Elsevier

Journal of Catalysis

Volume 267, Issue 2, 25 October 2009, Pages 181-187
Journal of Catalysis

The study of the oscillatory behavior during methane oxidation over Pd catalysts

https://doi.org/10.1016/j.jcat.2009.08.010Get rights and content

Abstract

The application of the thermogravimetric analysis (TGA) combined with on-line mass-spectrometry of the effluent gas mixture and temperature-programed oxidation (TPO) experiments allowed to demonstrate that significant variation of the carbon content in Pd catalysts occurred during the oscillatory methane oxidation. Together with the visual observation of the color changes due to the periodic variation of the Pd valence, these data indicate a very complicated mechanism associated with the oscillatory behavior, where both Pd oxidation–reduction processes and carbon deposition-removal play a crucial role. Depending on the phase of the oscillatory cycle, three states of metallic Pd with different reactivities were revealed: highly active, freshly reduced, metallic Pd; moderately active metallic Pd modified by carbon; and poorly active metallic Pd with adsorbed oxygen. It was found that hundreds of carbon monolayers could be periodically accumulated in the Pd powder catalyst during the oscillatory cycle. The accumulation of carbon on the Pd catalyst was the reason for the antiphase oscillations of the reaction products CO2 and H2O.

Graphical abstract

Significant variation of carbon and oxygen content in Pd catalysts during the oscillatory methane oxidation was observed. Four states of Pd with different activities were revealed.

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Introduction

Methane oxidation over palladium catalysts has been intensively studied by many investigators in recent years and a review of the obtained results can be found in [1]. Nevertheless the behavior of this important catalytic system is still not well understood. The actual nature of the active sites on palladium-based catalysts has been debated to a great extent in the literature. Some researchers [2], [3] reported that the metal phase was more active, while other studies demonstrated that PdO was the main active phase [4], [5]. Recently attempts had been made to solve this problem using the surface science technique. In situ X-ray photoelectron spectroscopy (XPS) measurements had been carried out during heating and cooling of the Pd(1 1 1) surface in the reactant mixture CH4:O2 = 1:5 [6]. It was demonstrated that the observed hysteresis phenomenon was connected with the variation of the surface oxidation state. The most active surface state in oxygen excess was found to be “PdO seeds + Pd5O4 oxide”.

A relatively small amount of studies has been devoted to the mechanism of the reaction under methane-rich reactant mixtures [7], [8]. However, these conditions are very interesting, because an oscillatory behavior can be observed only in the fuel-rich mixtures. König et al. were the first group who observed the oscillatory behavior during methane oxidation over a thick Pd-film catalyst [9]. Later the oscillatory behavior was observed by other groups over supported [10] and massive Pd catalysts [11]. Although it is generally accepted that transformations of the active Pd phase are the reasons for the oscillations during methane oxidation, there is still some disagreement in the literature concerning the reaction mechanism and particularly the Pd state that is most active. König et al. [9] and Zhang et al. [11] assumed that the Pd surface oscillates between a highly active oxide state and a less active metal-rich state, while Deng and Nevell identified PdO to be the less active catalytic phase [10].

In previous research, we investigated the oscillatory behavior during methane and ethane oxidation over Ni and Co catalysts [12], [13], [14]. The application of thermogravimetric analysis (TGA) in combination with on-line mass-spectrometry showed that a significant variation of surface oxygen content (several tens of monolayers) occurred during the oscillations of methane oxidation over Ni catalysts, verifying the periodic oxidation and reduction of the metal surface [12]. The visual observation of periodic color changes during the oscillatory cycle due to the variation of Ni and Co valence provided additional evidence that oxidation–reduction processes played a crucial role in the nature of the oscillatory behavior during methane and ethane oxidation over these catalysts [12], [13], [14]. The goal of this study was the application of TGA analysis in combination with on-line mass-spectrometry to investigate the mechanism of the oscillatory behavior during methane oxidation over Pd catalysts.

Section snippets

Experimental

The most stable and reproducible oscillations were obtained during methane oxidation over Pd catalysts in the form of a foil (3 × 3 × 0.35 mm) or a powder (Aldrich, 99.9+%, 1.05 μm average particle size). The study of the oscillatory behavior during methane oxidation over the Pd foil was carried out in a quartz flow reactor (i.d. 5 mm), operated at atmospheric pressure. The reactor was inserted into a furnace which allowed the observation and recording of the state of the sample surface by a video

Oscillatory behavior during methane oxidation over the Pd foil

Oxidation of methane over the Pd foil has been studied at flow rates from 10 to 40 ml/min at Tr temperatures of 300–485 °C (reaction mixture CH4:O2:Ar = 82:14:4). Table 1 demonstrates the effect of temperature on concentrations of reaction products at a flow rate of 20 ml/min. As the reactor temperature was increased stepwise from 30 °C to 350 °C the rate of methane oxidation was very low and only traces of CO2 product were detected at 350 °C. In Tr temperature range between 350 and 400 °C, only a

Discussion

The application of the thermogravimetric analysis combined with on-line mass-spectrometry of the effluent gas mixture together with the visual observation of the color changes due to the periodic variation of the Pd valence indicated a very complicated mechanism of the oscillatory behavior, where both the oxidation–reduction processes and carbon deposition-removal play essential roles. The mechanism of the oscillations can be presented as follows: at point F (see Fig. 4) the catalyst is in the

Conclusions

The experimental results presented in this paper demonstrate how the study of oscillatory behavior allows the extraction of the new information about the reaction mechanism. The application of TGA analysis in combination with on-line mass-spectrometry allowed to identify the variation of the Pd state during one cycle of the oscillations and to correlate the various states of Pd with a catalytic activity. It was demonstrated that the nature of the oscillations was connected with periodic

Acknowledgment

This work was supported by the Russian Foundation for Basic Researches (Grant No. 08-03-00364).

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