2D-COS of in situ μ-Raman and in situ IR spectra for structure evolution characterisation of NEP-deposited cobalt oxide catalyst during n-nonane combustion

Accepted Manuscript 2D-COS of in situ μ-Raman and in situ IR spectra for structure evolution characterisation of NEP-deposited cobalt oxide catalyst during n-nonane combustion

Damian K. Chlebda, Przemysław J. Jodłowski, Roman J. Jędrzejczyk, Joanna Łojewska PII: DOI: Reference:

S1386-1425(17)30469-9 doi: 10.1016/j.saa.2017.06.009 SAA 15225

To appear in:

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy

Received date: Revised date: Accepted date:

30 March 2017 1 June 2017 5 June 2017

Please cite this article as: Damian K. Chlebda, Przemysław J. Jodłowski, Roman J. Jędrzejczyk, Joanna Łojewska , 2D-COS of in situ μ-Raman and in situ IR spectra for structure evolution characterisation of NEP-deposited cobalt oxide catalyst during n-nonane combustion, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2017), doi: 10.1016/j.saa.2017.06.009

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ACCEPTED MANUSCRIPT

2D-COS of in situ µ-Raman and in situ IR spectra for structure evolution characterisation of NEP-deposited cobalt oxide catalyst during n-nonane combustion

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Jagiellonian University, Faculty of Chemistry, Ingardena 3, 30-060 Kraków, Poland Faculty of Chemical Engineering and Technology, Cracow University of Technology,

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Damian K. Chlebda1*, Przemysław J. Jodłowski2, Roman J. Jędrzejczyk3, Joanna Łojewska1

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Warszawska 24, 31-155 Kraków, Poland

Malopolska Centre of Biotechnology, Jagiellonian University, Gronostajowa 7A, 30-387

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Kraków, Poland

*corresponding author: [email protected]

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Keywords: n-nonane combustion, in situ spectroscopy, µ-Raman, FTIR, 2D-COS

Abstract

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New catalytic systems are still in development to meet the challenge of regulations concerning the emission of volatile organic compounds (VOCs). This is because such

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compounds have a significant impact on air quality and some of them are toxic to the environment and human beings. The catalytic combustion process of VOCs over non-noble metal catalysts is of great interest to researchers. The high conversion parameters and cost effective preparation makes them a valuable alternative to monoliths and noble metal catalysts. In this study, the cobalt catalyst was prepared by non-equilibrium plasma deposition of organic precursor on calcined kanthal steel. Thus prepared, cobalt oxide based microstructural short-channel reactors were tested for n-nonane combustion and the catalyst

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ACCEPTED MANUSCRIPT surfaces were examined by in situ µ-Raman spectroscopy and in situ infrared spectroscopy. The spectra collected at various temperatures were used in generalised two-dimensional correlation analysis to establish the sequential order of spectral intensity changes and correlate the simultaneous changes in bands selectively coupled by different interaction mechanisms. The 2D synchronous and asynchronous contour maps were proved to be a valuable extension

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to the standard analysis of the temperature dependent 1D spectra.

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1. Introduction

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Over the years, the catalytic combustion of hydrocarbons over supported catalysts has been

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under vigorous investigation [1–6]. The majority of catalysts are dedicated to the combustion of methane to carbon dioxide, while information about VOC abatement is scarce. The VOC

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emissions occur in all branches of industry, from the automotive sector to the chemical industry. VOCs are emitted by the energy sector in particular, especially in processes of

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converting flammable biomass/biogas to energy [7]. Depending on the technology used, the effluent gases containing

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energy production in biogas-fuelled turbines produces

hydrocarbons, nitrogen oxides, carbon monoxide, sulphur and halogen compounds [8]. The

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hydrocarbons present in effluent gases from biogas-fuelled engines are in the form of gases or vapours. The complexity of the exhaust gases requires high-throughput abatement systems,

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which would reduce the contaminants to the required level [9]. Of the broad range of technological methods of diluted effluent gas abatement, catalytic combustion seems to be the most efficient and economically justified. Cleaning systems currently used in industry can be successfully applied for biogas exhaust abatement. Ceramic monoliths employing noble metals for the active material are the most commonly used, due to their low flow resistance properties and high activity at moderate temperatures. Examples of the application of monoliths can be found elsewhere [10–12].

