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Perovskites as geo-inspired oxygen storage materials for chemical looping and three-way catalysis – a perspective Xing Zhu, Kongzhai Li, Luke M Neal, and Fanxing Li ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b01973 • Publication Date (Web): 24 Jul 2018 Downloaded from http://pubs.acs.org on July 25, 2018
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Perovskites as geo-inspired oxygen storage materials for chemical looping and three-way catalysis – a perspective Xing Zhu1,2, Kongzhai Li2, Luke Neal1, and Fanxing Li1* 1
Department of Chemical and Biomolecular Engineering, North Carolina State
University, 911 Partners Way, Raleigh, NC 27695-7905, USA. 2
State Key Laboratory of Complex Nonferrous Metal Resources Clean Utilization,
Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming 650093, China. * Correspondence author
[email protected] ABSTRACT: With highly tunable composition, structure, and chemical-physical properties, perovskite oxides represent a large family of mixed-oxide materials that finds many energy and environmental related applications. This perspective discusses the fundamentals and applications of perovskite oxides in the context of chemical looping and three-way catalysis (TWC). Both applications make use of perovskite oxides’ oxygen storage and donation properties (> 400 µmol O/gram) under macroscopic reduction-oxidation (redox) cycles and at elevated temperatures. While perovskite oxides have been investigated as oxygen storage materials (OSMs) and three-way catalysts for more than five decades, use of these oxides in chemical looping, as oxygen carriers or redox catalysts, is a relatively new topic. This article provides an account of the effects of compositional, structural, and surface properties of perovskites on their oxygen storage and donation properties as well as their interactions with various gaseous reactants. Design and optimization strategies of tailored perovskite oxygen storage materials for chemical looping and three-way catalysis are also discussed. Emerging applications of perovskite based redox catalysts for chemical looping partial oxidation are also covered. KEYWORDS: perovskite, chemical looping, three-way catalysis, oxygen storage material, redox, partial oxidation
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1 Introduction Discovered in the Ural Mountains in 1839, mineral forms of perovskite and perovskite derived structures represent some of, if not the, most important geologic compounds due to their high stability and versatility to accommodate various cations into their structures. The lower mantle is mainly composed of (Mg,Fe)SiO3 perovskite at a pressure of 130 GPa and a temperature of 3,500 °C. It maintains high thermal and chemical stability over long-term geologic evolution. The potential to achieve high stability for perovskites makes them promising for processes and applications under demanding conditions. Perovskite-structured oxides find many potential energy and environmental related applications due to their uniquely tunable bulk and surface properties for oxygen vacancy formation and elimination, mixed ion and electron conduction, surface/bulk oxygen evolution, band structure, and ability to accommodate large number of dopants in both the cationic (A/B) sites and the anionic sites. While there are several excellent recent reviews and perspectives on perovskites in heterogeneous catalysis,1-4 electrochemistry,1 and photocatalysis,5, 6 perovskite materials can be applied in other areas due to their mixed ionic-electronic conductivity and the ability to accommodate significant oxygen non-stoichiometry. For instance, perovskites have been reported to be highly effective for chemical looping processes, in which the lattice oxygen of transition metal oxides is used for carbonaceous fuel oxidation in absence of gaseous oxidants (Figure 1a). To date, over 150 articles have been published on the subject of perovskite
structured
materials
as
oxygen
carriers
for
chemical
looping
applications.7-28 The cumulative publication count on this topic by the end of 2017 increased by 30 fold compared to 2010. Similar to chemical looping, three-way catalysis also relies on cyclic reduction-oxidation (redox) reactions of oxygen carrier materials for exhaust gas treatment under varying air-fuel equivalence ratios during internal combustion engine operations (Figure 1b).1-4, 6 To complement the rich body of literature on perovskites, this perspective focuses on the fundamental and applications of perovskite materials involving macroscopic redox cycles, that is, reduction and oxidation reactions cycles with significant loss and gain of surface and ACS Paragon Plus Environment
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lattice oxygen species (often > 400 µmol O/gram). Examples for such applications include exhaust treatment for internal combustion engines (up to 862 µmol O/gram at 500 °C)29 and chemical looping (over 4,000 µmol O/gram reported at 850 °C).30, 31 Besides their excellent oxygen storage and donation properties, the high stability of perovskite materials makes them potentially suitable for chemical looping and three-way catalysis since both applications are operated under demanding environments in terms of both reaction temperature and redox stress exerted on the oxygen storage materials.2 The term “geo-inspired” in the title captures the motivation to explore perovskites in demanding applications. In the context of chemical looping and three way catalysis, this perspective aims to offer insights into the effects of structural and surface properties of perovskites on their oxygen storage and donation properties as well as their interactions with gaseous reactants. Emerging applications and design/optimization strategies for perovskite based oxygen storage materials are also discussed.
Figure 1. Schematics of perovskite based oxygen storage materials for chemical-looping (a) and three-way catalysis (b). MeOx and MeOx-y represents the oxidized and reduced states of perovskite, respectively. CLAS: chemical looping air separation, CLOU: chemical looping with oxygen uncoupling, CLC: chemical looping combustion, CLPOx: chemical looping partial oxidation. ACS Paragon Plus Environment
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2. Structure and Synthesis of Perovskite The perovskite structure family includes a large number of compounds with structures related to CaTiO3. While ideal perovskites are often referred to as ABX3 or ABO3 and considered to be cubic (Pm3m, Figure 2a), deviations from ideal compositions and distortions of the ideal lattice structure often occur. Since this article deals with oxygen storage materials, only oxygen anion (O2-) is considered in the following discussions, although anions other than O2- can be accommodated in the perovskite structure. Deviation from the ideal ABO3 stoichiometry can occur resulting from charge compensation (e.g. LaMnO3+δ)5 and/or exposure to highly oxidizing or reducing environments under elevated temperatures (e.g. Ca2Mn2O5)32. Such deviations can cause lattice distortion and, in many cases, vacancy ordering. The Brownmillerite structure, with a general composition of A2B2O5) represents a well-studied family of perovskite-related compounds with oxygen deficiency (lbm3, Figure 2d). It is noted that the ordering of oxygen vacancies for A2B2O5 can also take many other forms (Figure 2e,f). Another family of perovskite-related compounds, designed by Ruddlesden and Popper (R-P),31 has a general formula of AO(ABO3)n. Since AO takes a Rocksalt structure, R-P structured oxide materials exhibit a layered structure with alternating perovskite layers separated by Rocksalt layers (Figure 2g-i). The R-P oxide exhibits composition identical to that of ideal perovskite when n approaches infinity. Besides deviation in elemental compositions, lattice distortion in perovskites can result from ionic size or Jahn-Teller effects. A commonly adopted parameter to estimate the degree of lattice distortion for ABO3 perovskites is the Goldschmidt’s tolerance factor (t) as defined in Equation 1.
t=
√ ( )
Equation 1
An ideal perovskite has a tolerance factor of 1 (e.g. SrTiO3) whereas perovskite-type compounds can exhibit a t range of ~0.8 to 1.1 depending on the relative ionic radius. Deviation from a t of 1, however, leads to decreased symmetry of the crystal structure
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due to the tilting of the [BO6] octahedra. For instance, perovskites with t larger than 1 (and less than 1.1) typically adopts a rhombohedral structure whereas an orthorhombic structure can be anticipated for t smaller than 0.9. Further decrease of t leads to a hexagonal perovskite structure, cubic bixbyite structure, and, eventually, a hexagonal ilmenite structure when t drops below 0.75.2 It should be noted that the tolerance factor only provides a general indication of the perovskite structure and should not be used as strict demarcations.
