Total Oxidation of Lean Methane over Cobalt Spinel Nanocubes

Subscriber access provided by University of Newcastle, Australia

Article

Total Oxidation of Lean Methane over Cobalt Spinel Nanocubes Controlled by the Self-Adjusted Redox State of the Catalyst – Experimental and Theoretical Account for Interplay between the Langmuir-Hinshelwood and Mars-Van Krevelen Mechanisms Filip Zasada, Janusz Janas, Witold Piskorz, Magdalena Gorczy#ska, and Zbigniew Sojka ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b03139 • Publication Date (Web): 10 Mar 2017 Downloaded from http://pubs.acs.org on March 10, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Catalysis is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 50

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

ACS Catalysis

Total Oxidation of Lean Methane over Cobalt Spinel Nanocubes Controlled by the Self-Adjusted Redox State of the Catalyst – Experimental and Theoretical Account for Interplay between the Langmuir-Hinshelwood and Mars-Van Krevelen Mechanisms Filip Zasada*, Janusz Janas, Witold Piskorz, Magdalena Gorczyńska and Zbigniew Sojka Faculty of Chemistry, Jagiellonian University, ul. Ingardena 3, 30-060 Krakow, Poland

ACS Paragon Plus Environment

1

ACS Catalysis

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

Page 2 of 50

ABSTRACT

Involvement of suprafacial and intrafacial oxygen species in catalytic combustion of methane over the (100) faceted cobalt spinel was systematically examined as a function of temperature and CH4 conversion (XCH4). The clear-cut Co3O4 nanocubes of uniform size were synthetized by hydrothermal method, and characterized by XRD, RS, HR-TEM, XRF, TPSR (CH4/16/18O2) and by SSITKA (CH4/16/18O2) techniques. The experimental results were corroborated by first principles thermodynamic and DFT+U molecular modelling, providing a rational framework for detailed understanding the origin of a different redox comportment of the catalyst with the varying temperature, and its mechanistic implications. Three temperature/conversion stages of the methane oxidation reaction were distinguished, depending on involvement of the adsorbed or lattice oxygen and the redox state of the catalyst. A stoichiometric (100) surface region (300 °C < T < 450 °C, XCH4 < 25 %) is featured by the dominant suprafacial (Langmuir-Hinshelwood) mechanism of methane oxidation. A region of slightly defected surface (450 °C < T < 650 °C, 25% < XCH4 < 80%), in which oxygen vacancies produced upon CO2 and H2O release are virtually refilled by dioxygen, is characterized by coexistence of the suprafacial (LangmuirHinshelwood) and the intrafacial (Mars-van Krevelen) mechanistic steps. In a non-stoichiometric surface region (T > 650 °C, XCH4 > 80%), the oxygen vacancies are only partially refilled, the catalyst is significantly reduced, and methane is combusted according to the Mars-van Krevelen scheme. Molecular modelling revealed that the suprafacial Co–Oads adoxygen species are more active (∆Ea = 0.83 eV) than the intrafacial Co–Osurf surface sites (∆Ea = 1.11 eV) in the CH4 oxidation. The (100) surface state diagrams for the three distinguished conversion regions were


2

Page 3 of 50

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

ACS Catalysis

constructed to elucidate the catalyst thermodynamic behavior in those conditions. It was shown that the activity of cobalt spinel is maintained by redox auto-tuning of the catalyst, and dynamic adjustment of uneven participation of the suprafacial and intrafacial oxygen species in methane oxidation to the actual reaction conditions. These factors have important structural and mechanistic consequences for the catalytic CH4 combustion on cobalt spinel and related systems, controlling not only the sustainable versus the stoichiometric turnovers, but also for the prevalence or coexistence of the Langmuir-Hinshelwood and the Mars-van Krevelen mechanisms with the reaction progress.

KEYWORDS Co3O4, CH4 combustion, 18O2, reactive oxygen species, oxygen vacancies, surface phase diagram, DFT, atomistic thermodynamics, SSITKA


3

ACS Catalysis

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

Page 4 of 50

1.Introduction Abatement of methane atmospheric emissions belongs to one of the key challenges of the environmental

catalysis,

and

is

a

subject

of

intensive

fundamental

and

applied

investigations.1,2,3,4,5 The most simple solution for dropping the harmful environmental effect of methane release is the low temperature catalytic combustion, which is applicable for diluted air/methane streams. In such conditions concomitant production of undesired NOx is essentially reduced as well.6 Up to date, the catalytic combustion of methane has been extensively investigated over noble metals,7,8,9,10,11,12 transition metal oxides,13,14 and mixed metal oxides.15,16 Among the noble metals, Pd and Pt catalysts belong to the most active for methane combustion at low temperatures.17,18,19,4 However, despite of being expensive, they usually deteriorate due to sintering.20 Alternative oxide materials such as spinels21,22,23 and perovskites24,25 have been considered as promising catalysts for total oxidation of methane due to their high activity and distinctly lower costs when compared to the noble metals. Among them, cobalt spinel has been found to be one of the most active catalytic materials.26,20 Appropriate bulk doping27,23 and morphology engineering of Co3O4 have been used for improving its methane oxidation performance.28,29,30 Spectacular activity has recently been reported for Ni doped cobalt spinel, where full combustion of methane has been achieved below 350–400 °C.

31

However, despite

wide attention, a definite mechanism of the spinel catalysts operation in the reaction conditions varying dramatically with the progressing conversion has not been elucidated as yet. Furthermore, contradictory claims concerning involvement of lattice oxygen (intrafacial Marsvan Krevelen mechanism) or adsorbed oxygen (suprafacial Langmuir-Hinshelwood mechanism) in CH4 oxidation have been published in literature.23,31,32,33 The Mars-van Krevelen (MvK) mechanism in its immaculate form has been proposed in accounting for oxidation of hydrocarbons over d0 transition metal oxides such as V2O5, MoO3 or


4

Page 5 of 50

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

ACS Catalysis

WO3,34,35 where the empty conduction d-bands make direct activation of dioxygen via electron transfer impossible. However, in the case of dn>0 transition metal oxides, such as spinels or perovskites, the situation is more involved. The spinel valence and conduction bands are both of metal character, thus the requisite electrons are readily available. This allows for direct activation of dioxygen via electron transfer provided by the dn active centers. The resultant surface oxygen species such as O2–, O22– or O– may activate methane following the Langmuir-Hinshelwood (LH) route that often is preceded by an Eley-Rideal step, depending on the catalyst ability of methane capture prior to the actual C–H bond activation.36 This mechanistic pathway, however, may compete with an indirect oxidation process that involves lattice oxygen (MvK),37 and is controlled by dynamics of the oxygen vacancies formation.38 It is caused by the fact that incorporation of oxygen into the catalyst surface occurs via deep four electron reduction, and since the bare O2- adspecies are unstable oxygen vacancies are required for their successful accommodation. As a result, depending on the conditions, two alternate mechanistic pathways are possible, and their preference or coexistence is controlled by the actual redox state of the catalyst at various temperatures and reactants pressures, which until now has not been recognized properly. In order to address those mechanistic issues in more detail and clarify ambiguities concerning the pathways of lean methane oxidation over spinels, and conceivably other related 3dn oxides, we selected nanocubic Co3O4 as a well-defined model reference catalyst. A governing factor of high catalytic performance of cobalt spinel in hydrocarbon oxidation can be attributed to the occurrence and stability of various suprafacial reactive oxygen species (ROS), produced by associative/dissociative adsorption of dioxygen, and facile formation of oxygen vacancies.39,40,41 The ROS entities may be stabilized on the tetrahedral (CoT) and octahedral (CoO) cationic sites as well as on the anionic (O2–surf) surface centers (in the form of labile O22– anions).42 It has been


5

ACS Catalysis

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

Page 6 of 50

suggested that in catalytic oxidation processes such as CO oxidation43 the ROS intermediates play a key role in the catalytic performance of Co3O4 via the Langmuir-Hinshelwood mechanism.44 In turn, the oxygen vacancies generated during catalytic reactions (especially in the reducing conditions45) or produced spontaneously at elevated temperatures46 have been implicated in the Mars-van Krevelen redox cycles proposed, e.g., for high temperature oxidation of methane over cobalt spinel. 23,32,47 It is worth mentioning here that both LH and MvK catalytic turnovers may often provide similar rate equations that can fit rate data equally well, so they may not be recognized properly based on routine kinetic data only.48 Herein, we examine the interplay between the Langmuir-Hinshelwood and the Mars-van Krevelen mechanistic steps of lean methane oxidation on the model cobalt spinel catalysts of a nanocube morphology by means of temperature programed surface reaction (TPSR) measurements (using both 18O2 and 16O2), corroborated by steady state isotopic transient kinetic analyses (SSITKA). Experimental results were supported by spin resolved DFT+U calculations of the reaction energetics for the kinetically relevant steps of the methane combustion (C–H bond activation and CO2, H2O formation and desorption). The ab initio thermodynamic description of the exposed (100) surface state of Co3O4 at various methane conversion stages was used as a conceptual playground for quantitative interpretation of the catalytic data. A comprehensive description of the catalyst dynamic redox behavior and molecular-level account for ROS (Oads) versus surface lattice oxygen (Osurf) involvement in the CH4 oxidation allowed for coherent explication the pathways along which the reaction products are formed and released. The obtained results may also contribute to better understanding the complex surface redox state – reactivity relationships for other dn transition metal oxides, and clarify some past controversies as well.


