Recent Advances on Nitrous Oxide (N2O) Decomposition over Non

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Recent advances on nitrous oxide (N2O) decomposition over non–noble metal oxide catalysts: catalytic performance, mechanistic considerations and surface chemistry aspects Michalis I. Konsolakis ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b01605 • Publication Date (Web): 17 Sep 2015 Downloaded from http://pubs.acs.org on September 23, 2015

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Recent advances on nitrous oxide (N2O) decomposition over non– noble metal oxide catalysts: catalytic performance, mechanistic considerations and surface chemistry aspects

Michalis Konsolakis School of Production Engineering and Management, Technical University of Crete, GR–73100, Chania, Crete, Greece

To whom correspondence should be addressed. E–mail: [email protected]; Tel.: +30 28210 37682; URL: http://www.tuc.gr/konsolakis.html

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Abstract Nitrous oxide (N2O) is the largest stratospheric ozone depleting substance, being concomitantly the third most potent greenhouse gas. The direct catalytic decomposition of N2O (deN2O process) is one of the most promising remediation technologies for N2O emissions abatement. Although noble metals (NMs)–based catalysts demonstrate satisfactory deN2O performance, their high cost and sensitivity to various effluent stream components (e.g., water vapor, oxygen) limit their widespread industrial applications. Hence, the development of NMs–free catalysts of low cost and satisfactory deN2O performance is of paramount importance. This survey appraises the recent advances, which have been reported since 2000, on N2O decomposition over non–noble metal oxidic catalysts. Initially, a brief overview of N2O sources, environmental consequences and remediation technologies is provided. The literature related to deN2O process over NMs–free metal oxides (MOs) is categorized and critically discussed, as follows: (i) bare oxides, (ii) hexaluminates, (iii) hydrotalcites, (iv) spinels, (v) perovskites, and (iv) mixed metal oxides not belonging in the above categories. The review covers several aspects with respect to the reaction mechanisms, the structure-activity correlations, the role of various inhibitors (e.g., O2, NO, H2O) as well as the strategies followed to adjust the local surface structure of MOs. Fundamental insights towards fine tuning of surface chemistry of MOs by means of advanced preparation routes and/or electronic promotion are also provided, paving the way for real–life energy and environmental applications, beyond the deN2O process. Keywords: Nitrous oxide (N2O) decomposition; metal oxides; hexaluminates; hydrotalcites; spinels; perovskites;



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1.

Introduction

1.1 Nitrous Oxide (N2O): sources & environmental consequences Nitrous oxide (N2O) is the third most significant anthropogenic greenhouse gas and the largest stratospheric ozone depleting substance [1–6]. In particular, N2O has a global warming potential (GWP) of approximately 310 times higher compared to CO2, depleting concurrently ozone layer in a way similar to chlorofluorocarbons (CFCs) due to its long lifetime (114 years) in atmosphere (Table 1) [7,8]. More worryingly, the anthropogenic N2O emissions are rapidly increasing and are projected to almost double by 2050, unless mitigation strategies put forward by each country [6]. Therefore, the understanding of its origin and control should notably contribute to the stabilization of Earth’s climate. Table 1. Properties of N2O at 25 °C and standard pressure. Chemical Formula

N2 O

Molecular Shape

Linear

Solubility in water

0.111 g/dm3

Density (vapor)

1.8 g/dm3

Dipole moment

0.166 D

Standard enthalpy of formation

82.05 KJ mol−1

Standard molar entropy

219.96 J K−1 mol−1

Lifetime in atmosphere

114 years

Global Warming Potential (GWP–100 year) 310

N2O is emitted by both natural and anthropogenic sources. Natural emissions include terrestrial, marine and atmospheric sources and are estimated at about 11 Mt N2O–N/yr (megatons of N2O in equivalent nitrogen units per year), compared to about 6 Mt N2O–N/yr of anthropogenic emissions. Human activities that contribute to N2O emissions include mainly the biological transformation of fertilizer’s nitrogen ACS Catalysis_Revised MS_ cs-2015-01605m


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into N2O (agriculture), biomass burning, fossil fuels combustion, industrial activities, wastewater treatment, aquaculture, solvents use and oceans due to anthropogenic N deposition (Figure 1) [5, 6].

Ocean (0.2) Solvent and other product use (0.05) Anthropogenic (6.2 Mt N2O-N/yr))

Aquaculture (0.05)

N2O emission sources

Wastewaster (0.2) Fossil fuel combustion-Transposration (0.1) Fossil fuel combustion-Stationary (0.5) Industry (0.3) Biomass burning (0.7) Agricultural (4.1)

Terrestrial (6.6) Marine (3.8) Atmospheric (0.6)

0

2

4

6

8

Natural (11 Mt N2O-N/yr))

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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10

Mt N2O-N/yr

Figure 1. Current natural and anthropogenic N2O emissions in Mt N2O–N/yr. (megatons of N2O in equivalent nitrogen units per year) [5, 6].

Agriculture is by far the largest source of N2O emissions due to human activities, responsible for 4.1 Mt N2O–N/yr, which is equivalent to 66% of total gross anthropogenic emissions. N2O emissions from synthetic fertilizers, manure and crop residues are mainly involved in agricultural N2O emissions [6]. N2O emissions from biomass are mostly derived from forest fires, crop residue burning and biomass combustion for heating and cooking. Current biomass–derived N2O emissions are equivalent to 0.7 Mt N2O–N/yr, corresponding to 11% of total gross anthropogenic emissions. ACS Catalysis_Revised MS_ cs-2015-01605m


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Wastewater treatment along with aquaculture are responsible for the emissions of 0.25 Mt N2O–N/yr, or 4% of the total gross anthropogenic emissions. This category involves wastewater treatment processes as well as the discharge of nitrogen wastes to water. In particular, human wastes are usually treated in waste water facilities, where part of the nitrogen is removed through a denitrification processes. Initially, large materials are removed by a mechanical-oriented treatment, where about 10% of the nitrogen is removed. The second step involves the conversion of organic nitrogen to ammonia, nitrate and nitrite via the use of micro-organisms. This step results in a decrease of the nitrogen content of sewage influent by approximately 35%. Finally, a nitrogen removal by up to 80% is achieved by advanced techniques in tertiary treatment. N2O can be produced during the above processes as an intermediate product of nitrification and denitrification, with the latter to be mainly responsible for N2O emissions. Several parameters involving the concentration of dissolved oxygen, the pH and the concentration of reactive nitrogen in influents can notably affect the N2O emissions [1]. The industrial and fossil fuel combustion sector includes N2O emissions from nitric and adipic acid production as well as emissions from stationary and mobile combustion sources. The current emissions are approximately 0.9 Mt N2O–N/yr, equivalent to 15% of the total gross human sources. These emissions are expected to be increased to 1.0 and 1.4 Mt N2O–N/yr by 2020 and 2050, respectively [5, 6, 9]. Nitric and adipic acid production are the main N2O industrial sources. Nitric acid is the major feedstock in manufacturing processes related to the production of explosives, nitrogen–based fertilizers, adipic acid, etc. N2O is released as a by– product during the Pt–catalysed ammonia oxidation process [6, 9, 10]. The amount of the formed N2O depends on the catalyst type and age, as well as on the combustion ACS Catalysis_Revised MS_ cs-2015-01605m


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conditions in the oxidizing unit. The formation of N2O at nitric acid plants can be considered as the result of the following reactions: 2 NH3 + 2 O2 → N2O + 3 H2O

(1)

2 NH3 + 8 NO → 5 N2O + 3 H2O

(2)

4 NH3 + 4 NO + 3 O2 → 4 N2O + 6 H2O

(3)

Adipic acid is a dicarboxylic acid, (CH2)4(COOH)2, produced from the oxidation of a keton–alchocol mixture by nitric acid. It is the main feedstock in nylon production, employed in the manufacturing of various products such as plastics, synthetic fibres, lubricants, etc. N2O is emitted as a by–product during the oxidation of a keton–alchocol mixture by nitric acid [6, 11, 12]: (CH2)5CO

(Cyclohexanone)

+

(CH2)5CHOH

(Cyclohexanol)

(CH2)4(COOH)2 (Adipic Acid) + N2O + H2O

+

→

HNO3

(4)

N2O emissions from stationary combustion sources are produced by public and industrial power plants as well as by other combustion processes employing fossil– or bio–fuels as energy carriers. N2O is formed by the high temperature oxidation of nitrogen that exists either in atmospheric air or in fossil fuels (organic N). The amount of N2O released by fossil fuels combustion is strongly dependent on fuel type characteristics, combustion temperature and abatement technology [6]. N2O emissions from mobile sources are released primarily by three way catalytic converters (TWCs), which are worldwide employed to simultaneously abate nitrogen oxides (NOx), carbon monoxide (CO) and hydrocarbons (HCs). The amount of N2O released by the transportation sector is mainly affected by the adopted control technology, the driving cycle, the TWC operating temperature as well as by the TWC composition and aging [6, 12–15]. N2O is a by–product of the reactions taking place ACS Catalysis_Revised MS_ cs-2015-01605m


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in TWCs and its formation is notably favored during the ‘‘cold start’’ and ‘‘intermediate temperature’’ periods [12, 16]. Additionally, N2O emissions are in general increased as a result of catalyst deactivation and as the fuel sulfur is increased [12, 17]. 1.2. Effect of human activities in N2O atmospheric concentration Although N2O exists in Earth’s atmosphere since there has been life [18], only in the last 100-150 years its concentration has been rapidly increased (Figure 2). The stable concentration of N2O over thousands of years was due to the fact that the natural N2O emissions were counterbalanced by natural sinks, primarily by the chemical breakdown of N2O in the stratosphere. Nowadays, however, the concentration of N2O, as well as of other greenhouse gases, i.e., carbon dioxide (CO2) and methane (CH4), have increased unprecedentedly. In 2013 the concentration of these gases was 326 ppb, 395 ppm and 1893 ppb, exceeding the 1750 levels by 20, 40 and 170%, respectively (Table 2).



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340 ~326 ppb 320

N2O concentration (ppb)

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300

280 2013 260

240 1750

1800

1850

1900

1950

2000

Year

Figure 2. General trend of atmospheric N2O concentration in the 1750–2013 period (b) [6].

Table 2. Current (2013) concentration of greenhouse gases (N2O, CO2, CH4) in relation to pre–industrial period (1750) levels. 1750

2013

Percentage

concentration

concentration

increase (%)

N2O

275 ppb

326 ppb

19

[6]

CO2

280 ppm

395 ppm

41

[6]

CH4

700 ppb

1893 ppb

170

[19]

Greenhouse gas

Reference

The atmospheric concentration of N2O has notably increased since 1850 and much more rapidly in the last 60 years (Figure 2). This increase is undoubtedly associated with the extended utilization of fertilizers in the agricultural sector as well as with the ACS Catalysis_Revised MS_ cs-2015-01605m


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extensive use of fossil fuels in the energy sector. Nowadays, N2O concentration in the atmosphere exceeds to a large extent the highest concentrations recorded in ice cores during the past 800,000 years [20].

