Spectroscopic IR, EPR, and operando DRIFT insights into surface reaction pathways of selective...

This journal is c the Owner Societies 2012 Phys. Chem. Chem. Phys., 2012, 14, 2203–2215 2203

Cite this: Phys. Chem. Chem. Phys., 2012, 14, 2203–2215

Spectroscopic IR, EPR, and operando DRIFT insights into surface

reaction pathways of selective reduction of NO by propene over

the Co–BEA zeolite

Piotr Pietrzyk,*a Christophe Dujardin,b Kinga Gora-Marek,a Pascal Grangerb and

Zbigniew Sojkaa

Received 25th September 2011, Accepted 8th November 2011

DOI: 10.1039/c1cp23038g

Interaction of a Co–BEA catalyst with individual components (NO, C3H6, CO, O2) and mixtures

simulating the real feed of the selective catalytic reduction (SCR) of nitric oxide in static and pulse

experiments at variable temperatures was investigated by means of IR, EPR, and operando

DRIFT spectroscopy coupled with QMS/GC analysis of the products. Speciation of cobalt active

sites into Co(II), mono- and polynuclear oxo-cobalt species as well as CoO clusters was quantified

by IR using CO and NO as probe molecules. The key intermediates, by-products, and final

products of the SCR reaction were identified and their spectroscopic signatures ascertained.

Based on the spectroscopic operando results, a concise mechanistic scheme of the selective catalytic

reduction of nitric oxide by propene, triggered by a two-electron Co(II)/Co(0) redox couple, was

developed. It consists of a complex network of the sequential/parallel selective reduction steps that

are interlocked by the rival nonselective oxidation of the intermediates and their thermal

decomposition. It has been shown that the SCR process is initiated by the chemoselective capture

of NO from the reaction mixture by the cobalt active sites leading to the cobalt(II) dinitrosyls,

which in the excess of oxygen are partially oxidized to surface nitrates and nitrites. N2O is produced

by semi-decomposition of the dinitrosyl intermediates on the mononuclear centers, whereas NO2 via

NO oxidation on the polynuclear oxo-cobalt sites. Cyanide and isocyanate species, formed together

with propene oxygenates in the course of the CQC bond scission, are the mechanistically pivotal

reaction intermediates of C3H6 interaction with the dinitrosyles and NO3�/NO2

� surface species.

Dinitrogen is produced by three main reaction routes involving oxidation of cyanides by NO/NO2,

reduction of dinitrosyls, nitrates, and nitrites by propene oxygenates (medium temperature range)

or their reduction by carbon monoxide (high temperature range).

I. Introduction

Removal of NOx from exhaust gases by its selective catalytic

reduction (SCR) or direct decomposition (deNOx) over porous

materials functionalized with transition-metal ions (TMI) involves

active centers of specific electron and magnetic structure that are

able to constitute suitable redox couples triggered by electron and

spin transfer processes occurring within the NO–TMI moiety.1,2

Indeed, the chemical reactivity of the coordinated NO is featured

by removal of the unpaired electron from the 2p* antibonding

orbital to form a nitrosonium NO+ cation or addition of an

electron to the 2p* orbital, giving rise to a nitroside anion NO� of

distinctly different reactivity.3 The other pathway leading to the

changes in the valence state of the active sites consists of an

oxygen transfer step associated with the oxidation of NO to

NO2 or mild oxidation of the hydrocarbon reducing agent to

oxygenates by the active sites containing reactive extra-lattice

oxygen (ELO). Obviously the occurrence of both pathways

depends on the nature of the active site, and electron transfer is

characteristic of the bare cations, whereas in the case of the

mono or polynuclear oxo-cation species oxygen transfer plays

a decisive role.4 Pronounced spin redistribution on passing

from nitric oxide to dinitrogen is another noteworthy issue in

the mechanistic studies of NOx abatement, and this constraint

can be surmounted owing to the active sites constituted by

transition-metal ions (TMI).1,3

All those aspects of the catalytic chemistry of NOx can be

investigated using zeolites exchanged with TMI, which are

important model and practical materials extensively studied in

the SCR context.1,2,5–7 Zeolitic catalysts provide relatively

a Faculty of Chemistry, Jagiellonian University, ul. Ingardena 3,30-060 Krakow, Poland. E-mail: [email protected];Fax: +48 12 634-05-15; Tel: +48 12 663-22-24

bUnite de Catalyse et de Chimie du Solide, Universite de Lille1,Sciences et Technologies, UMR CNRS 8181, 59655 Villeneuved’Ascq Cedex, France

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2204 Phys. Chem. Chem. Phys., 2012, 14, 2203–2215 This journal is c the Owner Societies 2012

wide temperature windows for SCR and, depending on the

nature and nuclearity of the dispersed metal ions, high catalytic

activity. In particular, cobalt-exchanged high-silica zeolites,

such as ZSM-5 and BEA, have been recognized as the efficient

catalysts in SCR of NOx with methane7–9 and with larger

hydrocarbons.10–12

The nature of the cobalt active sites is, however, still debating.

In general two types of cobalt centers have been distinguished:

isolated mononuclear cobalt(II) cations located in the exchange-

able positions13–15 and cobalt mono- and polynuclear clusters

bearing extra-lattice oxygen of poorly defined stoichiometry

CoxOy.16–19 To some extent, the concentration of both types

of cobalt centers, i.e., the isolated and clustered ones, can be

controlled by adjusting the zeolite type, its Si/Al ratio, concen-

tration of cobalt ions, and the nature of the solvent used during

the ion exchange process. It has been recognized that isolated

and clustered oxo-centers differ in reactivity toward NOx

abatement. As shown by Smeets et al.,7 isolated Co(II) sites

have been the most active in N2O decomposition, whereas

oxo-forms have been found less active or even inactive.

From the practical point of view, removal of noxious gases

emitted by internal combustion engines operating under O2-rich

conditions (diesel and lean-burn gasoline engines), where the

noble-metal three way catalysts are not effective, is a major

challenge for environmental catalysis. The exhaust gases produced

under such conditions include nitrogen oxides (NOx), unburned

carbon monoxide, hydrocarbons and oxygenates. However,

despite the considerable progress made in the last decade,

development of an efficient and robust catalyst for SCR of

NOx by hydrocarbons (HC-SCR) still remains an open issue,

mainly due to the incomplete understanding of the reaction

mechanism.20 Mechanistic investigations of NOx-SCR over

Co-exchanged zeolites revealed the appearance of –CN and

–NCO species.8,21 They can be directly oxidized to N2 by

NOx8,21 or in an alternative contentious account, hydrolyzed

by water to NH3 and CO2 leading to higher SCR activity.12

Addition of oxygen leads to a significant enhancement of the

SCR efficiency by controlling formation of the highly reactive

oxygenates used as efficient reductants,22 yet this point has not

been elucidated definitely.