6

Page 7 of 50

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

ACS Catalysis

2.Experimental methods and computational schemes 2.1. Materials and Characterization. For synthesis of cobalt spinel nanocubes 0.01 mol of Co(NO3)2·6H2O (MERK) and 0.005 mol of NaOH were dissolved in 10 mL of deionized water. The mixture was next transferred to a 20 mL teflon-lined steel autoclave, heated at 180 °C for 5 h, and finally cooled to room temperature. The resulting product was separated by centrifugation, washed several times with absolute ethanol and distilled water, and finally dried at 60 °C in air overnight. The synthesized material was calcined at 500 °C for 5 h to produce well crystalline Co3O4 spinel. The phase composition of the samples was confirmed by X-ray diffraction with CuKα radiation using the Rigaku Miniflex X-ray diffractometer equipped with the DeTEX detector. Diffractograms were registered in the 2θ range of 15–85° with the resolution of 0.02°, and the spinel crystal phase of the samples was confirmed by reference to the AMCSD database. Raman spectra were recorded with an inVia Renishaw spectrometer equipped with the Leica microscope using the 785 nm laser excitation. Transmission electron microscopy (TEM) imaging was performed by means of a Tecnai Osiris microscope (FEI) operating at 200 kV. Prior to TEM analysis the samples were ultrasonically dispersed in methanol on a holey carbon film supported on a copper grid (400 mesh). The grid was dried for 45 min, and then surface contaminations were removed by plasma-cleaning. 2.2. Catalytic studies. The temperature programmed surface reactions (TPSR) were performed in the range of 25–800 °C, using a set-up operating in the LabView environment, equipped with a QMS detector (Hiden Analytical HPR20), Brooks mass controllers, and automatic Valco switching valves. The experiments were carried out using a quartz flow reactor and 150 mg of the catalyst (sieve fraction of 0.2–0.3 mm), with the feed flow of 30-50 ml·min−1, and the heating


7

ACS Catalysis

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

Page 8 of 50

rate of 10 °C·min−1. The methane conversion was calculated on the basis of the QMS signals calibrated against the helium gas balance. 2.3. SSITKA experiments. The steady-state isotopic transient kinetic analyses using labeled 18O2 with 99 % enrichment (CK Isotopes, UK) were performed in a flow quartz reactor with 150 mg of the catalyst (sieve fraction of 0.2–0.3 mm). The gas flow rate was equal to 50 ml·min−1 and the feed gas consisted of CH4 (1.0 % in He), 18O2 (5.0 % in He/Ar) and 16O2 (5.0 % in He/Kr). The reactions were carried out at 400 °C, 500 °C, and 600 °C under the atmospheric pressure. 2.4. Molecular modeling. DFT+U calculations were carried out using the VASP code with the Hubbard parameter U = 3.5 eV.49,50 The projector augmented plane wave method (PAW)51 together with the generalized gradient PW91 exchange-functional52 were employed. We used standard Monkhorst-Pack53 grid with the 5×5×5 sampling mesh for bulk and the 5×5×1 mesh for slab calculations. The cutoff energy was set to 500 eV and the SCF convergence criterion to 10– 5

eV. Systematic validation of such calculation scheme against the experimental data has been

provided in our previous paper.38 The bulk cobalt spinel unit cell was obtained by full optimization of the all internal degrees of freedom of the cubic (1×1×1) unit cell containing 56 ions (Co24O32). Following our recent work32 we investigated a dominant stoichiometric bare surface, labeled as (100)-S, and nonstoichiometric (100)-VO and (100)-2VO surfaces, disordered by the presence of oxygen vacancies. We used a (1×1) slab of 17 atomic layers (~16 Å) with the vacuum separation of 20 Å, and the supercell composition corresponding to Co48O64 (stoichiometric Co3O4) for the (100)-S plane, Co48O62 (Co3O3.875 stoichiometry) for the (100)VO, and Co48O60 (Co3O3.75) for the (100)-2VO termination. They are presented in Figure S1 in Supporting Information (SI). Atomic positions in the four top and four bottom layers were relaxed within the criterion of 1·10-4 eV·Å-1. The nudged elastic band method with Climbing Image correction included (cNEB)54,55 was used to calculate the transition states of the crucial


8

Page 9 of 50

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

ACS Catalysis

steps of the methane oxidation. For the each investigated step five to nine NEB images (including the initial and the final ones) were used. 2.5. Atomistic thermodynamics. In order to compare the thermodynamic stability of various cobalt spinel surface states that may appear in the course of the methane combustion at various p,T conditions, we used a first principles thermodynamic modelling (FPT). The total free tot. enthalpy of the surface, Gsurf . (T , pi , ni ) , interacting with the key gas reactants (O2, H2O and CO2)

may be described as a function of the number of the adsorbed molecules, niads, at given T and pi values in the following way56,57 tot . Gsurf . (T , pi , ni ) =

1  slab   G (T , pi , {niads }) − ∑ niads µi (T , pi ) . A i 

(eq. 1)

In this equation A denotes the area exposed by the slab, Gslab corresponds to its free enthalpy, whereas µi(T,pi) is the chemical potential of the reactant/product molecules, i = O2, H2O, CO2. As tot. it is commonly resolved, the minimum of Gsurf . (T , pi , ni ) is not searched for directly, but instead

several conceivable adsorption models that differ in the number and nature of the admolecules are compared, to find the most stable surface configuration at given conditions. The stoichiometric pristine (100)-S, and the non-stoichiometric (100)-VO surfaces were taken as the prime initial states of the spinel catalyst surface.56 Thus, the interaction of the reactants and products with the spinel surface may be formulated in the following way:

(100)-S + ½O2 ↔ (100)-S–O

suprafacial ROS formation

(100)-S–CO2 ↔ (100)-S + CO2

suprafacial decarboxylation of (100)-S

(100)-VO–CO2 ↔ (100)-VO + CO2

intrafacial decarboxylation of (100)-VO


9

ACS Catalysis

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

Page 10 of 50

(100)-S–H2O ↔ (100)-S + H2O

suprafacial dehydratation of (100)-S

(100)-VO–H2O ↔ (100)-VO + H2O

intrafacial dehydratation of (100)-VO

The corresponding free energy of the reactants adsorption can be expressed as: slab tot. (T , pi , ni ) – [ Gslab(T , pO2 ) + Gigas (T , pi ) ] Gsurf . (T , pi , ni ) = G

where G

slab

(eq. 2)

(T , pi , ni ) is the free enthalpy of the slab model of the spinel surface covered by ni (T , pO2 ) refers to the (100)-S or (100)-VO surface (see below),

slab

molecules of the reactant i, G gas

and Gi

(T , pi )

to the free enthalpy of the gas phase O2, H2O or CO2. Assuming that

tot. tot. Gsurf . ≅ Esurf . , we may factorize the free energy of adsorption in two parts: an electronic

contribution, ∆Eel, calculated as difference of the corresponding electronic DFT energies, and the change in the chemical potential of the molecule i upon the interaction with the surface, ni∆µi. tot . el Gsurf . (T , p, ni ) = ∆E + ni∆µi

(eq. 3)

In turn, changes in the chemical potential of the gas phase and the adsorbed reactant molecules are described as:

µi(pi,T) = µ°i(T) + RT·ln(pi/p°)

(eq. 4)

and can be computed using the standard statistical thermodynamics.58 ∆µ°(T) = ∆(EZPE + Eosc(0 → T) + Erot + Etrans + RT – T(Sosc + Srot + Strans) (eq. 5) Finally, for description of both the stoichiometric and the defected spinel surfaces, the

(T , pO2 )

slab

corresponding free energies, the G

term in equation 2, were calculated in the

following way:59


10

Page 11 of 50

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

ACS Catalysis

, = , , , −

&

#$ , − % − ' ( , ) ! "

(eq. 6)

#$ In this equation, and represent the free energies of the stoichiometric Co3O4 slab ! "

and the corresponding bulk unit, respectively, whereas NO and NCo refer to the actual number of the oxygen and cobalt ions in the investigated slab models. The free enthalpies of activation for the key elementary steps where calculated in an analogous way as a difference between the free enthalpies of the transition and the reactant states (∆Ga = GTS – GR).


11

ACS Catalysis

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

Page 12 of 50

3.Results 3.1. Characterization of Spinel Catalyst. The XRD diffractogram of the cobalt spinel catalyst together with the Miller index attribution of the most intensive peaks is shown in Figure 1a. The characteristic diffraction pattern indicates that the sample is highly crystalline, and the XRD lines can be indexed within the Fd-3m space group (24210-ICSD) in a straightforward way. The lattice constant, a = 8.083 Å, and the oxygen structure parameter, u = 0.2632, were determined by the Rietveld simulation. In order to eliminate the presence of any spurious impurity phases, the cobalt spinel sample was additionally examined by Raman techniques (Figure 1b). The Raman spectrum of the nanocubic Co3O4 is characterized by 5 well-resolved bands at 194 (F2g), 480 (Eg), 520 (F2g), 620 (F2g) and 690 (A1g) cm−1, as predicted by the factor group analysis,60 confirming the sample good purity and crystallinity. The absence of other minute impurities, such as incompletely leached Na ions, which may contaminate the spinel catalyst - obscuring thereby its reactivity, was verified by XRF measurements (see Table S1 in Supporting Information).


