RESEARCH PAPER

Removal of Greenhouse Gas (N2O) by Catalytic Decompositionon Natural Clinoptilolite Zeolites Impregnated with Cobalt

A. Ghahri1 • Farideh Golbabaei1 • L. Vafajoo2 • S. M. Mireskandari3 •

M. Yaseri4 • S. J. Shahtaheri1

Received: 4 January 2017 / Revised: 24 May 2017 / Accepted: 13 June 2017

� University of Tehran 2017

Abstract In this work, natural zeolite, clinoptilolite were

treated with acid (0.6 N HCl; code ‘‘Z-AM’’) and alkaline

solutions (1.5 N NaOH; code ‘‘Z-BM’’). Thereafter, non-

modified (as parent zeolite; code ‘‘Z-NM’’) and modified

zeolites were impregnated with cobalt using wet incipient

impregnation method (Codes: ‘‘Z-AM-Co-0.5,1,1.5’’, ‘‘Z-

BM-Co-0.5,1,1.5’’, ‘‘Z-NM-Co-0.5,1,1.5’’). The prepared

zeolites were characterized by XRD, ICP-OES, BET, NH3-

TPD and H2-TPR. Also, these materials were studied for

the catalytic decomposition of nitrous oxide (a greenhouse

gas) to nitrogen and oxygen. The obtained results showed

that the applied modifications had no significant influence

(destruction) on the main structure of the zeolites including

clinoptilolite, quartz and cristobalite. In addition results

showed that acid modification increases the nitrous oxide

decomposition because of surface area increment and the

higher amount of CO/Al as well as the strong acid sites of

this zeolite compared to other zeolites. Also, experiments

showed that the main active species in nitrous oxide

decomposition are mono-atomic (Co2? cations) and other

species exhibit much lower activity. In conclusion, natural

clinoptilolite zeolites treated with acid and impregnated

with Cobalt (Z-AM-CO-1.5) could be a very effective and

cost-benefit catalyst for reducing N2O as a greenhouse gas,

due to its very low price, high chemical stability and high

availability.

Keywords Greenhouse gas � N2O decomposition �Clinoptilolite � Cobalt � Natural zeolite � Acid modification

Introduction

In recent decades, one of the most important issues in

environment has been climate change, especially global

warming and ozone layer destruction. Global warming is

intensified by the increased concentrations of greenhouse

gases (GHG) such as carbon dioxide, methane, nitrous

oxide and halogenated hydrocarbons (Gregg and Sing

1982; Sulbaek et al. 2010; Rajska et al. 2016; Osvaldo et al.

2016; Rowland et al. 1995). Although N2O, among the

GHG, is present in low concentrations in the troposphere, it

is a potent greenhouse gas with a global warming potential

of 310 and 21 times that of CO2 and CH4, respectively

(Kannan and Swamy 1999; Labhsetwar et al. 2009;

Paschalia Taniou and Savvas 2013; Xie et al. 2014; Xue

et al. 2007). It is estimated that the role of nitrous oxide as

a greenhouse gas and its effects is approximately 6.5% by

volume (Rajska et al. 2016; Osvaldo et al. 2016). In

addition, occupational exposure to N2O affects the central

nervous system, cardiovascular, hepatic, hematopoietic,

and reproductive systems in humans (Marylou Austin

2013; Wronska-Nofer et al. 2012).

Microbial activity in the soil is the main source of

nitrous oxide (Todd et al. 1995) and secondary sources are

exhaust streams of industries include production and

& Farideh Golbabaei

Fgolbabaei@sina.tums.ac.ir

1 Department of Occupational Health, School of Public Health,

Tehran University of Medical Sciences, Tehran, Iran

2 Department of Chemical Engineering, South Tehran Branch,

Islamic Azad University, Tehran, Iran

3 Department of Anesthesiology and Critical Care, Imam

Khomeini Hospital Complex, Tehran University of Medical

Sciences, Tehran, Iran

4 Department of Epidemiology and Biostatistics, School of

Public Health, Tehran University of Medical Sciences,

Tehran, Iran

123

Int J Environ Res

DOI 10.1007/s41742-017-0030-6

transportation of N2O, healthcare (use as an anesthetic gas

in hospitals and clinics), whipped creams (as a foaming

agent), natural gas pipelines (as a leak-detecting agent),

production of chemicals such as adipic acid, nitric acid and

fertilizer, and fossil fuel combustion (Kapteijn et al. 1997;

Marylou Austin 2013; Xie et al. 2014). It should be noted

that the concentration of nitrous oxide in the atmosphere of

various industries or exhaust streams of industries is dif-

ferent. For example, the range of nitrous oxide concentra-

tion is about 50–400 ppm in exhaust streams of operating

rooms (hospitals) in Iran (Maroufi et al. 2011; Masoumeh

et al. 2014; Mortazavi et al. 2013).

