Elsevier

Journal of Catalysis

Volume 379, November 2019, Pages 18-32
Journal of Catalysis

Promotion of La(Cu0.7Mn0.3)0.98M0.02O3−δ (M = Pd, Pt, Ru and Rh) perovskite catalysts by noble metals for the reduction of NO by CO

https://doi.org/10.1016/j.jcat.2019.09.005Get rights and content

Highlights

  • Noble metal promotion in Cu-doped LaMnO3 catalysts changes electronic structure.

  • Promotion changes the reducibility of B-site cations Cu and Mn.

  • Promotion leads to strong surface depletion of noble metal deviating from bulk value.

  • Noble metal promotion strongly influences the redox properties of the perovskites.

  • Low amounts of noble metals in surface regions improve the NO reduction by CO.

Abstract

To evaluate the structural and spectroscopic steering factors of noble metal promotion in the catalytic reduction of NO by CO, a series of La(Cu0.7Mn0.3)0.98M0.02O3−δ (M = Pd, Pt, Ru, Rh) perovskite catalysts is investigated. The materials are synthesized by a sol-gel method and characterized by X-ray powder diffraction (XRD), electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). All metal-promoted perovskites exhibit a comparatively higher activity for catalytic reduction of NO by CO with respect to pure La(Cu0.7Mn0.3)O3−δ . Among all catalysts tested, the La(Cu0.7Mn0.3)0.98Pd0.02O3−δ perovskite shows the highest catalytic activity, which is tentatively related to a combined synergistic effect of improved oxygen vacancy activity and noble metals. Additionally, the redox chemistry of the catalysts in different reducing (H2) and oxidizing (NO, O2) atmospheres is tested. An enhanced kinetic reducibility, especially with Pd, was observed. All the H2-reduced catalysts are capable of reducing NO. At low and intermediate temperatures, the formation of N2O is observed, but at higher temperatures NO is exclusively converted to N2. The introduction of noble metals leads to new adsorption sites for NO. As XPS suggests a tendency for depletion of noble metals in the surface-near regions, while the catalytic activity in NO reduction at the same time appears much improved, directed noble metal promotion with modest amounts especially in surface-near regions during synthesis appears as an encouraging method to economize the use of the latter.

Introduction

Emissions of nitrogen oxides (NOx) are a great concern for the environment and human life likewise [1]. In addition to its toxicity, NOx causes the formation of tropospheric ozone as well as photochemical smog (alongside SO2), and also plays a major role in acid rain formation [1]. Therefore, control of NOx emissions has become a very important environmental issue. Many methods for NOx abatement exist, including thermal decomposition, adsorption and catalytic removal [1].

For spark ignition engines, three-way catalysts (TWC) are commonly used [2]. Although a number of different reactions take place on a three-way catalyst, in view of environmental aspects the most important reactions are the oxidation of hydrocarbons (HC) and CO (to CO2 and H2O), as well as the reduction of NOx (to N2) [3], [4], [5]. The focus of the present manuscript lies on the reduction of NO by CO (following the reaction 2NO + 2CO → N2 + 2CO2) [6], [7], [8]. Three-way catalysts thereby typically contain Pt, Pd (for oxidation of CO and hydrocarbons) as well as Rh (for the reductive conversion of NOx) [3]. As those metals are rare and expensive, alternative catalysts with reduced noble metal loading are highly requested [9].

Perovskites as versatile catalysts [10], [11] are shown as potential alternatives for conventional diesel oxidation catalysts, NOx storage-reduction, soot combustion and three-way catalysts, to name a few examples [12], [13], [14]. The general formula of an oxidic perovskite is ABO3, where A has a larger ionic radius than B. In the perovskite structure, A and B are coordinated by 12 and 6 oxygen anions, respectively. Rare earth metal cations are usually found in the A-site position, while transition metals tend to occupy the B-site in the structure. The defect chemistry of a perovskite can be controlled by partial substitution of A and/or B, leading to compounds with the general formula of A1−xA′xB1−yB′yO3−δ [11]. The catalytic properties of these materials can be tuned by changing the composition [9], [14]. For three-way catalysts, the oxygen storage capacity is a key reaction parameter and some perovskites provide reversible oxygen uptake and release in a similar way as the archetypical ceria-based materials [14], [15].