12

Page 13 of 50

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

ACS Catalysis

Figure 1: The results of the Co3O4 catalyst characterization. XRD pattern (a), Raman spectra (b), and TEM pictures at various magnification (c1-c3), which reveal a clear-cut cubic shape of the spinel nanocrystals and their uniform size.

The microscopic TEM imaging (Figure 1c1) indicates that the Co3O4 morphology is dominated by loose cubic particles. The higher magnification pictures (Figure 1c2 and 1c3) reveal their well-developed (100) faceted shape and a regular size of ~40–60 nm. Thus, the catalytic performance of the investigated spinel samples may definitely be associated with the (100) termination exposed by the nonocubes exclusively, which was then used for the thermodynamic and molecular modeling.

3.2. Catalytic studies. The performance of cobalt spinel in the catalytic lean (1%) CH4 combustion, in the presence and in the absence of oxygen, is presented in the top panels of Figure 2, in the form of standard conversion curves. In 2 % of O2 (Figure 2a) an appreciable methane conversion was observed above 300 °C, and at 700 °C it reaches asymptotically its maximum value of ~85 %, despite that the gas phase oxygen is still present (~10 % of the initial flow). To monitor possible oxygen retention/release during the course of the CH4 combustion we calculated the flow balances (∆Fi = Fi(in) – Fi(out), µmol·min-1) for oxygen, ∆FO2, and methane, ∆FCH4, at all investigated temperatures. Taking into account the reaction stoichiometry, we assessed the amount of oxygen that is retained on (positive values) or released from (negative values) the catalyst surface as δO2 = ∆FO2 – 2·∆FCH4. The results (blue line in Figure 2a) show that in the lower temperatures small, gradually vanishing, accommodation of oxygen takes place until 450 °C, which is associated with the formation of reactive oxygen species on the catalyst surface. Above this temperature the oxygen is released, and this effect significantly overwhelms the low temperature retention. It indicates that not only the suprafacial ROS but also the


13

ACS Catalysis

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

Page 14 of 50

intrafacial oxygen (O2–surf) must be liberated by formation of the oxygen vacancies. These effects are rather small, and the highest |δO2| values do not exceed 0.06 µmol (less than 0.2 % of the total oxygen inlet). The observed slight reduction of the catalyst indicates that, once the oxygen pressure decreases due to the reaction progress, the catalyst changes its redox state by generation of the oxygen vacancies to maintain the equilibrium with the actual gas phase composition (catalyst redox auto-tuning). The enhanced reduction is then beneficial for activation of dioxygen, compensating its steadily dropping pressure as the conversion increases.

Figure 2. Catalytic performance of Co3O4 in lean CH4 combustion in the presence (a and a1) and absence (b and b1) of dioxygen, presented as conversion profiles (top panels) and reactants flows (bottom panels). The varying redox state of the catalyst surface is revealed by the oxygen retention line (δO2 in panel a) and by the Co/O ratio curve (panel b, insert).

The observed incomplete conversion of methane may result from the scarcity of ROS at elevated temperatures, since at pO2 < 10–3 atm the chemical potential of dioxygen is actually too small even to maintain the surface stoichiometry (completely refill the oxygen vacancies).


14

Page 15 of 50

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

ACS Catalysis

Apparently, also the reactivity of the abundant O2–surf anions in those conditions is not sufficient to assure total conversion of methane. Indeed, in the absence of gaseous dioxygen (Figure 2b), the onset of the CH4 conversion is shifted toward 450 °C, showing that in the temperature range of 300–450 °C only the ROS species may activate methane. The maximum conversion, reached at 675 °C, does not exceed 75 %. Upon further increase of the temperature it dramatically drops due to gradual transformation of cobalt spinel into cobalt monoxide,61,62 revealed by rapid decline of the O/Co ratio, shown in the insert. Inspection of Figure 2a1 and 2b1 shows that in the course of methane combustion in the presence or absence of oxygen, essentially complete CH4 oxidation takes place (the amount of CO is very low). In the anoxic conditions the pronounced decrease of the H2O and CO2 flows (and concomitant increase of CH4) was caused by the already mentioned formation of the distinctly less active, oxygen depleted (Co2+(1-x))[Co3+(2-2x)Co2+3xO4-x] shell around the cobalt spinel catalyst nanocrystals. In both oxygen-rich and anoxic conditions water and carbon dioxide are released practically simultaneously. This indicates that there are no serious kinetic obstacles for their straightforward desorption once they are produced, in line with the results of the molecular and thermodynamic modelling described below.


15

ACS Catalysis

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

Page 16 of 50

Figure 3: Temperature profiles of the reactants flows (µmole·min-1) in the course of the CH4-18O2 reaction (a) together with an aggregated flow of the dioxygen labelled with 16O evolved during this process (a1). Evolution of H2O and CO2 products (b), and the corresponding oxygen 16O fractions (b1).

More detailed insight into the ROS vs O2–surf participation in the methane oxidation reaction can be provided by isotopic experiments summarized in Figure 3. The flows of the reactants during the CH4 combustion in 18O2 are presented in Figure 3a. The profiles corresponding to the 16/18

O2 and

16/16

O2 species are close to zero until 650 °C. Above this temperature a significant


16

Page 17 of 50

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

ACS Catalysis

increase of

16/18

O2(g) reflected in a parallel but weaker growth of

16/16

O2(g) were observed.

Involvement of the oxygen-16 in the methane oxidation reaction is better revealed in Figure 3a1, where the aggregated flow of the produced dioxygen, labelled with O-16 (∆F(16O2) + 0.5 ∆F(16O18O)), is shown. The shape of this curve indicates that in the lowest temperatures, release of the oxygen-16 can be associated with the isotopic exchange with surface oxygen. This is also reflected by a shallow recess (negative peak) in the

18

O2 profile due to the corresponding

oxygen-18 consumption, situated at 200–250 °C (Figure 3a). Indeed, parallel pulse

18

Co316O4 experiments revealed that in this temperature region small amounts of gaseous

16

and

16

O2 are always produced upon

18

O2 + O18O

O2 pulses (see Figure S2 in SI). The mechanism of such

exchange (explained in detail in our recent paper63) involves surface peroxy species, and the calculated barrier of the oxygen atom flipping within the (16Osurf18Oads)2– unit is of 0.7 eV only. With the onset of the CH4 combustion (T ~300 °C), the

16

O-labeled ROS are mostly spent on

methane oxidation, therefore the corresponding line is gradually decaying until 400 °C. Above 450 °C the

16

O2 evolution is steadily progressing, and upon passing 650 °C this process is

markedly accelerated. The obtained results enable us to distinguish three distinct temperature/conversion windows corresponding to three stages of the CH4 reaction (marked in color in Figure 3). At the lowest temperatures (from 300 °C to 450 °C, blue region) the reaction proceeds mainly by involvement of the thermodynamically stable ROS, which are produced upon the suprafacial activation of the gas phase

18

O2. In these conditions the catalyst stoichiometry is well preserved. As the surface

oxygen species became unstable with the increasing temperature (450 °C < T < 650 °C, green region), the O2–surf anions are gradually engaged in the methane oxidation, and the catalyst surface becomes slightly defected (reduced) by formation of the oxygen vacancies (Figure 2a). This process is significantly augmented above 650 °C (yellow region), where generation of the


17

ACS Catalysis

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

Page 18 of 50

oxygen vacancies is no more limited to the surface region, but it extends also into the bulk of the catalyst.38 Indeed, the oxygen balance indicates that the spinel catalyst is substantially reduced, yet to much smaller extent in comparison to the combustion in the absence of oxygen at the same temperatures (Figure 2b), showing that the created vacancies are refiled by oxygen in ~98 % (in T = 650 °C). This point will be accounted for by the thermodynamic modelling in more detail (see below). The temperature evolution of the differently labelled methane combustion products (H218O, H216O, C18O2, C16O2, C16/18O2) is presented in Figure 3b. In the lowest temperatures the O-18 labelled entities dominate, which confirms that the suprafacial methane oxidation is operating actually. The corresponding oxygen 16O fractions (α(16O)i values64), which can be taken as a first approach measure of the intrafacial mechanism involvement, illustrate this point more clearly. They were calculated according to equations 7 and 8, and their temperature dependence is presented in Figure 3b1.

