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Experimental Features to Report in Heterogeneous Photocatalysis Md University of Kentucky UKnowledge Chemistry Faculty Publications Chemistry 10-15-2018 Photocatalytic Activity: Experimental Features to Report in Heterogeneous Photocatalysis Md. Ariful Hoque University of Kentucky, [email protected] Marcelo I. Guzman University of Kentucky, [email protected] Right click to open a feedback form in a new tab to let us know how this document benefits oy u. Follow this and additional works at: https://uknowledge.uky.edu/chemistry_facpub Part of the Analytical Chemistry Commons, Catalysis and Reaction Engineering Commons, Engineering Science and Materials Commons, Environmental Chemistry Commons, Environmental Sciences Commons, Materials Chemistry Commons, Materials Science and Engineering Commons, and the Physical Chemistry Commons Repository Citation Hoque, Md. Ariful and Guzman, Marcelo I., "Photocatalytic Activity: Experimental Features to Report in Heterogeneous Photocatalysis" (2018). Chemistry Faculty Publications. 115. https://uknowledge.uky.edu/chemistry_facpub/115 This Article is brought to you for free and open access by the Chemistry at UKnowledge. It has been accepted for inclusion in Chemistry Faculty Publications by an authorized administrator of UKnowledge. For more information, please contact [email protected]. Photocatalytic Activity: Experimental Features to Report in Heterogeneous Photocatalysis Notes/Citation Information Published in Materials, v.. 11, issue 10, 1990, p. 1-11. © 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). Digital Object Identifier (DOI) https://doi.org/10.3390/ma11101990 This article is available at UKnowledge: https://uknowledge.uky.edu/chemistry_facpub/115 materials Concept Paper Photocatalytic Activity: Experimental Features to Report in Heterogeneous Photocatalysis Md. Ariful Hoque 1 and Marcelo I. Guzman 1,2,* 1 Department of Chemistry, University of Kentucky, Lexington, KY 40506, USA; [email protected] 2 Center for Applied Energy Research, University of Kentucky, Lexington, KY 40511, USA * Correspondence: [email protected]; Tel.: +1-859-323-2892 Received: 14 September 2018; Accepted: 10 October 2018; Published: 15 October 2018 Abstract: Heterogeneous photocatalysis is a prominent area of research with major applications in solar energy conversion, air pollution mitigation, and removal of contaminants from water. A large number of scientific papers related to the photocatalysis field and its environmental applications are published in different journals specializing in materials and nanomaterials. However, many problems exist in the conception of papers by authors unfamiliar with standard characterization methods of photocatalysts as well as with the procedures needed to determine photocatalytic activities based on the determination of “apparent quantum efficiencies” within a wavelength interval or “apparent quantum yields” in the case of using monochromatic light. In this regard, an astonishing number of recent research articles include claims of highly efficient (photo)catalysts or similar terms about materials with superior or enhanced efficiency for a given reaction without proper experimental support. Consequently, the comparison of the efficiencies of photocatalysts may result as being meaningless, especially when reports are only based on expressions determining (1) a reaction rate per weight of catalyst or its surface area, (2) quantum efficiencies or quantum yields, and (3) turnover frequencies or turnover numbers. Herein, we summarize the standards needed for reporting valuable data in photocatalysis and highlight some common discrepancies found in the literature. This work should inform researchers interested in reporting photocatalysis projects about the correct procedures for collecting experimental data and properly characterizing the materials by providing examples and key supporting literature. Keywords: heterogeneous photocatalysis; photocatalytic activity; photocatalytic efficiency; rate per weight; apparent quantum efficiency 1. Introduction The use of photocatalysis has attracted a great deal of attention in the nanomaterials community to solve problems related to the mitigation of air pollution and environmental remediation of contaminated waters (i.e., the degradation of dyes) [1]. More recently, photocatalysis has been implemented for the conversion from solar to chemical energy. For example, the photocatalytic reduction of carbon dioxide (CO2) into carbon monoxide (CO) and organic compounds with one (e.g., CH3OH, HCOOH, and CH4) and two carbon atoms (e.g., HOOCCOOH, HCOCOOH, and CH3CH2OH) is of interest for the storage of solar energy in chemical bonds [2–6]. Similarly, the power of