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
(i)
Figure 2. (a)-(c) ABO3 perovskite; (d)-(f) ordering of oxygen vacancies in A2B2O5; (g)-(i) Ruddlesden-Popper phase materials (AO(ABO3)n, with n = 1,2,3). Due to the unique and demanding operational environments, perovskites for chemical looping and three-way catalysis applications need to be stable under an oxidizing environment at elevated temperatures (1000 °C and above). As such, synthesis of most of these perovskties can be rather straightforward: desired perovskite phase can form spontaneously from well-mixed precursors in air at high temperature. An
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important consideration is in the uniform dispersion of various elements in the complex oxides, especially given that doubly doped perovskites (A1-xA’xByB’1-yO3-δ) are not uncommon in these applications. Therefore, most synthesis approaches address the precursor dispersion issues. Following the mixing, a high temperature annealing step is necessary to promote the perovskite phase formation, with the exception of high-energy ball-milling. It is noted that the terms “dopants” and “doping” in the context of perovskites are different from those in some of the recent publications related to first-row transition metal oxide based oxygen carriers in chemical looping.33, 34 In the latter case, the dopant concentrations are generally much lower than those in perovskites for chemical looping or TWC. Table 1 summarizes the frequently adopted synthesis approaches and representative perovskites in chemical looping and three-way catalysis. Readers are directed to a few review articles for the details of these synthesis methods.3-5 Perovskites are, typically, non-porous materials. Surface areas of perovskites decrease considerably under the high temperature annealing step considering their high stability and surface energy.6, 35 This, coupled with the necessity for annealing in most conventional synthesis methods, lead to perovskites with low surface areas (< 15 m2/g and often 1000 °C) over extended hours. Therefore, it is questionable whether high surface area perovskites would be practical for the specific applications covered in this article. Another interesting observation, from past and ongoing studies of the authors, is that the rate of redox reactions for many of the perovskites of interest are not surface area limited.36, 37 This topic will be revisited in section 3.5.
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Table 1. Summary of synthesis methods, compositions, surface areas and applications of perovskites as oxygen storage materials for chemical looping (perovskites in three-way catalysis are summarized in Section 3). Synthesis Method
Solid State Reaction
Complexation
Composition
Pre/post treatment conditions
Ca1-xAxMnO3−δ (A = Sr and Ba) Sr(Mn1-xNix)O3−δ CaTi0.85Fe0.15O3−δ LaFe0.7Sr0.3O3−δ BaCe0.7Fe0.3O3−δ (Ca,A)MnO3−δ, Ca(Mn,B)O3−δ (A=La, Sr) and (B=Fe, Zr) AMnxB1-xO3−δ (A=Ca, Ba; B=Fe, Ni) Ca1-xAxMnO3−δ (A = Sr and Ba) BaMn0.5Fe0.5O3−δ SrFe0.95Cu0.05O3-δ, Ca0.8Sr0.2MnO3-δ BaFeO3−δ, SrFeO3−δ CaMn1-xBxO3−δ (B = Al, V, Fe, Co, Ni) CoTiO3−δ LaFeO3−δ La1-xSrxFeO3−δ La1-xSrxCoO3−δ La0.75Sr0.25Co1-yFeyO3−δ La2-xMxNiO4-δ (M=Ca, Sr)
1200 ºC/12h 750 ºC/20h 800 ºC/6h 800 ºC/6h 800 ºC/6h 1300 ºC/6h
Surface Area (m2/g) or Particle Size (µm) 90–250 µm 41.6-250 µm 0.44 m2/g 0.58 m2/g 0.61 m2/g 0.42-0.95 m2/g
1200 ºC/12h
Applicationa Ref. CLOU CLC CLC CLC CLC CLC
38
0.76-1.22 m2/g
CLPOx
42
1200 ºC/12h 1200 ºC/12h Auto-combustion 1000 ºC/6h 1200 ºC/12hl
90–250 µm 90–300 µm 90 µm
CLOU CLPOx CLAS CLC CLOU
38
900 ºC/3h 900 ºC/6h 900 ºC/6h, 700/750 ºC/6h, 700/750 ºC/6h, Modified Pechini 800 ºC/2h, Microwave assisted
6.08 m2/g 4.6-6.3 m2/g 3-10 m2/g 0.4-1.3 m2/g -
CLC CLPOx CLPOx CLRWGS CLRWGS CLPOx
46
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39 40 40 40 41
43 21 44 45
47, 48 49 50 51 52
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combustion 1000 ºC/6h
30, 53-56 La2-xSrxFeCoO6-δ CLPOx LaSrFeCoO6-δ 57, 58 LaxSr2−xFeO4−δ 950 ºC/6h 1-6 m2/g CLODH 31 Sr3Fe2O7−δ, SrFeO3−δ 1200 ºC/12h 1
91
CaxLa1-xMn1-yMyO3-δ (M=Mg, Ti, Fe, Cu)
FB
900-1000
CH4, syngas
air, 5% O2 in N2
CLOU
0.5~1
66
CaxMn1−yMyO3−δ (M = Mg, Ti)
FB
900-1050
CH4/SO2
5% O2 in N2
CLOU
0.86~1.58
92
Ca0.9Mn0.5Ti0.5O3−δ
FxB
400-800
Syngas
air
CLC
3
93
CaMn0.9Mg0.1O3−δ
TGA
700-1000
CH4, H2, CO
O2
CLC
1.1
94
CaMn0.9Mg0.1O3−δ
FB
800-960
biochar
air
CLC
2.1
95
CaTixMn0.9-xMg0.1O3−δ
TGA, FB
800-1000
H2/CO2, CH4/CO2
air
CLC
~8
19
CaMn0.775Ti0.125Mg0.1O2.9−δ
TGA
700-1000
CH4, H2, CO, H2S
5-21% O2
CLC/CLOU
8.0~8.5(1.93cycled)
96, 97
CaMn1-xBxO3−δ
TGA, FB
650-950
N2/He
10% O2
CLOU
0.68
45
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(B = Al, V, Fe, Co, Ni) (Ca,A)MnO3−δ, Ca(Mn,B)O3−δ (A=Fe, Zr) and (B=La, Sr)
FB
800-950
CH4
8.5% O2
CLC
1.2~2.0
41
Ca1-xLixMnO3−δ (x=0.001, 0.01, 0.015)
TGA
850
CH4, H2
air
CLC
8.25-8.78
98
CoTiO3−δ
TGA
300-900
CH4, H2
air
CLC
10.2
46
Sr(Mn1-xNix)O3−δ
TGA
600-800
H2/Ar
20% O2
CLC
3.44~8.23
39
59 La1-xCexBO3−δ (B=Co,Mn) RS 650 CH4 air CLC 0.37~1.0 a FB: fluidized bed reactor, CFB: continuous fluidized bed reactors, TGA: thermogravimetric analyzer, FxB: Fixed-bed reactor, RS: riser simulator. bOAvl.: available oxygen capacity.