Regarding the high number of people exposed to N2O

and also its adverse effects on environment as well as the

annual increasing rate of 0.2–0.3% in atmosphere, it is

important to find economical and feasible reduction

methods of nitrous oxide as one of the major challenges in

environmental protection and occupational health efforts

(Kapteijn et al. 1997; Labhsetwar et al. 2009; Paschalia

Taniou and Savvas 2013; Xie et al. 2014).

Different methods for N2O removal have been described

in the literature, which include thermal decomposition,

non-selective catalytic reduction, direct catalytic decom-

position and biological process (Labhsetwar et al. 2009;

Osvaldo et al. 2016; Xie et al. 2014). The decomposition of

nitrous oxide would be a relatively simple, suitable,

preferable, most efficient and economic method for sta-

tionary sources (Centi et al. 1997; Goto et al. 2000; Xie

et al. 2014). Many catalysts have been used in the

decomposition of N2O, including supported noble metals,

pure and mixed oxides and metal-exchanged zeolites

(Kapteijn et al. 1997; Mauvezin et al. 1999; Mul et al.

2001; Labhsetwar et al. 2009; Nobukawa et al. 2002; Xie

et al. 2014; Xue et al. 2007).

One group of catalysts which has demonstrated high

activities for N2O decomposition to harmless product (ni-

trogen and oxygen) are metal oxides and mixed metal

oxides such as Rh, Ru, Pd, Cu, Co, Fe, Pt, Ni, and Mn.

Since the surface areas of these materials are very low, the

metal oxides are used as supporting materials, e.g., zeolites

to form metal-exchanged zeolites (Kapteijn et al.

1996, 1997; Kawi et al. 2001; Miller et al. 1998).

The activity of metal zeolites depends on the metal ion

and type of zeolite in N2O decomposition. For example,

copper and iron-ZSM-5 are not quite active under

hydrothermal conditions, while cobalt-ZSM-5, because of

its thermostability, is very active under the same condition

(Kapteijn et al. 1997). However, cobalt is not very active in

Y-zeolite. In general, due to the high redox properties,

cobalt has excellent activity in nitrous oxide decomposition

as well as on most zeolites such as ZSM-5, Beta, ZSM-11,

Ferrierite and Mordenite (MOR) (Ates et al. 2011; Kapteijn

et al. 1996, 1997).

Most of the zeolites used in investigation of N2O

decomposition are synthetic and few studies have addres-

sed this issue using natural zeolites (Ates 2015). The main

reasons for low application of natural zeolite include low

thermal and hydrothermal stability at high temperatures,

heterogeneity and high cost of homogenization and

purification of this material (Ates 2007, 2015). However,

Ayten Ates found that the modification of natural zeolites

by ion exchange and acid treatment due to the formation of

isolated iron and Fe–O–Al species improves their activity

in N2O decomposition (Ates 2015). Other studies have

shown that iron exchanged natural zeolites have high

activity in selective catalytic reduction of nitrous oxide

with NH3 (Ates et al. 2011).

One of the most common and most used natural zeolite

in the world is clinoptilolite. This natural zeolite has certain

characteristics such as high chemical stability, high avail-

ability and low extraction cost and owing to these char-

acteristics; it is extensively employed in environmental and

industrial applications (Alver and Sakizci 2015; Ates 2007;

Garcia-Basabe et al. 2010; Radosavljevic-Mihajlovic et al.

2004).

Based on the studies in the literature, despite the

excellent response of cobalt synthetic zeolites in the

decomposition of nitrous oxide, to date, clinoptilolite and

cobalt have not been used in this process. Hence, in this

study, the first natural zeolite, clinoptilolite was treated

with acid (0.6 N HCl) and alkaline solution (1.5 N NaOH).

Thereafter, non-modified (as parent zeolite) and modified

zeolites were impregnated with cobalt using wet incipient

impregnation method. In the following sections, effects of

these modifications on physicochemical properties of zeo-

lites in N2O decomposition are discussed by XRD, ICP-

OES, BET, NH3-TPD and H2-TPR.