The objectives of the present study comprise quantification

of the cobalt cationic and oxo species in the Co–BEA zeolite,

assessment of its reactivity with single and mixed gas reactants,

identification of the reaction intermediates appearing in the

selective catalytic reduction of NO with CO and propene, and

evaluation of their mechanistic role. Combination of static IR

and EPR studies complemented by operando (DRIFT/gas-phase

IR/QMS) measurements was employed to provide a detailed

insight into the chemical nature of both the metal active centers

and the adsorbed species.

II. Experimental

Co–BEA samples were obtained by a standard ion exchange

procedure from the parent ammonium form of the BEA

zeolite with Si/Al = 12 (Tricat Zeolites). The mixture of

0.1 M Co(NO3)2 water solution in proportion 1 g zeolite per

100 cm3 was stirred for 20 h at 323 K, and the final pH of the

solution varied around 5. After drying at 353 K the obtained

samples were characterized by an ICP method, revealing

2.04 wt% of Co and 2.45 wt% of Al content. Adsorption and

reaction under static conditions were monitored with a Bruker

ELEXSYS-E500 X-band EPR spectrometer and a Bruker

Tensor 27 FTIR spectrometer equipped with an MCT detector

and with a spectral resolution of 2 cm�1. Computer simulations of

the EPR spectra were carried out with the EPRsim32 software.23

Prior to adsorption the samples were evacuated in vacuum

(B10�5 Pa) at 773 K (6 K min�1) for 3 h. Before adsorption

the gas reactants, NO and NO2 (Aldrich, 98.5 and 99.5%,

respectively), were purified by the freeze–pump–thaw technique,

whereas O2, CO, and propene (Aldrich, 99.95%) were adsorbed

directly without any prior purification. Adsorption was carried

out at 77 K or at room temperature and the samples were next

exposed to various temperatures.

In the case of the operando DRIFT experiments, Co–BEA

samples (B20 mg) were treated in a helium flow (13 mL min�1)

with the heating rate 6 K min�1 followed by final activation at

673 K for 2 hours. Prior to the measurements, the samples were

cooled down to 423 K. A typical temperature-programmed

surface reaction (TPSR) experiment consisted of a sequence of

pulses of the reactants followed by flow of the gas mixture (30min)

at 423 K and 503 K. After switching off the reactant flux, the

temperature was raised up to 673 K (6 K min�1) and during

that time a TPD-QMS experiment was carried out using a

quadruple mass spectrometer (Balzers Omnistar). The sample

was cooled down again to 423 K and the adsorption sequence

was repeated. For the SCR reaction, the reactant gas mixture

(744 ppm NO, 1388 ppm C3H6, 1388 ppm CO, and 0–3–10%

O2) was passed through the sample, and temperature was

increased with the heating rate of 6 K min�1. In the course

of those experiments, IR spectra of the gas-phase and DRIFT

spectra of the catalyst surface were recorded with 4 cm�1

resolution using a Thermo 380 and a Thermo 460 Protege

instrument, respectively. Both spectrometers were equipped

with MCT detectors.

Additional temperature-programmed catalytic activity

measurements were conducted in a fixed bed flow reactor with

a temperature gradient of 5 K min�1. The same gas mixture

composition as in operando TPSR was used with variable

amounts of oxygen 0–3–10% O2. 360 mg of the catalyst in

powder form was exposed to a total flow rate of 18 L h�1. The

catalyst was dehydrated under He at 673 K prior to the catalytic

measurements. The inlet and outlet gas mixtures were analyzed

using a mGC Varian CP-4900 chromatograph equipped with

two thermal conductivity detectors. The reactants and products

were separated on 5 A molecular sieves and poraplot Q columns.

III. Results and discussion

1. Spectroscopic characterization of cobalt active centers and

their interaction with single SCR reactants

1.1. Speciation of cobalt active sites.The nature and diversity

of the cobalt sites were examined by IR using CO and NO as

probe molecules. The IR spectra recorded upon CO and NO

absorption on the activated Co–BEA sample are shown in Fig. 1.

Adsorption of the CO probe leads to a complex broad IR band

appearing in the range of 2220–2160 cm�1 with three clear

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maxima (Fig. 1A). These signals observed at low temperatures

(150 K) were attributed to the stretching vibrations of the CO

admolecule, interacting with the isolated cobalt(II) sites located

in the exchangeable positions (2206 cm�1) and cobalt(II)-oxo

sites with extra-lattice oxygen (ELO) in the exchangeable

positions (2195 cm�1), and a minor band (2180 cm�1) due to

the intrachannel CoO cluster species, and to CO adsorbed on

Lewis sites (2235 cm�1).18

Room temperature adsorption of the nitric oxide probe

(Fig. 1B) leads to IR bands, the intensities of which vary with

the NO coverage. Already admission of the first doses resulted

in the appearance of three distinct bands. The band located at

1935 cm�1, exhibiting higher intensity at low coverage, is

tentatively attributed to the intrazeolitic mononitrosyl of

cobalt(III)–ELO.18 With the increasing NO doses a new doublet

signal with the components at 1895 and 1815 cm�1 gradually

developed to become dominant. The positions of the doublet

peaks are characteristic of the geminal dinitrosyl CoII(NO)2adduct. Such surface species were previously recognized in

various Co-exchanged zeolites, and their formation is manifested

by the presence of two collective vibrations of the nitrosyl

ligands, giving rise to symmetric (1895 cm�1) and antisymmetric

(1815 cm�1) stretchings.10,18,24

Based on the procedure developed by one of us for the

Co–ZSM-5 and Co–FER zeolites,18 the concentrations of the

Co(II) and Co(II)–ELO ions in the isolated exchangeable

positions and the Co(III)–ELO species were calculated from

the intensities of the 2206 cm�1, 2195 cm�1, and 1935 cm�1

bands, respectively, using the following absorption coefficients:

0.35 cm2 mmol�1, 0.120 cm2 mmol�1, 0.129 cm2 mmol�1,

determined previously.18 The obtained concentrations are equal

to 154 mmol g�1 for the isolated Co(II) centers, 69 mmol g�1 for

the Co(II)–ELO centers, and 90 mmol g�1 for the Co(III)–ELO,

which gives the approximate ratio of Co(II)/Co(II)–ELO/

Co(III)–ELO = 5 : 2 : 3.