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3.3 Perovskites for complete oxidation and air separation Table 2 summarizes recently studied perovskites for CLOU/CLAS/CLC. As discussed in Section 3.1, a common requirement for these redox pairs is relatively high equilibrium PO2 to ensure complete fuel oxidation and/or spontaneous oxygen release (CLOU/CLAS). Both empirical and more rigorous approaches have been used to develop oxygen carriers that satisfy such thermodynamic prerequisite. Empirically, A(+2)B(+4)O3-δ with Co, Mn, and/or Fe occupying the B-site are likely to be spontaneous for oxygen release due to the instability of the +4 states of these first-row transition metal cations. Redox properties of the perovskite can be affected by the size and electronegativity of the A-site cation, which is often selected from alkali earth metals (with the exception of Mg2+ whose ionic radii is too small). Moreover, substituting B-site with aliovalent cations can also affect the redox properties and stability of the oxygen carrier. While the empirical approach has led to the development of many promising oxygen carriers, recent DFT studies, complemented with experiments, have provided valuable insights for more systematic oxygen carrier development and optimizations for CLOU and CLC applications. Lau et al. utilized the Materials Project database (www.materialsproject.com), developed via DFT calculations implemented with the Vienna ab initio Simulation Package (VASP), to estimate the equilibrium chemical potentials of over five thousand redox pairs for their chemical looping applications.44 The findings were quite useful for chemical looping material screening. Specifically, the ground state energies and structures of binary and ternary compounds containing non-radioactive elements were retrieved from the Materials Project database. Using such information, open phase diagrams containing two elements in addition to oxygen were developed as a function of µO2. Feasibility of the redox reactions (in a range of 10-3 – 10 atm and 150 – 1700 K) and the corresponding redox energies of the metal oxides were then determined. Although over 2000 compounds were found to be suitable for redox reactions in the aforementioned temperature and PO2 ranges, practical considerations (e.g. cost, toxicity, oxygen capacity) were used to narrow the materials to around 100. The authors concluded that ABO3 perovskites are particularly promising among all the ACS Paragon Plus Environment
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metal oxide combinations investigated. A few perovskite structured oxygen carriers, including BaBiO3, BaCoO3, and SrFeO3, were determined to be suitable oxygen carriers. Experimental data supported the computational findings. SrFeO3 was found to be particularly promising for intermediate temperature (~550 °C) redox cycles with spontaneous oxygen release (CLOU/CLAS). Although this study relies on a computational database and does not consider the effect of dopants, oxygen vacancy creation, and configurational entropy of the solids99, the results are nevertheless quite useful from a materials screening/optimization standpoint. The ability to investigate a large number of materials with such a high efficiency is particularly attractive. In addition to equilibrium (µO2/PO2) considerations, oxygen carrier development should consider practical factors such as cost, activity, and long-term stability. From a cost standpoint, CaMnO3 based oxygen carrier represents a promising OC material for CLOU/CLC and potentially CLAS.71 As such, CaMnO3 based oxygen carriers are one of the most extensively investigated perovskite families for chemical looping applications. Similar to typical perovskites, CaMnO3 releases varying amount of oxygen under different redox stresses: ~1 wt.% or less oxygen release100, in the form of gaseous oxygen, can be anticipated under an oxygen chemical potential swing between oxygen lean air (< 0.21 atm) and an “inert” (e.g. 5.0 He) in the context of CLOU and CLAS. Meanwhile, deeper reduction of CaMnO3 by a carbonaceous fuel, which leads to the formation of CaO and MnO phases, can result in up to 11 wt.% oxygen donation.85 Since most of the studies focused on the CLOU properties of CaMnO3 based oxygen carriers, this will be the focus in the following discussions. Although CaMnO3-δ is capable of spontaneous oxygen release, it also undergoes irreversible phase transition to Ca2MnO4 and CaMn2O4 under repeated redox cycles at high temperatures.101 This leads to gradual loss of oxygen storage capacity and is undesirable in chemical looping processes. Other challenges associated with CaMnO3 are its instability in the presence of sulfur and relatively slow oxygen release kinetics since CaSO4 formed from gas-solid reaction between CaO and SO2 is relatively stable at the temperatures lower than 1200°C.89, 92, 102, 103 These limitations for CaMnO3 can be addressed by substitution of A-site or B-site cations. Effective A-site dopants ACS Paragon Plus Environment
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include La, Ba, and Sr while B-site substitutions include Ti, Mg, and Fe, among others.12, 38, 41, 45, 66, 67, 89-92 Galinsky et al. performed detailed studies to evaluate the effect of A- and B-site dopants.38, 45 With regards to the A-site, the authors used Sr and Ba as dopants to replace a portion of the Ca in the oxygen carrier.38 It was determined that replacement of Ca with Ba inhibits CLOU properties of the oxygen carrier. Moreover, Ba-substituted CaMnO3, similar to undoped CaMnO3, decomposes into the spinel and R-P phases at high temperature. In contrast, Ca0.75Sr0.25MnO3 exhibited enhanced oxygen release at low temperatures and maintained high resistance towards undesirable phase transition. In a follow-up study, B-site dopants included Fe, Ni, Co, V, and Al were studied.45 Among these, moderate amount of Fe can be fully incorporated into the perovskite phase without forming secondary or ternary oxide phases. Doping of 5% of Fe into the B-site improved the oxygen carrier’s high temperature stability while improving the overall oxygen carrier CLOU activity. A recent DFT studies by Mishra et al. offered additional insights in terms of the effects of dopants.74 Stability of CaTiO3, a mineral perovskite, also inspired the efforts to increase
the
redox
stability
of
CaMnO3.
CaMn0.875Ti0.125O3-δ
and
CaMn0.775Ti0.125Mg0.1O3-δ were reported to be superior CLOU oxygen carriers for converting coal, petroleum coke, methane, syngas, hydrogen, and carbon dioxide (see in Table 2).89, 96 In terms of methane CLC, Ca0.9Sr0.1Mn0.9Fe0.1O3-δ perovskite showed higher oxygen storage capacity and sustainability during the CLC of methane.41
Figure 5. Phase stability of (doped) CaMnO3 oxygen carrierss characterized by in-situ X-Ray Diffraction (XRD) at 5 °C/min ramping rate in 5.0 Ar; (a) CaMnO3 decomposes at ~1100 °C; (b) Ca0.75Sr0.25MnO3 and (c) CaMn0.95Fe0.05O3 show excellent stability for the entire temperature range up to 1200 °C (110 plane was
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shown to highlight the phase transition).38, 45 Substituted CaMnO3 were also explored for CLAS. Vieten et al. investigated a number of perovskite based oxygen carriers for air separation. Ca0.8Sr0.2MnO3-δ was determined to exhibit the highest oxygen storage capacity among the various ABO3-δ (A= Ca, Sr; B=Mn, Fe, Co and Cu) perovskites investigated.21, 32 It could sustain high stability and reversibility with an OSC of 2 wt.% (reduction in Ar at 1200 ºC and oxidation in a synthetic air with an oxygen partial pressure of 0.16 bar). The improved redox performance was attributed to enhanced oxygen diffusivity with Sr-substitution 21, 32
. The Sr-doped CaMnO3 allowed reoxidation of the oxygen depleted carrier at
temperatures as low as 250 ºC (under PO2 of 0.16 bar) in CLAS process.21 The material showed high recyclability and stability over 100 redox cycles at 850 ºC for CLOU in the context of coal conversion.41 To summarize, perovskites have shown excellent potential for CLOU, CLAS, and CLC applications due to their high flexibility, tunability, and fast redox kinetics. Recent advances in computational material science not only offer mechanistic insights but also efficient screening tools for perovskite based oxygen carriers. Coupling the advanced computational and characterization tools, rational design of oxygen carriers with tailored redox properties, oxygen storage capacities, and heat of reactions can potentially be realized in the near future. From a practical standpoint, oxygen carrier development should consider their phase and redox stability, pollutant resistance, cost, fluidization properties, and mechanical strength. Doping of A or B-sites of the perovskites have been shown to be effective to improve one or more of these properties. Further understanding to the role of dopants and development of rationalized doping strategy can be very useful for perovskite oxygen carrier optimization.
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Table 3. Perovskites for CLPOx. Perovskites
Additives Optimal Composition
Reactora
T /ºC
Oxidizing
Appplication OAvl.c,
atmosphere
atmosphere
b
µmol/g
Reducing
Ref.
