catalytic oxidation of carbon monoxide over a platinum surface

Good practice report

Ryo Horikoshi*, Syota Nakajima, Saburo Hosokawa, Yoji Kobayashi andHiroshi Kageyama

Illustrating catalysis with a handmademolecular model set: catalytic oxidation ofcarbon monoxide over a platinum surfacehttps://doi.org/10.1515/cti-2021-0010Received March 20, 2021; accepted June 28, 2021; published online August 2, 2021

Abstract: Catalytic converters (automotive catalysts) and the chemical reactions they catalyze appear ingeneral and introductory chemistry textbooks. Although the detailed mechanisms of the chemical reactionsthat occur in catalytic converters have been clearly revealed via recent developments in surface and compu-tational chemistry research, the description and illustration of the catalysis are still ambiguous in textbooks. Inthis paper, wedescribe an extracurricular lecturewhereby ahandmade teaching aidwas employed to illustratethe basic principle of the catalytic oxidation of carbon monoxide over platinum surface, which is an essentialreaction occurring in catalytic converters. The teaching aid, constructed combining easily available materials,can illustrate the positions and motions of the molecules on the platinum surface during catalytic oxidation.The lecture was favorably received by non-chemistrymajors and high school students. Despite the difficulty ofthe topic, the audience displayed a relatively high level of understanding.

Keywords: catalyst; catalytic converter; platinum; teaching aid.

Introduction

Despite the importance of catalytic converters (automotive catalysts) for reducing the harmful emissions ofvehicles powered by internal combustion engines, the descriptions of their reaction mechanisms are oftenobscure in chemistry textbooks for non-chemistry majors and high school students (Atkins, Jones, & Laver-man, 2013; Davis, Frey, Sarquis, & Sarquis, 2009; McMurry & Fay, 2010; McQuarrie, Rock, & Gallogly, 2011). Tostimulate these students’ interest for chemical learning, more detailed descriptions of the catalytic reactionsare needed. Over the past three decades, the exact details of several heterogeneous catalytic reactions havebeen clearly demonstrated as a result of the rapid progress of surface and computational chemistry research(Dumeignil, Paul, & Paul, 2017; Kolasinski, 2016, 2012; Young, 2009). For example, the catalytic oxidation ofcarbonmonoxide over platinum-groupmetals, which is a key reaction taking place in catalytic converters, canbe explained by the Langmuir–Hinshelwood (L–H) mechanism (Alavi, Hu, Deutsch, Silvestrelli, & Hutter,1998; van Spronsen, Frenken, & Groot, 2017). In addition, the precise positions of the adsorbed atoms on the

*Corresponding author: Ryo Horikoshi, Department of Environmental Science and Technology, Faculty of Design Technology,Osaka Sangyo University, Nakagaito, Daito, Osaka 574-8530, Japan, E-mail: [email protected]. https://orcid.org/0000-0002-8609-9173Syota Nakajima, Department of Environmental Science and Technology, Faculty of Design Technology, Osaka Sangyo University,Nakagaito, Daito, Osaka 574-8530, JapanSaburo Hosokawa, Elements Strategy Initiative for Catalysts & Batteries (ESICB), Kyoto University, 1-30 Goryo-Ohara, Nishikyo-ku,Kyoto 615-8245, JapanYoji Kobayashi and Hiroshi Kageyama, Department of Energy & Hydrocarbon Chemistry, Graduate School of Engineering, KyotoUniversity, Nishikyo-ku, Kyoto 615-8510, Japan

Chemistry Teacher International 2021; 3(4): 431–439

Open Access. © 2021 Ryo Horikoshi et al., published by De Gruyter. This work is licensed under the Creative CommonsAttribution-NonCommercial-NoDerivatives 4.0 International License.

metal surface during the catalytic reaction have been elucidated by the rapid development of scanning probemicroscopy (van Spronsen et al., 2017; Wintterlin, Schuster, & Ertl, 1996; Zambelli, Barth, Wintterlin, & Ertl,1997). Although themechanism of the oxidation reaction has been obvious for a number of years, descriptionsof the aforementioned reaction remain ambiguous in current chemistry textbook for non-chemistry-majoruniversity students and high school students.

