On ethane ODH mechanism and nature of active sites over NiO-based catalysts via isotopic labeling and methanol sorption studies

Journal of Catalysis 322 (2015) 118–129

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On ethane ODH mechanism and nature of active sites over NiO-based catalysts via isotopic labeling and methanol sorption studies Z. Skoufa a, E. Heracleous b,c, A.A. Lemonidou a,c,⇑ a

Department of Chemical Engineering, Aristotle University of Thessaloniki, University Campus, 54124 Thessaloniki, Greece School of Science and Technology, International Hellenic University, 14th km Thessaloniki, Moudania, 57001 Thermi, Greece c Chemical Process and Energy Resources Institute (CPERI), Centre for Research and Technology Hellas (CERTH), 6th km Charilaou, Thermi Road, P.O. Box 361, 57001 Thessaloniki, Greece b

a r t i c l e

i n f o

Article history: Received 26 August 2014 Revised 27 November 2014 Accepted 28 November 2014

Keywords: Ethane oxidative dehydrogenation (ODH) NiO-based catalysts Isotopic labeling Kinetic isotope effect (KIE) Methanol sorption O2-temperature programmed desorption (TPD) Reaction mechanism Non-stoichiometric oxygen

a b s t r a c t In this paper, the ethane oxidative dehydrogenation (ODH) mechanism is thoroughly investigated via isotopic labeling and methanol sorption studies over NiO and highly selective Ni0.85Nb0.15Ox catalysts. ODH experiments with unlabeled and deuterium labeled ethane demonstrated the existence of strong kinetic isotope effect (KIE) over both NiO and Ni0.85Nb0.15Ox, indicating that CAH bond scission is the rate determining step in ethane ODH. Similar KIE values obtained for NiO and Ni0.85Nb0.15Ox mixed oxide indicate that both catalysts share similar active sites for ethane activation. Methanol adsorption/desorption followed by TGA, MS, and in situ DRIFTS showed that pure and Nb-doped nickel oxide surfaces primarily host the same redox active sites that differ in terms of abundance (i.e. surface concentration) and activity. O2-TPD studies of used catalysts verified the participation of non-stoichiometric oxygen species in the reaction, which proceeds via a redox mechanism. Based on the above, a detailed reaction mechanism is proposed. Ó 2014 Elsevier Inc. All rights reserved.

1. Introduction Ethylene constitutes a major building block for the petrochemical industry, serving as feedstock for a wide variety of products. It is currently produced by steam cracking of naphtha or ethane, a mature but energy intensive process. Ethane oxidative dehydrogenation (ODH) emerges as the most promising alternative technology for the production of ethylene. The key for successful industrial implementation of the ODH process is the development of an effective catalytic system, able to selectively activate ethane CAH bonds toward ethylene at relatively low temperatures and at the same time minimize the unselective total oxidation routes to COx. Open literature counts numerous studies of different catalytic systems for ethane ODH. The majority of the catalysts studied and developed are based on transition metal oxides, particularly vanadium (V), molybdenum (Mo), and nickel (Ni) oxide. Over the ⇑ Corresponding author at: Department of Chemical Engineering, Aristotle University of Thessaloniki, University Campus, 54124 Thessaloniki, Greece. Fax: +30 2310 996184. E-mail address: [email protected] (A.A. Lemonidou). http://dx.doi.org/10.1016/j.jcat.2014.11.014 0021-9517/Ó 2014 Elsevier Inc. All rights reserved.

last 15 years, NiO-based materials have received much attention as effective catalysts for the oxidative dehydrogenation of light alkanes, their main advantage being their activity at low temperature (T < 400 °C). Multicomponent nickel-based materials for ethane ODH have been patented by Symyx Technologies in early 2000s [1]. Since then, doping with a second metal, such as Nb [2–5], Ce [6], Sn [7], W [8], Zr [9], and Al [10], was found to enhance the catalytic properties of pure NiO toward selective oxidation. Specifically, NiANbAO mixed oxides constitute a low-temperature catalytic system exhibiting one of the best performances in open literature [2–4,11]. While pure NiO is an effective total oxidation catalyst presenting only 20% ethylene selectivity, Ni0.85Nb0.15Ox mixed oxide presents 90% ethylene selectivity and increased activity [2]. Advanced characterization studies showed that part of Nb inserts the NiO parent lattice forming a NiANb solid solution [12]. This structural effect drives an electronic effect, ultimately leading to the reduction of the electrophilic O species of non-stoichiometric NiO, and the suppression of the total ethane oxidation to CO2 [2]. A more recent publication [5] by our group confirmed and generalized the importance and role of the non-stoichiometric oxygen species. Correlations between ethane surface consumption rates and ethylene selectivity as function of the surface concentra-