LaFeO3−δ
-
-
FxB
780-900
CH4
O2
CLMR
1700
60, 61
LaFeO3−δ
-
3DOM-LaFeO3−δ
TGA, FxB
850
CH4
O2
CLMR
~2600
68
La1-xSrxFeO3−δ
-
x=0.3-0.5
TGA
850
CH4
O2
CLMR
~1800
49, 104
LaFe1-xCoxO3−δ
-
LaFe0.7Co0.3O3−δ
FxB
850
CH4
H2O
CLSMR
4000
30
La1-xSrxFeO3−δ
NiO,
La0.7Sr0.3FeO3−δ
FxB
1000
CH4
H2O
CLSMR
912
62
La0.7Sr0.3Cr0.1Fe0.9O3−δ
FxB, Pulse
1000
CH4
H2O
CLSMR
2000
63
BaMn0.5Fe0.5O3−δ
TGA, FxB
900
CH4
O2
CLMR
-
42
43
5wt.% La1-xSrxMyFe1-yO3−δ
NiO,
(M=Ni,Co,Cr,Cu)
5wt.%
AMnxB1-xO3−δ (A=Ca, Ba; B=Fe, Ni) BaMn0.5Fe0.5O3−δ
-
-
FB
950
CH4
H2O
CLSMR
-
La2-xSrxFeCoO6
-
La1.6Sr0.4FeCoO6
FxB
850
CH4
H2O
CLSMR
~10000
La2-xMxNiO4-δ (M=Ca, Sr)
-
-
FxB, Pulse
900
CH4
O2
CLMR
~4500
52
Sr3Fe2O7−δ, SrFeO3−δ
-
-
FxB
900
CH4
CO2
CLMDR
4000
31
Rh/CaMnO3−δ
-
-
TGA, FxB
600-900
CH4
O2
CLMR
2400-790
105
LaSrFeCoO6
0 La1-xSrxCoO3−δ La0.75Sr0.25Co1-yFeyO3−δ
-
x=0.25
FxB
650-850
H2
CO2
CLRWGS
y=1
FxB
550
H2
CO2
CLRWGS
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4032.8 -
50 51
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La0.6Sr0.4Co0.2Fe0.8O3−δ
-
-
MR
850
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CO
H2O
CLWGS
La0.7Sr0.3FeO3−δ Ba0.5Sr0.5Co0.8Fe0.2O3−δ
-
FxB
850
Coal+H2O
O2
CLG
La2-xMxNiO4-δ (M=Ca, Sr, -
-
FxB
600
C2H6O+H2O O2
CLR
Ce)
C2H6O -
La1.4Sr0.6NiO4-δ
FxB
650
C2H6O+H2O O2
CLR C2H6O
LaxSr2−xFeO4−δ
106
0 -
La2-xSrxNiO4-δ
1000-280
Li, Na, K La0.6Sr1.4FeO4−δ
FxB
700
C 2 H6
a
O2
CLODH
of 1565-440
53 64
0 of 1000-290
65
0 57, 58
FxB: Fixed-bed reactor, TGA: thermogravimetric analyzer, Pulse: Pulse test, FB: fluidized bed reactor, MR: Micro Reactor. CLR: chemical looping reforming, CLMR: chemical looping methane reforming, CLSMR: chemical looping steam methane reforming, CLMDR: chemical looping methane dry reforming, RWGS-CL: reverse water-gas-shift chemical looping, CLWGS: chemical looping water-gas-shift, CLG: chemical looping gasification, CLODH: chemical looping oxidative dehydrogenation.
b
c
OAvl.: available oxygen capacity.
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3.4 Chemical looping partial oxidation 3.4.1 Tailoring the bulk properties of perovskites As discussed in Section 3.1, redox pairs for CLPOx can be selected from a thermodynamic/redox energy standpoint. That is, tuning the equilibrium chemical potential of the oxygen carrier can limit over-oxidation of these products, resulting in high selectivity. While none of the monometallic oxides of first-row transition metals possess the desired thermodynamic properties, perovskites are uniquely suited for CLPOx due to their tunable redox properties. In general, perovskites’ redox energies or µO2 can be tailored by varying A/B site cations,42, 44, 107-109 substitution,30, 43, 49, 51 and changing A:B site cation ratios31. Perovskites with suitable POx properties for methane conversion are summarized in Table 5. In general, the A-site cation of these perovskites typically include La, Sr, Ba, and/or Ca. Fe is a typical B-site cation although Mn, Co, and Ni are often used either alone or as a dopant.42 Among them, LaxSr1-xFeyCo1-yO3-δ represents an interesting family of perovskites due to their high ionic conductivity, high activity, and syngas selectivity. When reacting with methane, fully oxidized LaxSr1-xFeyCo1-yO3-δ tends to generate CO2/H2O rich products during the initial stage of the reaction before reaching a high syngas yield region. This is attributed to the high oxygen coordination of B-site cations and abundance of electrophilic oxygen species in the fully oxidized sample.60, 61 Controlling the degree of the re-oxidation can inhibit the formation of the undesired CO2 and H2O products.61 The rate of methane oxidation and syngas selectivity tend to increase with increasing extent of reduction. However, carbon deposition occurs towards the depletion of active lattice oxygen and with the formation of significant amount of metallic iron or cobalt. Coking can be inhibited by Sr doping and timely reoxidation of the redox catalyst. It was reported that, for LaSr1-xFexO3-δ, a Sr doping level (x) of 0.3-0.5 is favorable.49, 62 In addition to regeneration with air, steam or CO2 can be used to reoxidize the reduced redox catalyst, producing CO or H2. La0.6Sr0.4Co0.2Fe0.8O3-δ showed a constant hydrogen production via water splitting for more than 100 cycles.106 La0.7Sr0.3Cr0.1Fe0.9O3-δ mixed with 5 wt.% NiO shows good stability and activity for periodic methane POx to syngas and water splitting to hydrogen.62, 63 ACS Paragon Plus Environment
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Alkali earth metal (Ca, Sr, Ba) containing perovskites were also investigated for methane POx.31,
42, 43
CaMnO3, with higher PO2 than the desired CLPOx region,
exhibited low syngas selectivity. Addition of Ni enhanced their redox activity and selectivity but increased coke formation.42 BaMn1-xFexO3-δ perovskites, on the other hand, exhibited tunable µO2 with varying Mn:Fe ratio and high syngas selectivity.43 µO2 can also be tailored with Sr and Fe containing perovskites by varying the Sr and Fe ratios to form ABO3 or A3B2O7 structures.31 These redox catalysts tend to deactivate over multiple redox cycles due to phase segregation. Dispersing the perovskite material into an inert matrix of CaO can significantly increase the redox kinetics and stability of these perovskite materials when operated under methane-CO2 redox cycles.31 Over 98% single pass conversion of CO2 to CO was reported with these perovskite redox catalysts. Melo et al. and Zhang et al. reported La2-xMxNiO4-δ (M = Ca and Sr) as effective redox catalysts for CLR of ethanol and methane.52, 64, 65 Related to methane POx, Kuhn et al. investigated, via both experiments and DFT calculations, the reverse water-gas-shift chemical looping (RWGS-CL) which performs H2 oxidation and CO2-splitting in the two redox steps.50, 51, 108, 109 Such a redox scheme requires a balance between H2 oxidation (Step 1) and CO2-splitting (Step 2) since the two reactions have comparable PO2 at typical chemical looping temperature ranges. Thus, the thermodynamic driving force for the overall reaction, which is the reverse water gas shift reaction, can be calculated. Since the thermodynamic requirements to achieve reasonable CO2-splitting conversions are generally consistent with the optimal region for methane POx31, the findings from Kuhn’s studies are also applicable to methane POx and water/CO2-splitting cycles. In their recent studies, oxygen vacancy formation energy calculated from DFT was used to successfully screen perovskite based redox catalysts for RWGS-CL. Moreover, the vacancy creation energy correlated well with the materials’ enthalpy of formation and metal-oxygen dissociation energies. The vacancy formation energy was also reported to affect the surface properties and activity of the perovskites.108 These findings can be very useful to guide the development and optimizations of the bulk redox properties of perovskite based redox catalysts. ACS Paragon Plus Environment
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ACS Catalysis
While perovskites with tailored µO2 can be highly effective for POx, their application is largely limited to syngas generation since CO and H2 are the more thermodynamically stable products under oxygen lean conditions. Care also needs to be taken to inspect the kinetic aspects of the redox catalysts – while over oxidation of syngas can be limited thermodynamically, slow reaction kinetics may require high gas-solids contact time to achieve high syngas yield. Although the rates for chemical looping reactions, which involves lattice oxygen transfer and donation, tend to be slower than conventional catalytic reforming reactions, reliable reactor and techno-economic models that weighs the potential benefits of CLPOx (higher efficiency, lower separation cost) against its limitations (slower kinetics) are highly desirable in order to guide the selection and optimization of economically attractive redox catalysts and reaction schemes. 3.4.2 Tailoring the surface properties of perovskites Although limiting oxides’ µO2 in the purple region of Figure 3 ensures high syngas selectivity, oxides with higher µO2 can also be selective. This can be achieved, similar to heterogeneous catalysis, through tailoring the surface properties of the perovskites. For instance, Mishra et al reported that substitution of 20% Mn with Fe or Ni in CaMnO3 significantly increased syngas selectivity and more than doubled the CO yield from methane oxidation.42 The increase in syngas selectivity and yield can be attributed to the presence of reduced Fe or Ni on the redox catalyst surface for (partially) reduced samples. The metallic Fe or Ni acts as active sites for methane activation, thereby enhancing the rate of methane conversion, syngas selectivity, and the extent of lattice oxygen extraction. While Ni and Fe can be quite active for methane activation in their metallic forms, their oxidized states are non-selective.36, 110 Significant amount of lattice oxygen needs to be removed prior to the formation of active metallic phases on the oxide surface.79 In addition, Ni and Fe were suppressed on the perovskite surface due to the enrichment of A-site cations.42 To address these limitations, Shafiefarhood et al. investigated the effect of impregnating 0.5 w.t.% Rh on CaMnO3 based redox catalysts for methane oxidation.105 Unlike Ni and Fe oxides, Rh was readily reducible for methane activation. XPS indicated surface enrichment of ACS Paragon Plus Environment