The EPR spectrum of the activated Co–BEA catalyst

(Fig. 2A) shows a broad signal with geff equal to 5.4 and 2.0.

The spectrum could only be observed below 30 K. The shape,

g values, and relaxation behavior of the signal indicate that the

bare cobalt(II) centers in the BEA zeolite exhibit a high-spin

state (S = 3/2) in accordance with our recent EPR and DFT

study on cobalt(II) complexes.25 As compared to the hydrated

Co–BEA sample (spectrum not shown) the intensity of the

EPR signal is significantly lower, which results from thermally

induced in situ hydrolysis and formation of the oxo-like

clusters (see below) shown by IR measurements of CO and

NO adsorption. Polynuclear cobalt(II) centers forming the

oxo-clusters are apparently antiferromagnetically coupled

leading to decrease of the intensity of the cobalt(II) EPR signal.

Adsorption of CO at 77 K led to the formation of mono-

carbonyl CoII–CO adducts (as already seen with IR), giving

rise to an EPR signal with the clear hyperfine structure due to

the 59Co nucleus (I = 7/2, 100%), characteristic of a low-spin

(S = 1/2) state of cobalt (Fig. 2B). This finding proves the

divalent state of the cobalt sites definitely. Computer simulation

allowed for distinguishing three different component signals of

the cobalt monocarbonyl adducts with g1xx = 2.234, g1yy =

2.179, g1zz = 2.016, |A1xx| = 4.4, |A1

yy| = 3.4, |A1zz| = 7.6 mT,

g2xx=2.259, g2yy =2.149, g2zz=1.986, |A2xx| = 3.5, |A2

yy| = 1.8,

|A2zz| = 5.8 mT, and g3xx = 2.239, g3yy = 2.155, g3zz = 2.066,

|A3xx| = 4.5, |A3

yy| = 3.7, |A3zz| = 7.6 mT, reflecting speciation

of cobalt into three closely related varieties, usually associated

with a, b, and g sitting sites.6,13 We may thus conclude that

upon CO adsorption the spin-ground state of the cobalt(II)

centers changes from the quartet to the doublet one, due to the

spin cross-over characteristic of the 3d7 electron configuration.

A similar experiment carried out with NO adsorption resulted

in the EPR spectrum (Fig. 2C) attributed to the paramagnetic

dinitrosyl CoII(NO)2 adducts. The following spin-Hamiltonian

parameters of the cobalt dinitrosyls: gxx = 2.081, gyy = 2.195,

Fig. 1 Static IR spectra of (A) CO adsorption at 150 K and (B) NO

adsorption at RT on the Co–BEA sample. For NO adsorption spectra

exhibiting maximum intensity of the 1935 cm�1 (bottom spectrum,

100 mmol NO per 1 g of sample) and 1815 cm�1 (excess of NO in

gas-phase, 10 Torr) peaks are shown.

Fig. 2 X-Band EPR spectra of (A) the activated Co–BEA sample

recorded at 10 K, (B) adsorption of 20 Torr of CO at 77 K, and

(C) adsorption of 5 Torr of NO at 77 K. Black lines represent

experimental spectra; green lines, simulated spectra.

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gzz=2.086, |Axx| = 18.2, |Ayy| = 10.2 and |Azz| = 3.1 mT were

obtained by computer simulation. Formation of this intrazeolitic

complex can be rationalized in terms of a spin pairing process

between the quartet 4CoII(3d7) sites and two spin doublet NO

(2P1/2) radical ligands: CoII (4A2) + 2 NO (2P1/2)-

2Co(NO)2.

The resultant spin density is largely located on the metal center

(r3d E 80%), implying the following magnetic structure of the

dinitrosyl intermediates NO(km)mCoII(mk)NO. The actual

oxidation state of cobalt depends on the localization of both

electron pairs (km) on Co or NO moieties, which is dictated by

the relative position of the 2p* antibonding orbital of NO with

respect to the 3d manifold of Co (vide infra).

1.2. Mononuclear oxo and dinuclear oxo-cobalt sites. Apart

from the isolated cobalt(II) ions in the exchangeable positions

of the BEA zeolite, polynuclear oxo-bridged cobalt species

detected by both EPR and IR low-temperature adsorption

experiments were identified (in agreement with H2-TPR10,26,27

and other adsorption18,28 studies) and quantified. Their molecular

models, obtained by cutting off the fragments from the model of

the BEA zeolite (Materials Studio, Accelrys) and saturating the

resulting dangling bonds with hydrogen atoms, are shown in

Fig. 3. Because of their high charge, the bare CoIII ions cannot be

accommodated in the exchangeable positions of the high-silica

zeolites such as BEA. Thus, they are most probably associated

with the mono- and polynuclear cobalt–ELO species giving rise

to CoxOyn+ (n = 1, 2) entities. Our earlier IR and DFT study29

showed that during dehydration and activation of cobalt-zeolite

samples, the intrazeolitic Co2+(H2O)6 complexes hydrolyze,

producing a hydroxo (CoOH)+aq and a H+ species, counter-

balancing separately the negative charges of the zeolite frame-

work (Si–O–Al)�, according to the Pauling local electroneutrality

principle. They are precursors for the formation of the mono-

nuclear CoO+ and polynuclear oxo-centers detected by low-

temperature CO adsorption (see above). Possible reactions leading

to the formation of dinuclear centers involve subsequent olation

and oxolation steps:

(CoII–OH)+ + CoII–OH2 - (CoII–OH–CoII)3+ + H2O,

(1)

(CoII–OH)+ + (CoII–OH)+ - (CoII–O–CoII)2+ + H2O.

(2)

With the temperature increase, intrazeolite redox processes

may occur to produce mixed-valence cobalt sites, according to

the reaction such as:

2(CoII–OH)+ - (HO–CoIII–O–CoII)2+ + 1/2 H2. (3)

Proton release improves the stabilization of both resultant

cationic species within the zeolite channels:

(HO–CoIII–O–CoII)2+ - (O–CoIII–O–CoII)+ + H+zeol, (4)

where (O–CoIII–O–CoII)+ actually represents the polynuclear

CoxOyn+ (n = 1, 2) entities of undefined structure at present.

