Synthesis, structure and three way catalytic activity of Ce1−xPtx/2Rhx/2O2−δ (x = 0.01 and 0.02) nano-crystallites: Synergistic effect in bimetal ionic catalysts

doi:10.1016/j.apcata.2006.09.015

Applied Catalysis A: General

Volume 315, 23 November 2006, Pages 135-146

https://doi.org/10.1016/j.apcata.2006.09.015 Get rights and content

Abstract

Bimetal ionic Ce_1−xPt_x/2Rh_x/2O_2−δ (x = 0.01 and 0.02) catalysts have been synthesized by a single step solution combustion method. CO and C₂H₄ oxidation and NO reduction activities of the bimetal ionic catalysts are compared with mono-metal ionic Ce_1−xPt_xO_2−δ (x = 0.01 and 0.02) and Ce_1−xRh_xO_2−δ (x = 0.01 and 0.02) catalysts. X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) studies show incorporation of Pt²⁺ and Rh³⁺ ions in CeO₂ lattice. Low temperature hydrogen uptake at 0 °C and below over bimetal ionic catalyst shows enhanced redox property over the corresponding mono-metal ionic analogues. The TOF for CO oxidation over Ce_0.99Pt_0.005Rh_0.005O_2−δ is 0.035 s⁻¹ at 120 °C, compared to 0.003 s⁻¹ over Ce_0.99Pt_0.01O_2−δ and 0.035 s⁻¹ over Ce_0.99Rh_0.01O_2−δ at 135 °C. Enhanced catalytic activity of Ce_1−xPt_x/2Rh_x/2O_2−δ (x = 0.01) bimetal ionic catalysts compared to mono-metal ionic catalysts, Ce_1−xPt_xO_2−δ and Ce_1−xRh_xO_2−δ, is attributed to synergism brought about by the easy reduction of Rh³⁺ ion by Pt²⁺ ion.

Graphical abstract

Bimetal ionic Ce_1−xPt_x/2Rh_x/2O_2−δ (x = 0.01 and 0.02) catalyst with Pt in 2+ and Rh in 3+ states is synthesized by a single step solution combustion method. The bimetal ionic catalyst induces reduction of Rh³⁺ at a lower temperature in presence of Pt²⁺ ion compared to the corresponding mono-metal ionic catalysts, Ce_1−xPt_xO_2−δ and Ce_1−xRh_xO_2−δ, showing enhanced catalytic activity as well as synergistic effect.

Introduction

Bimetallic catalysts often exhibit improved properties than either of the single metal catalysts. This is called synergistic effect. The presence of the synergistic activity of platinum and rhodium in bimetallic catalysts and possible reasons for the effects has been discussed in literatures [1], [2], [3], [4], [5], [6], [7]. The synergistic effects in Pt–Rh bimetallic catalysts have been shown over alumina-supported catalysts for CO oxidation and NO + CO reactions [8], [9], [10], [11], [12], [13], [14]. The presence of synergism in Pt–Rh/Al₂O₃ system is generally explained as due to Pt–Rh alloying effect. The decay in the activity of rhodium catalysts at high temperature is explained as due to: (a) formation of reduction resistive oxide layer on the alumina surface or (b) formation of rhodium aluminate. When Pt is present together with Rh, reduction of Rh is promoted, thereby leading to synergistic alloying effect.

Factors which contribute to the catalytic properties of a bimetallic catalyst include: (a) the surface composition of the noble metal and (b) the strength of interaction between the noble metal and the catalyst support. For alumina-supported platinum catalysts, Chu et al. [15] suggested the sintering of the dispersed particles as an important factor in catalyst deactivation, indicating weak interaction between platinum and alumina. Too strong metal–support interaction may also lead to decreased catalytic activity, as suggested for alumina-supported rhodium [16], [17], [18]. Mechanism of this deactivation is still unclear [19], [20], [21]. When both Pt and Rh are added to the support, the synergistic effect of platinum and rhodium is shown to be capable of stabilizing rhodium against the formation of a reduction-resistive oxide phase on alumina support as shown by Polvinen et al. for bimetallic Pt–Rh catalysts with pure alumina and CeZrO₂-modified alumina washcoat [22].

There are a few studies on bimetallic Pt–Rh/CeO₂ catalysts [22], [23], [24], [25], [26], [27]. The effects of high temperature redox ageing on this catalytic system have been studied. The nature of the active sites of the catalyst in this bimetallic system is debated.

