Promotional effect of Ca on the Pd/Ce–Zr/Al₂O₃ catalyst for low-temperature catalytic combustion of methane

Abstract

Promotional effect of Ca on the catalytic property of Pd/Ce–Zr/Al₂O₃ catalyst towards methane combustion is examined. The surface properties and the oxidation/reduction behavior of these catalysts are investigated by BET, TEM, XPS, TPR, TPO and TPSR techniques. Activity tests in methane combustion show that addition of Ca to Pd/Ce–Zr/Al₂O₃ can promote remarkably its low-temperature activity. The thermal stability of the Pd/Ce–Zr/Al₂O₃ catalyst to the exposure at high temperature is also enhanced by Ca loading. XPS and TEM results show that the addition of Ca to Pd/Ce–Zr/Al₂O₃ catalyst generates well-dispersed PdO particles on support. H₂–TPR, O₂–TPO and CH₄/O₂–TPSR experiments show that the addition of Ca improves the reduction/reoxidation properties and thermal stability of the active PdO species, which increases the catalytic activity and thermal stability of the Pd/Ce–Zr/Al₂O₃ catalyst.

Introduction

It is widely accepted that catalytic combustion is a promising method that enables high combustion efficiency as well as low emission of air pollutants such as NO _x, CO and unburned hydrocarbons [1], [2], [3], [4], [5], [6]. Numerous research works have been carried out in order to develop catalytic combustors for high-temperature applications, such as gas turbines and jet engines [7]. Depending on the design of the catalytic combustion system, catalysts are exposed to a wide temperature range, which is 300–1300 °C. Therefore, the catalyst should be active at low temperatures to ensure ignition as soon as the mixture reaches the catalyst inlet. On the other hand, the catalyst should also be thermostable. So the development of a practical catalyst that has both high-temperature stability and low-temperature activity is crucial [8], [9].

Palladium supported catalyst has been found to be the most active catalyst for the catalytic combustion of methane [10], [11], [12], [13], [14], [15], [16], [17], [18], [19]. PdO has also been identified to be the active palladium species over those developed palladium catalysts [20], [21]. Alumina-supported catalysts are undoubtedly the most studied systems. However, one of major problem of this catalyst system is the decrease of the catalytic activity caused by the decomposition of the active component PdO into Pd metal at the high temperature [22]. Some researchers have thus evaluated the modification of Pd/Al₂O₃ catalyst by adding stabilisers cations, which besides improving thermal stability [23], [24], also affect PdO phase, especially its redox cycle [25]. The addition of La₂O₃ [23] and BaO [24] stabilizes the surface area of alumina and the addition of CeO₂ [25] and NiO [26] prevents the transformation of PdO to Pd.

In a previous work performed at our laboratory [27], we noticed that the presence of Ce and Zr have a strong influence on the thermal stability of Pd/Al₂O₃ catalyst. Their role is to stabilize Pd in a high oxidation state and enhance the reoxidation of Pd into PdO under cooling conditions. So the Pd/Ce–Zr/Al₂O₃ catalyst has higher catalytic activity after calcined high temperature (1100 °C). However, when the catalysts calcined at lower temperature (500 °C), Pd/Ce–Zr/Al₂O₃ is less active than Pd/Al₂O₃ catalyst. The purpose of the present work is to investigate the possibility of preparing catalysts that has both high-temperature stability and low-temperature activity. The focus of the present study is to examine the influence of the addition of Ca to Pd/Ce–Zr/Al₂O₃ with respect to the catalytic activity in complete oxidation of methane. Moreover, N₂-adsorption, transmission electron microscopy (TEM), temperature programmed reduction (TPR) of hydrogen, temperature programmed oxidation (TPO) and temperature programmed surface reaction (TPSR) are used to study the relation between the catalytic activity and the physical and chemical properties of the supported palladium catalysts.