Similar steps have been suggested in the literature earlier.16,17

A molecular model of the last complex attached to the zeolite

wall is shown in Fig. 3C. Reactivity of the proposed dimeric

centers toward NO is discussed below.

2. Evaluation of putative partial reaction pathways of the SCR

process

2.1. Interaction with C3H6 and oxygen–carboxylic inter-

mediates. The interaction of the Co–BEA with C3H6 was

monitored after adsorption of propene at 298 K and sub-

sequent reaction with dioxygen at 673 K (Fig. 4). The main

feature in the IR spectrum of propene adsorbed in the

Co–BEA zeolite is a pronounced shift of the CQC stretching

band from the position of 1646 cm�1, typical for the gas-phase

C3H6, or 1630 cm�1, characteristic of propene interacting with

Si–(OH)–Al sites, into the region of 1595–1610 cm�1. This

effect is connected with p-backdonation between cobalt 3d

electrons and carbon–carbon p bond, proving a direct inter-

action between the propene CQC moiety and the cobalt active

centers. After reaction with O2 (Fig. 4B), the bands due to the

coordinated propene disappear, and a new intense band at

1590 cm�1 attributable to cobalt(II) carboxylate species can be

observed, indicating a straightforward catalytic oxidation of

the adsorbed propene by dioxygen to acetates or formates.30–32

The broad, intense band at 1675 cm�1 can be assigned to the

vibrations of the carboxyl groups and deformation vibration

of the water by-product formed upon oxidation of propene.

Its higher energy tail (1700 cm�1) is characteristic of

Fig. 3 Schematic representation of the molecular models of (A) the

isolated CoII site in the BEA zeolite (cluster model of [Si5Al2O8(OH)12]2�

stoichiometry cut-off from the structure of BEA and terminated with H

atoms), (B) (CoIII–O)+ species, and (C) one of the possible dimeric

oxide-like species with a terminal oxo moiety (O–CoIII–O–CoII)+

(cluster model of [Si8Al2O11(OH)17]2� stoichiometry).

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CQO vibration of the aldehydes interacting with cobalt(II)

sites.32 The band at 1380 cm�1 can be assigned to a symmetric

vibration of a COO� group. The IR spectra recorded after

lowering the temperature to 150 K (Fig. 4C) revealed the

presence of the gas-phase products of deep oxidation of C3H6

such as CO2 and CO, interacting with cobalt sites upon

cooling. At the same time subsequent adsorption of NO

revealed a decrease of the intensity of the 1935 cm�1 band and

a parallel increase of the doublet bands at 1895 and 1815 cm�1

(Fig. 1B). This is consistent with the reduction of the CoIII–ELO

centers to the bare cobalt(II) cations in the presence of C3H6/O2

mixture, proving involvement of the ELO species in propene

oxidation in concert with the gas-phase oxygen.

2.2. Reactivity of NO adspecies. To study the stability and

reactivity of the nitric oxide adsorption products in the

circumstances related to the conditions of working catalysts,

preliminary experiments in the flow regime were carried out

with sequential pulse admission of the 1% NO in He. The

DRIFT spectra obtained in the course of the adsorption pulses

Fig. 4 IR spectra recorded after (A) saturation of the Co–BEA with

propene at RT and removing of the excess gas phase, (B, C) after

co-adsorption of O2 (15 Torr) and reaction at 673 K for 20 minutes.

Spectra (A) and (B) recorded at 298 K, spectrum (C) obtained after

reaction with O2 recorded at 150 K.

Fig. 5 Formation, thermal stability, and reactivity of the dinitrosyl complexes in contact with gas-phase NO and O2. (A) Sequence of adsorption

of NO pulses followed by purge with He and static flow with 1% NO/He; (B) changes of the intensity of QMS signals (m/z = 30 and 44) during

TPD; (C) changes of the intensity of the 1925 cm�1, 1889 cm�1, and 1806 cm�1 bands (DRIFT) and the bands integrated over the area of NO, N2O

and NO2 (gas-phase) obtained after 2 pulses of NO and flow of 1% NO/He at 503 K; and (D) IR spectra recorded at RT of the reaction of

cobalt(II) dinitrosyls with O2 (5 Torr) at 300 K for increasing time and their final evolution at 573 K.

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at 423 K are shown in Fig. 5A. After the first pulse the only

band observed at 1932 cm�1 was assigned to the NO–CoIII–ELO

intrazeolite species. The intensity of this signal passed through a

maximum to gradually disappear at the end of the third pulse.

At the same time, the doublet signal of the dinitrosyl species with

the components at 1894 and 1810 cm�1 (identified in static IR

experiments) developed to become dominant. During the next

pulses their intensity decreased moderately, but at the end of the

pulse sequence they were the only remaining bands that are

associated with the adsorbed NO. Upon the helium flush the

band at 1932 cm�1 disappeared, in contrast to the doublet

signal, indicating that only the dinitrosyl species are stable under

such conditions.

Subsequent steady flow of the NO/He mixture led to a sharp

increase of the intensity of the dinitrosyl bands, whereas the

1932 cm�1 band recovered only 10–20% of its initial intensity.

Thermal stability of the cobalt(II) dinitrosyls was assessed in

the subsequent TPD-QMS-IR experiment. The maximum of

the NO desorption peak appeared at 503 K (Fig. 5B), whereas

at 593 K the dinitrosyls disappeared completely. During the

desorption no new bands such as that of cobalt(II) mononitrosyl

were produced at their expense. However, the gas-phase IR

and QMS showed the presence of N2O at 703 K—the product

of semi-decomposition of the dinitrosyls (in contrast to com-

plete decomposition leading to elemental N2), schematically

represented as

CoII(NO)2 - CoII–ELO + N2O. (5)

The sequence of the NO pulses followed by a continuous

flow of NO/He was repeated at the temperature of maximum

NO release (503 K) due to decomposition of the dinitrosyl

species (Fig. 5C). At this temperature, admission of the NO/

He pulses resulted in the formation of the bands characteristic

of the CoII(NO)2 and NO–CoIII–ELO species, with the dinitrosyl

signal being the predominant one. In the case of the

NO–CoIII–ELO band, a slight red-shift from 1932 to 1925 cm�1

was observed upon passing to the higher temperatures. It could

indicate a ‘‘pure’’ temperature effect on band position33 or thermal

activation of the coordinated NO reactant (change in the

metal–nitrosyl distance resulting in loosening of the N–O bond).