Although several excellent chemical demonstrations of the catalytic reactions have been reported in theliterature (Horikoshi, Takeiri, Kobayashi, & Kageyama, 2018; Jacobse, Vink, Wijngaarden, & Juurlink, 2017;Laan, Franke, van Lent, & Juurlink, 2019; Spierenburg et al., 2017), only a limited development of teaching aidshas taken place for illustrating the mechanisms of catalytic reactions (Horikoshi, 2015; Horikoshi, 2021;Horikoshi, Kobayashi, & Kageyama, 2013; Horikoshi, Kobayashi, & Kageyama, 2014). Therefore, the authors ofthe present article conceived the development of a handmade teaching aid that can illustrate the details of thecatalytic oxidation of carbon monoxide over platinum, including the behavior of adsorbed molecules on themetal surface. Since the behavior of adsorbed molecules is relatively straightforward yet interesting, a lecturefocusing on it would be of pedagogical value for non-chemistry majors and high school students. The authorgave an extracurricular lecture using a handmade teaching aid for non-chemistry majors (environmentalscience majors) and high school students to illustrate the catalytic oxidation taking place in catalytic con-verters. In our experience, topics comprising chemistry of some difficulty are popular in extracurricularlectures intended for the mentioned types of students. In fact, despite the difficulty of the topic, many of themshowed a relatively high level of understanding. By using this teaching aid, students can understand that themetal surface acts like a field of collision between the reactants, and it contributes to their activation.

Preparation

Detailed instructions for constructing the teaching aid are provided in the Supplementary Material. Thepreparation of one set of the teaching aid costs about 30 USD. The construction does not require advancedskills or special tools. It took three days to prepare the teaching material, including painting and adhesivedrying times. Notably, the teaching aid includes a Pt(111) surface model and molecular models for dioxygenand carbon monoxide, and it is manufactured starting from easily available materials, like spherical plasticcapsules, ping-pong balls, and neodymiummagnets. The molecular models can be easily separated into theiratomic components. The detachability of atomic parts from the molecular models is a suitable feature toillustrate the adsorption and dissociation of molecules on the surface of Pt and the dissociation of the oxygenmolecule into atomic oxygen. This structural model roughly reflects the empirical atomic sizes: the plastichemisphere representing the platinum atom has a diameter of 7 cm, and the spheres representing the oxygenand carbon atoms have diameters of 4 cm. The empirical diameters of the platinum, oxygen, and carbon atomsare ca. 0.27, 0.12, and 0.14 nm (Atomic radii of the elements, n.d.), respectively; hence, the value of thediameter of the platinum atom model is slightly smaller than the correct value of 8.3 cm.

Lecture

This lecture consists of the three topics summarized in Table 1, including a demonstration experiment, aslideshow explanation, and a description of catalytic reaction. The total time required for the lecture is about50 min. The lecture was conducted online for non-chemistry majors and face-to-face for high school students.A detailed illustration using a handmade teaching aid for the reaction mechanism of the catalytic oxidation ofcarbon monoxide on a platinum surface is described in the following paragraphs. A short YouTube video (Ptcatalyst, 2021) shows how the handmade teaching aid works.

432 R. Horikoshi et al.: Illustrating catalysis with a handmade molecular model set

Adsorption and dissociation of oxygen molecule on Pt(111) surface

The positions andmotions of themolecules adsorbed on themetal surface can be observed by scanning tunnelmicroscopy and be estimated by computational methods (Zambelli et al., 1997). In detail, an oxygen moleculegets adsorbed on a Pt surface, and it subsequently dissociates to produce two oxygen atoms that end upsettling into hollow sites of the Pt surface (Figure 1).

Adsorption behavior of carbon monoxide on Pt(111) surface

Carbon monoxide can coordinate to three sites on the Pt(111) surface, namely top, bridge, and hollow sites, asincreasing the coordination numbers (Figure 2). In all cases, carbonmonoxide coordinates perpendicularly tothe Pt(111) surface via its carbon atom. Notably, in the bridge coordination model, the carbon dioxide modelmust be supported by hand to prevent it from falling.

Table : Details of the lecture.

Topic Content Remark

Demonstrationexperiment

A hydrogen combustion on a platinum leaf was conducted. A teaching aid developed by Shikura wasused (Sikaura, n.d.).

Slideshow explanation Technical terms, including catalyst, catalytic converters,and automotive-related pollutants were explained.

PowerPoint lecture slides were summarizedin the Supplementary Material.

Illustration of thecatalytic reaction

By using the handmade teachingaid, themechanismof theoxidation reaction of CO over platinum surface isexplained.

A YouTube video of how the teaching aidworks was uploaded (Pt catalyst, ).

Figure 1: Representation of the reaction pathwaywhereby a dioxygenmolecule in the gas phase producesoxygen atoms adsorbed on a Pt(111) surface: (a) beforeadsorption, (b) collision, (c) dissociation, and(d) settling into hollow sites of the Pt(111) surface.

Figure 2: Carbon monoxide molecule adsorption sites on Pt(111) surface:(a) top, (b) bridge, and (c) hollow sites.