In numerous studies, an efficient perovskite formulation for NOx abatement has been being looked for. La-based perovskites are frequently used in studies for three-way catalysis [9], [14], [12]. Manganese and copper containing perovskites have also attracted attention for the reduction of NO by CO due to their pronounced oxygen nonstoichiometry [16], [17]. In order to further enhance the catalytic activity of the oxides, small amounts of noble metals can be introduced [18]. In fact, some works deal with the synthesis, characterization, and catalytic properties of perovskite-type oxides with partial substitution of the B-site cation by precious metals. Guilhaume and Primet tested a LaMn0.976Rh0.024O3+δ catalyst in the NO + CO reaction, revealing a high activity around 573 K) [19]. Teraoka et al. [20] studied the effect of simultaneous Cu/Ru substitution in the perovskite structure for CO oxidation and the NO + CO reaction and found that the catalytic activity of La0.8Sr0.2M3+1−2yCuyRuyO3 (M3+ = Al, Mn, Fe, Co) materials increased significantly after substitution as a function of the host M3+ cations. The benefits of adding Pd and Pt into perovskite formulations for reductive NOx removal, oxidation of CO and hydrocarbons have also been reported [21], [22], [23]. One particular concern in the use of noble-metal-doped perovskite systems for automotive emissions control is the long-term stability of the structures under realistic operating conditions. Nishihata et al. in 2002 reported on one particular promising Pd-containing catalyst LaFe0.57Co0.38Pd0.05O3, on which Pd sintering is suppressed during ageing and time-on-stream. Reversible Pd exsolution and incorporation has been accounted for this enhanced stability, despite the exact location of Pd being unclear with respect to surface and bulk [24]. To focus on this issue, in this work we report on fundamental studies on the location and influence of noble metals (M = Pd, Pt, Rh and Ru) on the reduction of NO by CO on a previously characterized LaCu0.7Mn0.3O3−δ (LCM) catalyst [25]. The choice of LCM is essentially fuelled by the better catalytic properties in the reduction of NO by CO in comparison with the corresponding Fe- and Co-containing systems [25]. We note that from a practical viewpoint with respect to the use as a catalyst for the automotive emissions control, the stability of the LCM systems under realistic operating conditions has yet to be determined. However, based on previous results it appears as the most promising model system from a fundamental scientific standpoint to clarify the tasks we focus upon in the present manuscript. As a consequence of these results, the effect of noble metal promotion on the structural stability and the activity for the NO + CO reaction of the LCM73 catalyst is investigated in the present paper. A series of perovskites with the general composition La(Cu0.7Mn0.3)0.98M0.02O3 (M = Pd, Pt, Ru and Rh) were prepared by sol–gel synthesis and subsequently characterized by temperature-programmed methods, X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and electron microscopy (TEM). The catalytic performance was investigated in the catalytic reduction of NO by CO to determine the influence and steer the use of noble metals.

The study begins by analyzing the effect of noble metal promotion on a parent La3+Mn3+O3−δ lattice, which is further altered by rather large amounts of Cu. In fact, addition of Cu – on the basis of the sum charge of all participating cation species – leads to a perovskite with strong deviation from the ideal oxygen stoichiometry (depending on the oxidation state of manganese, the non-stoichiometry δ will increase to >0.2). This will allow us to especially focus on the influence of the oxygen vacancy concentration on catalytic performance, which is quantitatively studied by volumetric methods. XPS will be particularly used to focus on the chemical nature of the participating phases, at least in the surface-near regions, but also to reveal substantial surface enrichment/depletion of noble metal species in the surface and/or bulk. This would have an huge effect as it allows the selection of the catalyst synthesis to steer the amount of noble metals into surface or bulk regions of the catalyst and to eventually economize and decrease the use of noble metals, while keeping a high catalytic activity.