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ACCEPTED MANUSCRIPT An alternative to the monolithic systems could be the application of so-called short channel structured reactors based on transition metal oxides [11,12]. These reactors have great properties because they work in the developing laminar flow region, considerably enhancing heat and mass transport properties [12,13]. However, correct optimisation of the active material used is also required if the reactor carrier is to be fully optimised.

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Nowadays, the use of in situ spectroscopic methods as a tool for the description of

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catalyst surfaces during the reaction is unarguably standard in catalyst evaluation [14,15].

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Fundamental knowledge on the relationship between the catalyst surface and its activity can be obtained using the advanced in situ and operando methods. Through the application of the

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probe molecules, the information gathered by using even the classical bulk sensitive methods is limited to the surface. These methods most often include a combination of spectroscopic

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techniques [16,17] such as FTIR, UV-Vis and Raman, which provide complementary information on the reaction intermediates and the catalyst structure, respectively. Simple

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spectroscopic analysis is commonly presented in the form of one-dimensional spectra, by

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plotting intensity/counts vs. wavenumbers. The complex systems exposed to the influence of the external perturbations that give hard to discern spectroscopic patterns may be analysed by

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the application of two-dimensional correlation spectroscopy (2D-COS) [18,19]. Canonical spectroscopic representation by 1D graphs provides general information about the system,

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while the 2D correlation contour maps provide a more detailed view of peak shapes and positions [18]. Moreover, the application of Noda`s rules for the asynchronous spectrum provides information about the sequential order of band changes during analysis [18,20]. In this study, in situ homospectral two-dimensional spectroscopy was used to evaluate catalytic combustion of n-nonane over a plasma-deposited cobalt oxide structured catalyst. This step allows the intermediates created on the catalyst surface during the reaction to be followed. Two conditions were applied, oxygen-less for investigating the desorption of

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ACCEPTED MANUSCRIPT n-nonane, and oxygen-rich for investigating the combustion process. The results were analysed in order to determine the reaction mechanism.

2. Experimental Cobalt oxide catalysts were prepared by non-equilibrium plasma deposition [6,21].

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Cyclopentadienyldicarbonyl-cobalt(I) (Stream Chemicals, Inc.) at 9 std. cm3min-1 flow rates

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was used as a precursor for the cobalt spinel catalyst. The deposition was carried out in a parallel-plate plasma reactor (13.56 MHz) using a total pressure of 45 Pa and a glow

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discharge power of 40 W, with an the argon flow as the carrier gas.

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The catalyst layers were deposited over precalcined kanthal steel (FeCr alloy) oxidised at 1100 °C for 24 h to obtain a thin alumina layer at the support surface. The

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catalysts were characterised by in situ µ-Raman spectroscopy. Raman spectra were collected using a µ-Raman confocal microscope (LabRAM HR, Horiba Jobinn Yvonne, France) based

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on Czerny-Turner's monochromator and equipped with a deeply depleted thermoelectrically

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cooled CCD array detector. During data collection, a long working distance objective of 50× was used. The sample was placed in a sample holder of a cold-wall CVD micro-reactor

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designed for studying catalytic reactions at high temperature and pressure (CCR1000, Linkam Scientific Instruments, fitted with quartz windows and accompanied with gas inlets and

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outlets for reaction gas flow). Raman measurements were taken using a visible 514.5-nm argon ion (Ar+) laser. The laser power that reached the samples during the measurements was around 1.0 mW. The Raman spectra of the pre-oxidised samples were collected after the calcination at 450 °C in an 20%O2/He flow (Air Products, calibration gas) of 25 cm3min-1 for 30 min and further dehydration of the samples at 110 °C in an He flow (Air Products, 5.2) of 25 cm3min-1 for 30 min. The spectra were taken during the temperature programmed reaction in conditions as described below for in situ IR experiments.

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ACCEPTED MANUSCRIPT The infrared spectra were collected using a Thermo Nicolet 5700 FTIR spectrometer equipped with a nitrogen cooled mercury-cadmium-telluride (MCT) detector and KBr optics. Additionally, the diffuse reflectance accessory (Praying Mantis, Harrick) with a hightemperature chamber was used to carry out in situ measurements. The instrument was controlled by OMNIC software (Thermo Nicolet Analytical Instruments, Madison, WI). The

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spectra were registered in the spectral range of 4000–650 cm−1, averaging 32 scans at a

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resolution of 4 cm−1. The presented spectra within this paper are IR difference spectra

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obtained by subtraction of the dehydrated spectrum (registered at 110 °C, before reaction) from the spectra taken during reaction).