Materials and Methods

Preparation of the Catalysts: The natural zeolite, clinop-

tilolite used in the present study was obtained from

deposits in Semnan province in the center of Iran. Prepa-

ration of modified and non-modified samples from this raw

material was carried out according to the following steps

(Fig. 1):

1. Non-modified zeolite (code ‘‘Z-NM’’): First, samples

were grinded and sieved through a diameter of

400–420 lm. Thereafter, this raw material was washed

with distilled water (three times) to remove soluble

impurities such as dust. Finally, dried samples (for

24 h in an oven at 60 �C) were calcinated at 450 �C in

air for 2 h at a heating rate of 10 �C/min and kept in a

desiccator for the next stage (modified zeolites).

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2. Modified zeolite with acid (code ‘‘Z-AM’’): Accord-

ing to studies (Alver and Sakizci 2015; Garcia-

Basabe et al. 2010; Radosavljevic-Mihajlovic et al.

2004), to minimize the framework damage of zeo-

lites, acid treatment of zeolites was conducted by

placing sufficient amount of zeolite (Z-NM) in a flask

containing 0.6 N HCl solution in a ratio of 1:10 w/v.

This mixture was shaken at 150 rpm using a shaking

incubator at 60 �C for 24 h. Beyond the specified

time for shaking, the separated zeolites from solution

were washed three times with 0.05 N HCl for 15 min

and then rinsed with distilled water several times to

obtain a negative reaction for Cl-. Assurance of

complete removal of chloride ions was accomplished

through volumetric analysis by AgNO3. The samples

obtained were dried and calcinated in a similar way

to the first stage.

3. Modified zeolite with base (code ‘‘Z-BM’’): Alkaline

treatment was carried out at 50 �C using 1.5 M NaOH

(30 mg/g of zeolite; Z-NM) for 4 h in the shaking

incubator with agitation rate of 150 rpm. Thereafter,

samples were washed, dried and calcinated in a similar

way to the first stage.

4. Modified zeolite with cobalt (code ‘‘Z-NM, BM and

AM-Co-%’’): In this stage, modified and non-modified

zeolites were impregnated with cobalt using wet

incipient impregnation method. In each experiment,

20 g of heated zeolite in an oven at 100 �C was added

to 200 ml of deionized water containing 1.5, 3 and

4.5 g of Co(NO3)2�6H2O to obtain Co-zeolites of 0.5, 1

and 1.5 wt.%, respectively (code ‘‘Z-NM,BM,AM-Co-

0.5,1,1.5’’). Thereafter, mixed solutions were placed in

the incubator at 150 rpm and 60 �C for 24 h or more

until the solution was completely dried. After impreg-

nation, the samples were re-slurried in hot water

(150 �C) for 1 h and then washed, dried and calcinated

in a similar way to the first stage.

Catalyst characterization: It should be noted that

according to preliminary studies, determination of catalysts

characterizations were performed on selected samples.

The specific area of modified zeolites and parent zeolites

was determined by nitrogen adsorption on a MicrotracBel-

Belsorp mini II Japanese instrument. The pore size distri-

bution of the sample was obtained according to Horvath–

Fig. 1 Preparation schema of the natural and modified zeolites

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Kawazoe equation. In addition, microporous volume was

determined using the t-plot method.

For determination of the catalysts crystalline phases,

X-ray diffraction (XRD) patterns of samples was performed

with an X-ray diffractometer, Model (Philips: PW-1830)

operating at 40 kV and 30 Ma, using Cu Ka radiation. In

addition, determination of sample compositions was inves-

tigated by ICP technique, Varian Vista 735 ICP-OES.

Hydrogen temperature-programmed reduction (H2-TPR)

of samples and NH3 temperature-programmed desorption

(NH3-TPD) experiments were performed using (CHEMBET-

3000) from Quanta Chrome Corporation equipped with

thermal conductivity (TCD) equipment for determining H2

consumption and NH3 desorbed. Before the measurements

(H2-TPR and NH3-TPD), zeolites (0.05 g) were pretreated at

400 �C for 60 min in helium, and then cooled to room tem-

perature in helium. The H2-TPR runs were performed at a

ramping rate of 10 �C/min from 50 to 950 �C with the flow

rate on reduction gas at 5% for H2 and 15 cm3/min for He. In

NH3-TPD, samples were first saturated with NH3 diluted with

He (10%) for 60 min at room temperature. Thereafter, NH3

was desorbed from catalysts using a He flow (10 cm3/min) up

to 800 �C at a ramping rate of 10 �C/min.