+

* (H O) = 0

+

* (CO ) = 0

0123 4

123 4 5 0126 4

(eq. 7)

2 0 5 023 4234 23 426 4

23 426 4 5 023 423 4 5 026 426 4

(eq. 8)

Inspection of Figure 3b1 shows that the three distinguished temperature reaction regions are now delineated in an explicit way by the clear turning points in the

16

α(H2O) and

16

α(CO2)

profiles. The both quantities initially increase with the growing temperature, reaching steady values in the second region of the reaction. In this temperature window 16α(H2O) = 0.45 shows that, on average, about 0.66 H216O per one H218O molecule is produced. The value of 16α(CO2) = 0.25, in turn, corresponds to 1 C18O2 : 0.5 C16/18O2 : 0.15 C16O2 ratio. A relatively high value of


18

Page 19 of 50

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

ACS Catalysis

16

α(H2O) suggests that during the methane oxidation, once the oxygenates are formed, the

remaining protons are abstracted by both the ROS and O2–surf species forming suprafacial and intrafacial hydroxyl groups. Since the former are preferentially labelled with

18

O and the latter

with 16O, the resultant desorbed water molecules reflect such isotopic labeling (partially smeared by scrambling). In the case of carbon dioxide, the oxygen-18 labelled species dominate, which points to higher involvement of ROS than the lattice oxygen in the formation of the C–O bonds. An appreciable presence of C16O2 may be explained by scrambling of the surface and adoxygen atoms within the already mentioned flipping mechanism.63 Following the molecular modelling, oxygen vacancies can be produced mainly during dehydroxylation and decarboxylation of the spinel surface, and at this conditions they are refilled by the gas phase dioxygen (see below). The obtained results show that water and carbon dioxide are produced along apparently different ways, featured by an uneven participation of the LH and MvK reaction steps acting concurrently. Although the information about the lattice oxygen involvement inferred form the isotopic studies may be partly disguised by the observed scrambling, the gross features of the surface oxygen dynamic behavior in methane combustion over the cobalt spinel catalyst are accounted for in an adequate way. To assess the participation of the suprafacial oxygen in the course of the CH4–O2 reaction we next compared the results of methane oxidation in the flow of 1:1 mixture of 16O2/18O2 with the corresponding 16O2/18O2 (1:1) isotopic exchange. The latter reaction probes directly the fraction of the activated monoatomic oxygen species, which next recombine into methane conversion is preceded by the formation of

16

16

O18O. The onset of

O18O, which reveals that dissociation of

dioxygen on the catalyst surface is already advanced at these conditions. The resultant ROS species may either activate methane or recombine (Figure 4a). Evolution of the

16

O18O signal

reaches its maximum value at 450 °C and then gradually decreases, indicating that initially only


19

ACS Catalysis

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

Page 20 of 50

a small fraction of the monoatomic ROS generated on the surface is consumed in the methane oxidation, while most of them recombine back into dioxygen. Above 450 °C, once the rate of the CH4 combustion accelerates, more of the produced monoatomic oxygen species are engaged in the CH4 oxidation than in the recombination process. The corresponding

16

O2/18O2 isotopic

exchange profile (Figure 4b) shows that the presence of CH4 does not influence the onset of the exchange reaction, implying that ROS are reacting with the gas phase methane. The isotopic equilibrium composition is reached at 450 °C, and is nearly constant until 650 °C. Above that temperature, the isotopic composition is significantly perturbed by

16

O2 released from the

catalyst due to rapid oxygen vacancy formation, in accordance with the previous observations (Figure 3). In order to estimate the fraction of ROS engaged in the methane oxidation we compared the

16

O18O profiles recorded in the presence and absence of methane, expressing the

results as a percentage of

16

O18O consumed in the methane combustion (dotted line). The

consumption of ROS rapidly increases from 0 to 50 % in the first temperature window (300– 450 °C), where the suprafacial methane combustion takes place. Above 450 °C the 16O2 and 18O2 profiles steadily diverge, and slight advantage of the

16

O2 isotopomer indicates the presence of

the hetero-exchange with the surface oxygen that accompanies the oxidation reaction, resulting in an isotopic enrichment of the catalyst in oxygen-18. This process is significantly increased upon passing 650 °C (third temperature window). Such behavior is in line with the participation of the intrafacial CH4 oxidation mechanism, whose importance grows with the increasing temperature. The estimated overall consumption of the dissociated oxygen in the methane oxidation reaches 80 % at 750 °C. It should be noted, however, that in this temperature window the O-18 species are indirectly used in methane oxidation via participation of the lattice O2–surf (MvK pathway).


20

Page 21 of 50

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

ACS Catalysis

Figure 4: Evolution of the reactants and products (µmole·min-1) of the CH4 + 18O2 + 16O2 reaction as a function of temperature (a). TPSR profiles of the oxygen isotopomers for the isotopic exchange (16O2 + 18 O2 216O18O) on the spinel (100) surface (solid lines), and fraction of the dissociated oxygen (ROS) consumed in the methane oxidation (dotted line) (b).

3.3. SSITKA experiments. In order to provide more insight into the surface oxygen dynamics during methane combustion at various temperatures we performed SSITKA experiments using CH4/(16O2/Ar) and CH4/(18O2/Kr) switches. Figure 5 shows the 16O2 transients (solid lines) at T = 400 °C, 500 °C and 600 °C, corresponding to the three stages of the methane combustion reaction, discussed above. The Ar-reference responses are shown as dotted lines with the same color coding. In order to determine the residence time of oxygen, τO, the normalized ?

experimental data were fitted to a standard F (t) = 1-exp %- @ ' decay.65,66 The results show 4

that with the rising temperature the average residence time of oxygen is significantly increased in the sequence: τO(400 °C) = 3±1 s, τO(500 °C) = 6±1 s, τO(600 °C) = 17±2 s (see also insert in Figure 5).


21

ACS Catalysis

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

Page 22 of 50

Figure 5: Transient responses of O2 and Ar, after switching from 16O2 (5.0% in He/Ar) to the 18O2 (5.0% in He/Kr) measured at different temperatures together with the corresponding oxygen retention times (insert).

The obtained data show clearly that the surface oxygen status and its reactivity are dramatically changed during methane combustion with the increasing temperature and methane conversion. Short residence time of the surface oxygen is associated with operation of the suprafacial oxidation of methane in the low temperature window (< 450 °C), which is tantamount with the Langmuir-Hinshelwood mechanism. A significant increase of the oxygen residence time implies, in turn, gradual expansion of the intrafacial methane oxidation with the involvement of oxygen vacancies, which can be associated with the operation of the Mars-van Krevelen competing with the Langmuir-Hinshelwood steps. Above 600 °C, diffusion of the labeled oxygen into catalyst subsurface region of the spinel explains its prolonged retention.


22

Page 23 of 50

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

ACS Catalysis

3.4. First principle thermodynamics. In order to provide a thermodynamic rational for involvement of the suprafacial versus intrafacial reaction steps in the CH4 combustion a FPT modelling was next carried out. In our approach the suprafacial mechanism was epitomized in the following way. ½O2(g) + (100)-S → (100)-S–Oads

(eq. 9)

CH4(g) + (100)-S–Oads → (100)-CH3–OadsH → {C–O bond formation, C–H bond cleavage, H–O bond formation steps} → + (100)-S-(CO2,H2O) → CO2(g) + H2O(g) + (100)-S

(eq. 10)

Equation 9 accounts for the formation of the reactive oxygen species, whereas equation 10 for the subsequent oxidation of methane. The latter reaction can be divided into activation of CH4 with the formation of surface stabilized –CH3 and –OadsH species, oxidation of methane derived oxygenates, leading to formation of the C–O and O–H bonds and cleavage of the remaining C–H bonds, and suprafacial desorption of the final reaction products (H2O and CO2). An analogous scheme can be proposed for the intrafacial mechanism. Yet, in this case methane is activated by the lattice oxygen, and desorption of H2O and CO2 leads to the formation of oxygen vacancies (eq. 11). As implied by the experimental results the later are refilled by the gaseous dioxygen (eq. 12). CH4(g) + (100)-S → (100)-S-CH3–OsurfH → {C–O bond formation, C–H bond cleavage, H–O bond formation steps} → (100)-VO-( CO2,H2O) → CO2(g) + H2O(g) + (100)-VO

(eq. 11)

½O2(g) + (100)-VO ↔ (100)-S

(eq. 12)


23

ACS Catalysis

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

Page 24 of 50

The intermediate steps shown in curly brackets, depending on the reaction conditions may create a quite complex network of parallel/consecutive events. However, they are kinetically irrelevant since the corresponding energy barriers are distinctly smaller than those calculated for the opening (first C–H bond cleavage) and closing (CO2/H2O desorption) steps of the catalytic cycle that are here considered explicitly. A comprehensive quantum chemical account of these intermediate steps is beyond the scope of this work, and is a subject of a separate detailed computational study (now in progress). Within the applied atomistic thermodynamic account, the surface state of the catalyst was then assumed to be controlled by the equilibria with dioxygen, water and carbon dioxide. As implied by equations 9 and 12, dioxygen may adsorb on the surface not only giving rise to the formation of ROS, but also in the oxygen vacancy formation and refill. In the similar way the reaction products, water and carbon dioxide, may take part in the suprafacial equilibria preserving the surface stoichiometry (eq. 10), or may be engaged in the oxygen vacancy formation (desorption) or annihilation (readsorption) processes (eq.11), depending on the temperature and the actual O2, H2O and CO2 pressures. We confined our FPT modelling by treating explicitly the interaction of the spinel surface with dioxygen, water and carbon dioxide in turn, while taking the chemical potentials of the remaining species as the external parameters, kept constant for the given case. Such approach provides a useful thermodynamic background for the methane oxidation reaction, capable to rationalize the obtained experimental results in a straightforward way. Following our previous work on the thermodynamic stability of the (100) surface of cobalt spinel, for the FPT modeling we initially selected two terminations: (100)-S (stoichiometric Co3O4) and (100)-VO (nonstoichiometric catalyst with one oxygen vacancy per slab, Co3O3.875). Their surface (1×1) unit cells (delineated by black square) together with the basic structural characteristics (coordination numbers of the surface ions and the most important bond lengths)