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tion of O species for NiANbAO mixed oxides with a wide range of Nb content showed clearly that as the excess oxygen per catalyst surface area decreases, the catalysts become less active in the activation of ethane, but more selective toward ethylene (Fig. 1). The same correlation was also established for NiAAlAO mixed oxides [10]. Based on product distribution [2,5], the reactions taking place over pure NiO, as well as over the optimum catalyst, Ni0.85Nb0.15Ox, are the selective dehydrogenation of ethane to ethylene, the primary total oxidation of ethane to carbon dioxide, and the secondary oxidation of ethene to CO2, as shown in Scheme 1. Addition of niobium significantly suppresses the nonselective oxidation reactions compared to NiO without, however, totally eliminating primary ethane oxidation [2,5]. Further improvement of the NiANbAO system is possible only via deep understanding of the intermediate reaction steps and the mechanism under which the reaction proceeds. Complying with the need for passing from massive catalyst screening to rational catalyst design, the main challenge is the development of tailor-made catalysts, which contain if possible only the selective sites for ethylene production. In-depth elucidation of the reaction mechanism and the functionality of the catalyst are prerequisite for the success of such efforts. The mechanism of ethane ODH has been extensively studied over vanadium and/or molybdenum supported catalysts. These studies include, among other, investigation of the rate determining step and the participation of the catalyst lattice oxygen [13,14], as well as investigation of the effect of support and structure of vanadium oxide species on activity/selectivity (see for example Refs. [15,16]), etc. Other studies reach even deeper into molecular level via spectroscopic techniques, effectively describing the nature of catalytic active centers and intermediate reaction products (e.g. [17–19]), or involve detailed DFT studies (e.g. [20]). On the other hand, significantly fewer studies deal with ethane ODH mechanism over the promising family of NiO-based systems. Indicatively, the two most recent reviews on the mechanism of oxidative dehydrogenation [21,22] report the lack of extensive mechanistic investigations of ethane ODH over NiO-based catalysts. On the quest toward elucidation of the reaction mechanism, important issues to be answered are the determination of the rate determining step, as well as the identification of the active catalytic site and redox pairs. A first attempt to systematically investigate the reaction mechanism over NiO-based catalysts was held in the past by our group [23]. Steady State Isotopic Transient Kinetic Analysis (SSITKA) experiments proved the participation of catalyst lattice oxygen in

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Scheme 1. Triangular schematic representation of ethane ODH reaction scheme over NiO-based catalysts.