Catalytic test: The catalytic activity of zeolites in N2O

decomposition were conducted in a fixed bed stainless steel

reactor of 1 cm internal diameter and 10 cm length and a

temperature range of 150–600 �C. For each experiment, a

4 g catalyst was packed into the reactor and a gas stream of

N2O (350–400 ppm) balanced with N2 was fed into the

reactor at a rate of 300 cm3/min to obtain a space velocity

(GHSV) of 4500 cm3/(h gcat).

The concentrations of N2O in the reactor inlet and outlet

were measured according to NIOSH 6600 method (Eller

1994); collection and analysis were done using bag and

portable IR spectrophotometer model 3010 Bacharach

Company, respectively. Before each measurement, the

apparatus was calibrated according to manufacturer’s

instructions. The conversion of nitrous oxide to N2 and O2

was calculated using the following equation:

X ¼ ðN2OÞin � ðN2OÞout

ðN2OÞin

� �� 100; ð1Þ

where X is the conversion percent, (N2O)in and (N2O)out are

the nitrous oxide concentration in the reactor inlet at room

temperature and the reactor outlet at an elevated tempera-

ture, respectively.

Result and Discussion

physicochemical properties for zeolites (XRD): To reveal

the influence of modifications on the crystal structure of

zeolites, the XRD patterns of all the samples NM, BM, AM

and NM, BM, AM-Co-0.5, 1, 1.5 were prepared. Figure 2

shows the major phases of all the zeolites, which includes

clinoptilolite at 2h = 9.88�,11.19�, 22.46�, 30.38� (Akgul

2014; Alver and Sakizci 2015; Treacy and Higgins 2007),

quartz at 2h = 20.86�, 26.65�, 39.49� and cristobalite at

2h = 22.00�, 31.44�, and 36.5� (Treacy and Higgins 2007).

In addition, the trace phase in Z-NM is dolomite, which

after alkaline and acid treatments disappeared (Ates and

Hardacre 2012). A quick survey of the obtained patterns

show that HCl and NaOH treatments and cobalt loading on

samples did not produce significant effect and the Z-NM

zeolites structure were well preserved. However, a

decrease and increase in intensity of the diffraction peaks

were observed for acid and alkaline treatments, respec-

tively. These results are in perfect agreement with recent

studies (Akkoca et al. 2013; Alver and Sakizci 2015;

Cheng et al. 2005). For example, Erdogan and Sakizci

showed that acid treatment using concentrations up to

0.5 M have no considerable effect on the crystallinity of

samples (Alver and Sakizci 2015). However, a slight

decrease in the intensity of the main clinoptilolite peaks as

observed in the current study, resulted in a loss in crys-

tallinity of the sample due to dealumination and the partial

collapse of the structure of the natural zeolite.

Diffraction peaks of cobalt spinel structure were at

2h = 31.2�, 36.8� and 59.3� on XRD patterns of zeolites,

but they were not clearly distinguishable. This may be due

to the high intensity of diffraction peaks of the major

phases (clinoptilolite, quartz and cristobalite) and/or weak

diffraction peaks of cobalt spinel as Shen et al. have

pointed out in their research (Shen et al. 2012). In addition,

after impregnation of zeolites, a slight decrease in peak

intensities observed, which was similar to the results of the

study of Murat Akgul (2014) might be due to the distortion

of the mesoporous channels caused by the collapse of pore

structure during the modification process.

Fig. 2 XRD patterns of non-modified and modified zeolites

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Compositions analysis by ICP: Zeolites are crystalline,

porous, hydrated aluminosilicates material with the pri-

mary building units (PBU) of AlO4 and SiO4 tetrahedra

(Alver and Sakizci 2015; Kowalczyk et al. 2006).

Clinoptilolite—a natural zeolite—has been diagnosed with

exchangeable cations of potassium and calcium of high

value, and magnesium and sodium of low values. In

addition, compounds such as iron and titanium occur as

impurities in their oxides (Alver and Sakizci 2015).

According to chemical analysis results of samples

(Table 1), the main compounds of zeolites Z-NM are those

of silica and aluminum (71.38 and 10.76 wt.%, respec-

tively). Calcium and sodium are the next highest compo-

nents at 4.49 and 1.77 wt.%, respectively. These results are

similar to results of the studies conducted by Ates et al.

(2007), Camacho et al. (2011). The only difference was in

the low levels of calcium observed in this study compared

to that of Ayten study (0.45–1.64% against 0.45–5.74%).

