Spectroscopic Microreactors for Heterogeneous Catalysis Benjamin A
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! ""# $%" &" '()# *+ , !# ', -'( #. '%' /0 ) 1234!5 -!6 #1, 7 8 $) !"#$ ! !$9 # 4 '$# "# !%&' %(!#$"!!"!)#$ 169$ ) ,$#! :#!#;.:) '$ : '')<%, ! "" # $%" &" === ) ,$#! : ( ##$ :$ )#'$) ': '$% '$$ ' #:''$'3 '$ : '')<%,&> ! ""# $%" &" *&* 169$ $ :) # % '$$ ' % #!$ ! '$ : '')<%,* ! "" # $%" &" = ! ""# $%" &" ?'$ Review Article: Spectroscopic microreactors for heterogeneous catalysis Benjamin A. Rizkin,a) Filip G. Popovic,a) and Ryan L. Hartmanb) Department of Chemical and Biomolecular Engineering, New York University, 6 MetroTech Center, Brooklyn, New York 11201 (Received 3 May 2019; accepted 8 July 2019; published 5 August 2019) Microfluidic reactors with in situ spectroscopy have enabled many new directions of research over the last two decades. The miniature nature of these systems enables several key advantages in het- erogeneous catalysis, which includes the reaction surface or interface accessible to spectroscopic equipment making the discovery of new catalytic materials possible. Devices fabricated with mate- rials that are transparent to electromagnetic radiation enable in situ and in operando spectroscopy such as Raman, UV-Vis, and IR directly at the point of the reaction, and thus high fidelity, transient information on the reaction chemistry is available. Innovative designs with NMR, elec- trochemical impedance spectroscopy, x-ray techniques, or terahertz imaging have also advanced the field of heterogeneous catalysis. These methods have been successfully engineered to make major breakthroughs in the design of catalytic materials for important classes of chemical reac- tions. In this review, the authors provide an overview of recent advances in the design of micro- reactors with in situ spectroscopy for the study of heterogeneous catalysis to raise awareness among the vacuum science community on techniques, tools, existing challenges, and emerging trends and opportunities. © 2019 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/ licenses/by/4.0/). https://doi.org/10.1116/1.5108901 I. INTRODUCTION perspective1 and review2 succinctly details the state of the In the last two decades, microchemical systems have art of microreaction technology. These works explore proven indispensable to many fields of research ranging from opportunities for a move towards incorporating automation fl fl catalysis to cellular biology. There are many advantages to and self-optimization in continuous ow micro uidics for fi miniaturization of experiments such as the prevalence of enabling new reaction and separation technologies in ne surface forces over body forces, enabling researchers to chemical and pharmaceutical manufacturing. Related fl fl exploit physical phenomena to enhance heat and mass trans- reviews on the use of ow in chemistry and micro uidics can be found covering specific topics such as photochemi- port in their systems, along with having a highly controlled, 3,4 5 fi cal reactions, cascade reactions, functional material syn- reproducible, and quanti able environment. These microde- 6 7 vices also have the benefit of creating much less chemical thesis, and nanomaterials, as well as more general fl fl 8 waste or exposure to hazardous/toxic compounds at aggres- micro uidic ow chemistry related reviews. Inline spec- sive conditions, since reactions are performed with microliter troscopic integration schemes with microreactors and new developments implementing this approach are not specifi- instead of milliliter to liter volumes. This has the added 3–8 benefit of being able to sample a wider experimental range cally addressed in these works. However, other reviews regarding microfluidics and technique-specificinlineinte- given the same amount of time and resources. Perhaps most 9 critically, however, miniaturization allows for novel ways to gration approaches have been published. The trend toward miniaturization in analytical chemistry analyze chemical reactions and processes in situ and in oper- 10 ando, giving scientists and engineers a broader picture of is comprehensively reviewed by Ríos and Zougagh, and physics and chemistry. the more niche topic on miniaturized total chemical-analysis systems has been addressed in an extensive review by Guijt Perhaps one area that is uniquely well suited for the 11 application of microreactors is automation and high- and Manz. Reviews relating Raman spectroscopy to micro- fl 12–14 fl throughput screening. Due to the highly-quantifiable nature uidics have also been discussed. For a nonmicro uidic treatment on catalytic processes in aqueous environments of these reactors combined with precision sensors, low res- 15 idence volumes, and isothermal operation, it