessbauer results summarized in Figure S17 and Table S5 and the TEM results in Figure S11d 
reveal the formation of Fe carbide phases, so certainly some of the missing carbon was deposited as 
carbide and not converted into hydrocarbons. The question is how much of the carbon from the 
reactants remains in the reactor in the form of carbides or carbon, and how much is transformed into 
the desired hydrocarbon products? 

7)Figure S18 needs larger scale bars 

8)The authors need to provide more information regarding the reusability experiment shown in Figure 
S6. Specifically, was any pretreatment performed before reusing the Fe-SCR? 

9)What is the channel volume of the different SCR designs, and why were these designs chosen? The 
authors only stated that they designed different SCRs to tune the inner surface (page 5). Next the 
authors speculate that the design will affect the contact time of FT reactants and products. The 
catalyst/reactor contact time will certainly change with the channel volume if the flow rate is not 
accordingly adjusted. The contact time can be easily calculated given the known flow rate and the 
channel volume from the CAD design. How does the SCR contact time compare to the contact time in 
the T316/metal powder experiments? May be this discussion can be used to generate some general 
design rules? 



Reviewer #2 (Remarks to the Author): 

The paper is describing use of Selective Laser Sintering (or Selective Laser Melting, SLM) method for 
preparation catalysts for petroleum refining. The basic idea here is combination of the functionality of 
the printed object, in this case the reaction vessel, with the functionality of the printing material, which 
in this case serve as catalytically active material. This is not a new idea but one of the emerging trends in 
3D printing. It is possible to have considerable synergetic advances when the object has dual 
functionality. It is possible to optimize the shape and size of the printed object and when it is printed 
with active material it is possible to select or optimize also the functionality of the material. Both of 
these are nicely presented in the paper. 
I think this paper deserves to be published in Nature Communication. However, I think that the authors 
should note it the text that use of SLM-technique for printing catalytically active objects is part of a new 
and exciting trend in 3D printing, which aims at dual functionality of the printed object. Printing catalysts 
itself is not new. There are examples where FDM or similar type extruding techniques has been used to 
build catalytically active flow through objects: DOI: 10.1016/j.jcat.2015.11.019, DOI: 
10.1016/j.cattod.2019.06.026, DOI: 10.1016/j.jclepro.2018.12.274, DOI: 10.1016/j.apcatb.2019.01.008, 

DOI: 10.1149/2.0341905jes, DOI: 10.1016/j.jcou.2019.07.013, DOI: 10.1021/acsapm.9b00598. 
In addition to these example also SLS printing has been used for building reactor parts. Similarly, idea of 
printing catalytic reactors or reactor parts are well known: DOI: 10.1039/C7CY00615B, DOI: 
10.1021/acscatal.7b02592, DOI: 10.1021/acsomega.9b00711. The last one is an example of use of SLS-
technique for printing mixer for a catalyst reactor, but even in that one it has been mentioned that it 
would be possible to use printing for preparing any shape of objects. This means that the catalytic 
activity could be added in any part of the reaction vessel, not just in the walls of the reactor as in the 
paper here. This approach goes far beyond the catalysis. Similar technique could be used to prepare all 
kind of multifunctional objects. In addition to the biological systems it has been applied in various 
different areas. For example, NLO active lenses, where optical properties of the lens have been 
combined with NLO activity of the printing material, have been printed by using SLA technique 
(10.1021/acsomega.8b01659). 

As a summary, I think the paper should be published in Nature communication. To my knowledge it is 
the first example of use of SLM printing to build catalytically active reactor vessels. It extends the use of 
3D printed chemically functional objects into area of high pressure high temperature tasks. However, it 
should also be kept in mind that despite the obvious benefits of using metal printing it also has its 
limitations. It is not suitable for direct printing molecular materials i.e. molecular catalysts and it is not 
likely to be the first choice if the catalysis require noble metals. Therefore, the authors should put this 
into the wider context of latest developments in 3D-printing: multifunctional objects. 