1.3 N2O abatement: remediation technologies N2O emissions abatement can be generally achieved in two different ways: a) by limiting the formation of N2O, or b) by employing after–treatment technologies. The choice of approach is strongly dependent on N2O source and its particular “exhaust” characteristics as well as on relevant economic aspects. Hence, taking into account the diffuse character of N2O emissions from the agricultural sector and non-controled biomass burning, the employment of end–of–pipe technologies should be considered unfeasible. In this case, the first approach, i.e., lowering of N2O formation, is the best solution to reduce N2O emissions. For this purpose, the following strategies can be employed [6, 21]: – reducing food loss and wastes, – improving nitrogen use efficiency (NUE), both in crop and animal production. On the other hand, the concentrated N2O emissions from fossil fuels combustion and chemical industry can be effectively controlled by end–of–pipe remediation technologies. Consequently, the effective control of N2O emissions from combustion and industrial sources is a challenging environmental issue [6, 21]. Table 3 presents the exhaust gas composition of various industrial and combustion N2O sources, in terms of pollutants nature and concentration range. The co–existence of other components, such as NOx, H2O and O2, in off–gases should be also emphasized. These components should be always taken into consideration in any after-treatment remediation technology for N2O elimination. ACS Catalysis_Revised MS_ cs-2015-01605m


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Table 3. Exhaust gas composition (vol.%) of industrial and combustion N2O sources. Source

N2 O

NOx

O2

H2 O

CO

Reference

Adipic acid

20–50

0.7

4

2–3

0.03

[21–23]

Nitric acid

0.03–0.35

0.01–0.35

1–4

0.3–2

–

[20, 21]

FBC1

0.005–0.015

0.01–0.03

2–10

10

0.001–0.1

[21, 24]

TWCs2

0–0.13

0–0.2

0–0.1

~10

0–0.4

[21]

1

Fluidized Bed Combustion (FBC).

2

Three–way Catalytic Converters (TWCs).

3

Fluctuations are related to operation conditions (mainly air–to–fuel ratio, drive cycle and temperature) as well as to TWC composition and aging. Several after–treatment technologies have been developed and adopted so far for

N2O emissions control in chemical & energy industry. These can be classified to: i) thermal decomposition, ii) non–selective catalytic reduction (NSCR), iii) selective catalytic reduction (SCR), and iv) direct catalytic decomposition (deN2O) [25–28]. The N2O catalytic decomposition is one of the most promising methods for N2O emissions control, due to its simplicity, high efficiency and low energy requirements [24, 25]. For this reason significant research efforts have been lately focused on the development of novel catalytic materials for N2O abatement. More than 1000 articles have been published on N2O catalytic elimination since 2000, revealing the intense interest in this topic. 2.

Catalytic decomposition of N2O

The catalytic decomposition of N2O to nitrogen and oxygen has been examined over a wide variety of catalysts, which can be generally classified into noble metal supported catalysts, metal oxides and zeolite-based catalysts. The state–of–the–art on N2O



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catalytic decomposition has been reported only in a few articles, in the last twenty years [21, 23, 29–33]. The N2O decomposition on perovskites was reviewed by Swamy and Christopher in 1992 [29]. Subsequently, in 2001, the chemical structure and performance of perovskite–type materials was extensively covered by Peña and Fierro [30]. The deN2O performance of hydrotalcite–type materials was comprehensively summarized by Kannan in 1996 [31]. In the same year (1996), Kapteijn et al. [21] published a survey study on N2O decomposition over metal–based catalysts, bare or mixed oxides as well as on zeolitic systems. Several aspects on the reaction mechanism and kinetics as well as on the inhibiting role of several substances, such as O2, H2O, NO, were addressed. A comprehensive survey on the application of clay–derived catalysts for the removal of nitrogen oxides (deNOx) was given by Serwicka in 2001 [32]. In 2003, Centi et al. [23] reported on the catalytic reduction of N2O over highly active Rh–based catalysts. It was revealed that Rh catalysts supported over modified zirconia–alumina carrier are among the most effective catalysts for the deN2O process [23]. Although NMs–based catalysts exhibit satisfactory activity at relatively low temperatures, their catalytic efficiency is notably hindered by the presence of O2, NO and H2O, which are usually co-existing in exhaust gas stream. Furthermore, the high cost of NMs in combination with their inadequate thermal stability restricts their widespread industrial applications. Therefore, the development of highly active and stable catalytic materials of low cost is of paramount importance for the effective control of N2O emissions from industrial and energy sources [21, 33–37].



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MOs represent one of the most important and widely employed categories of solid catalysts, due to their low cost in conjunction to their particular characteristics, such as excellent redox properties, thermal stability and adequate catalytic activity [35, 38– 46]. Interestingly, mixed MOs, consisting of two or more single oxides in a specific proportion, are extensively employed in heterogeneous catalysis since they exhibit structural, electronic and chemical properties which are completely different from those of parent oxides. The synergetic effect between the different counterparts usually offers unique physicochemical properties, which are then reflected on the catalytic performance [35, 38, 42, 43, 45, 46]. Among the MOs catalysts, transition metal–based oxides are of major importance in the field of heterogeneous catalysis, due to their peculiar chemisorptions properties, attributed to the presence of partially filled d–shells of metal ions [34, 46]. It is well established that the performance of these multi–functional materials is greatly influenced by certain features, such as composition, crystal structure, morphology and surface/interface properties [35, 38, 42, 46]. Under this perspective, significant efforts have been devoted on the rational design and tailoring of MOs by means of advanced synthesis routes and/or the utilization of structural/electronic promoters. Recent advances related to these significant issues, in relation to the deN2O process, will be summarized and discussed in the sequence. More specifically, in the present review article, the recent advances in the field of N2O decomposition over MOs will be summarized and critically discussed. The literature studies devoted on deN2O process over NMs–free MOs are classified as follows: (i)

bare oxides,



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(ii) hexaluminates, (iii) hydrotalcites, (iv) spinels, (v) perovskites, (vi) mixed oxides not belonging to the above categories. The review covers several aspects on the reaction mechanism, on the role of various substances which coexist in N2O off–gases (O2, NO, H2O) as well as on the strategies followed to adjust the local surface structure of MOs and consequently their deN2O performance. Fundamental insights towards fine tuning of the surface chemistry of MOs by means of advanced preparation routes and/or electronic promotion are also provided. Emphasis is given not only on the deN2O performance of the aforementioned oxides, but also on the correlation between the catalytic activity and the surface chemistry of MOs.

2.1 Bare oxides Many single oxides have been examined for N2O catalytic decomposition. An extended overview of the studies reported up to 1996 can be found in the comprehensive review of “Heterogeneous catalytic decomposition of nitrous oxide” by Kapteijn, Rodriguez–Mirasol and Moulijn [21]. In Table 4 the main studies devoted on N2O decomposition over bare oxides in the last fifteen years are summarized. For comparison purposes the temperature required for 50% N2O conversion (light off temperature, T50) is provided along with the corresponding reaction conditions. T50 values are those found in the related literature studies; if not provided they are approximately estimated by N2O conversion versus temperature profiles. Although the comparison on T50 basis is not straightforward due to the ACS Catalysis_Revised MS_ cs-2015-01605m


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different reaction conditions employed by each research group, a general overview can be obtained. A more reliable comparison on activity basis (e.g., by means of specific reaction rate (r, mol m-2 s-1)), which could reflect more accurately MOs’ instinct characteristics, is also provided, where relevant data are available. Table 4. Main literature studies dealing with the deN2O performance of bare oxides. Catalyst

Reaction conditions

Nanocrystalline

0.5% N2O;

β–MnO2

WHSV1=72,000 ml g–1 h–1

Cryptomelane type

1% N2O;

MnO2

WHSV=120,000 ml g–1 h–1

Co3O4

5% N2O+10% O2; GHSV2=80,000 h–1

CuO MnO2

[47]

500

[48]

710

[49]

100% N2O; GHSV=600 h–1

510 510

NiO

300

Co3O4

382

MgO

0.5% N2O + 2% O2; WHSV=12,000 ml g–1 h–1

365 500

>500

Al2O3, Cr2O3

>500

CuO

550

Al2O3, CaO, Co3O4, Cr2O3, Fe2O3, In2O3,

1% N2O; WHSV=6,000 ml g–1 h–1

MgO, NiO, SnO2, TiO2

[51]

500

F3O4, CeO2

La2O3

[50]

470 470

MnO2

2

380

ZrO2

CuO

1

Ref.

555

Fe2O3 Cr2O3

T50 (°C)

>600 –

[52]

– –

WHSV: Weight Hourly Space Velocity [=] ml g–1 h–1. GHSV: Gas Hourly Space Velocity [=] h–1.



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The N2O decomposition in the presence of O2 or CH4 was extensively investigated over several bare oxides (Al2O3, CaO, Co3O4, Cr2O3, CuO, Fe2O3, In2O3, La2O3, MgO, NiO, SnO2 and TiO2) by Satsuma et al. [52]. An increase of N2O decomposition rate was observed in the presence of CH4, which was attributed to the reduction of N2O by methane. In contrast, the deN2O performance was suppressed by oxygen due to the inhibition caused by the reversible adsorption of oxygen atoms [52]. A correlation between the heat of formation of MOs and N2O decomposition rate (r, nmol m-2 s-1) was revealed (Figure 3). Both the promotion by methane and the

MgO

Al2O3 TiO2

1

0.1 100

CaO

Cr2O3

Fe2O3

CuO

10

NiO

100

La2O3

-2 -1

Co3O4

1000

SnO2 In2O3

inhibition by oxygen were more intense in oxides with a lower heat of formation.

N2O decomposition rate (nmol m s )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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N2O N2O+O2 N2O+CH4 200

300

400

500 0

600

700

-1

Heat of formation, -∆Hf (kJ mol )

Figure 3. N2O decomposition rate at 500 °C in a stream of N2O or N2O + CH4 or N2O + O2 as a function of the heat of formation of metal oxides. Reaction conditions: 1% N2O, 1% CH4, 8% O2, WHSV=6,000 ml g–1 h–1[52].



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Considering that the heat of formation is a reliable diagnostic of metal–oxygen bond strength, the effect of oxygen and methane on N2O decomposition was rationalized on the basis of metal–oxygen bond [52]. A detrimental effect was also observed for water vapor, co-existing in N2O off-gases (Table 3). The deN2O performance of bare oxides, such as Co3O4, is suppressed by H2O, due to its competitive adsorption on active sites [49]. In most cases the influence of water is reversible, unless a structure destruction is caused, as in zeolite-based materials [21]. It is also worth noting that although the conversion performance of bare oxides is more or less similar (Table 4), significant differences can be observed when the comparison is made on activity basis (Figure 3). The latter reveals the impact of bare oxides intrinsic characteristics on the achieved decomposition rate, which should be always taken into account in practical applications. The deN2O performance of various MOs in the presence of oxygen excess was examined by Inoue and co–workers [51]. It was found that the catalytic efficiency increased in the following order: NiO > CuO > Co3O4 > MnO2 > Fe3O4 > Al2O3 ≈ CeO2 ≈ Cr2O3 [51] (Table 4). In a similar manner, ordered crystalline mesoporous MOs, including CeO2, Co3O4, Cr2O3, CuO, Fe2O3, β–MnO2, Mn2O3, Mn3O4 and NiO prepared by hard templating, were comparatively tested in the decomposition of N2O; Co3O4, β–ΜnO2 and NiO exhibited the optimum performance [53]. An extensive theoretical study on N2O decomposition over alkaline–earth MOs (MgO, CaO, SrO, BaO) has been reported by Karlsen et al. [54, 55]. They proposed that N2O decomposition reaction involves first the dissociative adsorption of N2O, followed by active sites (*) regeneration via recombination of adsorbed oxygen species:



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N2O + * → N2O*

(5)

N2O* → N2(g) + O*

(6)

O* + O* ↔ O2(g) + 2*

(7)

In the first step, N2O is adsorbed (reaction (5)) and subsequently decomposed to N2(g) and adsorbed oxygen (reaction (6)). In the second step (reaction (7)) the catalyst surface is regenerated via a Langmuir–Hinshelwood (LH) mechanism, which involves the recombination of adsorbed oxygen species [56]. However, the molecular oxygen desorption through the recombination of Oads is determined by the lattice parameters (e.g., O–O bond distance), which limits the efficiency of deN2O process for several MOs [55]. In this regard, an alternative route involving the N2O interaction with surface oxygen species toward O2 desorption has also been proposed [55, 57]: N2O + O* → N2(g) + O2(g) + *

(8)

Moreover, the investigation of the electronic properties of CaO by means of Highest Occupied and Lowest Unoccupied Molecular Orbitals (HOMO and LUMO) further indicated that the adsorbed O atoms formed by N2O decomposition can interact with N2O towards N2(g) formation and active sites regeneration (reaction (8)) [57]. The above reaction scheme has been recently verified by a comprehensive molecular modeling complemented by ab initio micro–kinetic studies on a series of alkaline earth oxides (MgO, CaO, and SrO) [58, 59]. It was shown that terrace sites are responsible for the steady state deN2O reactivity, whereas the highly active sites located on edges and corners are involved only on the initial stages of the reaction due to their progressive deactivation by oxygen.