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The in situ experiments in static and dynamic modes were performed using 0.002% n-C9/ He or 0.002% n-C9/ 20% O2 (balanced with helium) mixture (total flow rate of 25

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cm3min-1). After achieving stationary conditions in the reaction chamber (after 5 min), the spectrum was measured and then the temperature was increased to 450 °C by 50 °C steps at

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a heating rate of 2 °C/min. The procedure was similar for the investigation of formic acid

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adsorption, during which a mixture of 0.2%HCOOH/He was used in a total flow rate of 25 cm3min-1. The adsorption of CO probe molecules was performed with a 1%CO/He mixture

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(Air Products).

UV-Vis diffuse reflectance spectra (DRS) spectra were collected using AvaSpec-

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ULS3648 High-resolution spectrometer equipped with a Praying Mantis High-Temperature Reaction Chamber (Harrick Scientific Co., Ossining, NY) and a high-temperature reflection probe (FCR-7UV400-2-ME-HTX, 400 µm fibers). As a light source the AvaLight-D(H)-S Deuterium-Halogen Light Source was used. The spectra were registered in the frequency range 250-800 nm. The spectrum of each catalyst sample in particular temperature was the result of 32 co-added. The spectra were registered with the same procedure as for dynamic is situ FTIR measurements and converted to the Kubelka–Munk function.

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ACCEPTED MANUSCRIPT Before 2D correlation analysis, the differential IR spectra were pre-treated in terms of baseline correction by using multiplicative scatter correction methods in CHEMOFACE software [22]. The 2D correlation spectroscopy was performed using 2Dshige (c) software (Shigeaki Morita, Kwansei-Gakuin University, Nishinomiya, Japan). The reference spectrum used for calculation was obtained as the average of temperature dependent spectra for each

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particular sample and experiment.

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3. Results and discussion

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3.1 Investigation of surface structure by Raman spectroscopy

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The evolution of the deposited cobalt oxide film on a kanthal steel support upon increased temperature was examined by µ-Raman spectroscopy. The observed region (Figure 1)

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between 100 and 1000 cm−1 is characteristic for cobalt oxide structures [23,24]. In the process of high-temperature treatments during n-nonane combustion, the structure of this oxide

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exhibits small changes that are represented at a lower intensity, and shifting and broadening

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band characteristics can be observed. In the temperature range of 40-450 °C, five different Co3O4 bands can be identified on the Raman spectra, at 194, 470-482, 513-520, 610-620 and

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673-691 cm−1. These correspond to the F2g (194 cm−1), Eg (470-482 cm−1), F2g (513-520 cm−1), F2g (610-620 cm−1) and A1g (673-691 cm−1) vibrational modes of the crystalline phase

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[24,25]. According to Vuurman et. al. [26], the additional weak and very broad Raman band near 598 cm−1 can be assigned to the dispersed (amorphous) cobalt oxide species. The analysed catalyst seemed to be fairly stable within the temperature range used, but the observed changes may suggest the appearance of an additional phase during the catalyst process of n-nonane combustion. It is worth mentioning that the kanthal steel support used for the metal promoter may influence the small changes observed in the spectra. The preparation step that employs calcination at high temperature (1000 °C) leads to the creation of a thin

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ACCEPTED MANUSCRIPT layer of α-Al2O3 [12]. During the reaction, the interaction between this layer and cobalt promotor may lead to the creation of cobalt-alumina structures. Both synchronous and asynchronous 2D Raman correlation spectra are shown in Figure 2A-D, and clearly confirm the presence of peaks at 194, 482, 520, 598, 620 and 685 cm−1. These contour map representations indicate areas of positive correlations with shades of

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red, and negative correlations with blues. There is one very strong auto-peak on the diagonal

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of the synchronous spectra at Φ(685, 685) > 0, and others of lower intensity at previously

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mentioned frequencies. The strong auto-peaks are, according to the Noda theory [18], supposed to appear in a situation when any band within the analysed spectral region exposed

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to external perturbation changes in its intensity to a great extent. Cross-peaks are located at corresponding off-diagonal positions. According to the theory of generalised two-dimensional

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correlation spectroscopy, the results reveal that the band at 685 cm-1 is very sensitive to increased temperature, and every analysed band varied together in the same direction. The

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absence of the cross-peaks in the asynchronous spectrum suggests that the components of the

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analysed peaks decreased in phase as the temperature increased. The 2D Raman spectra gave similar results to those obtained from 1D spectra, confirming that, during the reaction, the

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structure of catalyst changes slightly at higher temperature. The results are in good agreement with the experiments both with and without oxygen in the reaction conditions.