To unravel the reactivity of the observed species, the evolution

of the 1925 cm�1 band intensity due to the NO–CoIII–ELO

species was compared with the changes in the gas-phase NO2

band intensity (integrated over the area of 1658–1567 cm�1)

registered after the NO/He flux at 503 K (Fig. 5C). The results

indicate that the production of NO2(g) is related to a decline of

the intrazeolitic NO–CoIII–ELO species, whereas the intensity of

the dinitrosyl bands remains nearly constant with the elapsing

time at this temperature. The integral intensity plots for NO2

and N2O show that these two by-products are apparently

formed with different rates, indicating that the oxidation of

NO(g) to NO2(g) by Co–ELO species is more difficult than the

semi-decomposition of CoII(NO)2 into N2O(g) and ELO.

The reactivity of the dinitrosyl species with dioxygen was

investigated in static IR measurements and the results are

shown in Fig. 5D. The bands of the dinitrosyl CoII(NO)2intermediates decrease, and apart from the bands at 1870 cm�1

and 1750 cm�1 due to gas phase N2O3 and N2O4, respectively,

new IR signals in the 1650–1350 cm�1 region assigned to

NO2� and NO3

� species develop.24 This indicates that the

nitrosyls can be easily oxidized to the surface nitrates and

nitrites upon contact with O2 already at 300 K. They were

stable under the evacuation even at higher temperatures. The

position of these bands strongly depends on the NO2�/NO3

�

structure (speciation into monodentate, bidentate, or bridged

entities), and on the kind of the cation to which they are

bonded. The bands in the region 1660–1560 cm�1 are supposed

to originate form the bidentate nitrates NO3�. The mono-

dentate NO2� species were detected at lower frequency (1528,

1510 cm�1).24 Heating the reaction mixture at 573 K led to

a complete disappearance of the dinitrosyl complexes and N2O3/

N2O4 nitrogen oxides, which was accompanied by a further

development of the IR bands due to NO2�/NO3

� species, with

the nitrites (1510 cm�1) peak being the dominant one.

The above observations show that the cobalt(II) dinitrosyl

adducts can easily be thermally semi-decomposed into nitrous

oxide and cobalt–ELO species, whereas in the presence of

oxygen they can be oxidized to the surface nitrates and nitrites.

The CoxOyn+ centers are the reactive sites towardNO oxidation

to NO2 via an oxygen transfer pathway. We can postulate the

following stoichiometry of the redox processes explaining the

observed cobalt reactivity leading to the formation of NO2:

(O–CoIII–O–CoIII)2+ + NO - (CoII–O–CoII)2+ + NO2,

(6)

or taking into account the well known equilibrium reaction

2NO + 1/2 O2 + 2H+zeol " 2NO+ + H2O,

(O–CoIII–O–CoII)+ + NO+ - (CoII–O–CoII)2+ + NO2.

(7)

The real mechanism depends obviously on the actual structure

of the CoxOyn+ centers. The mononuclear Co–ELO centers

most likely can oxidize NO in a similar way but this process is

expected to be energetically less favorable.

2.3. Interaction of C3H6 with dinitrosyls. The reactivity of

the intermediate dinitrosyl complexes of cobalt(II) was next

tested in the presence of C3H6, and monitored with in situ EPR

measurements. This allows for probing the reaction progress

from the cobalt center view point. The resultant spectra are

shown in Fig. 6. At first, a blank adsorption of propene on the

activated Co–BEA sample was carried out. No new EPR

signal was observed at room temperature (Fig. 6A). In the

next experiment the sample with the preadsorbed nitric oxide

(Fig. 6B) was evacuated at temperatures increasing from 373

to 573 K. The resulting spectrum showed destruction of the

dinitrosyl complex without formation of any new EPR signal

(Fig. 6C). After such treatment the sample was probed sepa-

rately with CO and C3H6, which gave rise to appearance of

EPR signals (Fig. 6D and E) completely different from that

previously observed for the reference cobalt(II) carbonyl

complex (Fig. 2B). Reaction of C3H6 (12 Torr, 443 K) with

the cobalt sites generated upon decomposition of the dinitrosyl

intermediates at 573 K led to a new EPR signal (Fig. 6D)

characterized by well-resolved cobalt hyperfine structure with

gxx = 2.096, gyy = 1.924, gzz = 2.297, |Axx| = 1.2, |Ayy| =

5.2, |Azz| = 9.9 mT. Such EPR parameters are similar to those

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already observed for the well established organometallic

cobalt(0) molecular complexes.34 In the case of the interaction

with CO (Fig. 6E), the spin-Hamiltonian parameters (gxx =

2.105, gyy = 2.199, gzz = 1.973, |Axx| = 4.8, |Ayy| = 12.5,

|Azz| = 3.5 mT) are, likewise, consistent with the cobalt(0)

carbonyl species, providing yet another indication of the

cobalt(0) state due to the interaction with CO upon previous

treatment with NO at elevated temperatures.

In situ reaction of the Co–BEA with a NO and C3H6

mixture (6 and 12 Torr, respectively) was monitored with

EPR in the temperature range of 295–773 K. The SCR

reaction starting at 573 K was perceived by the appearance

of the signal due to the cobalt(0) carbonyls shown in Fig. 6F,

proving that CO is a by-product of propene oxidation by NO.

Subsequent QMS analysis of the gas products evolved in the

course of the reaction revealed the presence of N2 and CO2/CO,

and the absence of NO/NO2, indicating that the SCR process

was accomplished successfully. The same reaction sequence

was observed when the Co–BEA sample was contacted with a

gas mixture containing NO, C3H6 and O2 simultaneously.

Thus, the obtained results showed clearly that under the

SCR conditions cobalt(II) was transformed into the dinitrosyl

complexes by chemo-selective capture of NO from the reaction

feed, which is a key requisite for their reduction to dinitrogen.

Additionally, EPR provided a direct evidence for the inner-

sphere reaction between the dinitrosyls and propene, which is

oxidized to CO/CO2 (even in the absence of oxygen), giving

rise to the diagnostic EPR signal of the Co(0) carbonyls, with

concomitant formation of N2. These experiments reveal

undoubtedly that the CoII/Co0 redox couple is involved in

the interaction of the coordinated NO with propene leading to

N2, COx, and H2O.