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Reaction between atomic oxygen and carbon monoxide on the Pt(111) surface

The generally acceptedmechanism for the oxidation of carbonmonoxide over platinum is of the L–H type. Themechanism proceeds via the following four steps: (i) dissociation of the dioxygen molecule into two oxygenatoms on the surface of Pt(111), (ii) adsorption of gas phase carbon monoxide onto the Pt(111) surface, (iii)collision of adsorbed atomic oxygen with carbon monoxide, and (iv) desorption of the thus generated carbondioxide to the gas phase. The generated carbon dioxide is bent temporary (Matsushima, Matsui, & Hashimoto,1984). Stage (iii) of this mechanism should be described in detail, because the positions and motions ofadsorbed molecules and atoms on the Pt(111) surface are somewhat complex and quite interesting (Figure 3).First, two oxygen atoms locate into hollow sites of the metal surface, and a carbon monoxide molecule getsadsorbed on a top site of the Pt(111) surface, respectively (Figure 3a). Next, the carbon monoxide moleculemoves to the adjacent top site via the formation of a bridge site (Figure 3b). At the same time, one oxygen atomleaves the hollow site for the adjacent bridging site (Figure 3c). Finally, carbon monoxide and atomic oxygencollide with each other around the hollow site (Figure 3d) to generate amolecule of carbon dioxide (Figure 3e).These collision and desorption processes are revealed in detail by a published ab initio density functionaltheory study (Alavi et al., 1998).

Notably, the demonstration experiment performed at the beginning of the lecture, which consisted of theoxidation (combustion) of molecular hydrogen on a Pt surface, can be also considered to proceed via an L–Hmechanism (Gorodetskii, Block, & Drachsel 1994); however, illustrating this reaction pathway with thedescribed handmade model would be difficult, because the reaction involves complex intermediates.

Instructiveness of the lecture

We estimated the instructiveness of the lecture conducted using the teaching aid based on the results of anonline test administered immediately after the online lecture in the case of the non-chemistry majors (envi-ronmental sciencemajors) and a comprehension test administered one week after the lecture in the case of thehigh school students. The results of these tests are summarized in Tables 2 and 3.

Figure 3: Pathway of the reaction between atomic oxygenand carbon monoxide on a Pt(111) surface:(a) adsorption of oxygen atoms and a carbon monoxidemolecule on the metal surface, (b–d) motions of atomicoxygen and carbon monoxide on the aforementionedsurface, and (e) formation and desorption of carbondioxide. The described motions of the molecules andatoms were reproduced based on the report by Alaviet al. (1998).

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Student comprehension for non-chemistry majors

An online lecture was carried out for second-year (fourth semester) environmental science majors of A uni-versity. Many students ofAuniversity do not take chemistry in high school and they study basic chemistry afterentering university. The reason for conducting the lecture in an online style has to do with the students beingrestricted from going to school due to measures against COVID-19. Sixty-three students, including eightwomen, attended the online lecture. Shortly after the 50-min lecture, students took an online comprehensiontest. The test took 20min, and it consisted of a total of 12 questions, including four questionnaires (Table 2). Theinstructiveness of the lecture was estimated based on the result of the comprehension test. Among the par-ticipants, 76% had taken Introductory Chemistry in their first year at University (second semester), and 21%

Table : Questions and percentage of correct answers in the comprehension test administered to non-chemistry majorsimmediately after the lecture.

No. Question Percentage of correct answers

Did you take the “Introductory Chemistry (nd Semester)” class at the university?□ Yes; □ No

Yes_% (/)

Are you aiming to get a science teacher's license now? □ Yes; □ No Yes_% (/) Do you like chemistry? □ Yes; □ No Yes_% (/) Did you enjoy today's lecture? Strongly agree disagree _% (/)

_% (/)_% (/)

Briefly explain what catalytic converter (automotive catalyst) is. % (/) Complete the chemical reaction equation for complete combustion of carbon

monoxide. (a) CO + (b) O → (c) CO

% (/)

Which component is contained in the exhaust froma gasoline-engine when the airsupplied to the engine is low? Not necessarily one. Answer everything.□ gasoline component [HC]; □ carbon monoxide CO; □ NOx x = or

% (/)

Which is the correct mechanism for the catalytic oxidation of carbonmonoxide onplatinum surfaces?□ Oxygen molecules adsorb on the platinum surface, while carbon monoxidemolecules do not.□ Both oxygen molecules and carbon monoxide molecules are adsorbed on theplatinum surface.