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The different experiment conditions allow the temperature-oxidative response of catalyst surfaces, to be checked carefully. The in situ measurement conditions with controlled gas atmosphere avoid the oxidation of the surface by air. The registered spectra describe how the catalyst surface transforms upon changing conditions of methane combustion or desorption. As shown in the literature, increasing temperature leads to the progressive reduction of cobalt under the reaction conditions. The spectra obtained under oxidative and non-oxidative conditions, and corresponding 2D correlation spectra, show a slight broadening of the bands,

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ACCEPTED MANUSCRIPT especially the band around 520 cm-1 (Figure 1 and 2). This may suggest the creation of a cobalt-alumina spinel structure, thus a thick layer of Al2O3 is present on a surface of the kanthal steel as a result of the high temperature calcination process but it can be excluded in our samples according to the results presented by [27]. The formation of the mixed spinel oxide, is prevented by the corundum type α-Al2O3. However, when comparing the two

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correlation spectra for different environments, the differences in the responses to temperature

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are clear. Figure 2C-D suggests that the registered spectra react strongly to temperature, and

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band broadening is more emphasised than for oxygen-less conditions (Figure 1A-B). This may suggest the creation of other phases. To follow the oxidation state of cobalt DRS UV-Vis

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was employed. The results are presented in Figure 3. In situ UV-Vis spectroscopy probes electronic structures and allows for observation of cobalt oxidation states (both fully oxidized

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and reduced). Concerning the dehydrated catalysts the spectra indicate absorption bands at ~700, 530 and 400 nm due to the formation of Co3O4. During the reaction progress the

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difference spectra show three band that become more intense at higher temperatures (bands at

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620, 580, 540 nm). Such bands correspond to the presence of Co2+ ions in the tetrahedral coordination, that may suggest the partial reduction of catalysts at higher temperatures.

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However the Co3O4 is dominant at all temperatures. The TPD results presented by Gao et al. [28] show that Co3O4 undergoes a two-step reduction (Co3O4 → CoO → Co0). Due to the

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high signal from the cobalt spinel, the signals from CoO may be masked (Co0 shows no Raman bands).

3.2 Investigation of surface species by in situ DRIFT spectroscopy The prepared samples of catalytic materials were tested at various temperatures in the catalytic combustion of n-nonane, using DRIFT spectroscopy. The in situ DRIFT spectra were acquired at temperatures in the range of 40-450 °C. Figure 4 presents the differential DRIFT spectra of the Co3O4 plasma-deposited catalyst. It can be noted that the absorption 8

ACCEPTED MANUSCRIPT bands appear in three different spectral ranges. The first range includes four bands of different spectral intensity, at 2971, 2934, 2864, and 2850 cm-1. At higher temperatures, those four bands overlap greatly. Those located at 2971 and 2864 cm−1 can be assigned to the asymmetric and symmetric stretching bands of the methyl group, respectively, while bands at 2934 and 2850 cm-1 are specific for the methylene group. The next region can be described by

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absorption bands that are characteristic of the C=O stretching mode (bands in the spectral

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range of 2300–2400 cm-1) that have their origin in the gaseous carbon dioxide molecules

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created [29]. As shown in Figure 4, the intensity of this band increases simultaneously with the temperature. However, in the Figure 4B the spectrum at 100 °C has higher bands in this

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region in comparison to sequent registered at higher temperatures, it can be due to registering the spectrum not into stationary state or some atmospheric carbon dioxide leak into

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measurement accessory. Notwithstanding, the increasing intensity tendency is preserved for the rest spectra. The third spectral region refers mainly to the intermediate species of adsorbed

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carbonate or other species, that appear during combustion. The bands associated with n-

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nonane molecules are also present at 1442 and 1394 cm-1, and correspond to the vibration of the methylene group. The broad band with the maximum intensity at 1442 cm-1 can be