2.4. Pulse experiments with C3H6/CO. As discussed above,

the intrazeolite cobalt sites include not only the isolated bare

cobalt(II) ions but also the mono- or dinuclear cobalt–ELO

centers (Fig. 3). The reactivity of such species toward reductants

(C3H6/CO) was examined in a series of pulse TPSR experiments.

The QMS signals of the reactants and products evolved in

the reaction course of the Co–BEA sample with C3H6/CO are

shown in Fig. 7A. During the temperature ramp from 423 to

703 K a new signal due to the total oxidation to CO2 and

enhancement of the CO signal due to the partial oxidation of

propene were observed. Both products result from the oxidation

of propene by ELO species, and this effect was confirmed by a

decrease of the cobalt(III)–ELO concentration revealed by IR

investigations using NO as the probe molecule (the diagnostic

band at 1935 cm�1 due to the mononitrosyl adduct was used to

monitor this process). As demonstrated in the preceding section,

the oxidation of propene leads to the surface carboxylic acids and

aldehydes with the characteristic IR bands at 1675 and 1590 cm�1

Fig. 6 In situ EPR spectra of NO reaction with cobalt sites and its

reduction with C3H6 over the Co–BEA zeolite. Particular spectra recorded

after quenching at 77 K represent (A) state after thermal activation and

adsorption of C3H6, (B) adsorption of 5 Torr NO, (C) thermal decom-

position (573 K at vacuum) of dinitrosyls, (D) adsorption of 12 Torr of

C3H6 after decomposition, (E) adsorption of 10 Torr of CO after decom-

position, and (F) reduction of 5 Torr NO with 12 Torr C3H6 at 573 K.

Black lines represent experimental spectra, green lines, simulated spectra.

Fig. 7 Changes of the (A) intensity of the QMS signals during the

TPSR sequence of pulses and flux of the C3H6/CO mixture recorded

after TPD of NO experiment and (B) gas-phase IR intensity of the

signals corresponding to SCR reaction of Co–BEAwith NO (744 ppm),

C3H6 (1390 ppm), CO (1390 ppm), and O2 (3%) mixture.

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(Fig. 4B). At elevated temperatures such species can easily

undergo decarboxylation leading to the secondary gas-phase

by-products such as dihydrogen and methane observed by QMS

and IR, following the reactions: CH3COOH - CH4 + CO2

and H2CO - H2 + CO (Fig. 7A and B). The literature data

indicate that the decarboxylation rates at 473 K (1.81� 10�8 s�1)

and 573 K (8.17 � 10�8 s�1) are sufficiently high owing to the

low activation energy of 8.1 kcal mole�1 only.35 Both by-products

(H2 and CH4) were detected during the pulse TPSR experiments,

whereas methane was also observed during the reaction with the

SCR mixture composed of NO and C3H6/CO with O2 (Fig. 7B).

In the latter case the absence of H2 and the decrease in CO

concentration are obviously connected with their oxidation by

dioxygen, proving again that both, the gas-phase and ELO

oxygens, are involved in the SCR process.

3. DRIFT operando studies

The SCR catalytic performance of the Co–BEA sample was

examined at various oxygen contents (744 ppm NO, 1388 ppm

C3H6, 1388 ppm CO, and 0–3–10% of O2) in the temperature-

programmed DRIFT operando with IR/QMS detection and

TPSR GC measurements. The TPSR GC curves of NO

conversion, N2/N2O yields, and COx/C3H6 concentrations

during SCR are presented in Fig. 8. The NO conversion to

nitrogen starts above 573 K, whereas N2O formation progres-

sively develops with the increasing temperature reaching

Fig. 8 NO conversions (violet squares), yields of N2 (blue squares) and N2O (red circles) production, and C3H6/CO concentrations during SCR

reaction with 0% O2 (A and B), 3% O2 (C and D), and 10% O2 (E, F).

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the level of 20%. The NO conversion remains incomplete

(65%) at 773 K in the absence of oxygen (Fig. 8A). Addition

of O2 changes dramatically the overall SCR behavior, and

plays

a significant beneficial role for NO conversion. The N2O

production starts atB573 K and upon reaching the maximum

at 673–743 K (depending on the oxygen content) declines

rapidly, which is accompanied by the parallel increase of the

N2 yield (Fig. 8C and E). Inspection of the GC profiles

indicates that conversion into N2O vanishes with depletion

of C3H6 from the reaction mixture. Once N2O and propene

disappeared, formation of N2 was accompanied by dramatic

decline of the CO concentration. The latter was initially

produced by decomposition of the propene oxygenates (see

below), and then upon reaching the maximum content at the

zero propene level, oxidized in two different consecutive

processes inferred from two slopes shown in Fig. 8D and F.

Thus, we may conclude that at the highest temperatures CO is

the main reductant. The lack of C3H6 in the feed triggers

reduction of the surface sites by CO via oxygen transfer

resulting in pronounced CO2 formation, as it can be deduced

from the increased slope of the corresponding concentration

profile and simultaneous sharp decline of the

CO profile (Fig. 8D and F). The amount of oxygen available

during SCR shifts the local maximum of NO conversion to

desired N2 by controlling the concentration of the C3H6

reductor and its oxygenated derivatives. Higher the O2 level

faster the oxidation of C3H6 and enhanced conversion to N2 at

the high temperature stage.

3.1. SCR in the absence of O2. In the case of reduction of

NO with a C3H6/CO mixture without oxygen predominant

formation of N2O is related to the already observed decom-

position of the cobalt(II) dinitrosyl adducts, and production of

ELO species (eqn (5)). The latter are important in the high-

temperature stage of the reaction (above 633 K) where they are

responsible for the oxidation of CO as revealed by the decline

of the carbon monoxide content (Fig. 8B). At the highest

temperatures, above 723 K, formation of N2O by semi-

decomposition of the dinitrosyls is accelerated with respect to

SCR conversion into N2 (Fig. 8A). Formation of N2 is connected

with the interaction of the dinitrosyls with propene and CO. The

concentration of C3H6 drops already at the beginning of the

reaction mostly due to the sorption by the catalyst but the

minimum at 573 K, correlated with the onset of the N2 appear-

ance, is clearly connected with its chemical transformation.