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Wu et al. [60] studied the N2O decomposition over CaS surface by using density functional theory (DFT) calculations. An energy barrier of approximately 1.23 eV was calculated for N2O decomposition to N2(g) and Oads. However, the removal of Oads, either by binding with a neighboring Oads or by reacting with the N2O molecule, required much more energy, i.e., 1.88 and 1.86 eV, respectively. It is therefore concluded that the removal of the adsorbed O atoms is the rate-determining step. Interestingly, the presence of CO accelerated the removal of Oads, notably improving the deN2O performance. In a similar manner, Satsuma et al. [61] concluded that N2O decomposition over CaO is a structure sensitive reaction demanding coordinately unsaturated sites [61]. Highly unsaturated sites are very active in N2O decomposition, but rapidly poisoned by oxygen species. Conversely, poorly unsaturated sites are responsible for the high deN2O activity [61]. In the light of the above findings, it can be deduced that relatively high temperatures are required in order the deN2O process to proceed promptly over bare oxides. This can be mainly attributed to the limited regeneration of active sites at low temperatures through the recombination of adsorbed oxygen species. Moreover, the presence of H2O in the feed stream results in a further inhibition due to its competitive adsorption on active sites. Therefore, bare oxides can be considered as possible candidates for high temperature deN2O process. Significant efforts should be focused in the enhancement of the low temperature deN2O performance of bare oxides by adjusting their surface and redox characteristics. The recent developments towards this direction are described in the sequence.



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2.2 Hexaaluminates Metal–substituted hexaaluminates have recently attracted great attention in heterogeneous catalysis due to their remarkable resistance to sintering and thermal shock up to 1600oC. In this regard, metal-substituted hexaaluminates can be considered as promising candidates for the high temperature N2O decomposition. Hexaaluminates can be represented by the general formula AMxAl12−xO19, where A stands for alkali, alkaline–earth or rare–earth cation, whereas M represents a transition or noble metal ion in Al crystallographic site. The unique physicochemical properties of hexaaluminates are derived from their peculiar layered structure, which consists of alternate stacking along the c–axis of Al2O3 blocks and mirror planes in which the cations are located [62-74]. More importantly, the substitution of Al3+ ions in hexaaluminate structure by transition metal ions, notably affects the redox properties of hexaaluminates, which is then reflected in the catalytic performance. Depending on the charge and radius of the cations located in mirror planes, β–alumina or magnetoplumbite–like (MP) structures can be obtained. A typical structure of β-Al2O3 and MP–type Βa–hexaaluminate is depicted in Figure 4 [74].



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Figure 4. The structure of Ba-hexaaluminate: (a) β-Al2O3 and (b) MP. Numbers in parentheses refer to the different Al sites: Al(1), octahedral site; Al(2), tetrahedral site; Al(3) in β-Al2O3, tetrahedral site; Al(3) in MP, octahedral site; Al(4), octahedral site; Al(5) in β-Al2O3, tetrahedral site; Al(5) in MP, trigonal bipyramid site. Reproduced with permission from reference [74]. Copyright © 2011, Elsevier.

Several metal–substituted hexaaluminates have been studied for N2O catalytic decomposition. Lanthanum and barium hexaaluminates substituted by transition metals (Mn, Fe, Ni), have been primarily employed for N2O decomposition. In Table 5 the main studies devoted on deN2O over hexaaluminates in the last fifteen years are summarized. For brevity’s shake, only the optimal deN2O performance of various substituted AMxAl12–xO19 hexaaluminates is provided, which is attained at a specific molar content (x).



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Table 5. Main literature studies on N2O decomposition over hexaaluminates. Catalyst

Reaction conditions

BaRu0.2FexAl11.8–xO19

30 % N2O; GHSV= 30,000 h–1 30 % N2O;

BaIrxFe12–xO19

GHSV= 30,000 h–1 0.15% N2O;

LaFeAl11O19

WHSV=60,000 ml g–1 h–1

LaFexAl12–xO19 ABAl11O19

30 % N2O; GHSV= 30,000 h–1

T50 (°C)*

Ref.

475 (x=1)

[66]

550 (x=0.6)

[67]

530

[68]

550 (x=5)

[69, 70]

0.15% N2O;

620

WHSV=30,000 ml g–1 h–1

(A=Ba, B=Fe)

LaMnxAl12–xO19

30 % N2O;

630 (x=1)

BaMnxAl12–xO19

GHSV= 30,000 h–1

580 (x=1)

A=La, Ba, B=Mn, Fe, Ni

[65]

[71]

*optimum deN2O performance obtained at a specific composition (values in parenthesis). The deN2O performance of various metal substituted hexaluminates has been extensively investigated by Zhang and co–workers [66, 67, 69–74]. Various aspects related to the impact of the preparation procedure, phase composition, and crystal structure on the deN2O performance, were thoroughly examined. In particular, Mn– substituted

La–hexaaluminates

(LaMnxAl12–xO19)

and

Ba–hexaaluminates

(BaMnxAl12–xO19) were tested for N2O decomposition [71]. Both La– and Ba– hexaluminates were found to be more active than Mn/Al2O3 reference sample [71]. The superior performance of Ba–hexaaluminate with a β–Al2O3 structure was ascribed to a larger fraction of octahedral Mn3+ in relation to La–hexaaluminate with a MP structure [71]. More specifically, for La-hexaaluminate with a MP structure, Mn preferentially enters tetrahedral Al(2) sites at a low Mn content (x = 0.5) as Mn2+, which is inactive for N2O decomposition. At higher Mn contents, the substitution of ACS Catalysis_Revised MS_ cs-2015-01605m


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octahedral Al(1) sites with highly active Mn3+ species is taking place, resulting in the enhancement of the activity. However, in the case of Ba-hexaaluminate with a βAl2O3 structure, Mn3+ can replace the octahedral Al(1) sites even at a low Mn content (x = 0.5), resulting in much higher activity compared to the La-hexaaluminate [71]. Incorporation

of

Fe

ions

in

Ru–substituted

barium

hexaalumimates

(BaRu0.2FexAl10.8–xO19, x = 0, 0.5, 0.8, and 1) suppressed Ru evaporation under high temperature conditions (1100–1200 °C) through transformation of easily volatile RuO2 to stable BaRuO3 phase. The latter was considered responsible for the improved catalytic performance of BaRu0.2FeAl10.8O19 hexaaluminates [66]. Similarly, it was found that BaIrxFe12−xO19 offered a better dispersion of iridium through its incorporation into the hexaferrite structure [67]. Recently, the same group [69]

showed that Fe–substituted La–hexaaluminate catalysts exhibited higher N2O decomposition rates compared to Ba–hexaaluminates with equal Fe content. In particular, Fe-substituted La-hexaaluminate catalysts exhibited higher rate values as compared to Ba-hexaaluminates (2.8−16.4 × 10−7 mol m−2 s−1 versus 0.8−2.5 × 10−7 mol m−2 s−1) in N2O decomposition at 500 °C. The correlation of the specific rates with Fe occupation in Al sites of the MP phase indicated that Fe3+ ions in Al(3) and Al(5) sites in the minor plane can be accounted for the high activity of Lahexaaluminates [69]. It should be also underlined that higher rate values can be in general obtained over hexaaluminates compared to bare oxides (Figure 3). Santiago et al. [68] found that the utilization of carbon templates during the synthesis of LaFeAl11O19 hexaaluminates results in increased surface area and improved deN2O performance. A quasi-linear correlation between the N2O decomposition rate and the BET surface area was revealed. Furthermore, carbon–



22

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templated LaFeAl11O19 demonstrated high and stable catalytic performance under simulated exhaust conditions containing N2O, NO, O2, and H2O [68]. The N2O decomposition has also been examined by the same group [65] over a series of metal– substituted hexaaluminates with the general formula ABAl11O19 (A= La, Ba and B=Mn, Fe, Ni). Fe– and Mn–hexaaluminates demonstrated the highest deN2O performance, whereas the catalytic activity was not affected by the A cation (Figure 5). Interestingly, Fe– and Mn–substituted hexaluminates exhibited high deN2O performance in ammonia oxidation simulated exhaust conditions (mixture of N2O, NO, O2 and H2O) [65].

1000

Ba-Fe-Al

La-Fe-Al

600

Ba-Mn-Al

La-Mn-Al

Ba-Ni-Al

Ba-Al

La-Ni-Al

700

La-Al

800

Al2O3

o

900

Blank

ABAl11O19hexaaluminates (A=La, Ba & B=Mn, Fe, Ni)

Light off temperature, T50 ( C)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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500

Catalyst type

Figure 5. N2O conversion performance of ABAl11O19 hexaaluminates (A= La, Ba and B=Mn, Fe, Ni), in terms of light off temperature (T50). Conditions: 5000 ppm N2O in He; P= 1 bar; WHSV= 30,000 ml g–1 h–1 [65]. The high temperature catalytic decomposition of N2O was also studied in calcium aluminate (12CaO·7Al2O3, mayenite), a material with extra-framework oxygen anions ACS Catalysis_Revised MS_ cs-2015-01605m


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[75]. The mayenite exhibited excellent decomposition efficiency and stability at very high temperatures (~780 °C) for at least 10 h in a pilot plant using real process gas feed (N2O+NO+NO2+O2+H2O). These results demonstrated the ability of mayenite materials for industrial applications in nitric acid plants. The impact of substituted metal ions in the hexaaluminate structure was thoroughly investigated by Zhang and co-workers by means of several sophisticated techniques [72-74]. In particular, the mechanism of stabilization of Fe ions in β-alumina and magnetoplumbite-type (MP) substituted Ba hexaaluminates (BaFexAl12–xO19) was investigated by means of Rietvelt refinement and Fe Mössbauer spectroscopy [72, 74]. It was found that Fe3+ ions at low concentration localized at tethrahedral Al(5) sites of β-Al2O3, whereas at high concentrations occupied the trigonal bipyramidal Al(5) and octahedral Al(3) sites in MP phase (Figure 4). These findings, in relation to deN2O performance of BaFexAl12–xO19 materials, lead to the conclusion that Fe ions in Al(5) sites in β-Al2O3 and Al(3) sites in MP structure are highly active for N2O decomposition. Fe ions incorporated to these sites are more easily accessible to N2O adsorption, compared to those in the deep positions (Al(2) in β-Al2O3 and MP, Al(4) in MP). Furthermore, Mössbauer results indicated that the oxygen atoms in these specific sites are more easily moved and diffused, greatly enhancing the oxygen desorption and in turn the deN2O activity [72, 74].