photocatalytic water splitting for producing molecular hydrogen (H2) and oxygen (O2) can also serve to produce chemical fuel [7,8]. Thus, the synthesis of advanced semiconductor materials with valuable photocatalytic properties is continuously generating new catalysts for driving a variety of reactions more efficiently [9,10]. The produced new catalysts are often tested against a specific reaction such as the degradation kinetics of a dye, resulting in works that claim to have improved the photocatalytic efficiency (or enhanced the activity) of the process relative to previous Materials 2018, 11, 1990; doi:10.3390/ma11101990 www.mdpi.com/journal/materials Materials 2018, 11, 1990 2 of 11 materials, modified stoichiometric ratios, variable geometries, or single components integrated into nanocompositeMaterials 2018, 11 heterostructures, x FOR PEER REVIEW [ 11 ,12]. However, such measurements are insufficient2 toof 11 report photocatalytic activities because they lack a mechanism to account for the number of photons driving the processmodified [13 ,stoichiometric14]. ratios, variable geometries, or single components integrated into Innanocomposite addition, a heterostructures trained expert [11,12]. in photocatalysis However, such should measurements find it concerning are insufficient that to a significantreport numberphotocatalytic of articles activities could because be published they lack with a mechanism poorly characterizedto account for the materials number of [ 15photons]. The driving common the process [13,14]. doubly problematic case corresponds to articles that neither explain the experimental details, In addition, a trained expert in photocatalysis should find it concerning that a significant number conditions,of articles and procedurescould be published followed with when poorly synthesizing characterized and characterizingmaterials [15]. theThe materials, common nordoubly consider appropriateproblematic protocols case corresponds for reporting to articles measurements that neither explain of photocatalytic the experimental activity. details, conditions, Various practices and can beprocedures found in followed the literature when synthesizing which have and been characterizing applied forthe comparingmaterials, nor the consider activity appropriate or reactivity of theprotocols photocatalysts, for reporting which measurements include reporting of photocatalytic (1) the activity. rate of Various formation practices of a can product be found or lossin of a reactantthe literature per gram which (or have alternatively been applied per forsurface comparing area) the ofactivity catalyst, or reactivity and (2) of the the turnoverphotocatalysts, numbers (or thewhich turnover include frequencies) reporting (1) [13 the]. However,rate of formation as mentioned of a product above, or theloss principal of a reactant concern per gram in a reaction(or promotedalternatively by light per issurface to account area) of forcatalyst, the and numbers (2) the ofturnover photons numbers driving (or the it andturnover not frequencies) the amount of photocatalyst[13]. However, participating as mentioned in the abov process.e, the Therefore, principal concern attempts in toa reportreaction the promoted activity by of photocatalystslight is to account for the numbers of photons driving it and not the amount of photocatalyst participating in in any reaction that omit computing the number of photons cannot be justified and are strongly the process. Therefore, attempts to report the activity of photocatalysts in any reaction that omit discouraged.computing Several the number special of considerationsphotons cannot be must justified be taken and are into strongly account discouraged. for comparing Several the special efficiency of theconsiderations photocatalysts. must These be taken experimental into account features for comparing are needed the efficiency to report of andthe photocatalysts. compare the activityThese of photocatalystsexperimental in termsfeatures of are apparent needed quantumto report and efficiencies compare (AQE)the activity or apparent of photocatalysts quantum in yieldterms (AQY),of not simplyapparent on quantum reaction efficiencies rates, are presented(AQE) or apparent below. quantum Other important yield (AQY), outlines not simply will beon reaction briefly addressedrates, to guideare nonexpertspresented below. to correctly Other important determine outlines photocatalytic will be briefly performances. addressed to guide nonexperts to Figurecorrectly1 summarizes determine photocatalytic the four critical performances. features that need to be reported in a photocatalysis paper that are describedFigure 1 summarizes in the next sections:the four critical (1)
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