the reaction, much like a typical Mars-Van Krevelen redox mechanism, both over pure NiO as well over mixed Ni0.85Nb0.15Ox oxide [23]. The scope of the present study is to shed more light on the mechanistic aspects of ethane ODH over NiO-based catalysts. The identification of the rate determining step for ethane activation, as well as for ethylene secondary oxidation, is attempted via isotopic studies and determination of kinetic isotope effects. Moreover, additional studies aiming at determining the active redox pair participating in the reaction mechanism and further elucidating the role of non-stoichiometric oxygen species are conducted. Finally, methanol is used as a probe molecule in in situ DRIFTS and temperature-programmed desorption studies followed by TGA and MS. All the above aspects are approached in view of the important changes in physicochemical and catalytic properties of pure NiO induced by the presence of niobium in the selective Ni0.85Nb0.15Ox mixed oxide. Based on the new results and past studies, a more detailed reaction mechanism is proposed. 2. Experimental 2.1. Catalyst preparation, physicochemical characterization, and catalytic performance Ni0.85Nb0.15Ox mixed oxide was prepared by the evaporation method according to the procedure previously reported [2]. Namely, aqueous solutions containing the appropriate amounts of nickel nitrate hexahydrate (Merck) and ammonium niobate oxalate (Aldrich) were mixed and heated at 70 °C under continuous stirring for 1 h to ensure complete dissolution and good mixing of the starting compounds. The solvent was then removed by evaporation under reduced pressure, and the resulting solid was dried overnight at 120 °C and calcined in synthetic air at 450 °C for 5 h. Pure NiO was obtained from direct decomposition of nickel nitrate hexahydrate (Ni(NO3)26H2O, Merck) at 450 °C for 5 h in synthetic air. The solid catalysts were characterized as to their specific surface area by means of N2 adsorption at 77 K using the multipoint BET analysis method. The BET surface area for pure NiO and Ni0.85Nb0.15Ox was found equal to 12 m2/g and 82 m2/g, respectively [5]. Further details on the crystal structure and additional physicochemical properties of the two catalysts could be sought in previous publications [2,5]. The catalytic performance of the samples in ethane ODH was measured in a fixed-bed quartz reactor with a 10% C2H6/5% O2/85% He flow at 300–425 °C and W/F ratio 0.24 g s/cm3. Details on the experimental setup and the conversion/selectivity obtained have been reported in [5]. 2.2. Isotopic studies The isotopic tracer studies were conducted in a homemade flow apparatus, specially designed to allow fast response times (<10 s). The experiments were performed in a U-shaped quartz reactor. The internal diameter of the reactor tube in the catalytic zone was

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9 mm, whereas the internal diameter in the pre-catalytic and postcatalytic sections was 4 mm. The temperature of the catalyst was measured by a thermocouple placed in a quartz capillary well located in the middle of the catalytic bed. The reactor was situated in a cylindrical furnace controlled by a programmable temperature controller. Experiments with deuterated ethane C2D6 (2% C2D6/He, CortecNet; 99% atom enrichment) and ethylene C2D4 (2% C2D4/He, CortecNet; 99% atom enrichment) were conducted over NiO and Ni0.85Nb0.15Ox in order to investigate the existence of kinetic isotope effect. For exploring ethane activation, experiments with 1%C2D6/1%O2/He, 1%C2H6/1%O2/He, and 0.5%C2D6/0.5%C2H6/1%O2/ He feed were carried out. Additional isothermal experiments were also conducted over both samples that involved switching from 1%C2H6/1%O2/He feed to 1%C2D6/1%O2/He feed. Temperature-programmed experiments with 1%C2D4/1%O2/He, 1%C2H4/1%O2/He, and 0.5%C2D4/0.5%C2H4/1%O2/He feeds were conducted to study ethylene conversion pathways. The kinetic isotope effects were calculated as the ratio of normal hydrocarbon (C2H6 or C2H4) consumption rate to deuterated hydrocarbon (C2H6 or C2D4) consumption rate under identical conditions. The experimental procedure involved catalyst pretreatment under 10% O2/He flow at 450 °C for 30 min, followed by subsequent cooling to room temperature under He flow. Finally, the temperature was raised to 450 °C at a heating rate of 10 °C/min under a total flow of 50 cm3/min of the appropriate composition. The reactor exit was monitored online by mass spectrometry (Omnistar, Balzers) by following m/z signals: 4 (He), 36 (18O2, C2D6), 34 (18O16O), 32 (16O2), 30 (C2H6), 24 (C2H4), 18 (H2O), 44 (CO2), 26 (C2D4). Overlapping fragmentation contributions of various gas compounds were taken into account. 2.3. CH3OH sorption–desorption experiments Methanol sorption/desorption experiments were conducted in a TGA setup (SDT Q600, TA Instruments). A nitrogen flow was saturated with methanol by being passed through a methanol-containing condenser (CH3OH, 99.8% PanReac) kept at appropriate temperature in order to obtain 10% v/v methanol concentration. During the sorption step, the methanol-saturated stream was passed over the catalytic sample at 100 °C for 60 min. Subsequently, the sample was purged with N2 for 60 min to remove physisorbed methanol. Finally, temperature-programmed desorption (TPD) was performed by heating the sample to 450 °C with a heating rate of 10 °C/min under nitrogen flow. Temperature-programmed methanol desorption was additionally studied in the homemade flow apparatus described above. The catalytic samples were placed in a U-shaped quartz reactor and MeOH adsorption was performed at 100 °C via passing a 10% v/v methanol-saturated helium stream for 1 h. After flushing with He until room temperature, the catalyst sample was heated at a constant rate of 10 °C/min under helium flow, and the reactor exit composition was constantly monitored by online mass spectrometry following m/z signals 4 (He), 15 (CH4), 18 (H2O), 29 (CH2O), 31 (CH3OH), 44 (CO2), 46 (DME), 60 (methyl formate). Overlapping fragmentation contributions of various gas compounds were taken into account. Finally, in situ DRIFT spectroscopy was employed for investigating surface species during methanol adsorption over NiO and Ni0.85Nb0.15Ox catalysts. The powdered samples were placed inside a Zn/Se window chamber (Specac), and in situ DRIFT spectra were collected by a Bruker Tensor 27 FT-IR spectrometer. The chamber temperature was controlled by a Specac temperature controller. At each temperature studied (20, 50, 100, 150, 200, and 250 °C), after acquisition of stable reference spectra under He flow, gas phase methanol was introduced to the environmental chamber via a MeOH-saturated helium flow. In situ DRIFT spectra were