2.3 Hydrotalcite–based catalysts Transition metal–based mixed oxides are employed in many heterogeneous catalytic processes due to their adequate activity and thermal stability. These oxides are mainly obtained by a controlled thermal decomposition of various precursors like hydroxides, nitrates, etc. Mixed oxides prepared by the thermal decomposition of layered double



24

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hydroxides (LDHs), also known as hydtrotalcite–like compounds (HTLs) or anionic clays (ACs), represent one of the most important classes of mixed oxide–based catalysts [76–81]. The chemical composition of HTLs can be represented by the general formula [M(II)1− x M(III)x (OH) 2 ]x + [A nx/n− ] ⋅ mH 2 O , where M(II) and M(III) correspond to divalent and trivalent metal cations, respectively, A to n–valent anion and x usually obtains values in the range of 0.25 to 0.35 [78, 79]. Thermal decomposition of HTLs results in finely dispersed MOs of M(II) and M(III) with large surface area and good thermal stability [77]. The recent studies devoted on N2O decomposition over hydtrotalcite–based compounds are summarized in Table 6. Table 6. Recent studies on N2O decomposition over hydtrotalcite–based compounds. Catalyst Co4MnAl

Reaction conditions 0.1 % N2O; WHSV=60,000 ml g–1 h–1

T50 (°C)

Ref.

400

[76, 82]

450 (x=0) Co4Mn2–xAlx

0.1 % N2O; WHSV=60,000 ml g–1 h–1

400 (x=0.5) 380 (x=1.0)

[77,83,84]

420 (x=1.5) 460 (x=2.0)

Ni4–xMgxAl2

Ni4–xMgxMn2

0.1 % N2O;

425 (x=0)

WHSV=60,000 ml g–1 h–1

>450 (x>0)

0.1 % N2O; WHSV=60,000 ml g–1 h–1

Co2Mg2Mn2 Co2Mg2MnAl Co2Mg2Al2 CoAl Co2Al Co3Al

0.1 % N2O; WHSV=60,000 ml g–1 h–1

0.5–1.0 % N2O; GHSV=30,000 h–1

Co4LaAl

>450

[85]

[85]

450 450

[77, 84]

435 330 390 410

[86]

280



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Co6LaAl Ni2Al

390 0.1 % N2O; WHSV=60,000 ml g–1 h–1

380

Cu10/Mg61/Al29

480

Co10/Mg61/Al29

580

Fe10/Mg61/Al29 Ni10/Mg61/Al29

0.5 % N2O + 4.5% O2; –1

–1

WHSV=30,000 ml g h

640 >650

Co10C10/Mg61/Al29

425

Co10C10/Mg61/Al29/K

400

[87]

[88]

The N2O decomposition over catalysts derived from HTLs was expansively investigated by Kannan and co-workers [31, 78, 80, 81]. Various calcined hydrotalcites (CHTs) were prepared by precipitation followed by thermal decomposition of metal nitrates precursors, where M(II) and M(III) were divalent (Mg, Ni, Co, Cu, Zn) and trivalent (Al, Cr, Fe) cations, respectively [78]. Ni–Al and Co–Al materials exhibited the optimum deN2O performance, followed by Cu-, Mgand Zn-based CHTs [78, 80, 81]. Among the catalysts examined, Co–Mg–Al samples offered superior activity, even under water- and oxygen–rich conditions (N2O + O2 + H2O). Complete conversion of N2O was achieved at 450 °C under both oxygen deficient (0.1% N2O) and O2 excess (0.1% N2O + 2.5% O2) conditions, whereas a slight degradation was observed when N2O co–exists with both O2 and water (0.1% N2O + 2.5% O2 + 2% H2O) [78]. The observed trend in catalytic activity was rationalized on the basis of the N2O reaction mechanism over CHTs in conjunction to the electronic state of active metal ions. The adsorption of N2O is initially taking place on active metal ions through electron back-donation from metal d-orbitals into the π* orbitals of N2O. This step is followed by N2O decomposition to N2 and Oads. The recombination of Oads is then ACS Catalysis_Revised MS_ cs-2015-01605m


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occurring towards the formation of molecular oxygen. In this mechanistic sequence, both the number and the oxidation state of metal entities as well as the surface oxygen mobility are expected to play a key role on deN2O process [78, 80, 81]. In this regard, the increase in activity with increasing cobalt loading was attributed to high dispersion of Co2+ ions [81]. In a similar manner, the inferior deN2O performance of Zn-Al and Mg-Al as compared to Cu-Al CHTs, was attributed to the difficulty of electron donation from the filled orbitals of Zn2+ and Co2+ to the anti-bonding orbitals of N2O. The higher activity of Cu was ascribed to its multiple oxidation states which facilitate electron donation [78]. Valuable contribution on N2O decomposition over CHTs has been also performed by Obalová and co–workers [76, 77, 82–85, 87]. N2O decomposition has been comprehensively studied over Co4Mn2–xAlx hydrotalcites [76, 77, 82–84]. The best performance was obtained for Co4MnAl composites, due to their optimum surface composition and redox properties [83]. More specifically, Co4MnAl catalyst contains the optimum surface amount of cobalt and manganese, which are highly reducible. However, any deviation from this specific stroichiometry results in an inferior deN2O performance, due to the non optimum geometric arrangement of the active sites as well as to the strongly bonded surface oxygen species which hinder the molecular O2 desorption [83]. The detrimental role of O2 was verified by the same group by means of a kinetic analysis [86]; the high concentration of oxygen in the feed stream results in active sites coverage, and consequently in a degradation of the deN2O efficiency. The impact of Co4Mn2–xAlx characteristics (composition, surface area, etc) on the deN2O activity can be more accurately revealed by the estimation of the kinetic constant k (mol s-1 m-2 Pa-1) (Table 7). The constant k decreased in the order: Co4MnAl > Co4Mn1.5Al0.5 > Co4Mn0.5A1.5 > Co4Mn2 > Co4Al2, revealing the ACS Catalysis_Revised MS_ cs-2015-01605m


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Page 28 of 87

enhanced surface reactivity of Co4MnAl mixed oxides [83]. It should also be mentioned that the kinetic constant is not analogous to the surface area of metal oxides, implying the key role of intrinsic characteristics (instead of textural characteristics) on the deN2O rate of Co4Mn2–xAlx hydrotalcites. Table 7. Kinetic constant of N2O decomposition at 420 °C over hydtrotalcite–based compounds assuming first order kinetics. Surface area

Kinetic constant, k

(m2 g-1)

(x1011 mol s-1 m-2 Pa-1)

Co4Mn2

43.6

4.3

Co4Mn1.5Al0.5

89.9

10.8

Co4MnAl

92.7

15.6

Co4Mn0.5Al1.5

103.2

6.6

Co4Al2

82.1

2.2

Catalyst

Τhe impact of carbon monoxide and/or oxygen on the deN2O performance was also examined oven Co–Mn–Al oxides prepared by calcination of hydrotalcite–like precursors [82]. It was found that in the absence of oxygen in the feed stream, CO strongly enhanced N2O conversion (Figure 6), by facilitating the removal of strongly adsorbed oxygen atoms. However, in the presence of oxygen, CO acts as a non– selective reductant, inhibiting N2O abatement (Figure 6). The decrease of N2O consumption rate in the co–presence of CO and O2 was ascribed to the limited active sites for N2O adsorption/decomposition [82]. An analogous behavior, in relation to the impact of CO and/or O2 on N2O decomposition, has been observed by Chang et

al. [89] over CHTs obtained by co–precipitation of cobalt, aluminum and transition metal (Rh, Pd) nitrates. Similarly, in recent studies on N2O decomposition over noble ACS Catalysis_Revised MS_ cs-2015-01605m


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metal–based structured catalysts, the pronounced effect of CO was clearly revealed under oxygen deficient conditions [90, 91]. In addition, the existence of oxygen inhibits the deN2O performance, even in the presence of CO [90, 91].

100 N2O+CO N2O

80

N2O conversion (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

N2O+O2 60

40

20

0 250

N2O+CO+O2

N2O N2O+O2 N2O+CO N2O+CO+O2

300

350

400

450

500

o

Temperature ( C)

Figure 6. Effect of CO and/or O2 on N2O decomposition over Co–Mn–Al hydrotalcite-based compounds. Reaction conditions: 0.1 mol % N2O balanced by He, WHSV=60,000 ml g–1 h–1 [82]. Kovanda et al. [87] investigated the effect of hydrothermal treatment on the properties of Ni–Al layered double hydroxides with different Ni/Al molar ratios (2, 3 and 4). The hydrothermal treatment notably improved the deN2O performance of Ni2Al oxides, due to its positive impact on the textural characteristics. More specifically, the enhanced catalytic activity of hydrothermally treated samples was related with the decreased amount of amorphous component in CHTs and the increased incorporation of Al cations into a NiO-like structure. Moreover, a close ACS Catalysis_Revised MS_ cs-2015-01605m


29

ACS Catalysis

correlation between the pore size distribution and the catalytic activity was observed, revealing the key role of diffusion processes on the deN2O performance of CHTs. The influence of thermal treatment conditions on the N2O decomposition was also examined over magnesium–aluminum hydrotalcite–based materials containing additionally Co, Cu, Ni, Fe [88]. The optimum deN2O performance among the samples calcined at 600 °C was obtained for the copper–containing catalysts (Figure 7).

100

80 N2O conversion (%)

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Page 30 of 87

o

450 C o 550 C o 650 C

60

40

20

0 Ni

Fe

Co

Cu

Metal type (M) in M-Mg-Al hytrodalcites

Figure 7. N2O decomposition performance at 450, 550 and 650 °C for Cu–, Co–, Fe– and Ni–Mg–Al CHTs at 600oC. Conditions: 5% N2O + 4.5% O2; WHSV=30,000 ml g–1 h–1 [88]. An additional improvement on deN2O performance was obtained by combining higher calcination temperatures (700 and 800 °C) with alkali promotion (potassium). The light off temperature (T50) was decreased to 400 oC over the multifunctional ACS Catalysis_Revised MS_ cs-2015-01605m


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material Co10C10/Mg61/Al29/K calcined at 800 °C and promoted with 0.9 wt.% K (Table 6) [88]. The mode of action of alkali promoters in the deN2O process over mixed oxides is of particular importance, and will be further discussed in the sequence.

2.4 Spinels Spinel–type oxides based on 3d transition metals have attracted increasing interest, both in theory and practice, due to their adequate thermal stability and catalytic activity. Spinels can be represented by the chemical formula AB2O4, where A is a divalent cation (such as Mg, Ca, Mn, Co, Ni, Cu, Cr, Fe, Zn) in tetrahedral sites and B is a trivalent cation (such as Cr, Fe, Co) in octahedral sites. The A component of the spinel structure is often partially substituted by other divalent 3d metals (such as Mn, Ni, Cu, Cr, Zn and alkaline earths) in order to modify the physicochemical properties of spinels. In this regard, various catalysts prepared by calcination of hydrotalcites (CHTs) have similar composition with spinel-type oxides. Hence, numerous spinel– type oxides can be obtained with specific surface and redox properties for several catalytic applications. In this framework, several spinel–type oxides, summarized in Table 8, have been recently studied for N2O decomposition. Table 8. Recent studies on N2O decomposition over spinel–type oxides. Catalyst CuxCo1–xCo2O4


T50 (°C)

Ref.

