A2B2O7 pyrochlore compounds: A category of potential materials for clean energy and environment protection catalysis

doi:10.1016/j.jre.2020.01.002

Journal of Rare Earths

Volume 38, Issue 8, August 2020, Pages 840-849

https://doi.org/10.1016/j.jre.2020.01.002 Get rights and content

Abstract

A₂B₂O₇ pyrochlore is a kind of important functional materials for different purposes, which has been investigated extensively by crystallographers and material scientists. However, the catalytic chemistry of this type of special compounds has rarely been documented, though a few researchers have tried to synthesize some pyrochlore compounds with different chemical compositions for a variety of green energy production and air pollution control reactions in the history. With the expectation to help catalysis scientists to get better acquaintance with, and gain deeper understanding on this type of compounds as heterogeneous catalysts, the major publications over the past several decades have been screened and reviewed in this paper, based also on our own experience of studying on this type of catalytic materials. The crystalline phase transformations of the compounds with the change of the A and B site cations, the phase change's influences on the surface and bulk properties, and their subsequent impact on the catalytic performance for different reactions have been summarized. Furthermore, the future work which needs to be performed to perceive in depth this kind of important materials as catalysts has been proposed and suggested. We trust that this short review contains valuable information, which will provide great help for people to get better cognition for A₂B₂O₇ pyrochlore compounds, and assist them to develop better catalysts for various reactions.

Graphical abstract

Pyrochlores are a type of potential and promising catalysts for clean energy production and environment protection catalysis. The structure and reactivity can be finely tuned by changing the A and B site cations. From the point of view of fundamental research and industrial application, it deserves further study on the preparation and properties of this type of interesting composite oxides.

Introduction

Pyrochlores are complex metal oxides with a general formula of A₂B₂O₇, where A site represents an 8-fold coordinated metals, such as Ca, Sr, Sn, Pb, Bi, Y, La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, Lu, and B site represents 6-fold coordinated metals such as Sn, Ti, Nb, Ta, Zr, Hf, Ru, Ce.¹ The A site cation generally has an oxidation state of +2 or +3, and the B site cation has a corresponding oxidation state of +5 or +4. Therefore, for a typical A₂B₂O₇ compound, the metal cations will possess a total valence state of +14, which will be balanced by 7 lattice O²⁻ anions.¹ It has been reported that the stable crystalline phase structure of an A₂B₂O₇ compound is decided by the r_A/r_B radius ratio.²

(1)
If this ratio is above 1.78, a monoclinic layered perovskite phase will be formed, which belongs to the space group P2₁.³ This phase structure can be assigned to a homologous series of A_nB_nO_3n+2 (n = 4) compounds, which possess parallel layers of four distorted BO₆ octahedra connected by corners.⁴ Via a cleavage plane along the c-axis, two types of A cations can be observed. As shown in Scheme 1(a), the one is topped with corner-sharing distorted BO₆ octahedra, and the other is located in the interstice between the two octahedra layers.⁴
(2)
If the radius ratio is in the range of 1.46–1.78, an ordered pyrochlore phase will be formed which belongs to the space group of Fd-3m.¹^,²^,⁵^,⁶ As shown in Scheme 1(b), the structure of the ordered pyrochlore phase can be described as an ordered defective fluorite, in which 1/8 of the oxygen anions are unoccupied.¹^,²^,⁵^,⁶ The general formula of a pyrochlore structured A₂B₂O₇ can also be written as A₂B₂O₆O′. The A and B cations occupy the 16d (1/2, 1/2, 1/2) and 16c (0, 0, 0) sites, respectively, and the O and O′ oxygen anions are in 48f (x, 1/8, 1/8) and 8b (3/8, 3/8, 3/8) sites, respectively.¹^,²^,⁵^,⁶ Compared with a typical cubic fluorite phase, an 8a (1/8, 1/8, 1/8) site is unoccupied, thus resulting in an ordered inherent 8a oxygen vacancy in its 1/8 unit cell, which is able to improve the oxygen mobility of the lattice oxygen anions.¹^,²^,⁵^,⁶ To maintain charge neutrality, it results in a face-centered cubic pyrochlore structure with doubled side lengths compared with a cubic fluorite structure.¹^,²^,⁵^,⁶ According to the local coordination environments, four sublattices can be observed: AO₆O₂′, BO₆, OA₂B₂, O′A₄. In the AO₆O₂′ sublattice, the 8-fold coordinated A site cation is located within a disordered cube and coordinated to six O anions spaced equally and two O′ anions, in which the bond length of A－O′ is slightly shorter than that of A－O.¹^,²^,5, 6, 7 In the BO₆ sublattice, the smaller 6-fold coordinated B-site cation surrounded by two 8a inherent oxygen vacancies is located within a distorted octahedral and coordinated to six O anions at equal distance.¹^,²^,⁵^,⁶ In an OA₂B₂ sublattice, one O anion is coordinated to two A sites and two B sites.¹^,²^,⁵^,⁶ In an O′A₄ sublattice, one O′ anion is coordinated to four A sites.¹^,²^,⁵^,⁶ The pyrochlore structure can also be described as a network consisting of corner linked BO₆ octahedra with A cations filling the interstices.¹^,²^,⁵^,⁶
(3)
If the radius ratio is below 1.46, a disordered defective fluorite phase will be formed, which belongs to the space group Fm-3m.¹^,²^,⁵^,⁶ As shown in Scheme 1(c), compared with an ordered pyrochlore phase, A and B cations distribute randomly in their sublattices, and the oxygen vacancies in the anion sublattices are more disordered.¹^,²^,⁵^,⁶^,⁸ It has been reported that both A and B site cations exhibit an average coordination number of 7 in the disordered defect fluorite structure, and some results have proven that the coordination number of A site cations is still bigger than that of the B site cation, implying that the B site cation is surrounded by the inherent 8a oxygen vacancies.⁹ Because of the simultaneous disorders in both the cation and anion sublattices, in comparison to an ordered pyrochlore phase, the oxygen mobility has been further enhanced.
(4)
If the radius ratio is close to 1.17, rare earth C-type phase will be formed. The structure is similar to Y₂O₃ whose A and B cations distribute even more randomly in the sublattices.¹^,²^,⁵^,⁶^,¹⁰