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Rh compared to the bulk average by 3 or 5 times for oxidized and reduced sample.105 Rh impregnation on CaMnO3 led to more than 300 °C decrease in the onset temperature for methane POx. Although CaMnO3 exhibited less than 5% syngas selectivity, Rh impregnated CaMnO3 was 93% selective at 600 °C. Transient pulse and isotope exchange experiments indicated that Rh promoted dissociative adsorption of methane. The higher concentration of CHx species on the surface are highly effective for oxygen removal from the surface, which leads to increased driving force for O2- conduction from the bulk. The apparent activation energy for the surface reaction decreased by 95% with Rh promotion whereas the apparent activation energy for the overall reaction, which includes bulk oxygen transport to the surface, decreased by 50%. The effectiveness of impregnating or doping active metals to perovskite redox catalysts were also explored in a few other studies.63, 79, 83, 111, 112 These studies highlighted the critical role of the surface for the redox reactions: (i) the rate of lattice oxygen removal during the reduction step can be significantly affected by the surface of the redox catalyst since a catalytically active surface promotes surface oxygen removal and hence increases the driving force for O2- transport from the bulk; (ii) tuning the redox catalyst surface is effective to change product selectivity. Meanwhile, the nature of the redox catalyst surface is highly dynamic and is dependent upon the degree of reduction of the redox catalyst, reducibility of the active species, and a balance between the rate of O2- transport from the bulk and the rate of surface oxygen removal. In addition to impregnating the surface with active metals, synergistic effects among oxides were also reported. Zheng et al dispersed CeO2 nanoparticles on three-dimensional ordered macroporous (3DOM) LaFeO3-δ for methane partial oxidation.69 High activity and yield for methane POx, with 3,380 µmol/g oxygen storage capacity, was reported in the methane POx/water splitting redox process.69 Authors attributed the high activity to the interfacial effects between LaFeO3-δ and CeO2. The coexistence of Ce4+/Ce3+ and Fe3+/Fe2+ couples on the interfaces led to abundant oxygen vacancies, offering the active sites for methane adsorption and activation as well as high surface and bulk oxygen mobility. The high mobility of ACS Paragon Plus Environment
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ACS Catalysis
oxygen also inhibits coke formation. While syngas generation from methane is the most frequently studied subject for CLPOx, a few recent studies extended the applications of perovskite based redox catalysts beyond methane POx. Gao et al. reported Ruddlesden-Popper structured LaxSr2-xFeO4-δ (LaSrFe) redox catalysts for ethylene production via chemical looping oxidative dehydrogenation (CL-ODH).57,
58
In the CL-ODH scheme, the redox
catalyst functions as both a catalyst for C-H bond activation in ethane and an oxygen donor for hydrogen oxidation. The reduced redox catalyst is subsequently reoxidized with air. The overall reaction, C2H6 + 1/2O2 → C2H4 + H2O is identical to conventional, heterogeneously catalyzed oxidative dehydrogenation with ethane and O2 co-feed. As such, ethylene production via CL-ODH is net exothermic and is not subject to equilibrium limitation.113 The findings of this study contrasts with the methane POx studies discussed above. Instead of attempting to increase the activity of the surface and hence accelerate the O2- removal from the bulk, authors showed that it is important to decelerate O2- flux from the perovskite lattice. In fact, the high O2- flux from the as-prepared LaxSr2-xFeO4-δ samples resulted in abundance of electrophilic surface oxygen species (O2- → O- → O22- → O2-). The latter are highly nonselective, leading to undesirable COx products. To address this issue, alkali metal oxides were used to “promote” (or suppress to be exact) the redox catalyst. It was determined that alkali metal oxides, such as Li2O, formed a surface layer on the redox catalyst and decreased the O2- flux from the bulk by 300%. The surface oxygen exchange rate was also significantly inhibited. As a result, the nonselective electrophilic oxygen species on the redox catalyst surface was greatly inhibited, leading to significantly improved ethylene selectivity ( 1) lead to NOx emissions due to a lack of reductants. OSMs are therefore critical to buffer the fluctuations in λ due to their ability to store oxygen under fuel lean conditions (oxygen excess) and to release oxygen under fuel rich conditions (oxygen lean), enlarging the operating window of the air–fuel equivalence ratio.127-131 In commercial TWCs, the platinum group metals (PGMs) containing heterogeneous catalysts and OSM are the most important components. It is noted that the redox/PO2 swing in typical ICE exhaust is narrower than that in typical chemical looping processes. Moreover, the typical operating ACS Paragon Plus Environment
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temperatures in chemical looping processes (> 700 °C) are significantly higher than those for TWCs (< 500 °C), with the exception of the fuel cut-off stage. While many of the research findings in chemical looping oxygen carriers can be very relevant to OSMs and vice versa, one should keep in mind the different operating conditions and requirements for these applications. CeO2-based oxides (e.g., CeO2-ZrO2 solid solution, CZO) are the most widely adopted OSMs in TWC systems.128, 132-134 However, one challenge with ceria-based OSMs is the relatively limited availability and high cost of ceria. Additionally, ceria based oxides often face thermal stability issues due to loss of surface area at high temperatures (800 °C or higher)133. Owing to the limited oxygen storage capacities ( 1,000 °C) lean oxidative condition.135 Perovskite oxides, with a general formula of ABO3, are alternative OSMs with excellent potential for lowering the cost and improving thermal stability and oxygen storage capacity.1, 126, 136-142 Moreover, perovskites can be quite active for both CO and hydrocarbons (HCs) oxidation and nitric oxide (NOx) reduction even without the presence of PGMs.139 For instance, Co and Mn containing perovskite catalysts exhibit high activity for CO/HC oxidation in a reducing environment and superior activity for the reduction of NOx in an oxidizing environment.139,
140
In addition to being
standalone OSM/catalysts, perovskite oxides also show excellent performance in stabilizing PGMs (e.g., Pt, Pd, Rh and Ru). The PGMs, in turn, can improve the resistance of perovskties to sulfur poisoning.1, 136, 139, 141 Although perovskite based OSMs have yet to be widely utilized in commercial TWCs due to the relatively low sulfur resistance, difficulty for preparing structured catalysts, and the possible reactions with other components in the TWC converters126,
143-146
, their unique
properties make them promising next generation OSMs – a topic this is being actively ACS Paragon Plus Environment
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investigated. Tables 5 and 6 summarize some of the most commonly investigated perovskite OSM/catalysts both without and with PGMs. The catalytic performance of non-PGM containing perovskites can be affected by the nature of the metal-oxygen bonds at the surface, oxygen vacancy concentration, and the mobility of lattice oxygen in the perovskite.1,5 In the oxidation reactions over perovskites without precious metals, the reaction rate is affected by both gas phase composition and the activity of lattice oxygen in the catalysts.147 Numerous literature reports indicate that lattice oxygen and vacancies in the perovskites play roles in CO oxidation though rapid exchange/evolution of oxygen from the bulk and surface.138, 148, 149
This is in general agreement with the Mars van Krevelen mechanism. Oxygen
vacancies are also crucial for the activation of O2. The rate of O2 chemisorption was reported to be very rapid in the initial reaction stage, but it can become a rate-determining step thereafter.150 Catalytic oxidation of HCs also shows similar phenomena.138, 151, 152 NOx can be reduced by H2, CO, and/or HCs over the perovskite catalysts. Irrespective to the reducing agent, oxygen vacancies play a critical role in NOx reduction. It was proposed that NO was first adsorbed on the oxygen vacancies and then release its oxygen to form N2. Thereafter, the oxygen vacancies can be restored by a gaseous reducing agent (H2, CO, or HCs).2, 139 This, in effect, represents a reverse Mars van Krevelen mechanism. A simplified schematic of perovskite oxides in three-way catalysis is shown in Figure 7.