Section snippets

Preparation of the perovskite catalysts

The perovskites were prepared by sol-gel synthesis using metal nitrates La(NO3)3·6H2O, Mn(NO3)2·4H2O, Cu(NO3)2·3H2O, Pd(NO3)2·2H2O, Pt(NH3)4(NO3)2, Rh(NO3)3·2H2O, Ru(NO3)2, and citric acid as precursors. The latter has been shown to be a particular promising complexing agent for metal ions with respect to the homogeneous distribution in the solution [26]. The required amounts of metal nitrates were dissolved in 50 mL diluted nitric acid. As a complexing agent, citric acid was added and the

Results and discussion

The specific surface area values of the perovskite catalysts determined by BET are listed in Table 1. Those of the noble-metal promoted specimens are found in the range of 11–14 m2 g−1, indicating that the addition of the noble metals to LaMn0.7Cu0.3O3−δ decreases the respective surface area. (pure LCM73: 32 m2 g−1)

Conclusion

The catalytic activity of La(Cu0.7Mn0.3)0.98M0.02O3−δ (M = Pd, Pt, Ru and Rh) perovskite catalysts was evaluated for the catalytic reduction of NO by CO. It has been shown, that the addition of especially Pd to the surface-near regions (either present as very small particle clusters ≤1 nm or highly dispersed in surface-near layers) leads to an enhanced kinetic reducibility of the parent perovskite lattice. The catalyst with the formula La(Cu0.7Mn0.3)0.98Pd0.02O3−δ shows by far the highest

Acknowledgements

This work was performed within the framework of the funding programme IMPULSE Iran Austria, financed by funds of the OeAD fonds and of the Ministry of Science, Research and Technology of the Islamic Republic of Iran. We also thank the SFB F45-N16 special research program for financial support. This work was performed within the framework of the research platform “Materials and Nanoscience” and the special PhD program “Reactivity and Catalysis”, both at the University of Innsbruck.

References (46)

  • B. Gao et al.

    Mesoporous LaFeO3 catalysts for the oxidation of toluene and carbon monoxide

    Chin. J. Catal.

    (2013)
  • L.G. Tejuca et al.

    Structure and reactivity of perovskite-type oxides

    Adv. Catal.

    (1989)
  • Y. Liu et al.

    Significant performance enhancement of yttrium-doped barium cerate proton conductor as electrolyte for solid oxide fuel cells through a Pd ingress–egress approach

    J. Power Sources

    (2014)
  • R. Wang et al.

    Role of redox couples of Rh0/Rhδ+ and Ce4+/Ce3+ in CH4/CO2 reforming over Rh–CeO2/Al2O3 catalyst

    Appl. Catal., A

    (2006)
  • J.S. Yoon et al.

    Catalytic activity of perovskite-type doped La0.08Sr0.92Ti1−xMxO3−δ (M = Mn, Fe, and Co) oxides for methane oxidation

    Int. J. Hydrogen Energy

    (2014)
  • M. Shen et al.

    Effects of calcium substitute in LaMnO3 perovskites for NO catalytic oxidation

    J. Rare Earths

    (2013)
  • S.S. Maluf et al.

    Effects of the partial replacement of La by M (M=Ce, Ca and Sr) in La2xMxCuO4 perovskites on catalysis of the water-gas shift reaction

    J. Nat. Gas Chem.

    (2010)
  • M. Jo et al.

    Auger electron peaks of Cu in XPS

    Appl. Surf. Sci.

    (1996)
  • H. Tanaka et al.

    Advances in designing perovskite catalysts

    Curr. Opin. Solid State Mater. Sci.

    (2001)
  • A.E. Giannakas et al.

    Preparation, characterization and investigation of catalytic activity for NO+CO reaction of LaMnO3 and LaFeO3 perovskites prepared via microemulsion method

    Appl. Catal., B

    (2004)
  • A.K. Ladavos et al.

    Mechanistic aspects of NO+CO reaction on La2−xSrxNiO4−δ (x=0.00–1.50) perovskite-type oxides

    Appl. Catal., A

    (1997)
  • V.C. Belessi et al.

    Catalytic behavior of La–Sr–Ce–Fe–O mixed oxidic/perovskitic systems for the NO+CO and NO+CH4+O2 (lean-NOx) reactions

    Catal. Today

    (2000)
  • A. Srinivasan et al.

    Review of chemical reactions in the NO reduction by CO on rhodium/alumina catalysts

    Catal. Rev.

    (2010)
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