Illustrating catalysis with a handmade molecular model set: catalytic oxidation of carbon monoxide over a platinum surface

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

The positions and motions of the molecules adsorbed on the metal surface can be observed by scanning tunnel microscopy and be estimated by computational methods (Zambelli et al., 1997). In detail, an oxygen molecule gets adsorbed on a Pt surface, and it subsequently dissociates to produce two oxygen atoms that end up settling into hollow sites of the Pt surface (Figure 1).

Figure 1:

Representation of the reaction pathway whereby a dioxygen molecule in the gas phase produces oxygen atoms adsorbed on a Pt(111) surface: (a) before adsorption, (b) collision, (c) dissociation, and (d) settling into hollow sites of the Pt(111) surface.

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, as increasing the coordination numbers (Figure 2). In all cases, carbon monoxide coordinates perpendicularly to the Pt(111) surface via its carbon atom. Notably, in the bridge coordination model, the carbon dioxide model must be supported by hand to prevent it from falling.

Figure 2:

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

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

The generally accepted mechanism for the oxidation of carbon monoxide over platinum is of the L–H type. The mechanism proceeds via the following four steps: (i) dissociation of the dioxygen molecule into two oxygen atoms 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 carbon dioxide 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 of adsorbed 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 gets adsorbed on a top site of the Pt(111) surface, respectively (Figure 3a). Next, the carbon monoxide molecule moves to the adjacent top site via the formation of a bridge site (Figure 3b). At the same time, one oxygen atom leaves the hollow site for the adjacent bridging site (Figure 3c). Finally, carbon monoxide and atomic oxygen collide with each other around the hollow site (Figure 3d) to generate a molecule of carbon dioxide (Figure 3e). These collision and desorption processes are revealed in detail by a published ab initio density functional theory study (Alavi et al., 1998).

Figure 3:

Pathway of the reaction between atomic oxygen and carbon monoxide on a Pt(111) surface: (a) adsorption of oxygen atoms and a carbon monoxide molecule on the metal surface, (b–d) motions of atomic oxygen and carbon monoxide on the aforementioned surface, and (e) formation and desorption of carbon dioxide. The described motions of the molecules and atoms were reproduced based on the report by Alavi et al. (1998).

Notably, the demonstration experiment performed at the beginning of the lecture, which consisted of the oxidation (combustion) of molecular hydrogen on a Pt surface, can be also considered to proceed via an L–H mechanism (Gorodetskii, Block, & Drachsel 1994); however, illustrating this reaction pathway with the described 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 an online test administered immediately after the online lecture in the case of the non-chemistry majors (environmental science majors) and a comprehension test administered one week after the lecture in the case of the high school students. The results of these tests are summarized in Tables 2 and 3.

Student comprehension for non-chemistry majors

An online lecture was carried out for second-year (fourth semester) environmental science majors of A university. Many students of A university do not take chemistry in high school and they study basic chemistry after entering university. The reason for conducting the lecture in an online style has to do with the students being restricted from going to school due to measures against COVID-19. Sixty-three students, including eight women, attended the online lecture. Shortly after the 50-min lecture, students took an online comprehension test. The test took 20 min, and it consisted of a total of 12 questions, including four questionnaires (Table 2). The instructiveness of the lecture was estimated based on the result of the comprehension test. Among the participants, 76% had taken Introductory Chemistry in their first year at University (second semester), and 21% 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 aids which 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, their test scores after the lecture were relatively high. Of course, this good result was not due only to the usefulness of the teaching aid but also to the efforts made by the non-chemistry majors. Five students incorrectly answered the question pertaining to the definition of catalytic converter (Q5 in Table 2). One of them copied and pasted the contents of a corresponding Wikipedia page. The remaining four gave completely irrelevant answers. The accuracy rate of the answers to the question in which students were asked to balance the chemical equation representing the combustion of carbon monoxide (Q6) was 95%, a higher than expected value. This outcome may derive from the fact that lecture participants had studied basic chemistry thoroughly in an Introductory Chemistry class. The percentage of correct answers to the other questions was also high.

Student comprehension for high school students

As described above, the lecture was conducted online for non-chemistry majors; however, a face-to-face lecture 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) who aimed 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 50 min. The students sat in groups of three and listened to the lecture while moving the molecular models on the distributed teaching aid. A worksheet including the nine questions listed in Table 3 was distributed at the beginning of the lecture, and participants listened to the lecture while filling it out. One week after the lecture, the students were also administered a comprehension test including mostly the same questions included in the worksheet. Based on 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 the comprehension test administered one week later decreased for some questions. The average rate of correct answers for all the questions was lower than 70%. To question 4 (Table 3), many students answered that carbon monoxide did not get adsorbed on the platinum surface. The instructor moved the carbon monoxide model to demonstrate its adsorption on the platinum surface model before colliding with atomic oxygen, but the students mistakenly recalled that the carbon monoxide molecule had not undergone adsorption on the platinum 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 are unfamiliar technical terms for them. With only 50 min of lecture time, high school students might have felt that the lecture was progressing too quickly. Therefore, the teaching aid should be moved more carefully and explained in more detail in the lecture intended for high school students with respect to the lecture intended for the non-chemistry majors.