becomes pos- and their in situ monitoring, see Shi et al. There has also sible to control these systems with extreme precision and been a review published on heterogeneous electrocatalysis very quickly. This new area of research and application has by Kalz et al. who covered aspects of catalyst design ranging from critical length scales to different methods of catalyst shown promise in the research and discovery of new 16 physics and materials in broad fields such as macromolecu- and reaction characterization. fi lar science and electrochemistry. An outstanding recent A sizeable number of signi cant contributions cover the basic principles of the operando methodology.17–19 This methodology aims to uncover both structure and activity a)B. A. Rizkin and F. G. Popovic contributed equally to this work. information to better understand the relationship of the two by b)Electronic mail: [email protected] concurrently evaluating catalyst performance and surface 050801-1 J. Vac. Sci. Technol. A 37(5), Sep/Oct 2019 0734-2101/2019/37(5)/050801/22 © Author(s) 2019. 050801-1 050801-2 Rizkin, Popovic, and Hartman: Review Article: Spectroscopic microreactors for heterogeneous catalysis 050801-2 interactions during catalytic processes under technically rele- The scope of the present review is not intended to be an vant reaction conditions.20 For appropriate implementation of exhaustive examination of the literature but instead is meant the methodology, it is important to characterize multiple to be an update from 2013 onward of the major trends in the phases in a space- and time-resolved multitechnique fashion field of microreactors integrated with in situ spectroscopy for and understand the relations of phenomena at multiple the study of heterogeneous catalysis. For the state-of-the-art scales.21 There are reviews that cover the use of in situ or prior to 2013, you may find the excellent review by operando methodologies for specific reaction systems, such as Yue et al.32 instructive. Outstanding reviews on microfluidic the review by Rodriguez et al.22 on water-gas shift reaction synthesis of nanomaterials, including catalysts, can also be on metal oxide catalysts and Newton’s23 on time-resolved found by Marre and Jensen33 and by Song et al.34 Another operando x-ray techniques for the study of CO oxidation over primary objective of the present review is to raise awareness platinum. For a more general review of x-ray spectroscopy for amongst the vacuum science community on techniques, heterogeneous catalysis, see the work of Frenkel and van tools, emerging trends, and existing challenges within the Bokhoven.24 Unified multistep continuous flow platforms field of heterogeneous catalysis for the retrieval of high fidel- have found the use of different types of microfluidic and spec- ity transient catalytic information. Our review includes con- troscopic pairings as essential for their operation, whether it cepts on the design of microreactors and their integration be the utilization of in-line or in situ analysis techniques.25 with in situ spectroscopic techniques for those interested in A recent review by Al-Rifai et al.26 provides a perspective learning more about the field or applying the techniques in on how to use microreaction technology to help facilitate cat- their own investigations. alytic process development. From a kinetic-experimentation standpoint, the authors address how the expanded reaction space provided by microfluidic technology allows exploring II. DESIGN kinetics of novel processes windows, applications that suc- There are many design considerations to take into account cessfully extracted intrinsic kinetics and the challenges asso- when planning a new microchemical system for in situ ciated with this approach. Most importantly, for the purpose spectroscopic applications, including the flow regime, of this review, they provide what a microreactor requires for chemical compatibility, heat transfer, and transparency to appropriate integration with an analytical technique for in electromagnetic (EM) waves. By selecting the right material situ operation. Broadly speaking, the requirements are (i) suf- and tuning properties of the system, it becomes possible to ficient understanding of the flow behavior in the system that optimize the system for a given chemistry and spectroscopic allows for the accurate determination of the intrinsic reaction technique used. It is important to consider the critical kinetics, (ii) having the capability to dissipate the heat gener- length scales of the phenomena being analyzed, as well as ated by the electromagnetic beam of the chosen analytical the time scales and material transparency. These consider- technique and thus keep isothermal conditions, and (iii) dis- ations are shown in Fig.