Parallel DRIFT measurements (Fig. 9A) show disappearance

of the dinitrosyl doublet at 703 K and lack of the bands (1675

and 1590 cm�1) associated with the oxidation of propene by

ELO to carboxylic species (Section 2.1), indicating higher

reactivity of the dinitrosyl intermediates with propene in

comparison to the ELO species. Because of the oxygen absence,

oxidation of C3H6 proceeds via direct interaction between the

olefin and the dinitrosyls, as it was also inferred from the

parallel EPR data (Fig. 6). This interaction involves formation

of cyanide intermediates (CN), clearly seen in the DRIFT

spectra (Fig. 9B). The characteristic bands located at 2313

and 2283 cm�1 are attributed to cyanide bound to the Lewis

and Brønsted sites (denoted as L–CN and B–CN),9,12,21

whereas the weaker peak at 2173 cm�1, appearing at the highest

temperature, is responsible for cobalt(II)-bound cyanide species

CoII–CN.8 The broad, intense band centered at 1633 cm�1,

declining rapidly with temperature, was assigned to the defor-

mation vibration of the water by-product. The shoulder in the

1633 cm�1 peak observed in the range 1690–1700 cm�1 can be

attributed to the carbonyl vibration of aldehydes (formaldehyde,

acetic aldehyde), as already observed for the cobalt-exchanged

zeolites (see below).32

Depending on the orientation of the propene molecule,

formation of the cyanide and aldehyde intermediates can be

proposed to occur via two reaction pathways (Scheme 1). The

main intermediate species, cyanides, can be next oxidized by

NO or NO2 (produced on ELO) in a reaction such as NO +

CN- N2 + CO and NO2 + CN - N2 + CO2, proposed by

Bell et al.8 The reported rate constant of these processes

measured from IR data at 723 K for Co–ZSM-5 was equal to

3.7 � 10�7 s�1 ppm�1 and 2.9 � 10�6 s�1 ppm�1, respectively.8

Since in the DRIFT spectra we did not observe any features

attributable to amides or ammonia, we think that the routes of

N2 formation proposed by Bell are mechanistically more likely

than the more intricate pathway involving CN hydrolysis

followed by multiple NH3-SCR steps.36

Fig. 9 DRIFT spectra of adsorbed species on the Co–BEA catalyst

in the flow of NO (744 ppm) + C3H6/CO (1388/1388 ppm). Spectra

recorded in a temperature range of 423 (a) –703 K (f) with temperature

increase as indicated by the arrows.

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Summarizing, in the absence of oxygen the reaction starts

with formation of the key cobalt(II) dinitrosyl intermediates.

They can be semi-decomposed into N2O and ELO or they can

attack the double bond of the C3H6 reductant to produce

aldehyde and cyanide. The latter reduces NO/NO2 to dinitrogen,

whereas the ELO species are depleted by CO at elevated

temperatures restoring the active sites. Methane and hydrogen

indicated in Scheme 1, detected in the TPSR and partial

reaction experiments (Section 2.4), are diagnostic products

for thermal decomposition of CH3CHO and H2CO.

3.2. SCR in the presence of O2.OperandoDRIFT spectra of

Co–BEA after exposure to NO (744 ppm), C3H6 (1390 ppm),

CO (1390 ppm), and 3% O2 mixture (in He balance) were

collected at temperatures increasing from 423 K up to 703 K,

and then the reaction was monitored under the steady-state

conditions. The spectra in the range characteristic of the

nitrosyls and the products of their oxidation, nitrites and

nitrates (2000–1400 cm�1), are shown in Fig. 10A. At the

temperatures up to 540 K, the growing bands at 1807 and

1890 cm�1 of the CoII(NO)2 intermediates were observed.

Under these conditions, the band due to NO–CoIII–ELO was

not detected. At the same time the signals located at 1575, 1500,

and 1464 cm�1, indicating formation of the surface nitrates and

nitrites, were observed. Part of this spectral region was obscured

by the 1633 cm�1 bending band of water, and as expected, its

intensity declined with temperature disappearing at 550 K (see

Section 3.1). At higher temperatures a new, relatively broad,

intense signal centered at 1668 cm�1 developed, and its presence

is associated with the surface oxygenates of propene, such as

acetates and formates (see the reference spectrum in Fig. 4). In

the earlier studies of nitromethane decomposition over

Co–ZSM-5,10,37,38 the band at 1665 cm�1 has been assigned

to the species responsible for catalyst’s deactivation such as

s-triazine and melamine.37 This peak has also been assigned to

oxime intermediates by Beutel et al.39 but involvement of such

odd species leads to an excessive complexity of the postulated

reaction mechanism and is in variance with recent spectroscopic

results.36

In the spectral range of 2150–2350 cm�1 characteristic of

NQCO and CRN stretching vibrations,30 operando DRIFT

spectra exhibited well developed multicomponent bands

shown in Fig. 10B. A distinct peak observed at 2173 cm�1 is

characteristic of cyanide CoII–CN species.8 It has been found

previously that during NO reduction with CH4 they were

the most abundant N-containing intermediates.8 This peak

was associated with the additional bands at the positions 2314

and 2287 cm�1, assigned to the cyanides bound to the Lewis

and Brønsted sites, respectively.9,12,21 The cyanides attached to

the acidic sites appeared to be more stable and less reactive

than those attached to cobalt, since their diagnostic bands

were still present under the steady-state conditions at 703 K

(Fig. 10B). This observation is in agreement with the recent

Scheme 1

Fig. 10 DRIFT spectra in the steady state of adsorbed species on

CoBEA catalyst in the flow of NO (744 ppm), C3H6/CO (1388/1388 ppm),

and O2 (3%). Spectra recorded in a temperature range of 423 K

(a)–703 K (g) as indicated by the arrows.

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kinetic IR studies showing that the consumption rate of the

cyanide intermediates during CH4-SCR follows the order of

k(CoII–CN) 4 k(L–CN) 4 k(B–CN).21 In addition, the rate of

the reaction of CoII–CN with NO2 has been found to be an

order of magnitude higher than the reaction rate of NO with

O2.8 For heavier hydrocarbons, like propene, isocyanate species

were also observed during SCR, and their two characteristic

bands at 2262 cm�1 and 2239 cm�1 can be seen easily in

Fig. 10B. The first one was attributed to isocyanates bound to

the Lewis sites (L–NCO).8,12,37,40 The second one was situated

in the spectral range 2235–2190 cm�1 that can be connected with

the presence of the cobalt-bound isocyanates CoII–NCO.12,37,41,42

The position of this band is very sensitive to the valence state of

the metal, as it has been shown for Cu–ZSM-5 zeolites,40 for

which reduction of the metal site leads to the pronounced (up

to 50 cm�1) blue-shift. Typical positions of the CoII–NCO

band are around 2200 cm�1,12,37 which in comparison to

2235 cm�1 found in this study can indicate reduction of the

cobalt sites in agreement with EPR experiments (see above).