24

Page 25 of 50

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

ACS Catalysis

are presented in Figure 6a and 6b. As shown in Figure 6a, the (1×1) cell of the (100)-S termination contains one protruding tetrahedral CoT2c ion (violet) with two dangling bonds, four singly truncated octahedral ions, CoO5c (blue), and two recessed tetrahedral cobalt ions, CoT4c (green). The anionic part (oxygens marked red) consists of four oxygen ions of the 3-fold coordination that are linked to two CoO and to one CoT ion (denoted as O2O,1T), two 3-fold oxygen anions linked to CoO exclusively (O3O), and four O3O,1T species of the full coordination. In the case of the (100)-VO termination, the oxygen vacancy is formed by removal of the most labile O2O,1T. The resultant structure relaxation is now more pronounced (Figure 6b, the vacancy is marked as yellow cube), and entails shift of the exposed CoT2c cation toward the neighboring octahedral empty site, accompanied by elongation of the Co–O bonds in the vicinity of the vacancy. The (100)-VO surface is constituted by the following ions {1CoT2c, 1CoT3c 3CoO5c, 1CoO4c; 4O2O,1T, 2O3O, 2O3O,1T}. In the highly reducing conditions, where higher vacancy concentrations are expected, we also considered a more highly defected (100)-2VO termination with two oxygen vacancies per slab and the Co3O3.75 stoichiometry.38 As discussed in detail in our previous work,32 among various reactive oxygen species formed on the stoichiometric (100)-S surface in oxygen rich conditions, the most stable are the monooxygen species, CoT2c–Oads, attached to the tetrahedral cobalt centers (Figure 6c1). They are characterized by the quite short Co–O bond length of 1.66 Å, and may be involved in the suprafacial C–H bond activation (see below). Such species are produced upon dissociative adsorption of O2, and the remaining oxygen moiety is stabilized in the form of CoO5c–Oads adducts with dCo–O = 1.66 Å (Figure 6c2). A more comprehensive molecular description of gas phase oxygen activation leading to formation of such adspecies is provided in Supporting Information (Figure S3). In the case of the (100)-VO termination, adsorption of O2 may also occur at the oxygen vacancies with the formation of diamagnetic surface peroxy anions (Osurf–


25

ACS Catalysis

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

Page 26 of 50

Oads)2–, characterized by dO–O = 1.48 Å (Figure 6c3). All these oxygen varieties can easily be transformed into each other due to their facile surface diffusion.63 For further modelling we selected the termination with the most stable CoT2c–Oads species, labelled as (100)-S-O. It is involved in the equilibria with the stoichiometric, (eq. 9), and defected, (eq. 12), cobalt spinel surfaces.

Figure 6: Perspective views of various cobalt spinel (100) terminations. The stoichiometric bare (100)-S termination (a) and the oxygen defected (100)-VO surface with an oxygen vacancy in the O2O,1T position (b). The (100) terminations with an oxygen adatom stabilized on the CoT2c (c1), CoO5c (c2) and O3c (c3) active sites. Color coding: CoO5c - blue; CoT2c - purple; CoT4c – green, spinel O - red; oxygen adatom – orange; oxygen vacancy - yellow cube.


26

Page 27 of 50

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

ACS Catalysis

Figure 7: Perspective view of various cobalt spinel terminations resulting from the interaction of (100)-S and (100)-VO surfaces with water and carbon dioxide. The hydroxylated stoichiometric (a) and the defected (a1) terminations are presented in the top pictures, whereas their carboxylated equivalents (b for (100)-S and b1 and for (100)-VO) are shown in the bottom panels. Color coding: CoO5c - blue; CoT2c purple; CoT4c – green, spinel O - red; carbon – grey; oxygen adatom – orange.

As already mentioned, water and carbon dioxide produced in the course of the methane oxidation reaction can be released with or without oxygen vacancy formation. The investigated hydroxylated and carboxylated cobalt spinel (100) planes engaged in both pathways are presented in Figure 7. Successive activation of the C–H bonds of methane due to the interaction with the surface adoxygen results in the formation of the surface hydroxyl groups (Figure 7a). The corresponding surface is coded as (100)-S-H2O, and is involved in a vacancy-free desorption of water, with restoration of the bare (100)-S termination (eq. 10). In the absence of the suprafacial ROS, accommodation of the protons released upon the C–H bonds scission is achieved by the lattice oxygen only, giving rise to the termination presented in Figure 7a1. Since desorption of water entails formation of an oxygen vacancy, this process can be described as


27

ACS Catalysis

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

Page 28 of 50

(100)-VO-H2O ↔ (100)-VO + H2O. Analogous terminations for CO2 desorption without and with vacancy formation are illustrated in Figure 7b and 7b1. They are labelled (100)-S-CO2 and (100)-VO-CO2, respectively, and the related desorption processes can be formulated as (100)-SCO2 ↔ (100)-S + CO2, and (100)-VO-CO2 ↔ (100)-VO + CO2. The energies of the O2, H2O and CO2 interaction with the (100)-S and (100)-VO surfaces, which were next used for the thermodynamic modeling, are collated in Table 1.

Table 1: The energies of O2, H2O and CO2 interaction with the (100)-S and (100)-VO terminations.

reactant adsorption site

Eads / eV

related termination

molecule CoT

–0.72

(100)-S-OCoT

CoO

0.15

(100)-S-OCoO

O2O,1T

0.42

(100)-S-OO

CoO, Olattice

–1.02

(100)-S-H2O

VO, Olattice

–1.45

(100)-VO-H2O

Olattice

–0.15

(100)-S-CO2

VO

–0.55

(100)-VO-CO2

defect type

locus

Evac. / eV

related termination

VO

O2O,1T

0.91

(100)-VO

O2

H2O

CO2

Due to large number of the variables (T, pO2, pH2O, pCO2), the results of the FPT modeling are presented for selected thermodynamic conditions, corresponding to the previously distinguished three temperature regions of low (XCH4 ~5%), medium (XCH4 ~35 %) and high (XCH4 ~80 %)


28

Page 29 of 50

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

ACS Catalysis

methane conversion in the TPSR reaction. The pO2, pH2O and pCO2 pressures were, therefore, accordingly set to the specific mean values corresponding to those conversion regions (see Table 2), and the associated free surface enthalpies were plotted against the temperature. Such approximation is justified by weaker sensitivity of the chemical potential to pressure (logarithmic dependence) than to the temperature (linear dependence).

Table 2: The mean pressures of the gas reactants, corresponding to selected conversion regions used in the atomistic thermodynamic modelling.

XCH4 / % pi/p0

pO2 pCO2 pH2O

~5

~35

~80

1·10-2 5·10-5 1·10-4

5·10-3 2.5·10-3 5·10-3

1·10-4 0.5·10-2 1·10-2

The results are presented in the left panels of Figure 8 for the whole temperature range with the conversion regions of interest marked in color. For the sake of clarity and further discussion their expanded versions are shown also in the right panels. In the low conversion regime (Figures 8a1, 8a2), the most stable termination is the ROS covered (100)-S-O surface (violet line), which at higher temperatures (> 370 °C) merges with the (100)-S bare surface (blue horizontal line). Thus, in those conditions cobalt spinel surface is covered by the abundant ROS species that may trigger the suprafacial methane oxidation. The surface energies of the stoichiometric terminations covered with water (100)-S-H2O (navy blue) and carbon dioxide (100)-S-CO2 (pale blue) are situated distinctly above the bare (100)-S plane, providing a strong thermodynamic driving force for the experimentally observed facile decarboxylation and dehydration of the stoichiometric (100) surface, beneficial for closing the catalyst cycle. The defected (100)-VO surface (lime green) around 380 °C is slightly less stable than its


29

ACS Catalysis

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

Page 30 of 50

stoichiometric and ROS covered counterparts, so they all may coexist, and release of the surface oxygen (O2–surf) is thus plausible at such conditions. The (100)-VO termination, in turn, is much more stable than its carboxylated (100)-VO-CO2 form, therefore the surface CO2 species produced during the methane oxidation may readily be desorbed regardless the temperature. In the case of the (100)-VO-H2O surface the situation is more involved. As it is implied by Figure 8a2, below 325 °C the intrafacial hydroxyls are stable, yet above this temperature they may be converted into gas phase water, leaving an oxygen vacancy behind. The latter, however, can be filled by oxygen since the (100)-VO surface is less stable than the (100)-S one. This provides a favorable thermodynamic playground for sustainable turnover above 330 °C and preservation of the spinel surface stoichiometry, as it was really observed experimentally (see the reaction onset in Figures 2–4).


30

Page 31 of 50

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

ACS Catalysis

Figure 8: The results of the FPT modeling showing the stability of different cobalt spinel terminations as a function of temperature for different thermodynamic conditions corresponding to low (XCH4 ~5%, a1 and a2), medium (XCH4 ~35 %, b1 and b2) and high (XCH4 ~80 %, c1 and c2) conversion of methane. Left side panels (a1, b1, c1) show the whole temperature range of the TPSR reaction, whereas in the right side panels (a2, b2, c2) an expanded version of the regions corresponding to particular conversion range are presented.

