Editorial Cite This: Chem. Mater. 2018, 30, 3599−3600
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Common Pitfalls of Catalysis Manuscripts Submitted to Chemistry of Materials
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it could be viewed as intent to deceive, something authors would want to avoid. The definitions of TOF and TON deserve more attention than space allows in this editorial, and authors are encouraged to consult other sources.1 The editors at Chemistry of Materials, however, would look favorably on manuscripts the comply with the standards described above.
any science and engineering journals focus on catalysis research, sometimes with “Catalysis” contained within the name of the journal. Chemistry of Materials also covers aspects of catalysis, as stated in its Scope Statement: “Among the areas of interest are inorganic and organic solid-state chemistry, composite materials, nanomaterials, biomaterials, thin films and polymers, especially when focused on the creation or innovative development of materials with novel and potentially useful optical, electrical, magnetic, catalytic, or mechanical properties”. Importantly, this statement emphasizes a focus on materials with innovative and potentially useful catalytic properties, not the catalytic reaction itself. As such, many authors of catalysis-relevant manuscripts submitted to Chemistry of Materials would not regard themselves as strongly affiliated with the core catalysis community as much as other chemistry subdisciplines. Consequently, these authors may not be familiar with the reporting standards in catalysis research, or they may not have experienced common pitfalls related to reporting catalytic properties of materials. Chemistry of Materials does not expect the same level of detail and depth when reporting catalytic data as journals that specialize in catalysis. There are some basic tenets authors should consider when submitting catalysis-relevant manuscripts to Chemistry of Materials, however, as failing to do so will likely lead to rejection, or at least a request for modification before further consideration of the manuscript. Some frequent issues experienced by the journal editors are addressed below, in brief, as guidance to authors for improving the quality of their manuscripts and the chances for a smoother review process.
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DEACTIVATION STUDIES AT FULL OR AT EQUILIBRIUM CONVERSION Long-term catalyst stability can be measured in a continuous flow configuration or in successive batch reactions. One of the most frequent mistakes we have observed is the analysis of deactivation at complete conversion, or at a conversion when equilibrium concentrations of all compounds have been achieved. The observation of full conversion for extended periods of time is then often used mistakenly as an indicator that a catalyst is stable under reaction conditions. At full conversion only a minimum catalytic activity can be determined, because the available amount of reagents limits conversion; the catalyst could in fact be much more active. Thus, any deactivation from a potentially higher activity level to the minimum activity, corresponding to full conversion, would not be detected. Stability of catalysts therefore should not be analyzed at full conversion, but at intermediate conversion levels in continuous flow experiments, for example, by reducing the mass of catalyst or increasing the flow rate of reagents. In batch experiments, one should select reaction times or catalyst amounts that are sufficiently short or low, respectively, in order to avoid complete conversion. Only then can a deactivation process be detected reliably.
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IMPROPER CALCULATION OF TURNOVER FREQUENCIES (TOFs) AND TURNOVER NUMBERS (TONs) If the number of active sites for a catalyst is known (frequently not the case!), TOFs can be calculated as a measure of rate molecules converted per site and time. A TOF is valid only for a specific reactant concentration, which should be reported and compared among catalysts only at the beginning of a reaction when the reactant concentration is not yet depleted and the TOF is more likely to be constant. Often, however, authors report TOFs by dividing the total number of converted molecules over the entire time span of the reaction, which then affords an ill-defined average TOF. Instead, authors should report TOFs measured at an early stage of the reaction, as defined by a specific % conversion. TON is a measure of catalyst stability, defined as the number of transformations a site can sustain until it is deactivated completely. Accurate determination of TON thus requires measurements until the activity of the catalyst is completely lost. Many manuscripts submitted to the journal report TON as the number of transformations effected during some specified reaction time, irrespective of whether the catalyst is active or not at that time. TON measured in this way is arbirtrary as it depends on whether the reaction time is extended or shortened. Moreover, © 2018 American Chemical Society
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COMPARISON OF SELECTIVITIES AT DIFFERENT CONVERSION LEVELS High product selectivity is almost always critical in a catalytic reaction, but selectivity often can depend on catalyst activity and % conversion, with the potential for misinterpretation of actual selectivity. For example, consider a consecutive reaction A → B → C, in which the first reaction is much faster than the second. If the rate of A → B is fast relative to that of B → C, the intermediate product B will accumulate during the initial stages of the reaction and the selectivity for B will appear higher than for C. At longer reaction times, as the conversion of A approaches completion, some of the intermediate B will have reacted to produce C, whose concentration will then grow further with time. Consequently, the apparent selectivity will differ if measured at at different conversion levels of A.2 In such cases, either reaction times (contact times in continuous experiments) or catalyst amounts should be adjusted so that conversion is identical for differently active catalysts. Published: June 12, 2018 3599
DOI: 10.1021/acs.chemmater.8b01831 Chem. Mater. 2018, 30, 3599−3600
Chemistry of Materials
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Editorial
NEGLECT OF MASS TRANSFER LIMITATIONS For fast reactions, the observed conversion might be affected by mass transfer limitations rather than catalyst activity, thereby masking the actual differences between different catalysts and often leading to the conclusion that the catalyst activities are similar or identical. Simple diagnostic tests can be used to exclude mass transfer limitations, such as doubling the flow rate after doubling the catalyst amount in a gas phase reaction. This test probes for film diffusion, and should lead to identical conversion, if film diffusion limitations are absent. The influence of pore diffusion can be assessed by changing the catalyst particle size, which again should lead to identical conversion. In a liquid phase reaction, measuring at different stirring rates probes for both, film diffusion and/or gas dissolution limitation in three-phase reactions. Also here identical conversion should be reached at different stirring rates, if these mass transfer limitations are absent. The reader should consult reference 2 for a more thorough exposition and examples of diagnostic tests for mass transfer limitations.
