Elsevier

Catalysis Today

Volume 65, Issue 1, 1 February 2001, Pages 77-89
Catalysis Today

The role of Pd precursors in the oxidation of carbon monoxide over Pd/Al2O3 and Pd/CeO2/Al2O3 catalysts

https://doi.org/10.1016/S0920-5861(00)00547-2Get rights and content

Abstract

The role of palladium precursors (e.g. chloride — PdCl2; acetylacetonate — Pd(acac)2; nitrate — Pd(NO3)2) in the catalytic properties of Pd/Al2O3 and Pd/CeO2/Al2O3 catalysts toward CO oxidation was herein investigated. The characterization techniques used for mapping the Pd sites were H2 and CO chemisorption, infrared spectroscopy (FTIR) of CO adsorbed, and temperature-programmed desorption (TPD). Unsteady-state CO oxidation was carried out by temperature-programmed surface reaction (TPSR). The nature of palladium precursors and their interaction with CeO2 affected the metallic dispersion and the site morphologies. Highly dispersed metal particles (d>50%) were obtained by using palladium chloride and acetylacetonate precursors on Pd/Al2O3 catalysts. Pd(1 0 0) and Pd(1 1 1) were the major palladium crystallite orientations in these samples, but a larger amount of low coordination sites located on Pd(1 0 0) faces was observed for the ex-chloride sample. These sites accounted for the oxidation of CO at very low temperatures on Pd-Cl catalyst. In the presence of ceria, the Pd dispersion was a function of the way in which each Pd precursors interacted with CeO2. A two-fold decrease of dispersion was obtained to ex-chloride sample, while a two-fold increase to ex-nitrate and the same dispersion to ex-acetylacetonate samples were otherwise observed. The metallic redispersion may be the result of the occupancy of ceria oxygen vacancies by the palladium crystallites. Ultimately, the interaction with ceria redispersed Pd crystallites in a more organized bi-dimensional structure with the predominance of (1 1 1) orientation. Due to the transient conditions of the reaction, ceria did not promote, as expected, the oxidation of CO. Ceria reduced species (Ce3+) were not able to help CO oxidize at lower temperatures due to lack of oxygen into its lattice. Thus, the highest rates for CO oxidation were only observed at higher temperatures for the Pd/CeO2/Al2O3 catalysts, a result of a combination of strong competition for oxygen molecules, which have replenished the ceria lattice, and the low activity of Pd(1 1 1) sites.

Introduction

Carbon monoxide oxidation on platinum metal group has been widely studied due to its extensive use in pollution control devices, such as catalytic converters and gas sensors [1], [2]. Early studies [3], [4] have reported that the oxidation of CO on palladium surfaces is a structure-insensitive reaction and proceeds through a Langmuir–Hinshelwood kinetic. Reaction rates are affected by CO coverage, showing an inverse first-order dependence on CO concentration. More recently, Haruta et al. [5] reported that highly dispersed gold deposited on reducible metal oxides has a remarkable activity for CO oxidation at low temperatures, and the reaction was independent on the partial pressures of CO and O2 and structure sensitive. Exploring the possibilities of metal–support interaction by changing Pd site properties toward CO oxidation, Pavlova et al. [6], [7] have also observed structure sensitivity at low temperatures on small ensembles of Pd supported over Al2O3, TiO2 and SiO2 carriers.

Palladium active sites are influenced by several factors, as including the particle mean size, the support interaction, the nature of precursor salts utilized in the preparation, and so forth. The particle size plays an important role for structure-sensitive reactions since either the site coordination, such as kink, step, and terrace atoms, or the crystal orientation affect the catalytic reactivity [8]. The metal–support interaction also contributes to change Pd sites, especially when they are supported on reducible transition metal oxides (TiO2, CeO2, Nb2O5 and La2O3). Epitaxial alignment with restructuring of catalytic sites at the interface, electronic transfer or encapsulation of metal particles by reduced support species, are some of the explanations that are possible to find in literature to define metal–support interaction nature [9], [10], [11].

In this study the nature of Pd active sites on Pd/Al2O3 and Pd/CeO2/Al2O3 catalysts was investigated. The main goals were to determine whether different Pd precursors interact distinctively with CeO2, and to understand how the interaction mechanism affects the Pd catalytic properties for CO oxidation. The palladium sites were characterized by H2 and CO chemisorption, temperature-programmed desorption (TPD), and infrared spectroscopy (FTIR) of CO adsorbed. The probing reaction was the oxidation of carbon monoxide carried out under transient conditions by temperature-programmed surface reaction (TPSR).

Section snippets

Preparation of catalysts

Catalysts with 1 wt.% Pd were prepared by impregnation over either γ-Al2O3 (BET area of 208 m2 g−1) or CeO2/Al2O3 (BET area of 193 m2 g−1) carriers. The γ-Al2O3 (AL-3916P, Engelhard Corp.) was previously calcined at 823 K for 16 h in an aerated muffle at a heating rate of 2 K min−1. The CeO2/Al2O3 system was prepared from the grafting reaction between a cerium acetylacetonate precursor — Ce(acac)3 (Aldrich Co.) — and alumina surface hydroxyl groups as described elsewhere [12]. The grafting reaction,

Metallic dispersion

Table 1 shows CO chemisorption and metallic dispersion measured by H2 adsorption using volumetric and pulse techniques. No significant changing in the chemisorption values were found in either method. The influence of Pd precursors in the metallic dispersion of the catalysts is very clear. Pd-Cl and Pd-acac samples showed the same dispersion values (∼51%), well above of the value obtained for Pd-N sample (∼16%). Chloride and acetylacetonate precursors are known for interacting with hydroxyl

Conclusions

The role of palladium precursors in the catalytic properties toward CO oxidation for Pd/Al2O3 and Pd/CeO2/Al2O3 catalysts was investigated in this study. Pd(1 0 0) and Pd(1 1 1) were the major palladium crystallite orientations in these samples, and the distribution and proportion of such surfaces were affected by the Pd precursor nature and the presence of CeO2. Highly dispersed metal particles obtained by using palladium chloride and acetylacetonate precursors were the most active sites in CO

Acknowledgements

The authors would like to acknowledge Ruth L. Martins, Leila Merat, Sidnei Joaquim and Victor T. Santos for technical support during the characterization work. One of the authors (R.S. Monteiro) would like to thank CAPES (Ministry of Education, Brazil) for financial aid.

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    Present address: Department of Chemical Engineering, Worcester Polytechnic Institute, 100 Institute Road, Worcester, MA 01609, USA.

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