Analysis of catalytic oxidation of aromatic hydrocarbons over supported palladium catalyst with different pretreatments based on heterogeneous adsorption properties

https://doi.org/10.1016/j.apcatb.2008.03.011Get rights and content

Abstract

We investigated the catalytic oxidation and adsorption of benzene, toluene, and o-xylene (BTX) by using the palladium (Pd)-based catalyst and its pretreated (pre-oxidized and pre-reduced) catalysts. The physico-chemical properties of the catalysts were characterized and confirmed by using several reliable methods such as the Brunauer Emmett Teller (BET) surface area analysis, gravimetric adsorption analysis, light-off curves analysis, and X-ray photoelectron spectroscopy (XPS). The adsorption and catalytic activities of the catalysts were found to be closely connected with the pretreatment methods and temperatures. Especially, the hydrogen treated catalysts with a largely metallic form enhanced the adsorption ability and catalytic activity of toluene compared to that of the parent and air treated catalysts. In addition, the adsorption equilibrium isotherms of BTX on pre-reduced catalyst at three different temperatures were analyzed successfully with the two sites localized Langmuir (L2m) isotherm equation. Moreover, the strong correlations between the catalytic behavior and the adsorption properties of BTX were explained in terms of adsorption affinity, isosteric heat of adsorption, and adsorption energy distributions.

Graphical abstract

The influence of pretreatments (air and hydrogen) on the Pd based catalyst for the adsorption and catalytic oxidation of different aromatic hydrocarbons (BTX) were investigated. The strong correlation between the catalytic oxidation and the adsorption was explained in terms of adsorption affinity, isosteric heat of adsorption, and adsorption energy distribution.

Introduction

Developing reliable methods of controlling the emission of volatile organic compounds (VOCs) is of great importance because of their harmful effects on human health and the environment. In general, the catalytic oxidation of VOCs has been considered as one of the well-established technologies available such as oxidation, adsorption, absorption, etc. Compared to the common thermal oxidation, the distinguished characteristics of catalytic oxidation are (1) low thermal NOx emissions, (2) high destructive efficiency, (3) low energy cost and (4) a relatively high flexibility [1], [2], [3], [4].

On the other hand, the proper selection and development of the catalyst is essential for successful removal of VOCs. Currently, various kinds of catalysts such as supported noble metals, metal oxides, and mixtures of noble metals and metal oxides are being extensively used for the complete oxidation of VOCs with different advantages and limitations. Considering the activity, selectivity and stability of the catalysts for catalytic oxidation, supported noble metals have been generally regarded as the most desirable catalysts despite their low resistance to deactivation and high cost for pure component. In addition, previous studies have shown that the main advantages of palladium (Pd)-based catalysts are high activity, thermal stability, performance and low cost compared to the platinum (Pt)-based catalysts, although they have a relatively higher susceptibility to sulfur containing pollutants [5], [6], [7], [8], [9], [10], [11].

Recently, many researchers have reported that the performance of Pd supported catalysts for the oxidation of VOCs is highly dependent on the oxidation state of Pd. In other words, pre-reduced Pd catalysts largely having metallic species (Pd0) are more active than the pre-oxidized Pd catalysts existing mainly in the oxide form (PdO/Pd2+) [5], [9], [10], [12], [13], [14], [15], [16]. Until now, considerable research effort has been mostly concentrated on the understanding of the role of the active Pd phase on the catalytic activity. However, little attention has been given to the investigation of the influence of the surface state on adsorption properties and the relationship between the adsorption and the catalytic activities of VOCs [16], [17], [18], [19]. It has been known that the catalytic activities of catalysts for aromatic molecules are closely connected with the interrelation of the reactant and the catalyst. The main factors in controlling the activity sequence are the strength of adsorption (adsorption affinity and heat of adsorption), the order of ionization potentials (or Debye dipole moment), and the strength of the weakest Csingle bondH bond in the molecule [15], [20], [21], [22], [23], [24], [25]. It seems, therefore, that a systematic approach to adsorption and catalytic oxidation of the Pd-based catalyst may provide some information on the role of the oxidation state of supported Pd with respect to adsorption equilibrium and catalytic oxidation of VOCs. Thus, in this work, we investigated the influence of pretreatments (air and hydrogen) on the adsorption and catalytic activities of different aromatic hydrocarbons over the Pd-based catalyst. Especially, benzene, toluene and o-xylene (BTX) were chosen as model compounds for their toxic, carcinogenic and different molecular properties. Finally, we present our interpretation of thermodynamic parameters such as isosteric heat of adsorption, adsorption affinity and adsorption energy distribution as well as its light-off (or ignition) curves and X-ray photoelectron spectroscopy (XPS) analysis.

Section snippets

Catalyst preparation and characterization techniques

The catalyst was prepared by using a conventional incipient wetness impregnation method with an aqueous solution of Pd(NO3)2 (Johnson Matthey). δ-Al2O3 (Showa Aluminum) was used as support for Pd-based catalyst. The catalyst contained 0.3 wt% Pd and was denoted as Pd-0.30. The impregnated supports were dried at 120 °C overnight and calcined at 400 °C for 4 h in a crucible. The prepared catalysts were pelletized, crushed and then finally sieved to obtain the particles with a diameter of 0.2–0.3 mm

Catalytic oxidation and adsorption of toluene

To understand the influence of surface treatments on the adsorption and catalytic activities of BTX over the catalyst, two different gases of air and hydrogen were used. Table 2 compares the BET surface areas of Pd-0.30 and its air and hydrogen pretreated catalysts. As presented in Table 2, the negligible differences between the BET surface areas indicated the insignificant effect of the pretreatment temperatures and the pretreatment methods on the textural properties.

The light-off (or

Conclusions

In this work, the influence of catalyst pretreatments on the adsorption and catalytic activities of BTX over the supported Pd and its pretreated catalysts was studied. The catalytic behaviors and adsorption properties of BTX were found to be highly dependent on the pretreatment conditions. The oxidation state and the surface energy characteristics of Pd on the support play a major role in the adsorption and catalytic system. Pre-reduced Pd/Al2O3 catalysts presented a higher catalytic activity

Acknowledgement

This work is supported by the Ministry of Environment as “The Eco-technopia 21 project”.

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