Figure 7. A simplified reaction scheme of PGMs-containing or non-PGM perovskite oxides in three-way catalysis.
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Table 5 Summary of non-PGM containing perovskite OSM/catalysts for CO/HCs oxidation or NOx reduction. Perovskites
La0.2Sr0.8Co0.6Fe0.4O3
Synthesis method Evaporation
Specific surface area (m2/g)
OSC (µmol O/g)
Reaction conditions
3.8
785(O2TPD,T600, fuel rich
375(C3H6) c
380,
Stoichiometric condition: 0.7 vol% CO, 100ppm N2, 240 c 0.78% O2, 450 ppm C3H6, 230 ppm C3H8, 230 ppm CH4, 233 ppm H2, 15 vol% CO2, 10% H2O, He balance, GHSV 245 c
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375
148
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ACS Catalysis
La0.5Sr0.5Co0.5Cu0.5O3
12
-
LaFeO3
13.9
-
LaFe0.9Cu0.1O3
18.4
LaFe0.8Cu0.2O3
13.8
= 60000 h-1; Fuel rich condition: 0.9 vol% CO, 100 ppm NO, 0.61% O2, 600 ppm C3H6, 300 ppm C3H8, 300 ppm CH4, 300 ppm H2, 15 vol% CO2, 10% H2O, He balance, GHSV = 60000 h-1
470(C3H8) 202
c
310(C3H6) 460(C3H8)
fuel rich c
350, fuel rich
157
158
438
463(C3H6)
-d
-
305
469(C3H6) -(C3H8) d
486
-
294
497(C3H6) -(C3H8) c
-d
a
Temperature at 50% conversion. the OSC values are calculated from the amount of CO2 produced in a series of alternated pulses: CO–O2–CO–O2–CO–O2 c estimation from the light-off curves of CO/HCs/NOx conversion in the references. d Conversion lower than 50 %. b
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Table 6 Summary of PGM-containing perovskites for CO/HCs oxidation and/or NOx reduction. Perovskites
CaCo0.5Zr0.5O3-δ
Synthesis method
Pechini
Specific surface area (m2/g) 2,300 µmol O/g) at 500 °C for in a fully reversible manner. Li et al.
171
observed that the
well-defined superstructure of Sr2Mg1−xMnxMoO6−δ double perovskites with valency pairs of (Mn2+-Mo6+) and (Mn3+-Mo5+) showing high oxygen vacancy concentration, favoring the activation of gaseous oxygen. Lin et al.173 reported that Ba and Ti/Nb co-substituted (Ba0.15Sr0.85)(B0.15Co0.85)O3−δ (B=Ti,Nb) perovskite-type oxides show high oxygen sorption/desorption performance in the temperature range of 300-950 °C. While double-perovskites and A/B site co-doped perovskites exhibited promising oxygen storage properties, preparation of these materials are more challenging. ACS Paragon Plus Environment
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Moreover, they are more likely to experience stability issues at high operating temperatures.172, 174 For example, the exclusion of cobalt oxide from the PrBaCo2O6−δ lattice was observed at high temperatures.174 4.1.2 Morphology and size effects The performance of perovskite oxides can be affected by their particle size, texture and morphology. In general, the bulk-surface oxygen exchange of perovskite oxides can be improved by increasing the specific surface area/porosity and reducing the crystallite size.2, 175, 176 However, the high specific surface area is difficult to survive considering the accelerated aging requirements (>1,000 °C) for practical TWC applications.175 Bonne et al. reported that lattice oxygen mobility of LaCoO3 doubled when its particle size was reduced to lower than 5 nm.177 Ran et al. reported that the structural defects and reducibility of La0.7Sr0.3Mn0.7Cu0.3O3+δ can be controlled by changing the macrostructure of materials.155 The formation of three-dimensionally ordered macroporous structure for the perovskite catalysts can significantly increase the concentration of active surface oxygen species163, 178, 179, improve the activity of lattice oxygen162, 180 and enhance the reducibility of the catalysts178,
179, 181
, which can explain the enhancement of the
catalytic activity for CO or HCs oxidations. Enhanced reducibility and catalytic activity were also observed on the mesoporous perovskite oxides prepared by the organic template decomposition method or vinyl silica template method. This was attributed to the increased number of exposed active sites and improved mass transfer rate due to the increased gas-solid contact area182, 183. However, these strategies for surface area enhancement may not be practical due to the complex preparation procedure and potential stability issues under demanding operating conditions. 4.1.3 Hybrid structure effects It was also observed that composite oxide catalysts with the presence of transition metal oxides in addition to the perovskite phase (La1-xSrxMny(Fe/Cu)zO3, y+z>1) exhibit greater capacity to reversibly donate lattice oxygen than the stoichiometric one.184 For example, the resulting metal-oxide composite for a nominal composition of LaFe2.4Cu0.4Ox shows superior oxygen storage capacity relative to single phase ACS Paragon Plus Environment
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ACS Catalysis
perovskite oxide.184 Composite forming from over-stoichiometric B-site metals also promotes structural defects in the perovskite phase, which improves the redox performance and enhances the catalytic activity for CO/HCs oxidation and NO reduction.156,
185
The synergistic effect of isolated oxide particles formation on
perovskite was also reported in the stoichiometric LaCo1-xCuxO3 system; the presence of isolated copper oxides causes the decrease of Co3+ reduction temperature, and the formation of vacancies in the perovskite enhances the oxygen mobility.148 These two factor, in turn, strongly improve the catalytic activity. Neal et al. reported that the formation of MeOx@LaySr1-yFeO3 core-shell structure significantly improved the redox stability and kinetics relative to isolated/supported transition metals while improving oxygen storage capacity relative to pure perovskites.186 4.2 PGM-containing perovskites The previous sections discuss non-PGM containing perovskites for three-way catalysis. Meanwhile, PGMs such as Rh, Pd, and Pt are important for commercial TWC/OSM systems due to their excellent catalytic activity. PGM-containing perovskites, proposed as “intelligent catalysts”, have been commercially used in TWCs for gasoline engine by Daihatsu Motor in Toyota Group since 2002.187 Due to their promising oxygen storage and catalytic properties, many perovskites exhibit unique properties as support or host structures for PGMs – PGMs can be incorporated into perovskite structures, dispersed on the surface of the perovskite oxides, and reversibly incorporate into and segregate from the perovskite host structure. This self-regenerative property helps to further improve their catalytic activity, stability and oxygen storage/release properties.187 Guilhaume et al.137, 138 reported that LaMn0.976Rh0.024O3.15 form by incorporating a small amount of rhodium into the LaMnO3.15 perovskite. The structure of the resulting perovskite can change, reversibly, between La(MnRh)O3.15 and La(MnRh)O3.00 at relatively low temperatures (~400 °C). The over-stoichiometric oxygen (or cation defects) can be easily removed or recovered under reduction (CO and HCs) or oxidation (O2 or NOx) conditions. He et al. investigated La1-xSrxMO3 (M = Co0.77Bi0.20Pd0.03) perovskite as three-way catalysts.159 It was reported that the ACS Paragon Plus Environment