Upon moving to higher temperatures (450 °C < T < 550 °C) and increased conversions (Figure 8b1, 8b2), stability of the crucial terminations is substantially modified by the actual pressures of the reactants (Table 2). As shown in Figure 8b1, the most stable are now the (100)S and (100)-VO surfaces, and the corresponding close lying lines show that they both may coexist. The former is slightly more stable in lower (T < 480 °C), whereas the latter in higher (T


31

ACS Catalysis

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

Page 32 of 50

> 480 °C) temperatures. This indicates that the equilibrium (100)-VO + ½O2 ↔ (100)-S may easily be established, and since filling of the vacancies is performed by dioxygen it leads to the transient appearance of metastable, yet highly reactive, surface peroxide species (O2 + VO = (Oads–Osurf)2–). Their involvement in the CH4–O2 reaction is controlled by the relative rates of the interaction with various methane oxygenate intermediates and recombination into gas phase dioxygen. The lines corresponding to the carbon dioxide desorption are well separated showing that the decarboxylation of the spinel (100) surface is thermodynamically favorable for both the suprafacial and the intrafacial pathway. Similar situation is observed for the suprafacial desorption of water, whereas the low temperature proximity of the (100)-VO and (100)-VO-H2O energies implies that in such conditions the intrafacial methane oxidation should be moisture sensitive. As a result, in this temperature region the LH and MvK mechanisms may operate simultaneously, and the catalyst surface state is slightly defected due to the persistent presence of oxygen vacancies, in accordance with the experiment (see Figure 2a). The reduced state of the spinel catalyst is beneficial for its performance, since the ROS covered surface is unstable at such conditions, however, the LH steps may still be triggered by the transient peroxy species, produced in the incipient stage of the vacancy filling by dioxygen (see above). In the region of the highest conversions (600–750 °C), the results of the FPT modeling using the actual reactants pressures (Table 2) indicate that the most stable are now the defected (100)-VO and (100)-2VO surfaces (Figure 8c1,c2). At such conditions the oxygen chemical potential is apparently too low to maintain the spinel stoichiometry (the surface energy of (100)S is substantially higher than that of (100)-VO), providing a suitable platform for the vacancy involving mechanism of the CH4 combustion. Furthermore, there is also a strong thermodynamic incentive for the intrafacial decarboxylation and dehydroxylation of the catalyst. Overall, such conditions are advantageous for operation of the Mars-van Krevelen mechanism. However, the


32

Page 33 of 50

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

ACS Catalysis

cobalt spinel catalyst becomes thermodynamically unstable, and is substantially reduced by progressive generation of oxygen vacancies, as revealed by the oxygen balance (Figure 2) and isotopic studies (Figure 3). As a result, the CH4 oxidation reaction is accompanied by the concurrent partial reduction of cobalt spinel occurring in the background of the catalytic cycles, in order to adjust the catalyst state to the current redox conditions. Such auto-tuning of the cobalt spinel catalyst allows for activation of oxygen by vacancy mechanism, in the temperature window where the ability for its suprafacial activation is lost.


33

ACS Catalysis

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

Page 34 of 50

3.5. Molecular modeling: To complement the description of methane combustion on the (100) surface of Co3O4, the presented atomistic thermodynamic modeling was corroborated by DFT calculations of the reaction barriers for the reaction key steps (eq. 9–12). Activation of the C–H bond involves two surface sites for accommodation of –CH3 and –H fragments, as the methyl moiety can be attached to both cationic (CoT2c, CoO5c) and anionic sites (–Oads, O3c, O2O,1T), whereas, hydrogen to anionic centers only. In order to emphasize the dual nature of the active sites we have introduced a double site notation [site1–site2] were the first site refers to the methyl group accommodation center, whereas the second site to the hydrogen adsorption site. Activation of the C–H bond was examined for all possible [site1–site2] variants of the attachment. The energetic barriers and the most important features of the considered reaction steps are collected in Table S2, and the corresponding optimal geometries are presented in Figure S3 in Supporting Information. Brief analysis of these data shows that the lowest energy of the first C–H bond scission, ∆Ea = 0.83 eV, was obtained for the (100)-S–O surface exposing [CoT2c–Oads] active sites, whereas in the case of the bare (100)-S surface the lowest energy barrier was associated with the C–H activation on the [CoO5c–O3c] sites (∆Ea = 1.11), therefore only those steps were selected for further detailed discussion. For operation of the LH mechanism further oxidation of the –CH3 fragments should rely on application of adoxygen atoms only. The experimental results show that in the temperature range where this mechanism operates, the cobalt spinel (100) surface is covered by the abundant oxygen adspecies (Figure 4 and 5). In our calculations we have taken all these experimental evidences into account. Due to easy activation of O2 (0.68 eV), and fast surface diffusion of the resultant monoatomic oxygen adspecies (with small barrier of 0.7 eV), the latter in the reaction conditions are always present in excess with respect to the intermediate methane oxygenates, and are capable to accomplish all the successive steps of their suprafacial


34

Page 35 of 50

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

ACS Catalysis

oxidation, implicated by the LH mechanism. More detailed description of dioxygen activation on the (100) surface of Co3O4 can be found in Supporting Information (Section S5, Figure S4).

Figure 9: Atomic composition of the (100) termination showing the consecutive steps of the methane C– H bond activation on a bare (a,a#, b,b’) and a ROS covered (c,c#, d,d’) surface. Color coding: cobalt oxide oxygen: red, oxygen adatom: yellow; exposed tetrahedral cobalt: violet, recessed tetrahedral cobalt: green, octahedral cobalt: blue, carbon: grey, hydrogen: white.

The resultant molecular level description of the C–H bond activation on the pristine (100)-S (top panel) and the ROS covered (100)-S–O (bottom panel) surfaces is presented in Figure 9. In the case of the CH4 attack on the [CoO5c–O3c] site (sequence a→a#→b, where

#

indicates the

transition state), the CoO5c–O3c distance equal to 1.91 Å (see Figure 9a) is favorable for a rather low activation barrier of 1.12 eV, since the released proton can be accommodated readily by the proximal O3O2– anion without excessive stretching of the activated C–H bond (dC-H = 1.25 Å, 9a#). The structures of the resultant fragments, the –CH3 group stabilized on the CoO5c site (dCoO– CH3

= 2.05 Å) and the surface hydroxyl group (dH–O = 0.98 Å) are shown in Figure 9b. They are

slightly less stable (by 0.17 eV) with respect to the gas phase CH4. However, upon transfer of the –CH3 moiety onto the adjacent O2O,1T site (with ∆Ea = 0.62 eV only), the resultant surface methoxy species (Figure 9b’) becomes more stable by 1.3 eV.


35

ACS Catalysis

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

Page 36 of 50

For the oxygen covered surface, an analogous C–H bond activation was modeled on the [CoT2c–Oads] sites involving monoatomic oxygen O–CoT species (Figure 9c), the most stable adoxygen form in the onset temperature of the CH4 combustion (300 °C < T < 400 °C). In the transition state of methane activation configuration (9c#), the incipient hydroxyl and CoT–CH3 bonds are formed with the dO–H distance of 1.02 Å and dCoO–CH3 of 2.25 Å, which accounts well for the quite low energy barrier associated with this step (∆Ea = 0.83 eV). The resultant structure of the hydroxyl and the –CH3 entities, both attached to the same CoT2c site (Figure 9d), is more stable with respect to gas methane by 0.11 eV. Again, the energetics of this process can be improved by 1.1 eV upon transferring the –CH3 fragment onto the adjacent O2O,1T anion (with ∆Ea = 0.65 eV), and formation of the surface Osurf–CH3 group (Figure 9d’). The obtained results show clearly that the suprafacial monoatomic oxygen (O–CoT) species are more effective in the C–H bond activation than the intrafacial O3O sites acting in tandem with CoO5c. As implied by the experimental data (Figure 4 and 5), in the low conversion region (below 375 °C) the spinel surface is covered by abundant ROS species. Thus, the subsequent reaction steps of the remaining C–H bonds scission and formation of the C–O and O–H bonds can be performed by the readily available Oads species, as required by the Langmuir-Hinshelwood mechanism. An example of molecular modeling of such events is presented in Supporting Information (Figure S5). The atomistic thermodynamic account for the dehydroxylation and decarboxylation processes of the spinel catalyst surface was complemented by calculation of the energy barriers for the water and carbon dioxide desorption (kinetic stability). In the suprafacial mechanism of the catalyst dehydration, water is produced by association of the adjacent Co–OH and Oads–H species (Figure 10a) without extraction of the lattice oxygen atoms. Transition state for this mechanism (Figure 10a#) shows that only slight shift of the hydrogen stabilized on the O2O,1T


36

Page 37 of 50

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

ACS Catalysis

anions together with a small elongation of the H–O2O,1T and Co–O bonds (from 0.99 Å to 1.19 Å and from 1.90 Å to 2.14 Å, respectively) are required to pass the energy barrier of ∆Ea = 1.15 eV. The final state of this pathway is shown in Figure 10b, and the reaction energy equals 1.01 eV. Obviously, water produced along this route should be preferentially labelled with oxygen-18.

Figure 10: The atomic composition of the topmost (1 × 1) surface element showing subsequent steps of the (100) surface dehydroxylation, following the suprafacial (top panel) and the intrafacial (bottom panel) mechanisms. Left panels shows the initial structure, middle panels the transition state geometries, whereas right ones the final structures. Color coding: cobalt oxide oxygen: red, oxygen adatom: yellow; exposed tetrahedral Co: violet, recessed tetrahedral cobalt: green, octahedral cobalt blue, hydrogen: white.