318 (x=0.75)

[92]



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360 (Co3O4) A3–xBxO4 A, B = Mg, Al, Co

410 (MgCoAlO4)

5 % N2O;

460 (MgCo2O4)

GHSV=7,000 h–1

500 (CoAl2O4)

[93]

560 (Mg0.5Co0.5Al2O4) >700 (MgAl2O4) CoxFe3−xO4

pure N2O;

160 (x=0.6)

GHSV=15,000 h–1

MxFe3–xO4

0.5% N2O;

160 (M=Ni, x=0.75)

(M = Ni, Mg)

GHSV=20,000 h–1

170 (M=Mg, x=0.58)

MxFe1–xFe2O4

0.1% N2O

180 (M=Zn, x=0.6)

(M=Mn, Zn)

GHSV=20,000 h–1

140 (M=Mn, x=0.8)

[94]

[95]

[96]

397 (x=0) Co3–xFexO4

5 % N2O;

442 (x=1) –1

GHSV=7,000 h

560 (x=2)

[97]

502 (x=3) AB2O4 A = Mg, Ca, Mn, Co,

0.5 % N2O;

Ni, Cu, Cr, Fe, Zn

GHSV=80,000 h–1

440 (MgCo2O4)

[98]

130 (x=0.36)

[99]

B = Cr, Fe, Co ZnxCo1–xCo2O4 MxCo1–xCo2O4 (M=Ni, Mg) CoCexOδ (x=Ce/Co molar ratio) NixCo1–xCo2O4

MgCo2O4

0.1 % N2O; GHSV=15,000 h–1 0.1 % N2O;

130 (M=Ni, x=0.74) –1

GHSV=15,000 h

150 (M=Mg, x=0.54)

0.1 % N2O;

213 (x=0.5)

WHSV=18,000 ml g–1 h–1 0.05 % N2O;

260

WHSV=24,000 ml g–1 h–1

(x=0.5–0.75)

0.5 % N2O; WHSV=120,000 ml g–1 h–1



380

[100]

[101]

[102]

[103]

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Abu–Zied et al. [92] studied the decomposition of N2O over CuxCo1–xCo2O4 spinel–oxide catalysts. Partial replacement of Co2+ by Cu2+ in Co3O4 led to a significant improvement in the N2O decomposition. The optimum performance was obtained by Cu0.75Co0.25Co2O4 catalyst, which offers a T50 of 318 °C (Table 8). The deN2O performance was found to be controlled by a combination of several parameters, including the crystallite size, the surface area and the presence of residual promoting species (e.g., potassium) [92]. Stelmachowski et al. [93] explored the impact of Co2+ and Co3+ cations replacement in cobalt spinels by non–redox Mg2+and Al3+cations, respectively (Co3O4, MgCo2O4, MgCoAlO4, CoAl2O4, Mg0.5Co0.5Al2O4 and MgAl2O4). By means of several experimental and theoretical (DFT modeling) studies, it was revealed that the prime active sites for N2O decomposition are the octahedral Co3+ ions (Ea= 15–17 kcal mol−1), whereas the tetrahedral Co2+ ions were found to be much less active (Ea= 27–28 kcal mol−1). The redox dz2 orbitals of the Co3+ cations are considered responsible for the cleavage of N–O bond through electron transfer to N2O molecular orbitals. Moreover, the Co3+ cations are responsible for the recombination of surface peroxy species (O22–) into dioxygen, thus regenerating active sites [93]. Both factors are crucial for N2O elimination, taking into account the established redox mechanism of N2O decomposition (see reactions (5)–(8)). Following this perspective, a strong correlation between the deN2O perfromance and the work function of Co–Fe spinels was recently revealed (Figure 8) by means of Mössbauer spectroscopy and work function mesauremetns, verifying the key role of electronic factor in the deN2O process [97].



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650

5.0

T50 Work funtion

o

4.6

450

4.2

350

3.8

250

3.4

150

Work function, Φ (eV)

550

Light off temperature ( C)

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3.0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

x in Co3-xFexO4

Figure 8. The light off temperature of N2O (T50) and the work function (Φ) with respect to iron content (x) in Co3−xFexO4 spinels [97]. Amrousse et al. [94–96] demonstrated that the incorporation of Co, Ni, Mn and Mg cations into Fe3O4 spinel framework notably enhances the deN2O performance. For instance, the Co0.6Fe2.4O4 structure shifts the N2O conversion profiles to lower temperatures –by more than 100 °C– compared to Fe3O4 magnetite (Figure 9). The superior deN2O performance of Co-substituted Fe3O4 was mainly related to a synergistic effect between Co2+ and Fe2+ towards an enhanced N2O dissociation ability and diffusion of surface oxygen. Addition of oxygen or water excess in the feed stream slightly decelerated the N2O decomposition, due mainly to their competitive adsorption into the active sites. However, metal–substituted Fe3O4 substrate catalysts are capable of completely decomposing N2O in the presence of oxygen or water inhibitors at temperatures as



34

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low as 300 °C [94]. The superior deN2O performance of these composites, compared to that obtained by Maniak et al. [97] over the same type of catalysts (Figure 8), should be noticed. This discrepancy could be possibly attributed to the different synthesis routes, resulted in different textural/structural properties, as well as to the dissimilar reaction conditions employed.

400

N2O N2O+O2 N2O+H2O

350 o


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ACS Catalysis

300

250

200

150

100

Fe3O4

Co0.6Fe2.4O4

Figure 9. N2O conversion performance, in terms of T50, of Fe3O4 and Co0.6Fe2.4O4 catalysts in different feed compositions: pure N2O, N2O + 20 vol.% O2 and N2O + 10% H2O; GHSV=15,000 h–1 [94]. Russo et al. [98, 103] studied several spinel–type catalysts of the general formula AB2O4 (A = Mg, Ca, Mn, Co, Ni, Cu, Cr, Fe, Zn and B = Cr, Fe, Co) for N2O decomposition, both in the absence (0.5% N2O) and presence of oxygen (0.5% N2O + 5% O2). Spinel–type oxides containing Co at the B site were found to provide the best catalytic efficiency. In Table 9, adapted by [98], the deN2O performance of these



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oxides, in terms of T50, is depicted. The better activity of MgCoO4 catalyst was attributed to the high concentration on surface vacancies, having a key role on the deN2O process. Table 9. N2O decomposition performance of several AB2O4 spinels. Reaction conditions: 0.5% N2O, 0 or 5% O2; GHSV=80,000 h–1 [98]. Catalyst

T50 without O2 (oC) T50 with O2 (oC)

No catalyst

905

990

MgCr2O4

625

715

CaCr2O4

780

>800

MnCr2O4

645

725

CoCr2O4

550

685

NiCr2O4

630

725

CuCr2O4

630

745

MgFe2O4

525

540

CoFe2O4

580

600

CuFe2O4

575

650

MgCo2O4

440

470

CrCo2O4

568

630

MnCo2O4

650

735

FeCo2O4

600

675

CoCo2O4

475

510

NiCo2O4

505

545

CuCo2O4

700

>800

ZnCo2O4

475

500



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Fierro et al. [104–106] examined the impact of Cu–, Co–, Fe–doping on the deN2O performance of zinc manganite spinels (ZnMn2O4). It was reported that both undoped and doped catalysts exhibit almost the same catalytic activity regarding N2O decomposition. Yan et al. [99] explored the N2O decomposition reaction over ZnxCo1–xCo2O4 spinel catalysts. Partial substitution of Co2+ by Zn2+ in Co3O4 spinel oxide led to a significant enhancement in the catalytic activity, with the best performance to be obtained for Zn0.36Co0.64Co2O4 structure (Figure 10). On activity basis the Zn0.36Co0.64Co2O4 demonstrated an N2O decomposition rate at 200 °C of approximately 100 nmol m-2 s-1 as compared to ~2 nmol m-2 s-1 of bare Co3O4, revealing the pronounced effect of

Co2+ substitution by Zn2+ [99]. A similar

enhancement was achieved by the substitution of Co2+ by Ni2+ and Mg2+ in Co3O4 spinel;

the

optimum

behavior

was

observed

for

Ni0.74Co0.26Co2O4

and

Mg0.54Co0.46Co2O4 catalysts [100]. It is worth mentioning the inhibition caused by the presence of excess oxygen or water over ZnxCo1–xCo2O4 spinel catalysts (Figure 10) [99]. Although the O2 addition has a moderate deterioration effect on the decomposition of N2O, a more severe inhibition was obtained by water (Figure 10). This was mainly ascribed to the competitive adsorption of N2O with O2 or H2O on the same active sites. Moreover, the hydroxylation of active sites by H2O can be further accounted for the induced inhibition [99, 100].



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400

N2O N2O+O2 N2O+H2O N2O+O2+H2O

350

o


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300

250

200

150

100

Zn0.36Co0.64Co2O4

Co3O4

Figure 10. N2O conversion performance, in terms of T50, of Co3O4 and Zn0.36Co0.64Co2O4 catalysts in different feed compositions. Reaction conditions: 0.1% N2O; 0.1% N2O + 10% O2; 0.1% N2O + 5% H2O; 0.1% N2O + 10% O2 + 5% H2O; GHSV=15, 000 h–1 [99].

2.5 Perovskite–type oxides Perovskite–type

oxides,

with

the

general

formula

ABO3,

possess

unique

physicochemical properties (such as structural stability, chemisorption ability, acid–base characteristics, oxygen mobility and storage capacity), which render these materials , in conjunction with their low manufacturing cost, as ideal candidates for several catalytic processes [44, 107–109]. The larger A–site cation is usually a rare earth (La, Sm, Pr),

an alkaline earth (Sr, Ba, Ca), or an alkali (Na, K) metal cation. The B–site cation is a smaller transition metal cation, which primarily determines the catalytic activity of perovskites. The A–site cation is in general catalytically inactive, affecting however the oxidation state of B–sites and the creation of oxygen vacancies. Therefore, the ACS Catalysis_Revised MS_ cs-2015-01605m


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effect of A–sites in the catalytic performance is mainly indirect, by changing the oxidation state of B–site ion and/or by creating oxygen vacancies. Moreover, the partial substitution of A– and B–sites by a cation of different valence allows the valence state adjustment of A and B cations as well as the creation of anionic or cationic vacancies in perovskite oxides. Hence, the surface and redox properties of perovskites, and in consequence their catalytic performance, can be tailored by the incorporation of various metal ions into the structural framework [110– 115]. Due to the broad diversity of physicochemical properties exhibited by perovskite–type oxides, several review articles have been devoted to their properties and applications [e.g., 108, 115–118]. Perovskite–type oxides are promising catalysts for the high temperature deN2O process due to their low cost, thermal stability and adequate catalytic performance. In the last 15 years numerous perovskite–type materials have been reported for N2O decomposition [119–133]. The main studies on N2O catalytic decomposition over perovskite–type materials are summarized in Table 10. Table 10. Recent studies on N2O decomposition over perovskite–type oxides. Catalyst Pr1–xBaxMnO3

La1–xSrxMnO3

La1–xSrxFeO3

Reaction conditions 0. 5 % N2O; GHSV=7,500 h–1 0.15 % N2O; GHSV=30,800 h–1 0.15 % N2O; GHSV=30,800 h–1

T50 (°C)

Ref.

