Studies on self-sustained reaction-rate oscillations: II. The role of carbon and oxides in the oscillatory oxidation of carbon monoxide on platinum

doi:10.1016/0039-6028(87)90039-2

Surface Science

Volume 180, Issue 1, 1 February 1987, Pages 110-135

https://doi.org/10.1016/0039-6028(87)90039-2 Get rights and content

Abstract

CO oxidation on a platinum foil was studied in a high pressure flow cell (10²−10² Pa) and an UHV chamber (10⁻⁸ −5 × 10⁴ Pa) both interfaced to a surface infrared spectrometer. Real-time surface infrared and calorimetry experiments performed in the cell during oscillatory oxidation indicated a slow periodic variation (∼ 40%) in the number of active sites, the period of which was commensurate with that of the reaction-rate oscillations. Auger spectroscopy performed in the UHV chamber showed that surface carbon quantitatively accounted for the surface deactivation, as evidenced by the inverse correlation of the number of surface sites active towards CO adsorption with the surface carbon concentration and by the demonstration that, at the oscillation temperatures, carbon can diffuse from the bulk to the surface, oxygen can remove surface carbon and adsorbed CO can block carbon diffusion. Although silicon oxide was always detected on the surface with infrared spectroscopy, no periodic variation in it could be observed during the reaction-rate oscillations. Auger studies confirmed that the maximum and the variations in surface concentration of silicon oxide could not account for the variations in the number of active sites. A mechanism is therefore proposed in which carbon is driving the long-period self-sustained oscillations in the rate of CO oxidation on Pt.