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structural and defect properties of the perovskite can be tailored by the substituents of both A and B sites cations. It was proposed that co-incorporations of Sr2+ into the A sites and Bi5+ (along with Pd) into the B sites can create the coexistence of oxygen vacancies and over-stoichiometric oxygen.159 The term “over-stoichiometric oxygen” refers primarily to the loosely bonded or chemisorbed oxygen species. In addition to being a host structure, perovskite oxides have been explored as supports for PGMs. It was reported that perovskite supported PGMs (e.g. Pd/LaFe0.8Co0.2O3) can be more active and reducible than the PGM doped perovskite (e.g., LaFe0.77Co0.17Pd0.06O3).160 The strong metal-support interaction between PGMs and perovskite support significantly increases the PGM-containing catalysts’ resistance towards sintering, enhancing the catalytic performance while minimizing the noble metal content.188-192 A number of PGMs containing perovskites were touted as “intelligent catalysts” due to their self-regenerative properties. For instance, Pd in the LaFexCoyPd1-x-yO3-ẟ was reported to reversibly segregate from and re-incorporate into the perovskite structure under PO2 fluctuations in the ICE exhaust.141, 188, 193 Based on in-situ energy-dispersive X-ray absorption fine-structure (DXAFS) analysis, Uenishi et al. proposed that the migration of Pd into and out of the perovskite structure are very fast (~ 1s at 400 °C). This allows the catalyst be highly responsive to the frequent PO2 swings of the ICE exhaust.
194
It was also suggested that the facile segregation and re-incorporation of
the Pd species can inhibit the sintering of metallic Pd particles.141, 188, 195, 196 The reductive segregation of Pd0 on the surface layer of the catalysts can take place at low temperatures (~ 100 °C or higher). Meanwhile, oxidation of Pd0 to Pd2+ and its (partial) reincorporation into the perovskite framework occurs at approximately 200 °C in oxidative atmosphere.188 This unique redox property allows high catalytic activity at both oxidizing and reducing environments. Tanaka et al. investigated a number of PGMs-perovskite combinations and reported that PGMs can be at least partially incorporated into various perovskite host structures including LaFeO3 (for Pd, Rh, and Pt), LaAlO3 (for Rh), CaTiO3 (for Rh and Pt)193, etc. Similar phenomena were also reported in LaMnxPd1-xO3-δ by Tzimpilis et al.197 They proposed that the isolated ACS Paragon Plus Environment
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PdO instead of metallic Pd determines the redox property and activity of the catalyst, and the state of surface Pd species is controlled by the concentration of Mn in the B-site. Their work also indicated that excess Mn induces the formation of easily reducible PdO surface species that do not reincorporate into the lattice, resulting in lower activity. Conversely, Mn deficiency appears to inhibit PdO surface reduction, leading to improved activity. Using aberration-corrected transmission electron microscopy as illustrated in Figure 8, Katz et al.198 observed that the diffusion of Pd into and out of epitaxially grown LaFeO3 under high temperature oxidizing and reducing conditions. However, metallic Pd was not observed until reduction treatment at 700 °C in the presence of 5% H2. In addition, the authors suggested that the observed dissolution and segregation of Pd into and out of the LaFeO3 are significantly more limited than the results from Nishihata et al.141,
198
In terms of the Pt-CaTiO3 system, Katz et al. confirmed
reversible segregation and dissolution of Pt from/into the perovskite structure with high-angle annular dark-field imaging (HAADF).199 However, Pt-rich clusters were formed primarily within the oxide matrix. Moreover, the surface Pt clusters were observed to coarsen upon oxidation rather than incorporating into the perovskite phase.199 Although these studies casted doubts on the extent of self-regeneration behavior of PGM-containing perovskites in the context of three-way catalysis, further investigations of the same investigators still confirmed the satisfactory performance and redox stabilities of Pd doped LaFeO3.200 Moreover, the PGM-containing perovskites investigated, i.e. Pd containing LaFeO3 and Rh containing CaTiO3, were both highly resistant towards sintering (14 hours at 800 °C). Another potential advantage of incorporating PGM to perovskites is the enhancement of OSC. For instance,
OSC
of
thermally
aged
(at
900
°C
for
2h)
Pd(2wt%
over-stoichiometric)/YFeO3 catalyst was almost ten times higher than that of the aged YFeO3 without Pd. Both samples showed comparable specific surface area of 3.0 m2/g.201, 202 In another report, high-temperature hydrothermal treatment (950 °C for 3 h) was used to induce the migration of Pd out of the La0.91Mn0.85Ce0.24Pd0.05Oz and La1.034Mn0.966Pd0.05Oz perovskite lattice to form segregated PdO. This was reported to ACS Paragon Plus Environment
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be beneficial for improving the catalytic activity while maintaining high thermal stability of the catalysts.197 The self-regenerative perovskite-type catalyst Pd-LaFeO3 has been commercialized as a TWC component.187 Uenishi et al. reported that both the oxygen storage capacity (OSC) and oxygen release capacity (ORC) of the LaFe1-xPdxO3-δ (0.01≤x≤0.1) catalysts were improved by the presence of highly dispersed Pd nanoparticles on the oxide surface. These dispersed Pd0 particles on LaFePdOx acted as ‘‘breathing active sites’’ or a new type of catalytic active center that enable efficient oxygen exchanges, combining high catalytic activity and OSC/ORC properties.