The intensities of the cyanide and isocyanate bands showed

interesting evolution with the temperature ramp. Already at

423 K, at the beginning of the SCR reaction both intermediates,

CoII–CN and L–NCO, appeared. However, as the temperature

increased, the band centered at 2173 cm�1 (CoII–CN) evolved

passing through maximum intensity at 515 K to disappear

completely at 580 K, which is a typical behavior for the reaction

intermediates. At the same time the bands at 2262, 2287,

and 2239 cm�1 diagnostic for surface isocyanates developed,

showing the maximum intensity at about 673 K, and then

slightly declined.

Because stabilization of the –CN and –NCO groups involved

the acid sites of both Lewis and Brønsted nature, the corres-

ponding spectral changes in the region characteristic of –OH

groups were also investigated (Fig. 10C). Initial stages of the

SCR reaction led to the consumption of the OH groups

attached to extra-framework aluminium (3667 cm�1), which

next reacted with the cyanides and isocyanates giving rise to

the L–CN and L–NCO species (compare Fig. 10B). At 700 K

the free acidic Si–OH–Al groups (3599 cm�1) were partially

restored (Fig. 10C). Their appearance was additionally accom-

panied by the acid sites interacting with propene, as indicated

by the strongly red-shifted band centered at 3380 cm�1. The

C–H stretching vibrations of the propene admolecules can be

seen in the range 3000–2855 cm�1 (Fig. 10C). The broad band

located at 3225 cm�1, observed at the lowest temperatures

(i.e. 455 K), was attributed to the HNCO bonded with water

molecules43 inside the channels of the BEA zeolite.

The SCR process in the presence of oxygen is featured by the

simultaneous presence of the cobalt dinitrosyls and the nitrite

and nitrate surface species (Fig. 10A), which are the central

intermediates responsible for the initial oxidation of propene.

As shown in Scheme 2B, this process leads to the scission of

the CQC double bond and formation of CH3COOH and

CH3CHO oxygenates together with surface cyanide and iso-

cyanate species. The latter can be oxidized with O2, NO2, and

NO to N2, N2O, CO2, and CO (Scheme 2C). The oxygenates,

such as acetic or oxalic acids, interacting with the nitrates

and nitrites are the principal intermediates of the medium

temperature range of SCR (Scheme 2D) associated with the

local maximum of N2 yield (Fig. 8C and E). Parallel increase

Scheme 2

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of CO and CO2 observed in Fig. 8D and F can be associated

with the thermal decomposition of oxalic acid produced

according to Scheme 2D. The generated carbon monoxide is

used in the last stage of the SCR reaction as a terminal

reductant depleting residual nitrates, nitrites, and ELO oxygen

from the cobalt sites (Scheme 2E), which is illustrated by the

sudden increase of CO2 production at the expense of CO with

concomitant increase of the N2 yield (Fig. 8). The EPR spectra

shown in Fig. 6 provide direct evidence for CO generation in

the course of SCR and its coordination to the cobalt active

sites. The SCR pathways described above compete with two

non-selective routes (shown in Scheme 2F) of combustion of

the propene oxygenates and their thermal decomposition into

methane, CO, and CO2 fragments (detected by QMS/IR

techniques, Section 2.4).

As a result we may distinguish five principal steps of the

propene NO SCR process. (1) The key step initiating the whole

reaction is the formation of the dinitrosyl complexes by

selective capture of NO from the reaction mixture by divalent

cobalt centers. (2) The next step consists of the formation of

the carbon–nitrogen bond in a form of the CN and NCO

intermediates, and concomitant formation of the propene

oxygenates. (3) The CN and NCO species can be oxidized

by NO, NO2 (produced on polynuclear oxo-cobalt sites) or by

dioxygen to N2. (4) An alternative pathway of N2 formation,

dominant in the medium temperature range, consists of

reduction of the nitrates and nitrites by the oxygenates.

(5) At the highest temperatures reduction of nitrogen oxides,

residual NOx�, and Co–ELO sites involves CO as the principal

reductor.

IV. Conclusions

By means of combined use of IR, EPR, and operando DRIFT

spectroscopies and catalytic tests principal steps of the catalytic

selective reduction of NOx by propene over the Co–BEA

catalyst were explored in detail. Model studies of the inter-

actions of the cobalt active sites with individual reactants and

their mixtures provided spectroscopic signatures for the reaction

intermediates and identification of the main by-products and

final products. Static and operando measurements allowed for

establishing the principal mechanistic events of the SCR process,

triggered by the two-electron Co(II)/Co(0) redox couple. The

key step of the reaction consists of the prompt chemoselective

adsorption of NO on the cobalt(II) sites initiating the catalytic

cycle. In the absence of dioxygen, the dinitrosyl route leads to

the N2O intermediate reduced next by CO to N2. It is paralleled

by the reaction of propene with Co(NO)2 leading to the cyanide

(CN) and aldehyde (CH3CHO, H2CO) intermediates. They

are produced by oxidative insertion of the bound NO to the

CQC double bond. In the presence of dioxygen, the cobalt(II)

dinitrosyls are largely oxidized to the nitrates and nitrites

already in the low temperature range. The next step involves

formation of CN and NCO intermediates, associated with the

production of the propene oxygenates (carboxylic acids and

aldehydes), and occurs in the medium temperature range. In

this catalytic route dinitrogen is produced by direct interaction

of the CN and NCO species with NO and NO2. In parallel,

dinitrogen can also be produced during the reduction of the

nitrates and nitrites by the propene oxygenates. Upon depletion

of propene at the highest temperatures, carbon monoxide

becomes the main reducing agent for the remaining nitrates

and nitrites.

Acknowledgements

P.P. is grateful to the Universite des Sciences et Technologies de

Lille for the invited professorship. Partial financial support

from the statutory funds of Faculty of Chemistry, Jagiellonian

University, is acknowledged. The work was carried out within

the framework of The International Group of Research (GDRI).

Part of the equipment used in this study was purchased thanks to

the financial support of the European Regional Development

Fund in the framework of the Polish Innovation Economy

Operational Program (contract no. POIG.02.01.00-12-023/08).

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Spectroscopic IR, EPR, and operando DRIFT insights into surface reaction pathways of selective...

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