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Combined spatially resolved operando spectroscopy: New insights into kinetic oscillations of CO oxidation on Pd/Γ-Al<inf>2</inf>O<inf>3</inf>
2019, Journal of Catalysis
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However, a contradictory study by Newton et al. reports no evidence for the oxidation/reduction cycles of Rh during oscillating CO oxidation [14]. Instead the drive for spontaneous kinetic oscillations was suggested to result from the build-up of atomic carbon on the Rh surface, which was thought to be responsible for periodic deactivation of the catalyst and cause fluctuations in temperature upon its combustion [8,9,14]. It is clear from all of these investigations that the spontaneous kinetic oscillations over such catalytic systems is a complicated matter that requires further investigation.
Spatially resolved, combined energy dispersive EXAFS (EDE) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements have been performed over a fixed catalyst bed of Pd/γ-Al₂O₃ during kinetic oscillations of CO oxidation. The kinetic oscillations of CO oxidation over Pd (or for that matter Pt or Rh) catalysts are a complicated phenomenon that require characterisation techniques with high time resolution and spatial resolution in order to make links between catalyst structure and surface reactivity. By measuring the extent of Pd oxidation at the nanoparticle surface, from Pd K-edge EDE, and matching this with the CO coverage, from DRIFTS spectra, at multiple positions of the fixed bed reactor it is found that the majority of the catalyst undergoes a sharp transition from the CO poisoned catalyst to the highly active, oxidised Pd surface. This transition occurs initially at the end of the catalyst bed, nearest the outlet, and propagates upstream with increasing temperature of the reactor. The oscillations in Pd surface oxide formation and CO coverage are observed only in the first ∼1 mm of the bed, which gives rise to oscillations in CO₂ and O₂ concentrations observed by end-pipe mass spectrometry after the light-off temperature. The catalyst initially exists as less active, CO poisoned metallic Pd nanoparticles before light-off which transition to a highly active state after light-off when the Pd nanoparticle surface becomes dominated by chemisorbed oxygen. Kinetic oscillations only occur at the front of the catalyst bed where there is sufficient concentration of CO in the gas phase to compete with O₂ for adsorption sites at the catalyst surface. We demonstrate the complex nature of the evolving catalyst structure and surface reactivity during catalytic operation and the need for spatially resolved operando methods for understanding and optimising catalyst technologies.
What drives spontaneous oscillations during CO oxidation using O <inf>2</inf> over supported Rh/Al<inf>2</inf>O<inf>3</inf> catalysts?
2014, Journal of Catalysis
Citation Excerpt :
Unusually, therefore, within the sources of autonomous oscillatory phenomena in CO oxidation thus far observed in supported nanoparticulate noble metal catalysts, the behavior of the Rh/Al2O3 system is not at all founded upon that oxidation–reduction mechanism, originally due to Turner et al. [7,8], and so admirably demonstrated for the analogous Pt/Al2O3 cases using both Debye analysis of diffraction data [13] and latter time-resolved (quick scanning) XAFS [17,19]. Instead, this system seems to represent an example of a “carbon”-based model originally developed by Chabal and co-workers [10,11] as a possible explanation of spontaneous oscillatory chemistry observed during CO oxidation of Pt foils contaminated with carbon residues. These residues may periodically cause deactivation of the surface and therefore result in oscillatory behavior.
Spontaneous oscillations during CO oxidation by O₂ over Rh/Al₂O₃ catalysts have been investigated for stoichiometric (2CO:1O₂) and net oxidizing (1CO:6O₂) cases using parallel application of time-resolved Rh K edge energy dispersive X-ray absorption spectroscopy (XAFS), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and online mass spectrometry (MS). Whilst oscillatory chemistry is clearly visible over a range of temperatures and feedstock compositions, no evidence can be found for the participation of any IR active surface carbonyls to this chemistry. Equally, Rh K edge XAFS yields no evidence for reduction/oxidation cycles in the supported Rh as underlying the oscillatory behavior at the gas inlet (surface) of the sample bed or at varying axial positions below this point. In the stoichiometric case, variations in sample temperature are observed by both in DRIFTS and in a thermocouple inserted at the top of the catalyst bed. Oscillations occurring under net oxidizing conditions (378 K), however, yield no such detectable thermal variations during CO oxidation. These observations are discussed in terms of previously suggested mechanisms to explain this spontaneous behavior and in terms of the sensitivity of the applied probes to various aspects of the reactive system.
Nonlinear dynamics on catalytic surfaces: The contribution of surface science
2009, Surface Science
Rate oscillations and chemical wave patterns on catalytic surfaces have been intensely studied in the past two decades. Starting with rate oscillations as an exotic phenomenon and with only a few speculative ideas available about the mechanistic cause today a very high level of detailed mechanistic understanding has been reached and very sophisticated experiments are performed in this area. The development in this field over the past 25 years is reviewed briefly highlighting the contribution of the “single crystal approach” to the progress which has been achieved.
Oxidation of CO on a Pt/Al<inf>2</inf>O<inf>3</inf> catalyst: From the surface elementary steps to light-off tests: V. Experimental and kinetic model for light-off tests in excess of O<inf>2</inf>
2004, Journal of Catalysis