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assigned to the -CH2 bending vibration of the molecule chain (and the hidden band at a lower frequency is the asymmetric -CH3 bending band). The low intensity shoulder around 1380

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cm−1 is a symmetrical -CH3 bending band. Experiments using temperature-resolved DRIFT spectroscopy were carried out in order to establish the evolution of surface species under the reaction conditions. The spectrum collected at 100 °C is not complex and does not indicate the progress of the reaction (see Figure 4). The changes of the band at 1442 cm-1 are interesting because this band can be noticed on the spectra (see Figure 5), up to a temperature of 300 °C for oxygen-less conditions but only 250 °C in an oxygen rich environment. This is caused by the availability of oxygen

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ACCEPTED MANUSCRIPT as well as the process speed; if there is available oxygen, combustion is more likely to occur. Nevertheless, while the products of the oxidation process are CO2, CO and water molecules [30], different intermediates or non-complete combustion products may be present. The products of n-nonane oxidation were investigated by Rotavera et al. [31], who found that CO2, CO, H2O, CH4, CH2O, C2H4, and C3H6 were the most abundant. With increasing

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temperature new bands start to appear on both spectra for oxygen-less and oxygen-rich

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conditions. The weak and broad bands in the range of 2300-2000 and 1900-1800 cm-1 indicate carbon monoxide adsorbed onto the reduced Co catalyst support. The bands in the range of

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2200-2000 cm-1 are due to ν(CO) in Coδ+–CO and ν(CO) in Co0–CO [29,32] while the bands

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at lower frequency reveal the presence of CO adsorbed in a bridging form [29]. The spectral intensity of the described bands increases at higher temperatures, but the major change occurs

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for those shifted to a higher wavenumber. This suggests that more carbon monoxide molecules are connected with the surface active site as terminal groups. The evolution of

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bands above 150 °C reveals the existence of several bands at 1510, 1475, 1370, 1330, 1240,

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1194, 1335, 1075 and 1045 cm-1. After the disappearance of characteristic bands of n-nonane within the analysed range (1000-1800 cm-1), bands at 1730, 1610 and 1410 cm-1 start

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increasing. Temperature dependent spectra show that the bands at 1730, 1610, and 1330 cm−1 are developed synchronously as the reaction temperature increases. This suggests that those

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three bands arise from the same species that are created on the surface of the catalyst. A similar change is observed for gold catalysts on zeolites, as reported by Mohamed et al. [33]. The assignments of the bands at 1730 cm-1 to C=O stretching vibrations, at 1610 cm-1 (OCO) to symmetric stretching vibrations, and at 1330 cm-1 to (OCO) asymmetric stretching vibrations, indicate the presence of carboxylate species (acetates and formates). The geometric conformation of formates may be determined by the approach that uses the value of frequency separation � ν between the two highest ν(CO) bands [34]. The difference between

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ACCEPTED MANUSCRIPT the resulting structures can be ordered as monodentate formate > free formate > bidentate formate. The difference of the frequency separation that equals 280 cm −1 indicates the monodentate formate species [34]. Another band that can be recognised on the spectra is at 1370 cm-1 and can be associated with residual formate species [35]. The carbonate species have been identified by broad bands appearing at 1510 (weak) and 1410 cm−1 [35]. Also, the

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very poorly developed band over the cobalt catalyst at 1475 cm-1 suggests the presence of

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carbonate (CO3-) ions characteristic of adsorbed CO2 species [36]. These data can be taken as

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confirmation of the n-nonane dimerisation–cracking mechanism leading to low-carbon compounds (by the observed intermediate species) and their further oxidation to COx.

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In order to confirm the band assignment to the formate species, the adsorption of formic acid on catalyst surfaces was investigated. The registered temperature-dependent

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spectra after HCOOH adsorption are presented in Figure 6. Under both oxidative and helium flow conditions, similar bands are present. For oxidative conditions, the adsorption bands

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disappear at a lower temperature. The bands at 1730, 1370 and 1075 cm-1 are consistent with

with the catalyst surface.