Reviewer #3 (Remarks to the Author): 

In my opinion, this is a very interesting article since it addresses various scientific and technological 
aspects currently relevant. On the one hand, 3D-printing, an emerging technology. Particularly 
innovative is the technique based on selective laser sintering (SLS) using metallic powder, for the 
elaboration of new monolithic-type reactors. On the other hand, the catalytic activity presented, very 
efficient and relevant. Therefore, 3D-printing fabrication of metal-based catalytic devices is a field of 
much interest in catalysis today. 

The degree of novelty and interest in this article is high due to several reasons: 
The own technique used for 3D-printing (selective laser sintering, SLS), the composition of the reactors 
(Fe, Ni, Co) and the type of application for which you want to apply. On the other hand, although 3D-
printing technology is mostly applied to thermoplastic polymers or ceramic materials, little development 
still exists for metal 3D-printing, due to the high temperatures necessary in the process due to the high 
melting point of the metal. 

Although the references provided are quite illustrative and significant within the field, I think that some 
more reference could be added to the text, regarding the preparation and application of monoliths 
containing metallic species on the surface through 3D-printing, particularly those that incorporate metal 
on their surface. An example is our work related to the use of monoliths in multi-catalysis (Antonio S. 
Díaz-Marta, ACS-Catal., 2018, 8, 392-404). The reactor-catalysts presented in this work are in fact 3D 
monolitos. 

Although catalysts of this type (metal monolithics) have already been described for different catalytic 
transformations, in this work it is applied for industrial purposes in three different types of high value 
transformations: 
FT was performed on Fe-SCR and Co-SCR. 
Co2 hydrogenation on Fe-SCR. 
Co2 reforming of methane on Ni. 
The catalysts show a high level of reusability. 
That is why I consider this article as very interesting. 

The work seems to be very well executed. Two different centers, one in Japan and the other in China, 
have coordinated the manufacturing of these devices. One catalyst was prepared in Japan (Fe-SCR) and 
two in China (Ni-SCR and Co-SCR). 
Indeed, the design integrates the concept of catalyst and reactor in the same device. The 
characterization of the material is very complete. However, there are some aspects of this work that 
have generated in me some questions or doubts: 
-Since the specific surface is a key aspect in these reactors (basically it is for any type of reactor, ceramic, 
metal-ceramic or ultraporous) I wonder why the authors have not provided any data about BET area 
(specific surface). This would help the rest of the researchers in the field to have a clear idea regarding 
the catalytic surface of these manufactured materials. 
SEM images do indeed reveal a grainy surface, which is interesting from the point of view of a larger 
specific surface. Other data that accompany the characterization of the material such as XRD, RAMAN, 
Mössbauer, are very complete. 
In any case, I would like to know if they have considered measuring BET area or if they have an 
approximate idea of the value m2 / g of catalyst. 
-Related to the grainy nature of the surface, it would be positive to know if the researchers carried out 
some type of experiment to evaluate the possible leaching of metallic particles to the reaction medium, 
which could eventually contaminate the liquid-fuel (ICP or similar experiment). This is not an essential 
question anyway, although it does give an idea of the robustness of the monolith-reactor. 
- The variability in the catalytic activity as a consequence of the design in different geometries of the 
reactors is discussed in the article. The selectivity of Co-SCr-6 to generate gasoline, jet fuel or diesel fuel 
is particularly interesting. However, from the reading at the end of the article, it is not clear to me if this 
variation is as a consequence of an increase in the specific surface area of the reactor or rather the 
geometry of the reactor itself, a longer passage time of the reagents depending on the shape, or a 
combination of the two factors. 
-As for the virtual design (CAD) of the catalyst-reactor, the software they have used is not specified. 
- The supplied video is very illustrative of the SLS process. 
I highly recommend publishing this article.