A₂B₂O₇ pyrochlore compounds have attracted increasing attention as catalysts due to their open structure, good thermal stability, oxygen mobility and the possibility of tuning their properties by partial or full substitution of the A or B cation. Although the phase structures and transformation processes of this type of compounds have been vastly studied and reviewed by crystallographers and material scientists, their catalytic properties and applications as catalysts have been rarely investigated. Over the past decade, our group have been working on preparing and exploring A₂B₂O₇ pyrochlore compounds with varied chemical compositions for a variety of clean energy production and air pollution control reactions. During this process, a lot of related publications have been examined. We noticed that in comparison to other composite oxides with fixed formulas, such as ABO₃ perovskites, AB₂O₄ spinels, etc., the studies on the catalytic chemistry of A₂B₂O₇ compounds are much less documented. Although people have started to show some interest in this field over the past several decades, no related review has been devoted to this topic. Therefore, in this paper, we have summarized the main work on studying A₂B₂O₇ compounds as catalysts for different reactions, trying to give people a comprehensive understanding of the catalysis properties of this type of compounds, which have been achieved up to date. At the end of this paper, both the opportunities and challenges on using A₂B₂O₇ compounds as catalysts in clean energy production and environment protection catalysis are proposed and discussed.

Section snippets

Characterization of the surface and bulk properties

Former studies have proved that XRD and Raman spectroscopy are very effective to distinguish A₂B₂O₇ compounds with different phase structures.⁶^,10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21 As shown in Fig. 1⁶ compared with ordered pyrochlore, disordered defective fluorite and rare earth C-type phase, monoclinic layered perovskite exhibits more complex XRD diffraction peaks. Ordered pyrochlore, disordered defective fluorite and rare earth C-type phase exhibit similar XRD diffraction as cubic

Methane reforming and ethanol steam reforming

With the fast development of modern industry, people are seeking for new energy resources instead of fossil flues. The clean energies, such as hydrogen energy, have attracted tremendous attention due to their high energy density and clean combustion products free of any contamination.22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33 Methane and naphtha steam reforming is currently the commercialized technique, which has been successfully industrialized several decades ago, for large scale hydrogen

NO_x reduction and soot combustion

Nitrogen oxides (NO_x) coming from various combustion processes are harmful to the air atmosphere and human health.44, 45, 46, 47 The catalytic reduction NO_x by NH₃, CO and hydrocarbons as reductants has emerged as an efficient means of abating NO_x pollution.44, 45, 46, 47 Goodenough et al. adopted defective pyrochlores (Pb₂Pb_xRu_2-xO_6+δ) as active supports to disperse Pt for NO reduction.⁴⁴ They found that complete reduction of NO to N₂ was accomplished above 250 °C.⁴⁴ Kato et al. have studied a

Conclusions and perspectives

Efforts to systematically research and understand on the structure and reactivity relationship for pyrochlore materials for clean energy production and environment protection catalysis have been relatively scarce during the past decades. Based on the former studies, several important conclusions can be drawn here:

(1)
The change of crystalline phase and the order degree of the crystalline phase induced by the r_A/r_B ratios can significantly influence the catalytic performance for various reactions. A₂

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^☆: Foundation item: Project supported by the National Natural Science Foundation of China (21962009, 21567016, 21666020), Natural Science Foundation of Jiangxi Province (20181ACB20005, 20171BAB213013, 20181BAB203017) and Key Laboratory Foundation of Jiangxi Province for Environment and Energy Catalysis (20181BCD40004).

¹: These authors contributed equally to this work.

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A2B2O7 pyrochlore compounds: A category of potential materials for clean energy and environment protection catalysis☆

Abstract

Graphical abstract

Introduction

Section snippets

Characterization of the surface and bulk properties

Methane reforming and ethanol steam reforming

NOx reduction and soot combustion

Conclusions and perspectives

Prog Solid State Chem

Nucl Instrum Methods Phys Res, Sect B

Acta Mater

Acta Mater

Acta Mater

Appl Catal A

Chem Eng J

J Alloys Compd

Ultrason Sonochem

Solid State Ion

Dyes Pigments

Appl Catal A Gen

J. CO2 Util.

J Catal

Appl Catal A Gen

Int J Hydrogen Energy

J. Energy Chem.

Int J Hydrogen Energy

J. Energy Chem.

Mol. Catal.

Appl Catal A

Catal Today

Appl Catal B

Catal Today

Catal Today

J Catal

J Catal

J Solid State Chem

Mater Chem Phys

Procedia Eng

A₂B₂O₇ pyrochlore compounds: A category of potential materials for clean energy and environment protection catalysis☆

NO_x reduction and soot combustion

J. CO₂ Util.