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the discussion above, and describe different vibrations of the formate species that interact

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To prove the results and the above discussion, and to describe the dynamic of the changes during the process, two-dimensional correlation analysis was performed. This

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analysis helps to identify significant bands or spectral regions that are closely correlated, and to monitor the reaction process. Figure 6 shows the contour map representations of twodimensional IR correlation spectra generated from the temperature-dependent IR spectra of nnonane desorption and combustion over a cobalt-based catalyst. The maps constructed indicate areas of positive correlations with red shades, and negative correlations with blues. The synchronous maps (Figure 7A and C) for both oxygen-less and oxygen rich conditions are similar and indicate auto-peaks at Φ(1610, 1610) > 0, Φ(1410, 1410) > 0, Φ(1330, 1330) >

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ACCEPTED MANUSCRIPT 0, Φ(1135, 1135) > 0, Φ(1075, 1075) > 0, and Φ(1045, 1045) > 0. The peaks describe the simultaneous increase of spectral intensity as a response to increasing temperature. The corresponding positive cross-peaks were observed. The asynchronous spectrum (Figure 7B and D) provides better resolution than the synchronous spectrum for the observed peaks. The positive cross peaks at asynchronous maps occur at: Ψ(1570, 1355) > 0, Ψ(1570, 1130) > 0,

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Ψ(1570, 1075) > 0, Ψ(1565, 1075) > 0, Ψ(1570, 1045) > 0, Ψ(1560, 1045) > 0, Ψ(1440, 1350)

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> 0, Ψ(1440, 1265) > 0, Ψ(1440, 1130) > 0, Ψ(1440, 1075) > 0, Ψ(1440, 1050) > 0, Ψ(1440, 1010) > 0, Ψ(1340, 1263) > 0, Ψ(1140, 1126) > 0, and Ψ(1050, 1030) > 0, and the negative

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peaks at Ψ(1730, 1015) < 0, Ψ(1730, 1050) < 0, Ψ(1730, 1340) < 0, Ψ(1730, 1440) < 0,

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Ψ(1610, 1440) < 0, Ψ(1510, 1440) < 0, Ψ(1510, 1570) < 0, Ψ(1485, 1440) < 0, Ψ(1485, 1570) < 0, Ψ(1270, 1175) < 0, Ψ(1250, 1015) < 0, Ψ(1250, 1045) < 0, Ψ(1255, 1055) < 0, Ψ(1270,

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1015) < 0, Ψ(1270, 1015) < 0, Ψ(1270, 1050) < 0, Ψ(1110, 1050) < 0, Ψ(1130, 1015) < 0, and Ψ(1130, 1050) < 0. The rest of the peaks with opposite sign are located on the other side of

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the diagonal. All peaks listed above are most likely due to the different behaviour of the bands

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that correspond to the intermediate species, mostly of formate or carbonate origin. The asynchronous spectra reveal new bands that are not easily visible on the standard 1D spectra,

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at 1340, 1270 and 1250 cm-1. Those bands can refer to the various surface structure conformations created during the interaction of intermediate molecules with the catalyst

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surface. Notwithstanding this fact, the low spectral intensity of those bands suggests that the created connections are not preferable during this reaction. Based on the Noda theory [18], analysis of the band sign that appears on the synchronous and asynchronous maps leads to a conclusion about the sequential order of spectral intensity changes, and the relationship between them. Considering pairs of bands, it is possible to determine if changes occur synchronously or out of phase. For example, by introducing the Noda rules to the Ψ(1440, 1510) bands, it can be determined that the positive sign above the diagonal indicates that the

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ACCEPTED MANUSCRIPT bands at 1440 cm-1 and 1510 cm-1 varied out of phase with each other, and that the band at 1440 cm-1

changed prior to the band 1510 cm-1. The band characteristic of n-nonane

molecules disappear during the reaction, and the bands that characterise the interaction between one of the intermediate products of catalytic combustion (formate species) and the surface starts increasing in intensity. Similar can be said of other bands. The sequential order

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of spectral intensity changes was established from the signs of the 2D correlation cross peaks:

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1440 → 1570 → (1610, 1510, 1475, 1410, 1370) → (1730,1330) → 1250 cm-1

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In the detected sequence of events, the symbol "→" indicates that the given band v 1 varies earlier than following one v2. The calculated order is similar for both tested environments.