This is due to the low dolomite content in zeolite Z-NM.

It is well known that acid treatment of natural zeolites

causes the exchange of H? ion with exchangeable cations

in clinoptilolite and dealumination by hydrolysis of Al–O–

Si bonds (Akkoca et al. 2013; Alver and Sakizci 2015;

Kowalczyk et al. 2006; Rozic et al. 2005). Facilitation of

its dissolution by chloride ions via the formation of inner

sphere complexes with surface groups were also found, as

well as increase in the number of acidic sites and secondary

porosity (Akkoca et al. 2013; Alver and Sakizci 2015).

After HCl treatment of zeolite Z-NM, all elements were

reduced with the exception of silica and potassium due to

their high resistance to acid. The highest decrease was

recorded in calcium (52%). The presence of Ca can be

attributed to the dolomite and feldspar phase in the natural

zeolites as stated by Ayten Ates (2007). XRD results

(Fig. 2) showed that diffraction peaks of dolomite disap-

peared following treatment with HCl. Thus, it can be

adduced as the reason for calcium reduction. In addition,

dealumination value was about 8% in this study, which is

in consonance with the findings of other researchers (Ates

and Hardacre 2012; Segawa and Shimura 2000).

Several surveys on the alkaline treatment of zeolites

have shown that the wt.% of samples compositions were

changed (Lin et al. 2015). In this study, the results obtained

were similar to that reported by other investigators (Akgul

and Karabakan 2011; Akkoca et al. 2013); silica and

potassium decreased, but other compounds particularly

sodium increased.

Comparison of the percentage composition of samples

impregnated with cobalt and their parent zeolites showed

that as the amount of cobalt increased, wt.% of Na, K and

Fe decreased, but other elements were significantly

unchanged, which is similar to a study conducted in 2007

(Ates 2007). Cobalt to aluminum ratio was one of the

important parameters in the catalytic activity of transition

metal zeolites (Abu-Zied et al. 2008; Smeets et al. 2008;

Xie et al. 2015). The amount of this ratio was higher than

0.5 in most studies (Chen et al. 2004; Jentys et al. 1997;

Seyedeyn-Azad and Zhang 2001; Wang et al. 2000) and

was between 0.1 and 0.15 in the present study.

BET: Nitrogen adsorption and desorption of Z-NM,

Z-AM, Z-BM, Z-NM-Co-1.5, Z-BM-Co-1.5, Z-AM-Co-

0.5, 1.5, 2.5 are shown in Fig. 3 All isotherms were of the

type II according to IUPAC (Sing et al. 1985; Gregg and

Sing 1982). In these types of isotherms, samples’ structure

was composed from microporous and mesoporous materi-

als with multiple layers on zeolites. The isotherm curves in

Fig. 3 show that the modified zeolite with acid (Z-AM) is

able to absorb more nitrogen. The results of the influence

of acid and alkaline treatments and cobalt loading on the

specific surface area, total volume, and average pore

diameter are summarized in Table 2. As seen from the

results, modifying zeolites with HCl and NaOH have

opposite effect relative to each other on the specific surface

area. Acid treatment of zeolite Z-NM led to an abrupt

increase in specific surface area from 25.79 to 103.09 m2/g

and total volume from 5.92 to 23.68 cm3/g, non-significant

increase of micropore volume from 0.138 to 0.158 cm3/g

as well as significant decrease of average pore diameter

from 23.55 to 6.143 nm (Table 2).This considerable

growth in specific surface area and total volume of Z-AM

Table 1 Chemical

compositions (wt.%) and

textural properties of natural

and modified zeolites

Zeolite name Si/Al SiO2 Al2O3 BaO CaO F2O3 K2O Na2O TiO2 LOI Co Co/Al

Z-NM 6.63 71.38 10.76 0.08 0.97 0.74 4.49 1.77 0.19 8.5 NM –

Z-NM-1.5 6.51 70.68 10.85 0.09 0.82 0.78 4.4 1.08 0.2 8.78 1.2 0.11

Z-BM 5.79 68.86 11.89 0.07 1.15 0.82 3.88 2.87 0.22 9.12 NM –

Z-BM-Co-1.5 5.62 67.12 11.95 0.16 1.64 0.94 3.82 1.69 0.23 9.8 1.53 0.13

Z-AM 7.57 72.86 9.62 0.13 0.49 0.77 5.41 0.97 0.26 8.37 NM –

Z-AM-Co-0.5 7.77 73.75 9.49 0.12 0.49 0.47 4.38 0.75 0.23 8.75 0.45 0.05

Z-AM-Co-1 7.84 73.71 9.4 0.11 0.47 0.38 4.18 0.81 0.22 8.65 0.95 0.10

Z-AM-Co-1.5 7.91 73.68 9.32 0.12 0.46 0.29 3.98 0.89 0.22 8.42 1.5 0.16

Z-AM-Co-2.5 7.96 73.42 9.22 0.12 0.47 0.27 3.97 0.88 0.23 8.45 1.85 0.20

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compared to Z-NM can be attributed to dissolution of some