acquired 30 min after MeOH introduction. The samples were subsequently purged with He and heated up to the next higher temperature, and the experimental procedure was repeated. 2.4. Temperature-programmed oxygen desorption (O2-TPD) studies Oxygen desorption properties of fresh and used catalysts were studied by O2-TPD measurements. The catalyst sample (200 mg) was pretreated in a flow of He at 450 °C for 0.5 h and cooled to room temperature under helium flow. The system was subsequently flushed with He for 1 h and the temperature was raised to 850 °C at a heating rate of 15 °C/min in He (50 cm3/min). The reactor exit was monitored online by a quadrupole mass analyzer (Omnistar, Balzers) and the desorbed oxygen was detected by following the 32 (m/z) fragment. Calibration of the mass analyzer was performed with O2/He mixtures of known concentration. Quantitative estimation of the amount of desorbed oxygen was performed by integration of the corresponding oxygen flow rates with time. 3. Results and discussion 3.1. Isotopic studies 3.1.1. Kinetic isotope effects in C2H6/O2 reaction Two separate temperature-programmed experiments with normal (1%C2H6/1%O2/He) and deuterated (1%C2D6/1%O2/He) ethane were conducted over both NiO and Ni0.85Nb0.15Ox. Due to the large difference in surface area between the two catalysts, the sample weight was adjusted accordingly in order to achieve similar conversion levels. As expected [5], pure nickel oxide presents significantly higher specific surface activity than Ni0.85Nb0.15Ox. The main reaction products were ethylene (unlabeled and deuterated labeled), carbon dioxide, and water, with no important variation of the product distribution (selectivity) between conventional and isotopic feed within experimental error. However, the introduction of C2D6 in the feed led to significant changes in the temperature profiles of ethane surface consumption rate compared to C2H6, both over NiO and Ni0.85Nb0.15Ox as shown in Fig. 2a and b, respectively. Over both catalysts, the consumption of C2H6 commences at 260 °C and the consumption of C2D6 at 280 °C, indicating lower temperature of CAH (C2H6 activation) compared to CAD (C2D6 activation) bond cleavage. Provided that only reaction rates at differential conditions (C2H6/C2D6 conversion <10%) are used so that the reactants’ partial pressure remains almost constant, it is possible to calculate the apparent activation energy of the reaction with the Arrhenius equation. The calculated values for ethane and deuterated ethane are tabulated in Table 1. In accordance with previous results [5], similar apparent activation energies for C2H6/O2 reaction were found for NiO and Ni0.85Nb0.15Ox. Moreover, for both catalysts, the apparent activation energy for deuterated ethane consumption is higher than the values for normal ethane, revealing relatively easier activation of normal against deuterated ethane. The comparison of reaction rates obtained with C2H6/O2 mixture with those attained with C2D6/O2 at 330 °C (ethane conversion <10%) allowed the quantification of the kinetic isotope effect upon H/D isotopic switch. As tabulated in Table 1, normal kinetic isotope effects (KIE > 1) are observed for both catalysts. The strong kinetic isotope effect for ethane oxidative dehydrogenation, much larger than 1, indicates clearly that CAH bond cleavage is the rate determining step for ethane activation. Moreover, it is very interesting that similar KIE values were observed for pure nickel oxide and nickel–niobium mixed oxide, indicating that both catalysts share similar active sites. The calculated values are in good agreement with published results for ethane oxidative dehydrogenation over

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Table 1 Calculated apparent activation energies for C2H6/O2 and C2D6/O2 reactions and kinetic isotope effects (KIE) based on ethane consumption rates at 330 °C obtained in temperature-programmed experiments for NiO and Ni0.85Nb0.15Ox.