c
d
df
e
g
Figure 8. (a) XPS spectra of the LaFe0.95Pd0.05O3-δ pulsed-laser deposition thin film after thermal treatments. (b) High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image of a typical Pd particle on the LaFe0.95Pd0.05O3-δ film surface after aggressive reduction. (c) HAADF-STEM image of the oxidized Pd/LaFeO3 PLD thin film, with (d) an image at higher magnification of one of the partially embedded PdO particles (left) and an EELS line scan taken across the reaction zone (A→B) in (right). (e) HAADF-STEM image of a region of the oxidized Pd/LaFeO3 PLD thin film with a visibly distorted lattice, together with
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corresponding (f) Pd and (g) Fe X-ray energy dispersive spectroscopy (EDS) maps.198 Reproduced from reference 198. Copyright 2011, ACS. 4.3 Challenges and opportunities for perovskite based OSMs Perovskite oxides, which show high oxygen storage capacity and catalytic activity even at very low specific surface areas1,
139
, have been in the spotlight of TWC
research for more than 50 years. Although perovskite oxides were often referred to as promising catalysts with “limited commercial success”3, millions of PGM-containing perovskite-type TWCs have been successively utilized for gasoline engine in Toyota Group since 2002.187 Well-designed, non-PGM containing perovskites can also be used as efficient platinum substitutes in diesel oxidation and NO treatment.203 While perovskite structured OSMs/catalysts developed in the labs have demonstrated superior oxygen storage capacity and high activities for TWC reactions, a few practical limitations hinder their widespread utilizations. For instance, the complexity for some of the doped perovskites makes it challenging to prepare structured OSMs in commercial TWC systems: washcoated monolithic perovskite catalysts which usually contain complex components to obtain stable structure, perovskite oxides may react with the substrate, binder or primer materials, resulting decreased catalytic activity and/or oxygen storage capacity.126, 144, 145, 204 Other challenges include stability issues in the presence of sulfur for some perovskites. Self-regeneration of PGM can also be limited under certain practical exhaust conditions195; over-reduction can lower NO reduction activity; and, over oxidation of the materials can permanently lower activity for CO and HC oxidation reactions though sintering of perovskite particles. The complexity of advanced perovskite system is also an issue. Despite of the abovementioned challenges, recent research has revealed promising new directions that can potentially increase the competitiveness of perovskites for TWC applications. Additionally the adaptation of increasingly stringent emissions regulations combined with increasingly complex internal combustion technologies opens up new opportunities for perovskites research as more active and stable TWC/OSM with higher OSC are sought after.195 Advanced ICE technologies such as gasoline direct ignition (GDI), gasoline compression ignition (GCI), and exhaust gas ACS Paragon Plus Environment
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recirculation (EGR) all pose unique new challenges and opportunities for emissions control.205 In addition to the many of emerging and advanced perovskite based emissions catalysts detailed in this section, structurally controlled and rationally designed synthesis methods can also address stability and component interaction issues. By depositing LaFeO3 via atomic layer deposition on a porous MgAl2O4 support, followed by wet impregnation of Pd, Onn et al.206 demonstrated that it enabled a more practical form of self-regenerating catalyst than traditional, bulk perovskite oxides. This “smart” catalyst opens new perspectives for incorporating Pd into the LaFeO3 film in order to obtain a highly uniform initial Pd distribution, improving metal utilization and increasing thermal stability. PGM-free perovskite, which showed a superior performance for diesel exhaust treatment, may potentially be utilized for TWC applications. Under realistic conditions, La1-xSrxCoO3 displayed higher NO-to-NO2 conversions than a commercial Pt-based diesel oxidation (DOC) catalyst.203 Historical concerns about the stability of perovskite TWCs in SO2 are generally related to Co based perovskites139, but both PGM doped perovskite and the large design space of non-PGM containing perovskite materials show promise in addressing this issue.146 With the promising performance, large design space, and evolving environmental challenges, perovskites offer excellent opportunities for future development of commercially viable TWCs/OSMs. 5. Summary and outlook The unique and versatile structural and compositional properties of perovskite oxides make them intriguing materials for chemical looping and three-way catalysis applications. Continued research in the bulk and surface properties of perovskite oxides have resulted in new materials, methods, and mechanistic insights in tailoring the oxygen storage and catalytic properties of this important class of mixed oxides. While perovskite for TWC applications has been studied for more than five decades, the use of perovskite for chemical looping is a relatively new topic yet it offers significant new opportunities for a wide range of energy-related applications in terms of air separation, CO2 capture, water-splitting, as well as selective oxidations. ACS Paragon Plus Environment
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Since chemical looping processes make use of the bulk lattice oxygen in perovskites for cyclic redox operations, redox thermodynamics of the perovskites are of significant importance for designing suitable oxygen carrier materials, especially when high equilibrium oxygen partial pressures (PO2s) are desired, e.g. CLAS and CLOU. The bulk redox properties for perovskites often correlate well with vacancy formation energy and the latter, in conjunction with DFT calculation tools, can be used to efficiently screen perovskite based oxygen storage materials. The ever-expanding databases on oxide materials based on ab-initio calculations, e.g. the Materials Project, offer useful and quantitative information to guide perovskite material design and redox pair selection (when phase transition is anticipated). Oxygen carriers or redox catalysts for chemical looping partial oxidation can be optimized by tailoring both the bulk and surface properties of the perovskite oxides. In the case of syngas generation, over-oxidation of syngas products can be inhibited by limiting the equilibrium PO2 of structurally tailored perovskite oxides without relying on their surface properties. On the other hand, redox catalysts with high equilibrium oxygen chemical potentials can also produce partial oxidation products away from the thermodynamic equilibrium conditions. Under such a situation, the surface of the redox catalyst plays a critical role for the selectivity and activity. The surface properties of the perovskites are often subject to a complex and intricate balance between the bulk O2- migration and surface oxygen removal. Generally, a relatively more facile surface oxygen removal is needed to limit the non-selective oxygen species on the redox catalyst surface. This can be realized by promoting the surface with active sites for more facile activation of the feedstock or covering the surface with a layer of substantially different O2- conduction/evolution, and/or catalytic properties. From an application standpoint, further lowering the temperature for reversible lattice oxygen removal/uptake as well as utilization of the acid-base properties of the perovskite surfaces can open up additional opportunities for perovskite as redox catalysts in chemical looping partial oxidation. With the redox catalysts function as both a catalyst and oxygen separation agent, chemical looping partial oxidation represents an exciting area for process intensification. ACS Paragon Plus Environment
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Although detailed understanding of the redox catalyst reaction mechanism and design strategies are highly desirable, the very nature of chemical looping reactions, which involves removal or replenishment of significant fractions of bulk lattice oxygen from/to the perovskite based redox catalysts, poses significant challenges to study such reactions. This is due to the constant change in the bulk and surface properties of the redox catalysts throughout the looping reactions. In fact, substantial changes in reaction mechanisms, pathways, and product selectivities are often experienced within the same redox step (e.g. fuel oxidation). To address such challenges, state-of-the-art surface and material science characterization techniques as well as ab-initio calculation tools should be used for such studies. Due to the similarities in operating schemes and challenges between chemical looping and three-way catalysis, the fundamental insights to be generated from these studies will benefit both areas of research. In terms of three-way catalysis, the use of PGM-containing perovskites in commercial TWCs by the Toyota Group is very encouraging. However, further improvements in perovskites’ sulfur resistance, stability, ease in fabrication, and catalytic performance are important in order to meet the increasingly stringent emission regulations and the development/implementation of advanced ICE cycles. Balancing the complexity of the perovskite OSMs with their performance, stability, manufacturability and compatibility with other TWC components (e.g. substrate, binder or primer materials) are likely to be critical for their practical applications. With the promise of being standalone TWCs without PGMs and versatile oxygen carriers for chemical looping applications, perovskite oxides offer exciting possibilities and are too important to be overlooked.
Acknowledgements The authors acknowledge the fruitful discussions with and suggestions from Prof. Susannah Scott on the geo-inspired redox catalyst concept which played a critical role in developing of this perspective. This work was supported by the U.S. National
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Science
Foundation
(Award
No.
CBET-1604605,
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and
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Air
ACS Catalysis
CLAS
O2 N2
1 2 CLC + 3 CO CO22 4 Coal NG & Oil Biomass CO2 Sequestration 5 CLPOx MeO MeO Redox 6 x-y x 7 CL‐splitting 8 CO2 Chemicals H O 2 9 10 11 HCs CO H2O CO2 N2 O2 12 NO O x 2 13 MeOx: Perovskite 14 Treated by three‐way catalyst Exhaust gas from vehicles 15 16 17 18 19 Lower mantle 20 21 22 ACS Paragon Plus Environment 23 24 Perovskite phase Earth’s structure 25 26 27 28 29 30 31 32 33 34 35 36 37 38