442 (x=0.2)

[119]

725 (x=0.25)

[120, 121]

815 (x=0.6)

[122]

LaCo1–yO3

0.1 % N2O + 0.5% NO +

580 (y=0)

La1–xCoO3

6% O2 + 15% H2O;

588 (x=0.2)

La1–zCo0.8Fe0.2O3

GHSV=30,000 h–1

563 (z=0)



[123]

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0.1 % N2O + 0.1% NO + LaCoO3

3% O2 + 0.5% H2O;

467

[124]

–1

GHSV=10,000 h LaB1–xB΄xO3 CaB1–xCuxO3 B = Mn, Fe

18% Ν2Ο + ~10% Ο2; –1

GHSV=22,100 h

510 (LaFe0.7Cu0.3O3) 490 (CaMn0.6Cu0.4O3)

[125]

B΄ = Cu, Ni LaZrxCo1–xO3–δ

30 % N2O;

520 (LaZr0.05Co0.95O3–δ)

BaZrxCo1–xO3–δ

WHSV=30,000 ml g–1 h–1

520 (BaZr0.2Co0.8O3–δ)

La2Ti2(1−x)Fe2xO7–δ

10 % N2O + 10% CO; WHSV=24,000 ml g–1 h–1

LaBO3

0.5 % N2O;

B= Cr, Mn, Fe, Co

WHSV=120,000 ml g–1 h–1

[126]

370 (x=0.6)

[127]

455 (B=Co)

[129]

Kumar et al. [119] studied the N2O decomposition over Ba substituted PrMnO3 perovskite oxides (Pr1–xBaxMnO3). The Pr0.8Ba0.2MnO3 catalyst showed the optimum performance (T50=442 °C, Table 10) in direct catalytic decomposition of N2O. A moderate degradation was obtained in the presence of oxygen, which was exacerbated in the co-presence of NO (0.5% N2O + 5% O2 + 0.02% NO), resulting in a light off temperature of about 520 °C. The superior catalytic performance of Ba promoted perovskites was attributed to the facilitation of Mn4+/Mn3+ redox cycles as well as to the improved oxygen desorption, as inferred by XPS, TPR and O2-TPD studies [119]. More specifically, the Ba–induced modification on the deN2O performance of Pr1– xBaxMnO3 can

be well interpreted by taking into account the redox–type mechanism

of N2O decomposition over perovskite catalysts [119-121, 129], already described on the basis of reactions (5)–(8). The altered redox propertries (increase of Mn4+/Mn3+ ratio) of Pr1–xBaxMnO3 catalysts, linked to their improved oxygen mobility, was considered responsible for the improved catalytic activity [119]. ACS Catalysis_Revised MS_ cs-2015-01605m


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The key role of oxygen mobility on the deN2O performance of perovskites has been verified by means of various complimentary techniques. In a series of detailed studies by Ivanov et al. [120, 121], a direct correlation between the deN2O activity and oxygen mobility of Sr–subsituted LaMnO3 perovskites is established. It was revealed by means of steady–state isotopic transient kinetic analysis (SSITKA) that the active La0.3Sr0.7MnO3 exhibits higher rates of oxygen exchange on the surface and oxygen diffusion in the bulk, compared to inactive single–phase LaMnO3 [120]. In a more recent study by the same group [122] it was concluded that the activity of La1– xSrxMnO3

perovskites can be correlated with the oxygen surface exchange properties

rather than with the bulk oxygen mobility [122]. The key role of oxygen mobility on the deN2O performance was also verified by Granger and co–workers in a series of experiments with perovskites of different surface and bulk composition [123, 124, 130, 131]. In particular, the LaCo0.2Fe0.8O3 perovskite demonstrated the best performance amongst the Fe-doped LaCo1-xFexO3 perovskites under exhaust conditions representative of ammonia burner in nitric acid plants [131]. An increase of the intrinsic rate (moles N2O per unit time and surface Co atoms) from approximately 8 µmol s-1 to 60 µmol s-1 was recorded by Fe incorporation into LaCoO3, which was attributed to the reactivity of mobile oxygen species which govern the catalytic activity. In a similar manner, the introduction of Fe in the perovskite structure remarkably decreased the apparent activation energy from 222 kJ/mol over bare LaCoO3 to 144 kJ/mol over calcined LaCo0.2Fe0.8O3 perovskite, justifying the increase of intrinsic rate upon Fe doping [131]. Alini et al. [125] studied the direct decomposition of N2O over two different types of perovkites: (i) LaB1–xB΄xO3 and ii) CaB1–xCuxO3 (B =Mn, Fe and B΄=Cu, Ni). The



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best deN2O performance was obtained with CaMn0.6Cu0.4O3 catalyst calcined at 700 °C (Table 10). Interestingly, this catalyst demonstrated excellent stability (1400 h) in an industrial gas stream derived from an adipic acid plant [125]. In relation to structure-activity correlation, it was found that the calcination temperature notably affects the catalytic performance, whereas the reduction pre-treatment slightly influences the deN2O performance. By adjusting the calcination temperature (700 °C) and catalyst composition (Cu/Ca atomic ratio of 0.4), the formation of Ca3CuMnO6 phase was enhanced, revealing its significant impact on deN2O performance [125].

2.6 Ceria–based mixed oxides Over the last years, ceria–based oxides have received increasing attention in the field of heterogeneous catalysis, due to their exceptional physicochemical properties. The high oxygen storage–release capacity of ceria via Ce4+/Ce3+redox cycles is one of the special characteristics of CeO2, which makes this material remarkably effective in several catalytic processes. The catalytic properties of ceria and ceria–related materials have been reported by Trovarelli and co–workers in several comprehensive studies [134–138]. Moreover, transition metal–ceria interactions confer a synergistic effect, resulting in enhanced surface and redox properties and to a better catalytic performance [37]. Consequently, several CeO2–based transition metal catalysts (M/CeO2, M= Co, Cu, Fe, Zr,Ni) have been widely applied for N2O decomposition [139–146]. Table 11 summarizes the recent studies devoted on N2O catalytic decomposition over Ceria–based binary oxides.



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Table 11. Recent studies on N2O decomposition over ceria–based binary oxides. The optimum metal oxide composition is provided in parenthesis. Catalyst CuO/CeO2


CuO/CeO2

0.25% N2O; GHSV=45,000 h–1

Co3O4/CeO2

5 % N2O; GHSV=80,000 h–1

Fe2O3/CeO2

T50 (°C)

Ref.

380 (~25 mol% Cu)

[139]

440 (40 mol% Cu)

[140]

not provited

[141]

470 (50 at.% Fe)

[142]

660 (x=0.32)

[143]

430 (5 mol% Cu)

[144]

370 (67 mol% Cu)

[145]

0.45% N2O P=3 bar; –1

GHSV=23,800 h

0.2% N2O + 1.4% NO + CexZr1–xO2

1% O2 + 15%H2O; GHSV=36,000 h–1

CuO/CeO2 CuO/CeO2 NiO/CeO2

5.0 % N2O; WHSV=5,100 ml g–1 h–1 0.26% N2O; GHSV=19,000 h–1 0.26% N2O;

304 (90 mol %

P=3 bar; –1

GHSV=19,000 h

NiO)

[146]

Zabilskiy et al. [139] studied the N2O decomposition over Cu catalysts supported on CeO2 nanospheres, with particular emphasis on the impact of copper loading. It was found that Cu content greatly influenced the physicochemical, redox and catalytic properties. The optimum performance was obtained for 10 wt.% Cu/CeO2 (~25% mol% Cu) catalyst (Table 11), which offers the maximum amount of CuO nano– clusters. Copper content higher than 10wt.% negatively affects the Cu dispersion, leading to the formation of less active segregated CuO phase. In a similar study by the same group, the catalytic decomposition of N2O was studied in mesoporous CuO– CeO2 mixed oxides [140]. In this case the best activity was observed for the catalyst ACS Catalysis_Revised MS_ cs-2015-01605m


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containing 40 mol% Cu, which exhibited the highest reducibility and oxygen mobility [140]. The structure-activity correlation, in relation to the deN2O performance of Cu/CeO2 catalysts, can be interpreted on the basis of the following reactions [139, 140, 144, 145]: Cu+ + N2O → Cu+–ONN

(9)

Cu+–ONN → Cu+2–O– + N2

(10)

Cu2+–O– + Cu2+–O– → 2Cu+ + O2

(11)

N2O decomposition involves initially the adsorption of N2O on Cu+ sites (reaction (9)), followed by N–O bond scission and N2 desorption (reaction (10)). Recombination of neighbouring oxygen species toward oxygen desorption is finally taking place resulting in active sites regeneration (reaction (11)). The regeneration of active sites through mobile oxygen species derived from CeO2 has also been considered [139]: Cu2+–O– + Ce3+ → Cu+ + Ce4+–O–

(12)

2Ce4+–O– → 2Ce3+ + O2

(13)

The high deN2O activity of monomeric Cu+ species compared to dimeric Cu2+– Cu2+ pairs has been revealed, based on spectroscopic studies and DFT calculations [144, 145]. Dimeric copper species are responsible for the stabilization of oxygen intermediates, hindering active phase regeneration, in contrast to monomeric species, where oxygen intermediates are less stabilized [144]. The formation of Cu+1 active sites requires oxygen species recombination through the interaction of two neighbouring Cu2+–O– sites (reaction 11). However, in the case of isolated Cu2+–O– sites the recombination step is strictly hindered. In the last case,



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the formation of Cu1+ active sites can be only occur via mobile oxygen species originating from the CeO2 support (reactions (12) and (13)). In the light of these aspects the maximization of deN2O performance of Cu/CeO2 oxides at a certain Cu content is the result of the optimal dispersion of CuO and its synergistic interaction with neighbouring Cu2+ and/or Ce3+ species [139, 140, 145]. At low copper loading the formation of atomically dispersed CuO species hinders the interaction between them, whereas at adequately high contents the formation of segregated CuO phase is favored. In both cases the recombination step described above is notably hindered. The impact of NO and H2O on the deN2O performance of CeO2-based mixed oxides is of particular practical importance taking into account that these substances are usually present in N2O-containg gas steams. Generally, as in the case of other types of mixed oxides earlier described, the presence of NO or water in the feed stream has a detrimental effect on deN2O performance [139, 140]. NO affects the catalytic performance to minor extent, as compared to water, due to its competitive adsorption on Cu+ sites. On the other hand, water results in a more severe suppression because of the blockage of active sites and/or the formation of new inactive phases, such as CuO·3H2O. Nevertheless, the detrimental effect of water is fully reversible, thus, the catalytic activity is totally recovered under dry conditions. In summary, the deN2O performance of CeO2–based transition metals is greatly influenced by the concentration of metal (e.g., Fe, Co, Cu) and Ce3+ sites on catalyst surface and the extent of interaction between them. These factors are crucial for N2O decomposition, since they contribute to the facilitation of oxygen desorption from the catalyst surface and to the regeneration of active sites (reactions (11)–(13)) [132– 141]. The combination of transition metals (such as Cu, Co, Ni and Fe) with CeO2



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Page 46 of 87

facilitates oxygen species desorption, by altering the surface and redox properties of mixed oxides [94, 142]. The latter is responsible for the enhanced deN2O performance of mixed oxides compared to bare CeO2. The considerable synergy between CeO2 and transition metals can be more accurately revealed by comparing the apparent N2O decomposition rates of bare and mixed oxides. For instance, the N2O decomposition rate of Cu/CeO2 mixed oxides is up to one order of magnitude higher as compared to that recorded over bare CeO2 and CuO [145]. However, although the half-conversion temperature (T50) of Co3O4/CeO2 and Fe2O3/CeO2 mixed oxides is much lower as compared to that of bare oxides, different conclusions can be obtained on activity basis; the increase of reaction rate based on surface area (nmol m-2 s-1) is not very obvious when compared to that of bare oxides, implying that the increase of surface area upon the formation of mixed oxides is mainly governing the deN2O performance [101, 142].