For the suprafacial water desorption the activation barrier (1.15 eV) is higher than that calculated for the C–H bond activation energy (0.83 eV), although the latter is commonly recognized as a rate determining step of the CH4 combustion.67,68,69 However, in a more adequate account, upon inclusion of the temperature dependent (entropic) components to the DFT calculated electronic barriers, the situation is reversed above 75 °C, it is, much below the onset


37

ACS Catalysis

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

Page 38 of 50

of the methane combustion reaction (T = 300 °C), (see Figure S6 in Supporting Information and discussion therein). At this temperature free enthalpy of activation, ∆Gact, for methane activation equals to 0.83 + 0.51 = 1.34 eV, whereas for water desorption ∆Gact = 1.15 – 0.23 = 0.92 eV, in agreement with the experimentally identified rate determining step. In the intrafacial mechanism, formation of water involves extraction of a lattice oxygen atom as shown in Figure 10 (sequence c-c#-d) in more detail. Substantial reorganization of the bonds that leads to the transition state (c#) is energetically quite costly (∆Ea = 1.71 eV). The resultant atomic configuration of the relaxed defected surface after formation of a H2O molecule is shown in Figure 10d. The calculated reaction energy was equal to 1.45 eV with respect to the initial hydroxylated state of the (100) surface. This pathway of water release leads to formation of H216O molecules.

Figure 11: The atomic composition of the topmost (1 × 1) surface element showing subsequent steps of the (100) surface decarboxylation along the suprafacial (top panel) and the intrafacial (bottom panel) mechanism. Left panels show the initial structure, middle panels the transition state geometries, whereas right ones the final structures. Color coding: cobalt oxide oxygen: red, oxygen adatom: yellow; exposed


38

Page 39 of 50

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

ACS Catalysis

tetrahedral Co: violet, recessed tetrahedral cobalt: green, octahedral cobalt blue, carbon: grey, hydrogen: white.

The complementary suprafacial and intrafacial pathways of the (100) surface decarboxylation are presented in Figure 11. The structure of the surface carboxyl Osurf–COO species produced upon oxidation of the O2O,1T–CH3 fragment is presented in Figure 11a. The transition state configuration (11a#) is achieved by lengthening of the O2O,1T–C bond (from 1.75 to 1.81 Å), which is accompanied by the O–C–O angle flattening. This process requires rather small activation energy of ∆Ea = 0.24 eV only, but is slightly endothermic (∆Er = 0.15 eV). Such low activation barrier indicates that the nascent carbon dioxide molecule (11b) can be easily released (as C18O18O mainly). Taking into account additionally their small stabilization energy (– 0.15 eV) the surface CO2 species are both kinetically and thermodynamically very labile, therefore their liberation is irrelevant for maintaining the sustainable turnover. In the case of the intrafacial decarboxylation the initial attachment of the CO2 molecule is shown in Figure 11c, where a bidentate binding of the carbon atom to the lattice oxygen and the tetrahedral Co sites is responsible for its enhanced stabilization. In the transition state (11c#) the CoT–C and O2O,1T–CoO bonds are released, which is accompanied by strengthening of the both C–O bonds. The relaxed linear carbon dioxide molecule consists of one lattice and one suprafacial oxygen atom (C16O18O) as presented in Figure 11d. The activation energy for this process is equal to 1.13 eV, and the corresponding ∆Er value equals to 0.55 eV, indicating that the intrafacial CO2 species may be kinetically rested on the surface, but at low temperatures only since the desorption process is strongly favored by substantial gain in the translational entropy.


39

ACS Catalysis

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

Page 40 of 50

Summarizing, the obtained results reveal that in the case of water desorption along the suprafacial and intrafacial pathways the kinetic and thermodynamic barriers are very close (with differences < 0.25 eV). Thus, the surface behavior of water during methane combustion is essentially governed by the thermodynamics. Similar situation takes place for the suprafacial desorption of carbon dioxide (0.1 eV). Although, in the case of an intrafacial CO2 release, the kinetic barrier enhances the thermodynamic stability by 0.58 eV, we think that due to the high reaction temperature, where this mechanism operates (T > 450 °C), owing to the favorable entropic contribution the CO2 desorption the thermodynamic limit is not significantly obscured by the kinetics.


40

Page 41 of 50

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

ACS Catalysis

CONCLUSIONS Involvement of surface reactive oxygen species (suprafacial mechanism) and lattice oxygen (intrafacial mechanism) in the course of the catalytic combustion of methane over cobalt spinel nanocubes was explored by SITTKA and TPSR catalytic studies, first principles thermodynamics and molecular modelling. It was revealed that the catalyst redox state is continuously changing with the varying reaction conditions (T, pO2, pH2O, pCO2). From 300 °C to 450 °C, the suprafacial Langmuir-Hinshelwood mechanism operates, and the catalyst stoichiometry is preserved. Methane is activated by monoatomic oxygen O–CoT species (∆Ea = 0.83 eV), whereas facile decarboxylation and dehydroxylation leave the catalyst surface intact. In the temperature range 450 °C < T < 650 °C, also the O3O intrafacial sites acting in tandem with CoO5c are gradually engaged in the methane oxidation (with ∆Ea = 1.11 eV). The (100) surface becomes slightly reduced due to oxygen vacancy formation upon the intrafacial dehydroxylation and decarboxylation of the catalyst, and the vacancies are virtually refilled by gaseous oxygen. Since the stoichiometric and the oxygen vacancy defected terminations coexist in equilibrium in this temperature region, the Langmuir-Hinshelwood and Mars-van Krevelen mechanisms may operate simultaneously. Above 650 °C, generation of the vacancies extends into the bulk of cobalt spinel, and the stoichiometric steps of the catalyst reduction overwhelm the catalytic cycles. The high involvement of the lattice oxygen is indicative for the dominating Mars-van Krevelen mechanism. As a result the CH4 oxidation reaction is accompanied by the concurrent partial reduction of cobalt spinel occurring in the background of the catalytic steps, in order to adjust the catalyst redox state to the current thermodynamic conditions. The observed performance of cobalt spinel is governed by its high capability to redox auto-tuning controlled, at the thermodynamic limit, by equilibrium between the reactants, products and the catalyst surface.


41

ACS Catalysis

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

Page 42 of 50

In particular, it allows for reactive capturing of dioxygen in the temperature window where the spinel propensity for its suprafacial activation is lost.


42

Page 43 of 50

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

ACS Catalysis

AUTHOR INFORMATION Corresponding Author *[email protected], telephone number: +48 12 663 20 73

AUTHOR CONTRIBUTIONS The manuscript was written through contributions of all authors.

SUPPORTING INFORMATION 1.The (1×1) slab models of the (100)-S and (100)-VO terminations; 2.XRF results of nanocubes composition; 3.Pulse 18O2 + Co316O4 experiment results; 4.Oxygen activation and dissociation on (100) surface of Co3O4; 5.The energetic barriers and most important parameters for considered C–H activation steps; 6. Engagement of the monoatomic surface oxygen species in intermediate steps of CH4 combustion; 7. Free enthalpy of activation for key steps of methane combustion; This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGMENT The authors dedicate this article to the late Professor Adam Bielański. This work was supported by the financial support of Polish National Science Center grant no. DEC-2011/03/B/ST5/01564. The research was carried out with the equipment 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).


43

ACS Catalysis

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

Page 44 of 50

ABBREVIATIONS (100)-S; bare stoichiometric termination, (100)-S-O; ROS covered stoichiometric termination, (100)-VO; termination defected by surface oxygen vacancy, CoO; cobalt in octahedral site, CoT; cobalt in tetrahedral site. DFT+U; Density Functional Theory with Hubbard corrected functionals, ∆Ea; activation energy, ∆Er; reaction energy, ∆Ga; activation free enthalpy, ∆Gr; reaction free enthalpy, SI; Supporting Information, FPT; First Principles Thermodynamics, GGA; generalized gradient approximation, LH; Langmuir-Hinshelwood Mechanism, MVK; Mars-Van Krevelen Mechanism, NEB; nudged elastic band method, Oads; monoatomic suprafacial oxygen, Osurf; surface lattice oxygen, ROS; Reactive Oxygen Species, SSITKA; Steady State Isotopic Transient Kinetic Analysis, VO; oxygen vacancy,


44

Page 45 of 50

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

ACS Catalysis

REFERENCES

[1] Liotta, L. F.; Wu. H.; Panteleo, G.; Venezia, A. M. Catal. Sci. Technol. 2013, 3, 3085– 3102. [2] Cargnello, M.; Delgado Jaén, J. J.; Hernández G. J. C.; Bakhmutsky, K.; Montini, T.; Calvino G. J. J.; Gorte, R. J.; Fornasiero, P. Science, 2012, 337, 713–713. [3] Specchia, S.; Conti, F.; Specchia V.; Ind. Eng. Chem. Res. 2010, 49, 11101–11111. [4] Chin, Y.-H.; Buda, C.; Neurock, M.; Iglesia, E. J. Am. Chem. Soc. 2011, 133, 15958– 15978. [5] Chin, Y-H.; Buda, C.; Neurock, M.; Iglesia, E. J. Catal. 2011, 283, 10–24. [6] Choudhary, T. V.; Banerjee, S.; Choudhary, V. R.; Appl. Catal., A 2002, 234, 1–23. [7] Machocki, A.; Rotko, M.; Stasinska, B. Catal. Today 2008, 137, 312–317. [8] Chin, Y.-H.; Buda, C.; Neurock, M.; Iglesia, E. J. Am. Chem. Soc. 2013, 135, 15425– 15442. [9] Forzatti, P.; Groppi, G. Catal. Today 1999, 54, 165–180. [10] Janbey, A.; Clark, W.; Noordally, E.; Grimes, S.; Tahir, S.; Chemosphere 2003, 52, 1041– 1046. [11] Davis, M. B.; Pawson, M. D.; Veser, G.; Schmidt, L. D. Combust. Flame 2000, 123, 159– 174. [12] Kozlov, S. M.; Neyman, K. M. J. Catal. 2016, 337, 111–121.