% (/)

Which is the correct mechanism for the catalytic oxidation of carbonmonoxide onplatinum surfaces?□ Oxygen molecules adsorbed on the platinum surface dissociate into atomicoxygen.□ Carbon monoxide molecule adsorbed on the platinum surface dissociates intoatomic oxygen and atomic carbon.

% (/)

Which is the correct mechanism for the catalytic oxidation of carbonmonoxide onplatinum surfaces?□ Atomic oxygen settles at the top site of the platinum surface.□ Atomic oxygen settles at the hollow site on the platinum surface.

% (/)

Which is the correct mechanism for the catalytic oxidation of carbonmonoxide onplatinum surfaces?□ Carbon monoxide is adsorbed on the platinum surface through the oxygenatom.□ Carbon monoxide is adsorbed on the platinum surface through the carbonatom.

% (/)

Which is the correct mechanism for the catalytic oxidation of carbonmonoxide onplatinum surfaces?□ Carbonmonoxide molecule and atomic oxygen collide to form a carbon dioxidemolecule.□Atomic carbon and oxygenmolecules collide to forma carbon dioxidemolecule.

% (/)

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were aiming to become science teachers. Only 32% of the students answered they liked chemistry; however,89% of them answered that they had enjoyed the lecture. As expected, lectures using handmade teaching aidswhich also include demonstration experiments were also popular with college students.

In the case of the non-chemistry majors, the correct answer rate had a value over 80% for all questions.Notably, since the lecture on catalytic converters attracted the interest of environmental science majors, theirtest scores after the lecture were relatively high. Of course, this good result was not due only to the usefulnessof the teaching aid but also to the efforts made by the non-chemistry majors. Five students incorrectlyanswered the question pertaining to the definition of catalytic converter (Q5 in Table 2). One of themcopied andpasted the contents of a corresponding Wikipedia page. The remaining four gave completely irrelevant an-swers. The accuracy rate of the answers to the question in which students were asked to balance the chemicalequation representing the combustion of carbon monoxide (Q6) was 95%, a higher than expected value. Thisoutcome may derive from the fact that lecture participants had studied basic chemistry thoroughly in anIntroductory Chemistry class. The percentage of correct answers to the other questions was also high.

Table : Questions for high school students and percentage of correct answers.

No. Question Worksheet onlecture day

Comprehension testone after week the

lecturea

Do you like chemistry? □ Yes; □ No Yes_% (/) – Complete the chemical reaction equation for complete combustion of carbon

monoxide. (a) CO + (b) O → (c) CO

% (/) % (/)

Which component is contained in the exhaust from a gasoline-engine when theair supplied to the engine is low? Not necessarily one. Answer everything.□ gasoline component [HC]; □ carbon monoxide CO; □ NOx x = or

% (/) % (/)

Which is the correct mechanism for the catalytic oxidation of carbon monoxideon platinum surfaces?□ Oxygen molecules adsorb on the platinum surface, while carbon monoxidemolecules do not.□Both oxygenmolecules and carbonmonoxidemolecules are adsorbed on theplatinum surface.

% (/) % (/)

Which is the correct mechanism for the catalytic oxidation of carbon monoxideon platinum surfaces?□ Oxygen molecules adsorbed on the platinum surface dissociate into atomicoxygen.□ Carbon monoxide molecule adsorbed on the platinum surface dissociatesinto atomic oxygen and atomic carbon.

% (/) % (/)

Which is the correct mechanism for the catalytic oxidation of carbon monoxideon platinum surfaces?□ Atomic oxygen settles at the top site of the platinum surface.□ Atomic oxygen settles at the hollow site on the platinum surface.

% (/) % (/)

Which is the correct mechanism for the catalytic oxidation of carbon monoxideon platinum surfaces?□ Carbon monoxide is adsorbed on the platinum surface through the oxygenatom.□ Carbon monoxide is adsorbed on the platinum surface through the carbonatom.

% (/) % (/)

Which is the correct mechanism for the catalytic oxidation of carbon monoxideon platinum surfaces?□ Carbon monoxide molecule and atomic oxygen collide to form a carbondioxide molecule.□ Atomic carbon and oxygen molecules collide to form a carbon dioxidemolecule.

% (/) % (/)

Did you enjoy today's lecture? Strongly agree disagree _% (/) –

aFour students were absent when the comprehension test was administered.