2.7 Other binary or ternary oxides The MOs which do not belong to the above mentioned categories are summarized and discussed in this section (Table 12). These are mainly binary or ternary oxides based on transition MOs [147-160], such as Co3O4 [147–149], Fe2O3 [150, 151], NiO [36, 146, 152, 153], CuO [147, 154, 155], etc. Table 12. Recent studies on N2O decomposition over binary or ternary transition MOs. Values in parentheses indicate the composition in which the optimum deN2O performance is obtained. Catalyst

Reaction conditions

T50 (°C)

Ref. –2

M/ZrO2

0.4 % N2O;

400 (2.3 at Co nm )

(M=Fe, Co, Cu)

GHSV=24,000 h–1

410 (2.5 at Cu nm–2)



[147]

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450 (3.8 at Fe nm–2) 400 (10 wt.% MgO) Co/AxOy (A=Mg, Zn, Mn, Al, Ce)

410 (10 wt.% ZnO)

1.0 % N2O; GHSV=30,000 h–1

425 (10 wt.% Mn2O3)

[148]

450 (10 wt.% Al2O3) >480 (10 wt.% CeO2)

NiO/La2O3*

0.5 % N2O;

360

GHSV=2,400 h–1

(Ni/La molar ratio=4)

[36]

0.5% N2O + 0.5% O2 + Fe2O3/Al2O3

3.0% NO;

630

WHSV=60,000 ml g–1 h–1

(41.8 wt.% Fe2O3)

0.02% N2O + 0.14% NO+ Y2O3/ZrO2

1% O2+ 15% H2O; –1

GHSV=36,000 h Cu/SiO2–MxOy

0.15% N2O + 1.5% O2;

(M=Al, Ti, Zr)

τ (contact time)=36 s

730 (Zr/Y molar ratio=99)

[150]

[156]

530 (Al2O3) 560 (ZrO2)

[154]

580 (TiO2)

* Supported on cordierite ceramics (10 wt.% washcoat loading). Tuti et al. [147] studied the N2O decomposition over Co–, Cu–, and Fe–ZrO2 oxides. The following deN2O sequence was revealed: Co>Cu>>Fe, with the optimum deN2O performance, in terms of intrinsic reactivity, to be obtained for a metal content corresponding to 2–3 atoms/nm2 (Table 12). ZrO2–based catalysts also demonstrated excellent hydrothermal stability, in contrast to zeolites [140]. Shen et al. [148] explored the N2O decomposition ability of cobalt–based mixed oxides (Co/AxOy, A=Mg, Zn, Mn, Al, Ce). The optimum deN2O performance was obtained by Co/MgO (Figure 11). Their superior catalytic performance was attributed to a synergistic interaction between Co and MgO that enhanced the electron availability of active sites.



47

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The influence of some typical components, i.e., O2, H2O and NO, co-existing with N2O on the tail gas of a nitric acid plant, was also systematically investigated over Co/MgO catalysts [148]. The addition of 5% O2 in the feed stream shifted the N2O conversion profile to higher temperatures by about 15°C. Addition of 2% H2O on N2O + O2 stream resulted in a more severe inhibition, shifting the N2O conversion profiles by about 60 °C. This prohibiting effect is further exacerbated in the presence of 0.8% NO. The inhibition effect of various substances was related to their competitive adsorption on active sites. At this point, it is worth noticing that a completely analogous behavior, in relation to the inhibiting impact of NO, O2 and H2O on deN2O performance has been already described in almost all types of mixed oxides [e.g., 82, 94, 98, 99, 119, 139, 140].

o

400 C o 450 C o 490 C

100

80

N2O conversion (%)

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60

40

20

0 Co/MgO

Co/ZnO

Co/Mn2O3

Co/Al2O3

Co/CeO2

Binary oxides

Figure 11. N2O decomposition over Co–based binary oxides. Reaction conditions: 0.1 g catalyst, 1% N2O, GHSV= 30, 000 h–1, Co loading=10 wt.% [148].



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Giecko et al. [150] studied the decomposition of N2O over Fe2O3/Al2O3 oxides. The most active catalysts (41.8 wt.% Fe2O3) achieved 50% N2O conversion at about 630 °C (Table 12), in the presence of both NO and O2, with a slight activity degradation in the presence of H2O. The effectiveness of the catalyst in decomposing N2O in the nitric acid production plant was confirmed by the results of the 3,300 h real–plant experiments. Stelmachowski et al. [149] reported on deN2O performance of Co3O4/MgO oxides. It was suggested that the incorporation of Co ions into MgO strongly enhances the N2O decomposition, due to the change in the N2O activation step. However, the active sites for N2O decomposition over Co/MgO were hydroxylated to a great extent upon water addition in the feed, which in turn hindered the reaction rate. The beneficial impact of Co doping on the deN2O performance of MgO oxides was elaborated on the basis of the reaction scheme described below. In particular, the N–O bond scission over bare MgO proceeds through an anionic redox route involving surface oxygen transfer, in contrast to Co–doped MgO where the cationic redox route takes place through charge transfer from Co2+ sites into 3π* antibonding orbitals of N2O: Anionic route:

Cationic route:

N2O + O2–sur→ N2 + O22–sur

(14)

O22–sur + O22–sur→ 2O22–sur + O2(g)

(15)

Co2+ + N2O → Co3+ + N2 + O–

(16)

Co3+ + O– + O2–sur → Co2+ + O22–sur

(17)

Under this perspective, the Co2+cations may be considered as proper candidates for N2O activation over Co-based oxides [149]. However, recently obtained data on N2O decomposition over cobalt spinel catalysts demonstrated that the catalytic activity can



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be preserved upon the substitution of Co2+ cations by non-reducible Mg2+ [93]. This is obviously in contradiction to the assignment of Co2+cations as the active sites for N2O decomposition over Co3O4. In complete contrast, the replacement of Co3+ ions by Al3+ suppressed the deN2O performance, revealing their key role. The Co3+cations are considered responsible for oxidation of surface peroxy species (O22- → O2) by accepting the released electrons [149]. In view of these contradictory aspects, the facilitation of the redox cycles between Co2+ and Co3+, rather than the establishment of a certain oxidation state, is probably the key factor in deN2O process. This is in agreement with the aforementioned redox-type mechanism followed in the case of Cu-based catalysts, where the involvement of Cu2+/Cu+ redox pairs in the deN2O process is considered. In a similar manner, Scagnelli et al. [152] revealed by means of first principles DFT calculations, that Ni inclusion in MgO decreases the barrier for N2O decomposition from 1.37 eV over bare MgO to 1.19 eV over Ni–doped MgO. The latter explains the significant higher rate of N2O decomposition over the NiO/MgO sample, by about one order of magnitude, compared to pure MgO [153]. It was revealed by means of X-ray absorption fine structure spectroscopy that Ni2+ coordination sites distorted in the direction of Ni–Mg and Ni–Ni were responsible for the enhanced catalytic performance of Ni-doped MgO, implying the key role of neighboring interactions. Li et al. [36] investigated N2O decomposition in NiO/La2O3 mixed oxides supported on cordierite ceramics. The best deN2O performance was observed for a Ni/La molar ratio of 4.0 (Ni4La). Synergistic effects between NiO and LaNiO3 toward surface oxygen desorption and active phase (Ni2+) stabilization were considered responsible for the enhanced performance of mixed oxides compared to bare ones. ACS Catalysis_Revised MS_ cs-2015-01605m


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On intrinsic reaction rate basis, Ni4La catalyst was able to convert the largest number of N2O molecules at per unit time as compared to other NixLa composites, possessing thus the highest N2O conversion rate. The impact of oxides acidity on deN2O performance was investigated by Bennici

et al. [154]. N2O conversion increased in the following sequence: Cu/SiO2–Al2O3 > Cu/SiO2–ZrO2 > Cu/SiO2–TiO2 during the N2O decomposition in the presence of oxygen excess (Table 12). A more reliable comparison in terms of intrinsic reaction rate revealed that at 500 °C the activity of Cu/SiO2-Al2O3 was double and quadruple than the corresponding ones of ZrO2- and TiO2-based catalysts, respectively. The observed differences in activity were attributed to the support acidity, which affects copper distribution over the oxide support [154].

3.

Tailoring the local surface structure of mixed oxides (MOs)

Based on the research findings that were described in the previous sections it could be argued that the deN2O performance of MOs is strongly affected by acid/redox properties of the various components as well as by the synergistic effects between them. In almost all studies the N2O decomposition mechanism foresees adsorption of N2O on metal active sites followed by oxygen desorption. All mechanistic studies emphasize on the importance of both oxygen mobility (for active sites regeneration) and chemisorption ability (for N2O adsorption/dissociation). Therefore, it can be firmly conclude that the fine tuning of local surface structure of mixed oxides to ensure high N2O adsorption capacity and surface oxygen mobility is the key point for enhanced deN2O performance.



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In this regard, in this section the recent efforts devoted on surface chemistry modification of mixed oxides, in relation to deN2O process, are summarized. Two different approaches have been widely applied, either separately or simultaneously, for adjusting the surface and redox properties of MOs and in consequence the deN2O efficiency: (i) rational design of MOs by means of appropriate synthesis routes, ii) promotion of MOs by means of electronic promoters, mainly alkalis.

3.1 Rational design of MOs The ultimate target in the area of heterogeneous catalysis is the rational design of novel materials that can combine low cost with the desired physicochemical and catalytic properties. To meet this goal, the employed synthesis pathway must be cost– effective and able to produce a catalytic material with the appropriate textural, structural and surface properties in conjunction with adequate mechanical strength [38, 42, 45, 161]. The deN2O process over MOs, as it has been revealed so far, involves the dissociative adsorption of N2O on metal active sites followed by their regeneration through oxygen desorption. In this mechanistic sequence, both the surface oxygen mobility and the N2O chemisorption ability of active sites have a crucial role in the deN2O process. Furthermore, the sufficient thermal stability and “poisoning” tolerance is of great importance from the practical point of view. Hence, the synthesis route should be focused on the development of low cost, thermally stable materials with satisfactory catalytic performance. The latter should be adjusted by controlling the surface chemistry (active sites chemisorption ability) and redox properties (oxygen mobility) of mixed oxides through the appropriate combination of metal phases and the employment of a suitable preparation method. ACS Catalysis_Revised MS_ cs-2015-01605m


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Many synthetic pathways have been reported for the preparation of mixed oxides employed in the deN2O process. The main studies devoted on the influence of the preparation route on the deN2O performance of MOs are summarized in Table 13 and further discussed in the sequence. The optimum preparation method is also indicated in Table 13 along with its relative advantages. Table 13. Main studies devoted on the influence of synthesis method on deN2O performance of MOs.

Sample

MgCo2O4

Synthesis method

CP/high surface area;

(SCS);

high concentration of

co–precipitation (CP)

oxygen vacancies

(IWI); cellulose–templating (CT)

MgO

nanocasting

Ref.

route/main advantages

solution combustion synthesis

incipient wetness impregnation Na–CaO

Optimum preparation

[103]

CT/regularly shaped nano–sized particles; high

[162,

concentration of lower–

163]

coordinated lattice oxygen Highly porous solids;

[47]

Ordered structure GC/ formation of

La2Ti2(1–x)Fe2xO7–δ

gel combustion (GC);

nonstoichiometric single LaFeO3 phase; high

solid state reaction (SSR)

[127]

surface area; high chemisorption ability

sol–gel (SG); LaCoO3

reactive grinding (RG); colloidal crystal templating

RG/high surface area; high density of oxygen vacancies

(CCT) Y2O3/ZrO2

K–doped Co3O4

co–precipitation (CP); wet impregnation (WI) impregnation (I); precipitation (P);

[124]

CP/homogenous distribution

of

active

[156]

phases I/small crystallite size



[164]

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combustion (C); gradual oxidation (GO); hydrothermal synthesis (HS) P/excellent redox

impregnation (I); CuO-CeO2

precipitation (P); exotemplating (E)

properties linked to

[165]

Ce4+/Ce3+ and Cu2+/Cu+ redox pairs.

Ren et al. [47] studied the N2O decomposition over MnO2 samples synthesized by nanocasting from mesoporous silica SBA–15 and KIT–6, disordered mesoporous silica and colloidal silica. MnO2 samples that were prepared by employing hard templates demonstrated superior catalytic activity compared to commercial samples. Generally, it has been demonstrated that the hard–templating technique allows the preparation of porous materials on the nanometer scale with variable textural and redox properties [166, 167]. The deN2O performance of MgCo2O4 spinel catalysts, prepared either by solution combustion synthesis (SCS) or co–precipitation (CP), was comparatively studied by Zamudio et al. [103]. The superiority of catalysts prepared by CP method compared to those obtained by SCS was revealed (Figure 12). The latter was mainly ascribed to the high capacity of CP catalysts to enrich the surface with more reactive, weakly chemisorbed, oxygen species. This explanation is in line with TPD and XPS studies, both demonstrated the higher oxygen desorption capacity of CP catalysts when compared to SCS catalysts [103]. Moreover, CP method resulted to catalysts with higher surface area (143.8 m2/g) as compared to SCS (52.3 m2/g), which should be further accounted for the enhanced performance of CP catalysts.