impurities that block pores in the acids as well as the

almost complete replacement of the metal cations by H?

and opening of the windows formed from its replacement.

These results are in consonance with the results of other

researchers (Akkoca et al. 2013; Alver and Sakizci 2015;

Garcia-Basabe et al. 2010; Hernandez et al. 2013). In

addition, reduction of average pore diameter of Z-AM

zeolite was related to forming of secondary micropores by

free linkage. Results similar to this study were reported for

Clinoptilolites treated with acid solutions by Alver and

Sakizci (2015). According to the data in Table 3, modify-

ing Z-NM with NaOH solution had no significant effects on

the physical properties of zeolite. The results obtained from

this part of the study are in agreement with results from

studies by DicleBal Akkoca et al. (2013) and Jeong et al.

(2001).

Impregnation of Z-NM and Z-BM zeolites with cobalt

did induce an increase in total volume, micropore and

specific surface area and a decrease in average pore

diameter. As seen from the data of Table 3, SBET of Z-NM-

CO-1.5 and Z-BM-CO-1.5 are equal with 27.20 and

35.13 m2/g, respectively, indicating an increase in value

compared to their parent zeolites (Z-NM and Z-BM).

Similar changes after modifying zeolites with metals to

form metal/zeolite were also observed by other authors

(Akgul 2014; Liu et al. 2012). This increase in the BET

surface area of the modified Clinoptilolites is probably due

to the formation of surface cracks and defects as a result of

the collapse of pore structure during impregnation with

cobalt. Based on the results obtained (Table 2) as well as

the % increase in cobalt impregnation with Z-AM zeolites,

average pore diameter increased, whereas other parameters

including total volume, specific surface area and micropore

volume reduced in a similar way to previous studies (Akgul

2014; Rutkowska et al. 2014; Zhang et al. 2007). This

reduction in surface area after the modification can be

attributed to the occupation of the pores by cobalt oxides or

partial conglomeration of the samples crystallites.

TPR: The H2-TPR measurements carried out for selec-

ted zeolites (Z-NM-CO-1.5 and Z-AM-CO-1.5). According

to the H2-TPR profiles obtained in Fig. 4, three broad peaks

were characterized around 250–350, 350–450 and

700–900 �C for Z-NM-CO-1.5. In addition, in the case of

Z-AM-CO-1.5 addition to these broad peaks, a weak peak

was diagnosed around 500–600 �C. Several studies per-

formed on H2-TPR of cobalt-exchanged zeolites (Akkoca

et al. 2013; Liu et al. 2012; Xie et al. 2015) have shown

that up to 3 reduction regions can be distinguished;

(a) 200–400 �C (reduction Co3O4 to CoO and then to metal

cobalt in external surfaces of zeolites), (b) 400–700 �C(reduction CoOx in internal surfaces of zeolites) and

(c) 700–950 �C (reduction Co2? to Co? or Co0 ion-ex-

changed sites). In addition, Smeets reported that in

Fig. 3 N2 adsorption isotherms

of non-modified and modified

zeolites

Table 2 Textural properties of natural and modified zeolites

Zeolite name Vtotal (cm3/g) Vmicro SBET (m2/g) Dp (nm)

Z-NM 5.92 0.148 25.79 23.55

Z-NM-1.5 6.24 0.154 27.20 22.75

Z-BM 5.80 0.169 25.27 26.76

Z-BM-Co-1.5 8.07 0.176 35.13 20.07

Z-AM 23.68 0.158 103.09 6.143

Z-AM-Co-0.5 15.78 0.154 68.7 8.773

Z-AM-Co-1 15.2 0.149 67.5 8.976

Z-AM-Co-1.5 14.95 0.146 65.09 8.985

Z-AM-Co-2.5 11.65 0.149 50.72 11.780

BET specific surface area

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catalysts with low Co loadings (Co/Al \0.3), cobalt is

predominantly present as a mono-atomic Co species, and

higher Co loadings (Co/Al [0.5) culminate in the forma-

tion of different kinds of Co-oxides (Smeets et al. 2008).