45

ACS Catalysis

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

Page 46 of 50

[13] Zhang, Y.; Qin, Z.; Wang, G.; Zhu, H.; Dong, M.; Li, S.; Wu, Z.; Li, Z.; Wu, Z.; Zhang J.; Hu, T.; Fan, W.; Wang, J. Appl. Catal., B 2013, 129, 172–181. [14] Paredes, J. R.; Díaz, E.; Díez F. V. Ordóñez, S. Energy Fuels, 2009, 23, 86–93. [15] Bozo, C.; Guilhaume, N.; Garbowski, E.; Primet, M. Catal. Today 2000, 59, 33–45. [16] Yeh, T.–F.; Lee, H.–G.; Chu, K.–S.; Wang, C.–B. Mater. Sci. Eng., A 2004, 384, 324– 330. [17] Gelin, P.; Primet, M. Appl. Catal., B 2002, 39, 1–37. [18] Deutschmann, O.; Maier, L. I.; Riedel, U.; Stroemman, A. H.; Dibble, R.W. Catal. Today 2000, 59, 141–150. [19] Rotko, M.; Machocki, A.; Słowik, G. Appl. Catal., B 2014, 160–161, 298–306. [20] Liotta, L. F.; Di Carlo, G.; Pantaleo, G.; Deganello, G. Appl. Catal., B 2007, 70, 314–322. [21] Feng, S.; Yang, W.; Wang, Z. Mater. Sci. Eng., B 2011, 176, 1509–1512. [22] van Vegten, N.; Baidya, T.; Krumeich, F.; Kleist, W.; Baiker, A. Appl. Catal., B 2010, 97, 398–406. [23] Tao, F. F.; Shan, J.–J.; Nguyen, L.; Wang, Z.; Zhang, S.; Zhang, L.; Wu, Z.; Huang, W.; Zeng, S.; Hu, P. Nat. Commun. 2015, 6, 7798. [24] Marchetti, L.; Forni, L.; Appl. Catal., B 1998, 15, 179–187. [25] Ferri, D.; Forni, L. Appl. Catal., B 1998, 16, 119–126.


46

Page 47 of 50

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

ACS Catalysis

[26] Finocchio, E.; Busca, G.; Lorenzelli, V.; Escribano, V. S. J. Chem. Soc. Faraday Trans. 1996, 92, 1587–1593. [27] Bahlawane, N.; Herve, P.; Ngamou, T.; Vannier, V.; Kottke, T.; Heberle, J.; KohseHöinghaus K. Phys. Chem. Chem. Phys. 2009, 11, 9224–9232. [28] Hu, L.; Peng, Q.; Li, Y. J. Am. Chem. Soc. 2008, 130, 1613–16137. [29] Feng J. H.; Zeng, C. Chem. Mater. 2003, 15, 2829–2835. [30] Tang, X.; Li, J.; Hao, J. Mater. Res. Bull. 2008, 43, 2912–2918. [31] Hu, W.; Lan, J.; Guo, Y.; Cao, X.-M.; Hu, P. ACS Catalysis 2016, 6, 5508–5519. [32] Zasada, F.; Piskorz, W.; Janas, J.; Gryboś, J.; Indyka, P.; Sojka, Z. ACS Catalysis, 2015, 5, 6879–6892. [33] Bahlawane, N. Appl. Catal., B 2006, 67, 168–176. [34] Mars, P.; van Krevelen, D. W. Chem. Eng. Sci. 1954, 3, 41–59. [35] Grzybowska-Świerkosz, B.; Top. Catal. 2000, 11, 23–42. [36] Bielański, A.; Haber, J. Catal. Rev. 1979, 19, 1–41. [37] Hu, W.; Lan, J.; Guo, Y.; Cao, X.-M.; Hu, P. ACS Catalysis 2016, 6, 5508–5519. [38] Zasada, F.; Piskorz, W.; Sojka, Z. J. Phys. Chem. C 2015, 119, 19180–19191. [39] Sokolovskii, V. D. Catal. Rev.: Sci. Eng. 1990, 32, 1–49. [40] Takita, Y.; Tashiro T.; Saito, Y.; Hori, F. J. Catal. 1986, 97, 25–35.


47

ACS Catalysis

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

Page 48 of 50

[41] Maniak, G.; Stelmachowski, P.; Kotarba, A.; Sojka, Z.; Rico-Pérez, V.; Bueno-López, A. Appl. Catal., B 2013, 136–137, 302–307. [42] Stelmachowski, P.; Zasada, F.; Piskorz, W.; Kotarba, A.; Paul, J.-F.; Sojka, Z. Catalysis Today 2008, 137, 423–428. [43] Broqvist, P.; Panas, I.; Persson, H. J. Catal. 2002, 210, 198–206. [44] Stelmachowski, P.; Maniak, G.; Kaczmarczyk, J.; Zasada, F.; Piskorz, W.; Kotarba, A.; Sojka, Z. Appl. Catal., B 2014, 146, 105–111. [45] Liotta, L. F.; Ousmane, M.; Di, C. G.; Pantaleo, G.; Deganello, G.; Marci, G.; Retailleau, L.; Giroir-Fendler, A. Appl. Catal., A 2008, 347, 81–88. [46] Li, J.-B.; Jiang, Z.-Q.; Qian, K.; Huang, W.-X. Chin. J. Chem. Phys. 2012, 25, 103–109. [47] Hu, W.; Lan, J.; Guo, Y.; Cao, X.-M.; Hu, P. ACS Catalysis 2016, 6, 5508–5519. [48] Vannice, M. A. Catal. Today 2007, 123, 18–22. [49] Selcuk, S.; Selloni, A. J. Phys. Chem. C 2015, 119, 9973–9976. [50] Hafner, J. J. Comput. Chem. 2008, 29, 2046–2078. [51] Kresse, G.; Joubert. D. Phys. Rev. B 1999, 59, 1758–1775. [52] Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, 3865–3868. [53] Monkhorst, H. J.; Pack, J. D. Phys. Rev. B 1976, 13, 5188-5192. [54] Henkelman, G.; Uberuaga, B. P.; Jónsson, H. J. Chem. Phys. 2000, 113, 9901–9904.


48

Page 49 of 50

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

ACS Catalysis

[55] http://theory.cm.utexas.edu/vtsttools/ [56] Hamraoui, K.; Cristol, S.; Payen, E.; Paul, J. F. J. Phys. Chem. C 2007, 111, 3963–3972. [57] Piskorz, W., Gryboś, J. Zasada, F. Cristol, S. Paul, J.-F. Adamski, A. Sojka, Z. 2011, 115, 24274–24286. [58] Hirano, T. MOPAC Manual, 7th Edition; Stewart J. P., Ed.; 1993. [59] Reuter, K.; Scheffler, M. Phys. Rev. B 2001, 65, 035406. [60] Hadjiev, V. G.; Iliev, M. N.; Vergilov, I. V. J. Phys. C: Solid State Phys. 1988, 21, L199– L201. [61] Kaczmarczyk, J.; Zasada, F.; Janas, J.; Indyka, P.; Piskorz, W.; Kotarba, A.; Sojka, Z. ACS Catalysis 2016, 6, 1235–1246. [62] Soykal, I. I.; Sohn, H.; Miller, J. T.; Ozkan, U. S. Top. Catal. 2014, 57, 785–795. [63] Zasada, F.; Janas, J.; Piskorz, W.; Sojka, Z. Res. Chem. Intermed. 2016, doi:10.1007/s11164-016-2798-y [64] Martin, D.; Duprez, D. J. Phys. Chem. 1996, 100, 9429–9438. [65] Ledesma, C.; Yang, J.; Chen, D.; Holmen, A. ACS Catalysis 2014, 4, 4527–4547. [66] Efstathiou, A. M.; Gleaves, J. T.; Yablonsky, G. S. In Characterization of Solid Materials and Heterogeneous Catalysts: From Structure to Surface Reactivity; Che, M.; Védrine, J. Eds.; Wiley-VCH; 1999, Vol. 2, p 1013.


49

ACS Catalysis

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

Page 50 of 50

[67] van Santen, R.; Neurock, M. Molecular Heterogeneous Catalysis WILEY-VCH; 2006 [68] Au-Yeung, J.; Chen, K.; Bell, A. T.; Iglesia, E. J. Catal. 2001, 188, 132–139. [69] Van den Bossche, M.; Grönbeck, H.; J. Am. Chem. Soc. 2015, 137, 12035−12044.

TABLE OF CONTENTS


50

Total Oxidation of Lean Methane over Cobalt Spinel Nanocubes

Recommend Documents