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1000 MgCo2O4 spinel catalysts prepared by co-precipitation or solution combustion synthesis without catalyst

800

400

200

Solution combustion

600

co-precipitation

o


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ACS Catalysis

0

Figure 12. N2O conversion performance, in terms of T50, of MgCo2O4 spinel catalysts prepared by co–precipitation and solution combustion synthesis, in comparison to homogeneous N2O decomposition. Reaction conditions: 0.5% N2O, 5% O2; WHSV=120,000 ml g–1 h–1 [103]. Iron substituted lanthanum titanates (La2Ti2(1–x)Fe2xO7–δ) were synthesized by gel combustion (GC) and conventional solid state reaction (SSR) and were tested for N2O decomposition [127]. The catalyst prepared by the GC method exhibited better deN2O performance, which was attributed to the formation of non–stoichiometric single LaFeO3 phase with high surface area and chemisorption ability. Mössbauer spectroscopy in conjunction with TPR studies revealed that iron substitution results in an asymmetric environment around Fe ions, which is linked to the formation of anionic vacancies and improved reducibility.



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Granger and co–workers [124] showed that the preparation method of LaCoO3 perovskites notably affects the deN2O performance. LaCoO3 prepared by the reactive grinding (RG) method demonstrated the optimum deN2O efficiency, due to their higher specific area and higher density of oxygen vacancies. Moreover, Granger et al. [156] showed that the co–precipitation (CP) method can be employed to prepare homogeneous yttrium–doped ZrO2 samples. A synergistic effect toward high deN2O performance and stability was observed on ZrO2 with low yttrium (Y) content, prepared by CP method [156]. Interestingly, Y-doped samples prepared by CP are more resistant regarding the deactivation induced by NO, O2 and H2O, as compared to the bare ZrO2 sample. However, at high yttrium loadings an inhibition on N2O conversion was observed, irrespective of the preparation method. In that case the segregation of Y2O3 at the catalyst surface notably hindered the decomposition of N2O. In terms of activity rather than N2O conversion basis, the optimal promoted Ydoped samples demonstrated much lower apparent activation energy values (123 kJ/mol) as compared to bare ZrO2 (153 kJ/mol) and Y2O3 (251 kJ/mol), revealing that significant modifications in the adsorption properties and/or the reactivity of adsorbates on ZrO2 can be induced by Yttrium doping [156]. Kondratenko and co–workers [162,163] compared the deN2O performance of Na– CaO oxides prepared either by a cellulose–templating (CT) or a conventional insipient wetness impregnation (IWI) method. The CT method produces highly active materials with regularly shaped nano–sized particles, which facilitate the desorption of surface species blocking the active sites. In contrast, IWI-prepared catalysts have randomly shaped particles on a micron scale in which the desorption of inhibiting species is not favored. More importantly, a correlation between the catalysts particle size and their resistance to O2 and NO inhibitions was revealed, namely: the smaller the particles, ACS Catalysis_Revised MS_ cs-2015-01605m


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ACS Catalysis

the better the deN2O performance. The effect of the preparation method of Na-CaO composites on the deN2O performance was more clearly reflected in the N2O decomposition rates under differential conditions. The activity of CT-prepared catalysts was significantly higher (20 nmol m-2 s-1) than that of commercial CaO and IWI-prepared catalysts (~4.0 nmol m-2 s-1). CT synthesis enables homogeneous distribution of Na in CaO lattice, thus resulting in the formation of anionic vacancies which serve as active sites for N2O decomposition [162]. Inoue and co–workers examined the impact of the synthesis method on the deN2O performance of K–doped Co3O4 catalysts [164]. Materials were synthesized by employing impregnation, precipitation, combustion with glycine, gradual oxidation and hydrothermal methods. The catalyst prepared by impregnation of CoCO3 with an aqueous solution of KOH exhibited the optimum performance due to its superior textural and surface characteristics. The TPR/TPD studies revealed that the preparation method significantly influenced the extent of the reduction of Co3+ to Co2+, which crucially affect the N2O decomposition. Very recently, the impact of preparation method (impregnation, precipitation, exotemplating) on the deN2O performance of 20 wt.% Cu/CeO2 oxides was investigated by Konsolakis et al. [165]. The results revealed the superiority of Cu/CeO2 samples prepared by co–precipitation (Figure 13). More importantly, the catalysts prepared by precipitation exhibited about three times higher specific rates in comparison to catalysts prepared by impregnation (~2 nmol m-2 s-1 at 450 °C), revealing that the modifications in surface chemistry rather than in textural characteristics could be envisaged. The latter was verified by means of several characterization studies (XRD, XPS, H2–TPR, micro-Raman), which revealed that



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considerable modifications on the surface and redox properties of Cu–ceria oxides can be induced by the synthesis procedure [165]. The superiority of Cu/CeO2 samples prepared by precipitation was ascribed to their excellent reducibility as well as to the facilitation of the redox cycles between the Ce4+/Ce3+ and Cu2+/Cu+ pairs. Strong electronic metal-support interactions (EMSI effect) were considered responsible for their relative abundance in Cu+ and Ce3+ species (as indicated by XPS and microRaman analysis), which have a crucial role in the deN2O process, according to the already proposed reaction scheme (reactions (9)–(13)).

100

80

N2O conversion (%)

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Impregnation Exotemplating Precipitation

60

40

20

0

300

350

400

450

500

550

600

o

Temperature ( C)

Figure 13. N2O conversion performance of 20 wt.% Cu/CeO2 samples prepared by impregnation, precipitation and exotemplating methods. Reaction conditions: 0.1% N2O, GHSV=40,000 ml g–1 h–1. Recently, the effect of synthesis route on the deN2O performance of Co3O4 spinel catalyst was investigated [168]. Various samples were prepared by precipitation of cobalt nitrate in aqueous solutions employing different precipitation agents (NH3, ACS Catalysis_Revised MS_ cs-2015-01605m


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NaOH, Na2CO3) and reactions conditions (OH/Co molar ratio, aging time). Although only Co3O4 spinel-like oxide was obtained in all cases after calcination at 500 °C, a different N2O conversion performance was observed among the samples. The best deN2O activity was demonstrated by catalysts obtained from β-Co(OH)2 precursor. A direct correlation between the reduction characteristics and the catalytic performance was established, clearly revealing the key role of preparation procedure. The influence of the reducibility on the N2O decomposition of Co3O4 spinel catalysts can be well understood on the basis of the previously described redox cationic route (reactions (16), (17)). This at first involves the electron donation from Co2+ to N2O, followed by the formation of Co3+ species (reaction (16)). The Co3+ is then reduced to Co2+ by oxygen desorption (reaction (17)). In the light of the this mechanistic sequence, it is evident that the facilitation of the redox cycle between Co2+ and Co3+ is essential for the high reaction rate.

3.2 Electronic promotion of MOs Based on the aforementioned studies on N2O decomposition over metal oxide catalysts, one can affirm with certainty that the deN2O mechanism involves two main pathways: (i) adsorption and decomposition of N2O into metal active sites, and (ii) desorption of adsorbed oxygen species towards the regeneration of active sites. In this mechanistic sequence, both the surface redox properties and the oxidation state of active sites are expected to substantially affect the deN2O process. In this regard, particular attention has been focused on the tuning of MOs surface chemistry by means of bulk and/or surface promoters. More specifically, the improvement of the deN2O performance of mixed oxides can be achieved by a bulk modifier (e.g., alien cation) and/or by tuning the surface properties with an electropositive promoter, such as alkali or alkaline earth. ACS Catalysis_Revised MS_ cs-2015-01605m


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In general, promoters play a key role in heterogeneous catalysis due to their unique ability to adjust the physicochemical properties of catalysts [169]. Promoters can be divided into two subcategories: structural promoters and electronic or surface promoters. Structural promoters are mainly responsible for the stabilization of the active phase; this however does not exclude their impact on the chemical properties through metal–support interactions. On the other hand, electronic promoters can directly modify catalyst surface chemistry via direct or indirect interactions. The former denotes direct electrostatic interactions between reactants and promoter’s local electric field. The latter refers to the promoter–induced modification on metal Fermi level, which then is reflected in the strength of the chemisorption bonds of reactants [169]. The role of promoters in heterogeneous catalysis is the subject of several comprehensive studies [169–171]. The vast majority of studies on electronic promotion over metal oxide catalyst refer to alkali modifiers. Table 14 summarizes the main studies regarding the deN2O performance of alkali–promoted MOs. The impact of promoter species on the deN2O performance is illustrated via the parameter “∆Τ50”, which is defined as ∆Τ50=Τ50 (unpromoted catalyst) – T50 (promoted catalyst), and it is also included in Table 14.

Table 14. Effect of promoters on the deN2O performance of MOs T50 of un– Catalyst

Promoter

Reaction conditions

promoted catalyst (°C)

Co3O4

K

Co3O4

Cs

5.0 % N2O; GHSV=7,000 h–1 5.0 % N2O; GHSV=7,000 h–1

Promotional effecta Ref.

(∆Τ50, °C)

400

100 (2 at/nm2)b

[172]

400

250 (2–3 at/nm2)

[173]



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Co3O4 Mn3O4

K

Fe3O4

5.0 % N2O; GHSV=7,000 h–1

400

158 (2 at/nm2)

760

193 (8 at/nm2)

520

K

5.0 % N2O;

Na

GHSV=7,000 h–1

400

Cs

85 (3x10–4 mol Cs/g)

K

CuO

Cs

Co3O4

K

–1

410

–1

WHSV=60,000 ml g h

40 (3x10–4 mol K/g)

[176]

–4

10 (3x10 mol Na/g) –50 (3x10–4 mol Li/g)

0.25 % N2O; WHSV=7,200 ml g–1 h–1 0.5 % N2O + 2% O2; WHSV= 12,000 ml g–1 h–1

325

130 (Cs/Cu=0.1)

[177]

355

200 (K/Co=0.03)

[178]

K

159 (K/Co=0.05)

Cs

0.05 % N2O;

Na

WHSV= 24,000 ml g–1 h–1

467

Li K

[175]

60 (3x10–4 mol Rb/g)

0.1 % N2O;

Li

NiAl

104 (2 at Na/nm2) 5 (2 at Li/nm2)

Na

MgCo2O4

161 (2 at K/nm2)

Li

Rb Co4MnAlOx

40 (6 at/nm ) 185 (2 at Cs/nm2)

Cs Co3O4

[174]

2

135 (Cs/Co=0.05) 93 (Na/Co=0.05)

[179]

62 (Li/Co=0.05) 2 % N2O;

416

WHSV=8,400 ml g–1 h–1

55 (K/Ni=0.1)

a

expressed as ∆Τ50=Τ50 (unpromoted catalyst)–T50 (promoted catalyst);

b

optimal loading of alkali promoters. The effect of alkali (Li, Na, K, Cs) promotion on deN2O activity of Mn–, Fe–, and

Co–spinel catalysts was extensively studied by Kotarba and co–workers [172–175, 181, 182]. The mode of action of alkali modifiers was thoroughly explored by several complementary techniques, such as temperature programmed surface reaction (TPSR), pulse experiments of isotopically labeled

15

N218O, species resolved–thermal

alkali desorption (SR–TAD), Kelvin probe (work function measurements), DFT



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modeling, among other conventional methods. The results clearly revealed the pronounced effect of alkali addition on deN2O performance (Table 14). The beneficial effect of dopants follows, in general, the sequence: Li

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