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The results show a continuous decrease in intensity of the bands at 1440 cm−1. This bands react at first at a lower temperature, and then, as a result, the band at 1570 cm-1 also starts

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decreasing in intensity. Then, the bands that indicate the intermediates species appear. The bands at 1730, 1330 and 1250 cm-1 starts at very higher temperatures. Thus the determined

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sequence is adequate for both tested conditions, and it is more probable that reaction occurs

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even when no external oxygen is available. The first band that starts changing corresponds to the n-nonane molecule adsorbed on a catalyst surface (the band at 1442 cm-1 that describes the

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-CH2 bending vibration of the molecule chain). Increased temperature initiates the combustion reaction and the intermediates start appearing. The band at 1570 cm-1 can be assigned to the

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deformation vibration of hydrocarbon molecules δ(C-H). After this, the other bands at 1610, 1510, 1475, 1410 cm-1 react simultaneously. The last three bands correspond to the adsorbed carbon monoxide molecules and their interaction with the surface. The band at 1475 cm-1 describes the presence of free carbonate species [37–39]. The low intensity bands that start appearing may suggest the low concentration of species connected with carbonate molecules after n-nonane desorption. The band at 1610 cm-1 indicates the presence of the formate species. Furthermore, more formate species appear as the weak band at 1370 cm-1 at higher

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ACCEPTED MANUSCRIPT temperatures, as can be noticed on the spectra. The bands that react at higher temperatures (1730, 1330 and 1250 cm-1) correspond to the conformation of various formate species with support [40]. However, the band at 1250 cm-1 may also suggest the presence of monodentate carbonate species that are produced at higher temperatures. This suggests the presence at lower temperatures of bidentate species, which, with rising temperature, change to less

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bonded monodentates and finally leave the surface. The results described above suggest the

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presence of at least two types of active sites.

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The CO molecule was used as a selective probe molecule in order to obtain more detailed information on the chemical properties of the cobalt active centres. The DRIFT

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spectra in the 2000-2300 cm-1 spectral region were registered for analysed catalyst samples pre-treated at various temperatures after CO sorption at 30 oC, as shown in Figure 8. The

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spectrum after CO sorption reveals two bands observed at 2110 and 2174 cm-1, which is obviously due to the CO species adsorbed on the cobalt-based surface. Observations suggest

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that two types of cobalt ion site in the catalyst are responsible for CO adsorption at the initial

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stage. There are no peaks in this region for the sample at 50 oC and above, which suggests weak adsorption of CO on the surface. This means that the desorbed bands might be caused

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by weakly bound species. According to the literature, the two bands at 2110 and 2174 cm-1 can be assigned to dicarbonyl species (symmetric νCO and asymmetric νCO vibration) [41].

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After the material was heated, the intensities disappeared at 50 oC, which may be due to the reaction with the weakly bound active oxygen on Co3O4 [42] or lattice oxygen from the oxide. Based on the obtained results the more probable mechanism of this reaction is in accordance with the Mars-–van Krevelen rather than Langmuir–Hinshelwood mechanism. As shown in the literature, many reactions with the participation of oxides proceed with Marsvan Krevelen (MvK) mechanism [43]. In the foundation of this mechanism, the oxygen species introduced in the substrate come from the lattice, while the mobility of oxygen is

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ACCEPTED MANUSCRIPT associated with the reducibility of the catalyst [44]. Thus, in the oxygen-less environment, the result obtained during the desorption process is almost identical, and taking into account the behaviour of the n-nonane molecules and intermediates that appear on the surface the MvK mechanism seems to be more appropriate. However, this requires further investigation, for

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literature authors remain divided in their assessments of the correct reaction mechanism [44].

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4. Conclusions

The use of DRIFT and µ-Raman spectroscopy combined with two-dimensional correlation

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analysis represents a powerful tool for interpretation in catalytic research. The presented study

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demonstrates the use of 2D correlation spectroscopy as an extension for standard characterisation of Raman and DRIFT spectra. The µ-Raman study suggests that a cobalt

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layer deposited on kanthal steel by low-temperature plasma is fully covered with a Co3O4 spinel.

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The DRIFT and 2D-COS spectroscopic analysis revealed that carbonates and carboxylate

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species (acetates and formates) are the main intermediates of the catalytic combustion of nnonane. C9 molecules crack, producing methane, ethylene and other low-carbon

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hydrocarbons, which also undergo reactions leading to the formation of observed acetates and formates. These species could be mainly involved in the formation of CO and CO2 by simple

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decomposition. The CO sorption over the catalyst surface proved the existence of two types of active sites that may be responsible for the reaction path though carbonate and formate species. Moreover, the application of Noda`s rules revealed the sequential order of the combustion intermediates at the catalyst surface. The static (oxygen-less) in situ experiments show that the Mars-van Krevelen mechanism may describe the catalytic combustion of nnonane under oxygen-less conditions.