Ce–Zr–Ca/Al₂O₃ supports are prepared by co-impregnation of pseudo-boehmite (S_BET = 218 m²/g) with an aqueous solution of Ce, Zr and Ca nitrates, respectively. The samples are dried at 100 °C, and then calcined at 900 °C for 2 h. The total content of Ce and Zr as oxide state (CeO₂ + ZrO₂) is 18 wt.% and the molar ratio of Ce and Zr is 1:4. The content of Ca (M) is 0.2 wt.%, 0.4 wt.%, 0.8 wt.%, 1.6 wt.% for Ce–Zr–Ca/Al₂O₃, respectively. The corresponding supports are named as Ce–Zr–Cax/Al₂O₃ (x = 1, 2, 3, 4) as increasing the Ca wt.% content. A reference Ce–Zr/Al₂O₃ support is also prepared according to the same procedure followed in the Ce–Zr–Ca/Al₂O₃ systems.

Pd/Ce–Zr–Ca/Al₂O₃ and Pd/Ce–Zr/Al₂O₃ catalysts are prepared by conventional impregnation with an aqueous of H₂PdCl₄ as metal precursors. The impregnated samples are reduced by hydrazine hydrate, filtered and washed with large amount of water, dried at 100 °C for 12 h and then calcined at 500 °C for 2 h. In order to compare their thermal stability, the catalysts are calcined at 1100 °C for 4 h. The content of Pd for all catalysts is 0.5 wt.%.

Catalytic activity measurements are carried out in a fixed bed reactor (6 mm i.d.) filled with 120 mg catalyst powders. The catalysts are directly exposed to the reaction gas as the reaction temperature is reached without any pretreatment. The reaction gases for methane combustion consist of 1.5 vol.% CH₄ and 6.0% O₂ in N₂. The space velocity is 18,000 h^− 1. The analysis of the reactant and the organic compound in production are performed on a GC equipped with flame ionization detector (FID).

The surface areas of catalysts are obtained from N₂ adsorption isotherms (at the liquid nitrogen temperature) with the BET method, using a Coulter OMNISORP-100 apparatus. Prior to adsorption measurements, the samples are degassed under vacuum for 2 h at 200 °C.

The size of the metallic particles on the supported Pd catalysts is checked with transmission electron microscopy (TEM) using a JEM-2010 (HR) apparatus operated at 200 KV.

X-ray photoelectron spectroscopy (XPS) measurements are performed on a VG ESCALAB 2201-XL spectrometer. Non-monochro Mg-Kα radiation are used as a primary excitation. The binding energies are calibrated with the C_1s level of adventitious carbon (284.6 eV) as the internal standard reference.

The reduction properties of supported Pd catalysts are determined by means of the TPR technique. Prior to H₂-TPR measurement, a 50 mg catalyst is pretreated in air at 300 °C for 0.5 h. The reducing gas is a gas mixture of 5 vol.% H₂ in N₂, which is purified using deoxidizer and silica gel. The flow rate of the reducing gas is 40 ml/min. The temperature of the sample is programmed to rise at a constant rate of 10 °C/min. The amount of H₂ uptake during the reduction is measured by a TCD. The effluent H₂O formed during H₂-TPR is adsorbed with a 5A molecular sieve.

The reoxidation properties of the reduced catalysts are analysed by temperature-programmed oxidation (TPO). A 100 mg catalyst is used for each measurement. Prior to TPO experiments, the samples are reduced in H₂ atmosphere at 500 °C for 1 h. Then, the reactor is purged by pure He for 1 h at 500 °C and allowed to cool down to room temperature under He flow. Subsequently, TPO experiments are performed using a gas mixture of 5 vol.% O₂ in He with a flow rate of 40 ml/min, increasing the temperature from room temperature to 1000 °C at a heating rate of 20 °C/min. After reaching 1000 °C, the temperature is cooled down to 300 °C at a cold rate of 20 °C/min. Amount of O₂ consumption during the O₂-TPO is also measured by a thermal conductivity detector (TCD).

The temperature programmed surface reaction (TPSR) is carried out in the same apparatus as the catalytic activity tests analysis. For the reaction test, the feed composition is 0.5 vol.% methane, 2 vol.% oxygen and balance of nitrogen. The space velocity is 72000 h^− 1. The reaction temperature increases from room temperature to 1000 °C at a heating rate of 20 °C/min. After reaching 1000 °C, the temperature is cooled down to 300 °C at a cold rate of 20 °C/min.