forced closure of the hysteresis, which in turn leads to a sharp peak in the pore size distribution at a size of approximately 4 nm. This sharp peak is an artifact. Therefore, if pronounced closure of the hysteresis with a sudden drop around a pressure of 0.42 is observed, this should not be overinterpreted. Overinterpretation of BET Surface Areas for Microporous Materials. Likewise, BET surface areas are often determined from nitrogen sorption measurements. One crucial assumption of the BET method, however, is the unobstructed formation of adsorbed multilayers. In the case of microporous materials with pore sizes below 2 nm, this assumption is not justified, and BET surface area values for such materials become questionable, even meaningless. With proper care, one may extract meaningful surface area values from the isotherms,4,5 but this is rarely done, as unsually authors report only the values reported by the sorption equipment. Incidentally, here as for many other data, care should be taken to give a sensible number of digitsthe machines often provide five or more decimals, which is certainly far beyond the precision of the method.
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FAILURE TO STUDY THE CATALYST AFTER REACTION A substantial portion of the catalysis research reported in Chemistry of Materials describes highly sophisticated designer catalysts, including those with precise control of shape and elemental distribution. Differences in catalyst performance are then traced back to differences in these nanoscale features. Often, however, the materials are characterized prior to reaction, but not afterward. As it is well-known that materials can change substantially under reaction conditions, which sometimes are harsh, such conclusions may not be justified unless the characterization of the catalyst is performed in a manner that permits unequivocal side-by-side comparisons to its form before the reaction.
SUMMARY There are certainly many more pitfalls in analyzing and reporting catalysis work, but based on our experience, the points above represent a large majority of the technical flaws in manuscripts submitted to Chemistry of Materials in the field of catalysis. Proper reporting according to the advice above will reduce the frustration of editors, reviewers and authors, leading to a more streamlined publishing experience.
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Ferdi Schüth, Associate Editor Michael D. Ward, Associate Editor Jillian M. Buriak, Editor-in-Chief AUTHOR INFORMATION
ORCID
Ferdi Schüth: 0000-0003-3765-9848 Michael D. Ward: 0000-0002-2090-781X Jillian M. Buriak: 0000-0002-9567-4328
FAILURE TO CONSIDER DIFFERENCES IN SURFACE AREA OF CATALYSTS Unless specific factors come into play, such as the contribution of special edge or kink sites, catalytic activity of a solid scales with its active surface area. Rates measured on different samples will then be identical when normalized to surface area. The journal receives many manuscripts wherein differences in catalytic activity are attributed to specific differences in chemistry, but closer inspection reveals a trivial explanation that invokes a different active surface area. This can be a complex issue, as the identity and amount of active surface area may not be clear, what the active surface area is, but before elaborate explanations are given, authors should always consider that Occam’s Razor almost always wins.
Notes
Views expressed in this editorial are those of the authors and not necessarily the views of the ACS.
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REFERENCES
(1) Kozuch, S.; Martin, J. M. L. “Turning over” Definitions in Catalytic Cycles. ACS Catal. 2012, 2, 2787−2794. (2) Kapteijn, F.; Moulijn, J. A. Laboratory Testing of Solid Catalysts. In Handbook of Heterogeneous Catalysis, 2nd ed.; Ertl, G., Knözinger, H., Schüth, F., Weitkamp, J., Eds.; Wiley: Weinheim, 2008; pp 2019− 2045. (3) Thommes, M. Physical Adsorption Characterization of Nanoporous Materials. Chem. Ing. Tech. 2010, 82, 1059−1073. (4) Rouquerol, J.; Llewellyn, P.; Rouquerol, F. Is the BET equation applicable to microporous adsorbents? Stud. Surf. Sci. Catal. 2007, 160, 49−56. (5) Walton, K. S.; Snurr, R. Q. Applicability of the BET Method for Determining Surface Areas of Microporous Metal−Organic Frameworks. J. Am. Chem. Soc. 2007, 129, 8552−8556.
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THE ROLE OF POROSITY Proper analysis of porosity is not only relevant in catalysis, but also in other fields. These issues, among others, have been addressed in detail elsewhere, but we comment on two specific ones below.3 Artifacts in Pore Size Distributions Due to Forced Closure of Sorption Hysteresis. Nitrogen adsorption at 77 K is the standard method for surface area and pore size analysis. Pore size distributions typically are calculated from the desorption branch of the isotherm using the BJH algorithm. This normally works well, but the meniscus of liquid nitrogen becomes unstable at a relative pressure of 0.42, leading to 3600
DOI: 10.1021/acs.chemmater.8b01831 Chem. Mater. 2018, 30, 3599−3600