According to results of Fig. 4 (existence of one main

reduction region in both H2-TPR profiles) and Table 1 (Co/

Al ratio is 0.05–0.2), it can be concluded that the main

active species in nitrous oxide decomposition are Co2?

cations and other species which exhibit much lower

activity. This result is in complete agreement with the

results of other studies (Boron et al. 2015; Smeets et al.

2008; Xie et al. 2015).

TPD: The temperature-programmed desorption of

ammonia was conducted to obtain the strength and con-

centration of acid sites. NH3-TPD profiles of adsorbed

ammonia on the selected samples (Z-AM-CO-1.5 and

Z-NM-CO-1.5) are shown in Fig. 5. These results indicate

that zeolite of Z-AM-CO-1.5 is characterized by three

ammonia desorption peaks; the first at about 120 �C, the

second at about 450 �C and third at about 600 �C. Zeolite

of Z-NM-CO-1.5 also has two peaks of ammonia desorp-

tion at about 100 and 550 �C. The desorption above 300 �Cis related to decomposition of NH4

? resulting from NH3

reacting with strong acid sites (Ates et al. 2011; Rutkowska

et al. 2014; Shen et al. 2012). The low temperature peak

(below 400 �C) has been attributed to NH3 weakly absor-

bed by acid sites and NH3 linked to Na? or extra frame-

work of Al (Ates 2007; Ates and Hardacre 2012;

Rutkowska et al. 2014). Temperature of desorption peak on

zeolite of Z-AM-Co-1.5 was slightly higher than that of

Z-NM-CO-1.5 (600 vs. 550 �C). For this reason, we can

say that the acid sites in Z-ZM-Co-1.5 are slightly stronger

than that in Z-NM-CO-1.5.

Catalytic activities: The catalytic activities of natural

zeolite (Clinoptilolite) modified with 0.6 N HCl and 1.5 N

NaOH and impregnated with cobalt (different loading %)

as well as the parent zeolites were tested for nitrous oxide

(N2O) decomposition. Figure 6a shows that the modified

zeolites with HCl are more active than the parent zeolite

and the base zeolite in this catalytic process. Across these

zeolites including Z-AM, Z- BM and Z-NM, only 39, 34

and 32% N2O reduction occurred at 600 �C. It has been

reported that acid leaching of zeolites increases the

decomposition activity of N2O in comparison with parent

samples because of the formation of isolated iron and Fe–

O–Al species. In addition, the data of Table 2 reveal that

the specific surface area of Z-AM is higher than that of

other zeolites and this can be attributed to the more activity

of Z-AM in N2O conversion. These results are in conso-

nance with results obtained by other researchers (Kapteijn

et al. 1996; Miller et al. 1998). Figure 6b–d shows that

samples impregnated with cobalt are more active than the

parent zeolites in N2O decomposition. The results also

show that increase in conversion efficiency with increasing

amount of cobalt on zeolites was similar to the results of

Shen et al. (2012), Rutkowska et al. (2014). As seen from

Fig. 6d, an increase in cobalt loading on zeolites above

1.5% resulted in a decrease in activities. The reasons for

this phenomenon can be attributed to a smaller specific

surface area of samples containing higher metal loading,

and subsequently blocking the pores and channels of zeo-

lites, leaving only a small amount of cobalt oxides for

reaction and N2O decomposition. These results have also

been corroborated by other studies (Miller et al. 1998; Shen

et al. 2012).

The comparison of catalytic activity of the three zeolites

of Z-AM-CO-1.5, Z-BM-CO-1.5 and Z-NM-CO-1.5

(Fig. 6e) shows that samples modified with HCl had the

best performance in N2O direct decomposition. It has been

Fig. 4 H2-TPR profiles of selected catalysts

Fig. 5 NH3-TPD profiles of selected catalysts

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123

reported that metal loading does not affect its activity in

N2O direct decomposition (Guzman-Vargas et al. 2003)

and that an increase in the metal to aluminum ratio induces

a continuous increase in its activity (Li and Armor 1992;

Turek 1998). Co/Al ratio in Z-Am-Co-1.5 was slightly

more than that of the two other zeolites and it is responsible

for the high activity together with the high surface area

according to Table 1.

