The fate of platinum in Pt/Ba/CeO₂ and Pt/Ba/Al₂O₃ catalysts during thermal aging

Abstract

The behavior of Pt during aging of Pt/Ba/CeO₂ and Pt/Ba/Al₂O₃ NO_x storage-reduction catalysts (NSR) was studied using model catalysts with high and low Pt loadings. Pt composite formation, due to the possible reaction between BaO and platinum oxides, was observed in several cases and was elucidated by a series of analytical techniques, including X-ray diffraction (XRD), thermal analysis (TA), X-ray absorption spectroscopy (XANES, EXAFS), and electron microscopy. During calcination of mechanical mixtures of BaCO₃ and PtO₂, BaPtO₃ was formed at ca. 600 °C, transforming to BaPtO_2.38 above 800 °C. Investigation of Pt/Ba/CeO₂ and Pt/Ba/Al₂O₃ model catalysts with high and low Pt loadings revealed that in the case of Pt/Ba/CeO₂, the mixed oxide BaPtO₃ was formed at relatively low temperature (600–700 °C) in oxidizing atmosphere. Above 800 °C, BaPtO₃ reacted further with BaCeO₃ to form a double perovskite, Ba₂PtCeO₆. In contrast, for Pt/Ba/Al₂O₃, only the sintering of Pt, with no mixed Pt–Ba oxides, was found. The recovery of the catalytically active metallic Pt species could be achieved in the aged Pt/Ba/CeO₂ catalyst by reduction with hydrogen at relatively low temperature. Finally, investigation of the NO_x storage and reduction activity of the fresh, aged, and reduced catalyst confirmed that this treatment is beneficial for catalyst reactivation.

Introduction

The introduction of lean-burn engines with direct fuel injection is presently one of the most promising concepts for decreasing fuel consumption and thereby reducing associated CO₂ emissions [1], [2]. However, neither the conventional oxidation catalysts nor three-way catalysts are able to reduce NO_x emissions under lean conditions. The most promising strategy for reducing NO_x emissions is to use the NO_x storage-reduction (NSR) catalyst [1], [3], [4]. The NSR catalysts normally contain noble metals (Pt, Rh) for the oxidation of NO to NO₂ (under lean fuel conditions) and the reduction of stored NO_x (under rich conditions), and a storage component deposited on carrier oxides with a high surface area, such as La₂O₃-stabilized γ-Al₂O₃ or CeO₂ [3], [4], [5], [6]. The NSR performance of such catalysts depends on various factors, including the dispersion of the noble metal and the Ba constituent as well as its thermal stability, structural and textural properties. Deactivation of NSR catalysts is caused mainly by sulfur and thermal deterioration. Apart from the development of catalysts with higher tolerance for SO₂ or SO₂-derived species [7], one of the major challenges is therefore the improvement of the thermal stability and the development of possible reactivation procedures.

Generally, thermal deterioration induces sintering of catalytically active phases, the collapse of the pore structure of the support, and chemical transformations of catalytically active species, for example, during their reaction with the support. For automotive catalysts, the reaction of the catalytic phase with the support or other washcoat components [8], [9], particle growth of the precious metals [10], [11], sintering of the support and encapsulation of active metal particles [12], [13], and volatilization of active compounds [14] have been encountered. In addition, several studies of NO_x storage-reduction catalysts have been reported; these have focused on the formation of Ba-support composite oxides and sintering of noble metal particles [5], [8], [9], [15]. Also important is encapsulation of the noble metals by reducible supports and formation of rhodium aluminates [16], [17]. Surprisingly little attention has been given to the possible reaction between BaO and the platinum constituent, both of which are present in NSR catalysts. Despite its relative chemical inertness and stability, platinum reacts at high temperatures with alkali and alkaline earth metal to form mixed oxides. The reaction between BaCO₃ and PtO₂ or Pt black in oxidizing environments above 800 °C has been reported [18], [19], [20], [21]. The products of this reaction are BaPtO₃ [19], [21], Ba₄PtO₆ [18], and Ba₃Pt₂O₇, a solid solution with general formula Ba₃Pt⁴⁺_2+xO_7+2x [22], [23]. Using diffraction and anomalous fine-structure data, Vacinova and Hodeau [24] recently showed that the solid solution can be generally better described by the formula Ba_p(Ba_xPt²⁺_1−x)Pt⁴⁺_p−2O_3p−3, where p represents oxygen deficiency and x is a possible substitution of Pt²⁺ by Ba²⁺.

Platinum plays a double role in a NO_x storage-reduction cycle in the oxidation of NO to NO₂ (initiation of the storage process) and in regeneration, consisting of reduction of the stored NO_x species by H₂, CO, and hydrocarbons. Thus, the loss of Pt through the reaction with BaO could significantly affect the overall NO_x storage-reduction activity. Taking this scenario into account, in the present work we focused on the possible reaction between Pt- and Ba-containing species in Pt/Ba/CeO₂ and Pt/Ba/Al₂O₃ catalysts. For this purpose, we first investigated the processes occurring during calcination of BaCO₃ and PtO₂ mechanical mixtures and Pt/Ba/CeO₂ and Pt/Ba/Al₂O₃ model catalysts with high Pt loading, then studied the fate of Pt in model catalysts with low Pt loading, which have a greater practical relevance. Finally, we evaluated possible strategies for the reactivation of aged Pt/Ba/CeO₂ catalysts.