Effect of ceria on palladium supported catalysts for high temperature combustion of CH4 under lean conditions

doi:10.1016/S0920-5861(98)00518-5

Catalysis Today

Volume 50, Issue 2, 29 April 1999, Pages 399-412

https://doi.org/10.1016/S0920-5861(98)00518-5 Get rights and content

Abstract

The effects of CeO₂ and La₂O₃ addition to alumina-supported palladium catalysts have been investigated. Characterization of the promoted and unpromoted supports has been performed by XRD, TPR, L-Raman and surface area measurements. Supported Pd catalysts have been characterized by means of XRD and TG analyses.

TG analyses on palladium catalysts confirm the occurrence of reversible PdO⇔Pd⁰ transformation in the presence of O₂ and show that such palladium redox process is markedly affected by CeO₂. In presence of ceria, temperatures of reduction of PdO and reoxidation of Pd⁰ are both shifted 50–60°C above the corresponding temperature observed on unpromoted samples.

Activity tests in CH₄ combustion under lean conditions show that the addition of ceria on La₂O₃-stabilized alumina does not significantly affect light-off performances, but markedly affects the catalytic combustion behavior at high temperature in line with ceria effect of stabilization of active PdO species.

Introduction

In the last decades catalytic combustors for natural gas fueled gas turbines (GT) have attracted increasing attentions. Catalytica recently announced the commercialization of a system which guarantees NO_x emission below 3 ppm and CO and UHC emission below 10 ppm as certified in field tests on a 1.5 MW machine.

The development of suitable catalysts has played the main role in such achievement and will be still crucial in further developments. Catalytic combustor operates under adiabatic lean conditions. Air is delivered with extremely high flow rate (>100 kg/m² s) from the compressor (at temperatures, which depending on the pressure ratio, can be as low as 300°C) and is premixed with natural gas at equivalent ratios up to 0.4 which correspond to adiabatic reaction temperature of 1400°C. Accordingly, the catalyst must be extremely active to guarantee low temperature ignition, and able to withstand thermal stresses associated with high operating temperatures.

The following properties make mandatory the use of palladium-based catalysts in natural gas fuelled GT combustors:

1.
they have been known for a long time as the most active systems in methane combustion [1]; indeed, at short residence times of GT operations, they are the only catalysts able to provide ignition at temperatures close to that of the compressor discharge. This minimizes the preheating with NO_x-producing diffusive burner;
2.
they allow self-control of the catalyst temperature at 850–950°C during combustion of CH₄/air mixture with high adiabatic reaction temperature (1400°C), thus preventing the catalyst from too severe thermal stresses [2];
3.
all the Pd species that are present under reaction conditions (metal, oxide and hydroxide) exhibit negligible volatility at T<1000°C [3].

The property of temperature self-control has been associated with the reversible PdO⇔Pd⁰ transformation. Several studies have pointed out that the methane combustion activity of Pd oxide species is much more higher than that of metallic Pd 4, 5, 6, 7, 8. Accordingly, temperature self-control would occur through the following mechanism: at low temperature Pd is stable in the active oxide state and effectively ignites the reaction; the increase of the catalyst temperature due to combustion light-off causes the PdO→Pd⁰ reduction. Due to the low methane combustion activity of metallic Pd the reaction would switch-off thus decreasing the catalyst temperature; this results in the reoxidation of Pd to PdO that in turn re-ignites the combustion. At steady state a stable temperature would be reached which is related to the characteristics of PdO⇔Pd⁰ redox process and is well below the adiabatic reaction temperature.

Despite of the agreement on the general features of this mechanism many fundamental aspects deserve further investigation such as:

1.
the controlling factors of palladium redox process dynamics;
2.
the nature of the Pd species (bulk or skin oxide, PdO_x stoichiometry, dispersion of oxide and metal) and their associated activity and stability properties;
3.
the role of metal–support interactions.

Better comprehension of such fundamental aspects could provide a significant improvement of the catalyst performances, particularly with respect to minimization of catalyst overheating associated with local runaway which is described as a major challenge in the development of commercial application with satisfactory service life (8000 h) [2]. Pilot scale experiments [9]evidenced strong excursions of the catalyst temperature, particularly during start-up, that often result in permanent damage of the catalyst.

CeO₂ is a well known promoter in noble metal-based combustion catalysts. Indeed, palladium catalysts supported on CeO₂-promoted materials have been widely investigated and are currently used in catalytic mufflers operating under alternate reducing and oxidizing environment [10]. Studies on the role of CeO₂ in this kind of applications have been recently reviewed by Trovarelli [11]. However, much less work has been done to investigate the nature of palladium–CeO₂ interactions under lean conditions.

In this work the effect of CeO₂ addition on alumina supports for Pd-based combustion catalysts operating under lean condition is addressed. For this purpose binary and ternary supports have been prepared by impregnation of La₂O₃ and/or CeO₂ onto transition alumina and characterized by means of XRD and L-Raman spectroscopy, BET surface area and TPR experiments. Over palladium catalysts obtained with these supports, the characteristics of PdO⇔Pd⁰ transformation have been investigated by TG experiments with repeated heating/cooling cycles in air. Finally, activity tests in CH₄ combustion have been performed under both steady state and temperature cycled conditions. The major aim of the work has been to clarify the influence of CeO₂ on PdO⇔Pd⁰ transformation and its relevance with respect to CH₄ combustion properties.

Section snippets

Preparation

A γ-Al₂O₃ sample of 200 m²/g obtained by calcination at 700°C for 10 h of a commercial pseudobohemite powder precursor has been used as starting material. All the samples, supports and Pd-containing catalysts have been prepared via impregnation using the incipient wetness technique.

XRD and morphology

In Fig. 1 the XRD spectra of 5La-1000, 11.5Ce-1000 and 5La11.5Ce-1000 are reported, whereas Table 1 reports morphological data for all the supports calcined in the 1000–1100°C temperature range.

The 5La-1000 sample exhibits the reflections of transition δ- and θ-Al₂O₃ only (JCPDS 16-394 and 35-121, respectively,) along with a surface area of 124 m²/g. No reflections of α-Al₂O₃ are detected in the XRD spectra of this system upon calcination at 1050°C and 1100°C, that result in a surface area of 89

Conclusions

The effects of CeO₂ and La₂O₃ addition to alumina-supported palladium catalysts have been investigated through different characterization techniques and CH₄ combustion tests.

Characterization of the supports has evidenced that La₂O₃ is spread onto the alumina surface inhibiting its transition and sintering, whereas CeO₂ aggregates in crystals whose dimensions are controlled by the support porosity. A similar mechanism is likely responsible for the control of PdO particle dimensions.

In ternary La₂

Acknowledgements

Financial support for this work has been provided by Consiglio Nazionale delle Ricerche (CNR) – Roma (Italy). The author wish to thank prof. A. Trovarelli of Università di Udine for performing TPR experiments and providing assistance in their interpretation.

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Effect of ceria on palladium supported catalysts for high temperature combustion of CH4 under lean conditions

Abstract

Introduction

Section snippets

Preparation

XRD and morphology

Conclusions

Acknowledgements

Appl. Catal. A

J. Catal.

Appl. Catal. A

Catal. Today

J. Catal.

Combustion and Flame

Appl. Catal.

J. Catal.

Appl. Catal. A

Appl. Catal. A

Solid State Ionics

J. Catal.

J. Catal.

J. Catal.

Ind. Eng. Chem.

Effect of ceria on palladium supported catalysts for high temperature combustion of CH₄ under lean conditions