Based on the previous studies, several options has been

suggested for removal of N2O from exhaust streams of

artificial sources which include thermal decomposition,

catalytic decomposition, and decomposition by biological

process and purification (Vanisudha et al. 2003; Osvaldo

et al. 2016; Xie et al. 2014). Criteria for choosing the

options are Safety of the treatment, efficiency of the

technology, controllability and Investing and operating

cost. In following, the selected method (catalytic decom-

position) in the present study has been compared with other

methods. Since, the molecular size of N2O and O2 are

similar (N2O 1.6 A and O2 1.2 A), purification method is

very difficult to do and according to the low price of N2O,

this method isn’t cost-benefit in comparison with

Fig. 6 Decomposition of N2O over natural and modified zeolites. Experimental conditions: N2O concentration = 350–400 ppm balanced with

N2; rate flow = 300 cm3/min and space velocity (GHSV) = 4500 cm3/(h g cat)

Int J Environ Res

123

decomposition. Also, N2O decomposition by biological

process for the following reasons is not a feasible or cost-

benefit method in comparison with catalytic decomposi-

tion: (1) decomposition rate of N2O by microbes is low and

its efficiency is around 60–85% (Vanisudha et al. 2003),

while the method used in this study (catalytic decomposi-

tion of nitrous oxide by Zeolites Z-AM-CO-1.5) has high

rate in decomposition and efficiency of 100% at tempera-

ture of 400 �C (Fig. 6). (2) Some cultured microbes need

anaerobic environment for their growth. Absence of oxy-

gen is not possible in this case, as O2 is in the gaseous

mixture of exhaust streams of industries and difficult to be

separated from N2O, while catalytic decomposition of N2O

is feasible with high efficiency in the presence of O2 and

other agents such as CO2, humidity, NO, NO2, CO

(Chengyun Huang et al. 2017; Runhu Zhang et al. 2016;

Dann et al. 1995).

Thermal decomposition of N2O, due to the requirement

of high energy (temperature of 1500–1800 �C for conver-

sion around 90–95% or more), is not a cost-benefit method

in comparison with catalytic decomposition (Vanisudha

et al. 2003). In the present study, catalytic decomposition

of N2O on Zeolites Z-AM-CO-1.5 with efficiency around

90–95% or more is done at temperature 400–450 �C. It can

be concluded that the used method in present study can be

more cost-benefit than thermal decomposition.

Conclusion

In this study, natural zeolite (Clinoptilolite) treatment with

0.6 N HCl, 1.5 N NaOH and modified with cobalt by

impregnation method was used in direct N2O decomposi-

tion. From the experiments conducted for the non-modified

and modified zeolites by ICP, XRD, BET, TPR, TPD and

catalytic activity, it can be concluded that:

1. The applied modifications had no significant influence

(destruction) on the main structure of the zeolites

including clinoptilolite, quartz and cristobalite.

2. The alkaline treatment did not change the properties of

zeolites significantly, whereas acid leaching induced

abrupt increase in surface area and reduction in

average diameter of the pores. This could be an

excellent characteristic in absorption and consequently

in N2O decomposition.

3. Among the zeolites impregnated with cobalt (Z-AM-

CO-1.5, Z-BM-CO-1.5 and Z-NM-CO-1.5), sample

Z-AM-Co-1.5 had the highest CO/Al ratio, which is an

indication that it can be an important parameter in

nitrous oxide conversion in metal-zeolite catalyst. It

also demonstrated that increased cobalt loading is not

an indicator of continuous increase of the process.

4. TPR analysis showed that the main active species in

nitrous oxide decomposition are mono-atomic and

other species exhibit much lower activity.

5. Survey of the acid sites of samples showed that the

acid sites in Z—ZM-Co-1.5 were slightly stronger than

that in Z-NM-CO-1.5.

6. In this work, all samples used as catalyst were active in

N2O decomposition; however, zeolite Z-Am-Co-1.5

had the best performance in the process. This effect

could be attributed to the higher amount of CO/Al and

surface area as well as the strong acid sites of this

zeolite compared to other zeolites.

7. In conclusion, natural clinoptilolite zeolites treated

with acid and impregnated with Cobalt (Z-AM-CO-

1.5) could be a very effective and cost-benefit catalyst

for reducing N2O as a greenhouse gas, due to its very

low price, high chemical stability and high availability.

Acknowledgements The authors gratefully acknowledge the contri-

bution of Tehran University of Medical Sciences, Iran for funding this

research (Grant No. 28427).

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