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ACCEPTED MANUSCRIPT 5. Acknowledgments Financial support for this work was provided by the Polish National Science Centre – project no. 2015/19/N/ST8/00181 and partially within The National Centre for Research and

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Development project no. LIDER/204/L-6/14/NCBR/2015.

The authors would like also acknowledge prof. Jacek Tyczkowski (Technical University of

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Lodz, Faculty of Process and Environmental Engineering, Lodz, Poland) for catalysts

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preparation.

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Figures and tables:

Figure 1. Evolution with temperature of the in situ µ-Raman spectra over Co3O4 structured

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catalyst during n-nonane catalytic combustion: (A) helium flow (after adsorption step using 0.002% n-C9/ He at total flow of 25 cm3min-1) evolution with temperature; (B) evolution with

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temperature during reaction (0.002% n-C9/ 20% O2 balanced with helium; total flow of 25

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cm3min-1)

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Figure 2. Synchronous (A) and asynchronous (B) Raman correlation spectrum of n-nonane

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preadsorbed on a Co3O4 structured catalyst during dynamic temperature condition (40-450

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°C) under helium flow (25 cm3min-1) and synchronous and asynchronous catalytic Raman correlation spectrum of n-nonane catalytic combustion over Co3O4 structured catalyst at dynamic (40-450 °C) and oxygen-rich conditions (He 20 cm3min-1 / O2 5 cm3min-1).

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Figure 3. The differential in situ DRS UV-Vis spectra over Co3O4 structured catalyst during

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n-nonane catalytic combustion in a 250-800 nm spectral range; conditions: 0.002% n-C9/

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20% O2, balanced with helium at total flow of 25 cm3min-1.

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Figure 4. Evolution with temperature of the differential in situ DRIFT spectra over Co3O4

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structured catalyst during n-nonane catalytic combustion in a 4000-650 cm-1 spectral range:

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(A) helium flow (after adsorption step using 0.002% n-C9/ He at total flow of 25 cm3min-1) evolution with temperature; (B) evolution with temperature during reaction (0.002% n-C9/

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20% O2 balanced with helium; total flow of 25 cm3min-1)

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Figure 5. Evolution with temperature of the differential in situ DRIFT spectra over Co3O4 structured catalyst during n-nonane catalytic combustion in a 1800-1000 cm-1 spectral range:

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(A) helium flow (after adsorption step using 0.002% n-C9/ He at total flow of 25 cm3min-1) evolution with temperature; (B) evolution with temperature during reaction (0.002% n-C9/

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20% O2 balanced with helium; total flow of 25 cm3min-1)

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Figure 6. (A) DRIFT spectra of HCOOH preadsorbed on a Co3O4 structured catalyst during

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dynamic temperature condition (40-450 °C) under helium flow (25 cm3min-1); (B) DRIFT spectra of HCOOH preadsorbed on a Co3O4 structured catalyst at dynamic (40-450 °C) and

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oxygen-rich conditions (He 20 cm3min-1 / O2 5 cm3min-1).

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Figure 7. Synchronous (A) and asynchronous (B) DRIFT correlation spectrum of n-nonane preadsorbed on a Co3O4 structured catalyst during dynamic temperature condition (40-450

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°C) under helium flow (25 cm3min-1) and synchronous (C) and asynchronous (D) catalytic DRIFT correlation spectrum of n-nonane catalytic combustion over Co3O4 structured catalyst at dynamic (40-450 °C) and oxygen-rich conditions (He 20 cm3min-1 / O2 5 cm3min-1).

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Figure 8. DRIFT spectra of CO preadsorbed on a Co3O4 structured catalyst during dynamic

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Graphical abstract

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ACCEPTED MANUSCRIPT Highlights 1. 2DCoS represents a powerful tool in catalytic spectroscopy results interpretation. 2. Mars-van Krevelen mechanism may describe combustion of n-nonane on cobalt catalyst.

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3. As surface intermediates - acetates, formates and carbonate species was identified.

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