Reaction rate oscillations during CO oxidation over Pt/γ-Al2O3; experimental observations and mechanistic causes

doi:10.1016/0039-6028(85)90660-0

Surface Science

Volume 150, Issue 2, 2 February 1985, Pages 487-502

https://doi.org/10.1016/0039-6028(85)90660-0 Get rights and content

Abstract

In this paper we present experimental observations of reaction rate oscillations during CO oxidation over Pt/γ-Al₂O₃ at atmospheric pressures. Based on our experimental observations and prior experimental literature on this reaction we propose a mechanistic scheme, which we believe explains in qualitative terms the oscillatory behavior exhibited by this catalytic reaction system. This mechanistic scheme involves a Langmuir-Hinshelwood reaction between adsorbed CO and oxygen and a slow formation and reduction step of an inactive surface oxide species. Our experimental observations and mechanistic ideas, furthermore, support the existence of multiple time scale phenomena for this catalytic reaction system, an idea originally suggested by Chang and Aluko.

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The increase in the activation energy barrier with increasing O2/CO ratios (the inset of Fig. 4) may be explained as follows. Langmuir-Hinshelwood mechanism is generally proposed for CO oxidation over Pt surface sites [37,40–42]. The binding energies of CO and O change depending on the bonding configurations and are linearly correlated with the energy barriers for CO oxidation as has been reported in earlier theoretical calculations [32].
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The possibility of these two scenarios is discussed in more detail in the following sections. The band that we observed at 2123 cm−1 has been observed in many other studies [20,36–45] of CO adsorption on platinum, and has been assigned to CO adsorption on, or near, oxidized platinum sites. However, there are many different platinum-oxide phases (e.g. PtO, Pt3O4, PtO2), and it is not clear which phase is associated with the band at 2123 cm−1.
The kinetics of propane oxidation over Pt/Al₂O₃ are investigated in this work as a function of O₂/C₃H₈ ratio in the 150–300 °C temperature range. At high O₂/C₃H₈ ratios, the platinum nanoparticles are saturated with oxygen and the reaction rate is zero-order with respect to the oxygen partial pressures in this regime. As the oxygen coverage decreases with decreasing O₂/C₃H₈ ratio, the reaction rate increases and the reaction order changes from zero-order to negative-order in the oxygen partial pressure. The reaction rate is controlled to a large extent by the oxygen coverage on the platinum nanoparticles. However, at lower temperatures and higher oxygen pressures there is a slow deactivation of the catalyst that cannot be explained by a slow change in the oxygen coverage. Diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS) of adsorbed CO was performed to track the evolution of the nanoparticle structure over the course of the propane oxidation reaction and to determine whether the slow deactivation was caused by reconstruction of the platinum nanoparticles. We found that the platinum nanoparticles are significantly reconstructed during the course of the reaction, including the formation of a platinum oxide (PtO) which has a characteristic CO-DRIFTS band at 2123 cm⁻¹. The extent of PtO formation decreases with increasing temperature and, as a result, deactivation of the catalyst is less severe at higher temperatures. Unexpectedly, increasing the oxygen partial pressure resulted in less PtO formation. We believe that a different platinum oxide phase (e.g. PtO₂ or Pt₃O₄) is formed at higher oxygen pressures, which is reduced to metallic platinum during CO exposure at 25 °C, and therefore is not detectable by CO-DRIFTS. These results are unique because they show how the nanoparticle structure evolves over many hours of propane oxidation, and how the temperature and oxygen pressure influence the reconstruction of the nanoparticles, which has implications for a wide range of reactions not limited to propane oxidation.
Structural and chemical states of palladium in Pd/Al<inf>2</inf>O <inf>3</inf> catalysts under self-sustained oscillations in reaction of CO oxidation
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Single crystals [4,5], a metal wire [6] or metal powder granules [7,8] have been studied under conditions ranging from UHV [3,9] to atmospheric pressures [3,10–13], though it should be noted that the main data array was obtained for the bulk materials. Nevertheless, the number of publications devoted to self-sustained oscillation effects on the supported Pd [14–25] and Pt [26–34] catalysts is significant. Many of these works have focused on studying the kinetics of self-sustained oscillations and on the construction of kinetic models.
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The structure of alumina supported platinum catalysts has been studied using in situ time-resolved high-energy resolution fluorescence X-ray absorption spectroscopy (HERFD XAS), in situ infrared (IR) spectroscopy, and kinetic measurements during CO oxidation at O₂ to CO ratio of one. Regardless of the particle size, the CO oxidation occurred in two distinctive regimes, a high-activity regime and a low-activity regime, which have high and low rates of reaction respectively, as observed in previous studies. The size of the particles slightly affects the rate in the low-activity regime, which was more difficult to assess in the high-activity regime. The catalyst surface is poisoned by adsorbed CO in the low-activity regime, as probed by HERFD and IR, and in the high-activity regime, the catalyst exhibits higher activity paralleled with the formation of a surface oxide.
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The catalytically active species during the oxidation of carbon monoxide over a real alumina-supported platinum catalyst under atmospheric pressure are determined by combining in situ high-energy resolution fluorescence detection X-ray absorption spectroscopy (HERFD XAS) at the Pt L₃ edge and kinetic measurements. Catalysts were prepared by incipient wetness impregnation. The oxidation of carbon monoxide occurred in two distinctive regimes, a high-activity regime and a low-activity regime, which have high and low rates of reaction respectively. In the low-activity region, the catalyst is poisoned by carbon monoxide, limiting the dissociative adsorption of oxygen. In the high-activity regime, all CO is desorbed from the surface, increasing the rate of reaction. The low carbon monoxide concentration enables oxygen to react to the surface in this reaction regime generating a more reactive surface. HERFD showed that different phases are active depending on the reaction conditions in nano-sized catalyst particles.
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2009, Journal of Catalysis
Citation Excerpt :
The high activity regime occurs at high temperature and high carbon monoxide conversion. Similar results had been reported during carbon monoxide oxidation on various forms of platinum-based catalysts: [10–12,33,41–44]. Some studies on carbon monoxide oxidation have been made by varying the oxygen and carbon monoxide partial pressures at fixed temperatures [10,11,44].
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Reaction rate oscillations during CO oxidation over Pt/γ-Al2O3; experimental observations and mechanistic causes

Abstract

Chem. Eng. Sci.

J. Catalysis

J. Calalysis

J. Catalysis

Chem. Eng. Sci.

J. Catalysis

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Reaction rate oscillations during CO oxidation over Pt/γ-Al₂O₃; experimental observations and mechanistic causes