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Detecting and utilizing minority phases in heterogeneous catalysis

View Article: PubMed Central - PubMed

ABSTRACT

Highly active phases in carbon monoxide oxidation are known, however they are transient in nature. Here, we determined for the first time the structure of such a highly active phase on platinum nanoparticles in an actual reactor. Unlike generally assumed, the surface of this phase is virtually free of adsorbates and co-exists with carbon-monoxide covered and surface oxidized platinum. Understanding the relation between gas composition and catalyst structure at all times and locations within a reactor enabled the rational design of a reactor concept, which maximizes the amount of the highly active phase and minimizes the amount of platinum needed.

No MeSH data available.


Related in: MedlinePlus

Simulated gas phase concentrations (yellow: carbon monoxide; magenta: oxygen; cyan: carbon dioxide) and structure of the nanoparticles throughout the reactor obtained from the kinetic reactor model, as depicted below each of the graphs: chemisorbed carbon monoxide (green), surface oxide (red) and surface intermediate (dark blue) at t = −2 s (A), 2 s (B), 2.5 s (C), 3 s (D), 4 s (E) and 18 s (F). After the switch to catalytic conditions, carbon dioxide production starts, leading to a region near the outlet of the reactor where both carbon monoxide and oxygen are depleted and, thus, a significant amount of the surface shows a very low coverage (intermediate phase, B–F). With increasing oxygen concentration that surface gets oxidized, roughly in the middle of the reactor (D) and progressing downstream (E,F).
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f3: Simulated gas phase concentrations (yellow: carbon monoxide; magenta: oxygen; cyan: carbon dioxide) and structure of the nanoparticles throughout the reactor obtained from the kinetic reactor model, as depicted below each of the graphs: chemisorbed carbon monoxide (green), surface oxide (red) and surface intermediate (dark blue) at t = −2 s (A), 2 s (B), 2.5 s (C), 3 s (D), 4 s (E) and 18 s (F). After the switch to catalytic conditions, carbon dioxide production starts, leading to a region near the outlet of the reactor where both carbon monoxide and oxygen are depleted and, thus, a significant amount of the surface shows a very low coverage (intermediate phase, B–F). With increasing oxygen concentration that surface gets oxidized, roughly in the middle of the reactor (D) and progressing downstream (E,F).

Mentions: Figure 3 illustrates how surface structure and gas phase composition influence each other throughout the reactor at different snapshots in time. The catalyst structures are based on the XAS data (Fig. 1); the mass spectrometry data are modelled with finite element simulations using mechanistically established assumptions (Langmuir-type reaction mechanism including competitive adsorption and reaction and a fast desorption step) and a simplified mass-transfer-model (for details on the modelling approach, see the SI). The kinetic mechanistic model assumes that adsorbed carbon monoxide prevents oxygen from reacting with the surface. Desorption of carbon monoxide frees a site that enables oxygen activation. Surface oxygen may react with co-adsorbed carbon monoxide freeing sites for oxygen to adsorb and react either to carbon monoxide or to platinum to form a surface platinum oxide. This surface can reduce by reaction with surface carbon monoxide. Before the switch (Fig. 3A), the catalyst is carbon monoxide covered throughout the reactor, as the only reactant in the gas phase is carbon monoxide. Two seconds after the switch to catalytic conditions (Fig. 3B), carbon monoxide oxidation has started. This leads to a gradual increase in the carbon dioxide signal at the reactor exhaust and a decrease of oxygen and carbon monoxide further into the reactor. A surface covered in carbon monoxide is active after desorption of carbon monoxide, so that free surface is available for oxygen to dissociatively chemisorb and to react to adsorbed carbon monoxide33. Carbon monoxide and oxygen are depleted in the downstream parts of the reactor, resulting in decreased carbon monoxide coverage (Fig. 3C), especially near the outlet of the reactor where the concentrations are lowest. Not all oxygen is converted 2.5 s after the switch and is thus present for a short period after the switch in the exhaust of the catalyst, yielding the local maximum in concentration (Fig. 2). The appearance of this oxygen peak depends on the oxygen to carbon monoxide ratio and the total gas flow.