This article is the final part of a study on the CO/O₂ reaction over a 2.9% Pt/Al₂O₃ in line with the microkinetic approach of the heterogeneous gas–solid catalysis. Mainly, the kinetic parameters of each elementary step of two kinetic models (Models M1 and M2) determined previously are used to explain the evolution of the coverage of the adsorbed CO intermediate species (a strongly adsorbed linear CO species on Pt⁰, denoted by L) as well as the turnover frequency (TOF in s⁻¹) during light-off tests (increase in the reaction temperature T_r) using 1% CO/x% O₂/He gas mixtures with x⩽50. Model M1 involves a L-H elementary step between L CO species and a weakly adsorbed oxygen species (O_wads). It is operative (a) whatever T_r in excess CO and (b) only at low T_r values in excess O₂. Model M2 involves a L-H elementary step between a L CO and a strongly adsorbed oxygen species: O_sads is operative at high T_r values in excess O₂. It is shown that the switch for M1 to M2, at the ignition process, during the heating stage occurs for a high CO conversion (>60%) at a specific T_r value (denoted by T_i) depending on the oxygen partial pressure. Similar to the observations on Pt single crystals, it is shown that the ignition process is associated with a surface-phase transformation from a Pt surface mainly covered by L CO species (denoted by Pt–CO) to a Pt surface mainly covered by O_sads (denoted by Pt–O). The Pt–CO → Pt–O transformation is due to an oxidative removal of the adsorbed L CO species into CO₂ and not to a competitive chemisorption. The high CO conversion associated with the Pt–CO → Pt–O transformation indicates that mass-transfer processes contribute to the ignition process. During a cooling stage from T_r>T_i, the switch from M2 to M1 (extinction process) is associated with the surface-phase transformation Pt–O → Pt–CO at a reaction temperature T_e<T_i.
Monte Carlo simulations of oscillations, chaos and pattern formation in heterogeneous catalytic reactions
2002, Surface Science Reports
Citation Excerpt :
If this tendency was strong, one could observe segregation of C and CO (or O) during oscillations. The IRAS data obtained by Burrows et al. [133,134] appear to be in favour of this effect. Taking into account these points, it is of interest to explore the effect of C islands on kinetic oscillations.
Experimental studies employing surface science methods indicate that kinetic oscillations, chaos, and pattern formation in heterogeneous catalytic reactions often result from the interplay of rapid chemical reaction steps and relatively slow complementary processes such as oxide formation or adsorbate-induced surface restructuring. In general, the latter processes should be analysed in terms of theory of phase transitions. Therefore, the conventional mean-field reaction–diffusion equations widely used to describe oscillations in homogeneous reactions are strictly speaking not applicable. Under such circumstances, application of the Monte Carlo method becomes almost inevitable. In this review, we discuss the advantages and limitations of employing this technique and show what can be achieved in this way. Attention is focused on Monte Carlo simulations of CO oxidation on (1 0 0) and (1 1 0) single-crystal Pt and polycrystal Pt, Pd and Ir surfaces and of NO reduction by CO and H₂ on Pt(1 0 0). CO oxidation on supported nanometre-sized catalyst particles and NO reduction on composite catalysts are also discussed. The results show that with current computer facilities the MC technique has become an effective tool for analysing temporal oscillations and pattern formation on the nanometre scale in catalytic reactions occurring on both single crystals and supported particles.
Monte Carlo simulations of self-sustained oscillations of CO oxidation over non-isothermal supported catalysts
1998, Chemical Engineering Science
A new Monte Carlo simulation of CO oxidation over a non-isothermal supported catalyst is presented. It uses the Monte Carlo method applied to multiple 2D crystallites distributed on a flat support in combination with the partial differential energy balance to simulate the non-isothermal reaction on supported catalysts. The simulation predicts ignition and extinction transitions similar to those observed during a temperature programmed reaction experiment. However, no auto-oscillatory behavior is predicted when the surface is assumed to have constant activity. Stochastic fluctuations at each crystallite cannot be synchronized through the thermal coupling to form global oscillations. The model was extended to include a surface with variable activity. In this case an oxidation/reduction mechanism altering the oxygen sticking coefficient provides a bistable mechanism for auto-oscillations. This variable activity model predicts auto-oscillations and changes in oscillatory behavior with temperature and O₂/CO ratio similar to those experimentally observed. Simulation results also show that thermal communications among crystallites affect the shape of global oscillations. Irregular and chaotic oscillations are predicted when there are differences in the crystallite size, activity, and distribution of the crystallites on the support.

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^∗: Present address: Department of Chemical and Bio Engineering, Arizona State University, Tempe, AZ 85287, USA.

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Studies on self-sustained reaction-rate oscillations: II. The role of carbon and oxides in the oscillatory oxidation of carbon monoxide on platinum

Abstract

Phys. Rev. Letters

J. Chem. Phys.

J. Chem. Phys.

J. Electron Spectrosc. Related Phenomena

Phys. Rev.

Surface Sci.

Surface Sci.

Surface Sci.

J. Catalysis

J. Electron Spectrosc. Related Phenomena

Catalysis Rev. Sci. Eng.

Catalysis Rev. Sci. Eng.

Catalysis Rev. Sci. Eng.

Kinetika Kataliz

Dokl. Akad. Nauk SSSR

Surface Sci.

Phys. Rev. Letters

Surface Sci.