436 R. Horikoshi et al.: Illustrating catalysis with a handmade molecular model set

Student comprehension for high school students

As described above, the lecture was conducted online for non-chemistry majors; however, a face-to-facelecture was conducted for high school students. The lecture was conducted for second-year students (age: 17)as an extracurricular lecture in high school B. Participants were 18 students (nine male and nine female) whoaimed to advance to science college. Some of them plan on going to medical school after graduation. Notably,the participants have already studied the full range of high school chemistry. The lecture time was 50min. Thestudents sat in groups of three and listened to the lecturewhilemoving themolecularmodels on the distributedteaching aid. A worksheet including the nine questions listed in Table 3 was distributed at the beginning of thelecture, and participants listened to the lecturewhile filling it out. Oneweek after the lecture, the students werealso administered a comprehension test includingmostly the same questions included in theworksheet. Basedon the results of the worksheets and test, the instructor considered the instructiveness of the lecture.

The worksheets filled out on the day of the lecture were all correct, but the rate of correct answers in thecomprehension test administered one week later decreased for some questions. The average rate of correctanswers for all the questionswas lower than 70%. To question 4 (Table 3),many students answered that carbonmonoxide did not get adsorbed on the platinum surface. The instructor moved the carbon monoxide model todemonstrate its adsorption on the platinum surface model before colliding with atomic oxygen, but thestudents mistakenly recalled that the carbon monoxide molecule had not undergone adsorption on theplatinum surface. Half of the students answered that the site where atomic oxygen settled was the top site.Notably, it may have been difficult for the students to distinguish between top and hollow sites, which areunfamiliar technical terms for them.With only 50min of lecture time, high school studentsmight have felt thatthe lecture was progressing too quickly. Therefore, the teaching aid should be moved more carefully andexplained inmore detail in the lecture intended for high school studentswith respect to the lecture intended forthe non-chemistry majors.

Comparisons with previous works

Lectures for chemistry majors may need to involve computer graphics for representing catalyst structures andcatalytic reactions more accurately. On the other hand, the student-friendly teaching aids developed in thisstudy are convenient for non-chemistry majors and high school students. Several excellent student-friendlyteaching aids made from readily available materials have been developed in recent years (Elsworth, Li, & Ten,2017; Moreno, Alzate, Meneses, & Marín, 2018; Siodłak, 2017; Turner, 2016). These teaching aids are inex-pensive and lightweight, which makes them easy to use in classrooms. Moreover, constructing them in theclassroomwith students would be an enjoyable activity. The teaching aid we developed in this study is a littleexpensive at 30 USD, and it is somewhat bulky and hard to carry. Additionally, it takes three days to manu-facture it, so the instructor would not be able to construct it in the classroom together with the students.However, our catalyst model is one of the few teaching aids that can illustrate the mechanisms of catalyticreactions (Horikoshi, 2015; Horikoshi, 2021; Horikoshi et al., 2013; Horikoshi et al., 2014).

Conclusions

With the evolution of science and technology, details of chemical reactions have been revealed, and newmaterials have appeared. In extracurricular lectures for undergraduate andhigh school students, such cutting-edge topics are popular. Although these sorts of topics tend to be challenging for these students, the use ofeffective teaching aids can facilitate comprehension. Currently, catalytic converter systems comprise largeamounts of platinum-groupmetals (Pt, Pd, and Rh), and catalyst designs based on abundantmetals are highlydesirable from the viewpoint of element strategy initiative. Recently, the development of catalytic converters

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comprising mainly abundant metals (Fe, Mn, or Cu) has become an actively investigated research topic(Hosokawa et al., 2020; Ueda, Tsuji, Ohyama, & Satsuma, 2019). Since the mechanism of the catalytic reactionwhereby exhaust gas is purified by abundant metals has been also clarified, we will start the development ofteaching aids to illustrate the reaction mechanism in the future.

Hazards

Since hydrogen is an explosive gas, it should be used in small quantities. The neodymiummagnets inside thespherical plastic capsules and ping-pong balls may damage computers, mobile phones, and watches.

Acknowledgments: The authors are grateful to the students who participated to the extracurricular chemistryclass (Osaka Sangyo University and Tezukayama High School). R. H. thanks H. Nakajima (Tezukayama HighSchool) and H. Sikaura (KANAZAWA Science of GOLDMuseum) for their helpful discussions. R. H. also thanksEnago (www.enago.jp) for the English language review.Author contributions: All the authors have accepted responsibility for the entire content of this submittedmanuscript and approved submission.Research funding: This studywas supported by JSPS KAKENHI Grant Number JP19H04709 (Synthesis ofMixedAnion Compounds toward Novel Functionalities).Conflict of interest statement: The authors declare no conflicts of interest regarding this article.

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Supplementary Material: Instructions for constructing the structure models and lecture slides (PDF). The online version of thisarticle offers supplementary material (https://doi.org/10.1515/cti-2020-0010).

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