Detecting and utilizing minority phases in heterogeneous catalysis
Simulated gas phase concentrations (yellow: carbon monoxide; magenta: oxygen; cyan: carbon dioxide) and structure of the nanoparticles throughout the reactor obtained from the kinetic reactor model, as depicted below each of the graphs: chemisorbed carbon monoxide (green), surface oxide (red) and surface intermediate (dark blue) at t = −2 s (A), 2 s (B), 2.5 s (C), 3 s (D), 4 s (E) and 18 s (F). After the switch to catalytic conditions, carbon dioxide production starts, leading to a region near the outlet of the reactor where both carbon monoxide and oxygen are depleted and, thus, a significant amount of the surface shows a very low coverage (intermediate phase, B–F). With increasing oxygen concentration that surface gets oxidized, roughly in the middle of the reactor (D) and progressing downstream (E,F).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC5121607&req=5

f3: Simulated gas phase concentrations (yellow: carbon monoxide; magenta: oxygen; cyan: carbon dioxide) and structure of the nanoparticles throughout the reactor obtained from the kinetic reactor model, as depicted below each of the graphs: chemisorbed carbon monoxide (green), surface oxide (red) and surface intermediate (dark blue) at t = −2 s (A), 2 s (B), 2.5 s (C), 3 s (D), 4 s (E) and 18 s (F). After the switch to catalytic conditions, carbon dioxide production starts, leading to a region near the outlet of the reactor where both carbon monoxide and oxygen are depleted and, thus, a significant amount of the surface shows a very low coverage (intermediate phase, B–F). With increasing oxygen concentration that surface gets oxidized, roughly in the middle of the reactor (D) and progressing downstream (E,F).
Mentions: Figure 3 illustrates how surface structure and gas phase composition influence each other throughout the reactor at different snapshots in time. The catalyst structures are based on the XAS data (Fig. 1); the mass spectrometry data are modelled with finite element simulations using mechanistically established assumptions (Langmuir-type reaction mechanism including competitive adsorption and reaction and a fast desorption step) and a simplified mass-transfer-model (for details on the modelling approach, see the SI). The kinetic mechanistic model assumes that adsorbed carbon monoxide prevents oxygen from reacting with the surface. Desorption of carbon monoxide frees a site that enables oxygen activation. Surface oxygen may react with co-adsorbed carbon monoxide freeing sites for oxygen to adsorb and react either to carbon monoxide or to platinum to form a surface platinum oxide. This surface can reduce by reaction with surface carbon monoxide. Before the switch (Fig. 3A), the catalyst is carbon monoxide covered throughout the reactor, as the only reactant in the gas phase is carbon monoxide. Two seconds after the switch to catalytic conditions (Fig. 3B), carbon monoxide oxidation has started. This leads to a gradual increase in the carbon dioxide signal at the reactor exhaust and a decrease of oxygen and carbon monoxide further into the reactor. A surface covered in carbon monoxide is active after desorption of carbon monoxide, so that free surface is available for oxygen to dissociatively chemisorb and to react to adsorbed carbon monoxide33. Carbon monoxide and oxygen are depleted in the downstream parts of the reactor, resulting in decreased carbon monoxide coverage (Fig. 3C), especially near the outlet of the reactor where the concentrations are lowest. Not all oxygen is converted 2.5 s after the switch and is thus present for a short period after the switch in the exhaust of the catalyst, yielding the local maximum in concentration (Fig. 2). The appearance of this oxygen peak depends on the oxygen to carbon monoxide ratio and the total gas flow.

View Article: PubMed Central - PubMed

ABSTRACT

Highly active phases in carbon monoxide oxidation are known, however they are transient in nature. Here, we determined for the first time the structure of such a highly active phase on platinum nanoparticles in an actual reactor. Unlike generally assumed, the surface of this phase is virtually free of adsorbates and co-exists with carbon-monoxide covered and surface oxidized platinum. Understanding the relation between gas composition and catalyst structure at all times and locations within a reactor enabled the rational design of a reactor concept, which maximizes the amount of the highly active phase and minimizes the amount of platinum needed.

No MeSH data available.


Related in: MedlinePlus