In recent years, we have studied the single metal Pt and Rh ion substitution in ceria as in Ce_1−xPt_xO_2−δ and Ce_1−xRh_xO_2−δ (x = 0.01 and 0.02) catalysts, where Pt²⁺, Rh³⁺ ions are shown to be the active sites for adsorption [28], [29], [30]. These oxide solid solutions can be called mono-metal ionic catalysts. What can be the combined effect when both Pt and Rh are present as ions in the ceria supported catalyst? To understand this we have studied the structure, redox properties, CO and C₂H₄ oxidation, and NO reduction activity of Ce_1−xPt_x/2Rh_x/2O_2−δ (x = 0.01 and 0.02) bimetal ionic catalysts and compared them with their corresponding mono-metal ionic analogue, namely, Ce_1−xPt_xO_2−δ and Ce_1−xRh_xO_2−δ.

Section snippets

Experimental

The catalysts have been synthesized by the combustion of aqueous stoichiometric composition of (Pt, Rh) metal chloride, ceric ammonium nitrate with oxalyl dihydrazide (C₂H₆N₄O₂, ODH) as the fuel. Specifically, for the preparation of Ce_0.99Pt_0.005Rh_0.005O_2−δ, (NH₄)₂Ce(NO₃)₆, H₂PtCl₆, RhCl₃·xH₂O and ODH were taken in the mole ratio 0.99:0.005:0.005:2.376. A typical preparation consisted of dissolving 10 g of (NH₄)₂Ce(NO₃)₆ (E. Merck India Ltd., 99.9%), 4.77 ml of 1% H₂PtCl₆ (1 g H₂PtCl₆ dissolved in

XRD studies

Diffraction lines of the catalysts could be indexed to the fluorite structure of ceria. The sizes of the (Pt²⁺ and Rh³⁺ ion substituted) CeO₂ crystallites were obtained from the Scherrer formula. Crystallite sizes are in the range 40–50 nm (see Table 1). The XRD patterns of PtRhC1, PtRhC1(HT), PtRhC2 and PtRhC2(HT) catalysts, in the 2θ range 25–50° are magnified by 30 times in the Y scale with respect to CeO₂(1 1 1) peak and they are given in Fig. 1(a–d). XRD patterns of RhC1(HT) and PtC1(HT) are

Conclusions

In this study, activity of bimetal ionic catalyst is compared with the corresponding mono-metal ionic catalysts and the important conclusions are:

(i)
A single step solution combustion method can be employed to synthesize Ce_1−xPt_x/2Rh_x/2O_2−δ (x = 0.01 and 0.002).
(ii)
Hydrogen uptake study shows a lower temperature reduction of Rh³⁺ ion in the bimetal ionic catalysts compared to mono-metal ionic Ce_1−xRh_xO_2−δ catalyst.
(iii)
Synergistic effect of bimetal Pt²⁺, Rh³⁺ ion catalyst, Ce_1−xPt_x/2Rh_x/2O_2−δ, is shown to be

Acknowledgements

A. Gayen is thankful to Council of Scientific and Industrial Research (CSIR), Government of India for the award of a research fellowship. Thanks are due to C.R. Reddy, B.V. Kumar and N. Mahadeva, Bangalore Institute of Technology (BIT), for surface area measurement. Financial assistance from the Department of Science and Technology (DST), Government of India is gratefully acknowledged.

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¹: Present address: Department of Chemistry, Physical Chemistry Section, Jadavpur University, Kolkata 700032, India.

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Synthesis, structure and three way catalytic activity of Ce1−xPtx/2Rhx/2O2−δ (x = 0.01 and 0.02) nano-crystallites: Synergistic effect in bimetal ionic catalysts

Abstract

Graphical abstract

Introduction

Section snippets

Experimental

XRD studies

Conclusions

Acknowledgements

J. Catal.

J. Catal.

J. Catal.

J. Catal.

J. Catal.

J. Catal.

Appl. Catal. A: Gen.

J. Catal.

J. Catal.

J. Catal.

J. Catal.

J. Catal.

Appl. Catal. A

J. Catal.

J. Catal.

J. Catal.

Synthesis, structure and three way catalytic activity of Ce_1−xPt_x/2Rh_x/2O_2−δ (x = 0.01 and 0.02) nano-crystallites: Synergistic effect in bimetal ionic catalysts