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2.9 2.4

vanadium based catalysts [14], and with older studies on ethane total oxidation over NiO [24]. Using statistical thermodynamics and transition state theory [25], it is possible to theoretically estimate the maximum kinetic isotope effect. For ethane activation, the theoretical KIE at 330 °C was calculated equal to 6.7 and corresponds to the case of ethane activation taking place without catalyst oxygen participation, that is, the transition state involves only homogeneous dissociation of the CAH bond without the formation of bonds between ethane and the catalytic surface. The significantly lower values that were found experimentally suggest that the transition state for hydrogen abstraction during ethane ODH involves weakening of the CAH bond via catalyst oxygen participation. This conclusion is consistent with previous SSITKA studies with isotopically labeled oxygen over NiO and Ni0.85Nb0.15Ox that confirmed the participation of lattice oxygen in the reaction [23]. Aiming to validate the above results and gain further information, experiments using a mixed 0.5%C2H6/0.5%C2D6/1%O2/He feed were conducted over both oxides. The rate of deuterated ethane consumption was found significantly lower and equal to approximately 1/3 of the corresponding rate for normal ethane throughout the experiment, in complete agreement with the above presented results for the separate experiments. Favorable activation of unlabeled C2H6 against C2D6 can be further verified by following the profiles of the formation of water species, shown in Fig. 3 as MS signals corresponding to H2O, HDO, and D2O. Over both NiO and Ni0.85Nb0.15Ox, H2O is the first water species to be formed, as a result of C2H6 activation. As the reaction with deuterated ethane starts, cross-labeled water HDO is formed. Finally, the increase of deuterium atoms’ surface concentration leads to the formation of D2O resulting from the combination of two surface OAD species. It is worth to note that the above findings were verified during isothermal experiments at 330 °C (not shown) that involved switching from C2H6/O2 feed to C2D6/O2 feed. The calculation of KIE at equilibrium conditions, approximately at 30 min after introduction of the isotopic feed, resulted in similar values, KIE = 2.5 for NiO and KIE = 2.4 for Ni0.85Nb0.15Ox.

3.1.2. Kinetic isotope effects in C2H4/O2 reaction In order to investigate whether CAH bond cleavage is a kinetically-relevant elementary step in the activation of ethylene as well, we studied the existence of kinetic isotope effect for the ethylene total oxidation reaction over NiO and Ni0.85Nb0.15Ox. Temperature-programmed surface reaction experiments were conducted (feed composition: 1%C2H4/1%O2/He and 1%C2D4/1%O2/He) and the corresponding profiles are given in Fig. 4a and b for NiO and Ni0.85Nb0.15Ox, respectively. Over both catalysts, the reaction onset is at 250 °C and no significant differences are observed in the consumption rates of C2H4 and C2D4. KIE values equal to unity were calculated for both catalysts, thus indicating that CAH bond cleavage is not a kinetically-relevant step for ethylene oxidation reaction, unlike ethane activation. Similar results on the oxidation of ethylene over NiO have been reported by Yu Yao and Kummer [24], who found equal rates for the oxidation of C2H4 and C2D4. It has been proposed that the rate determining step for ethylene oxidation reaction is most likely ethylene chemisorption through its double bond or the subsequent attack at this bond [25]. In contrast to the above, Argyle et al. [14] concluded that CAH bond cleavage is the rate determining step in C2H4 oxidation over V2O5/Al2O3 catalyst, since a strong kinetic isotope effect was found for this reaction. The difference in bond energies between reactants and products in selective oxidation reactions has been reported as an important factor determining the maximum achievable selectivity at certain reactant conversion [26,27]. In simple words, in the case of ODH, these differences depict the different activity of the alkane substrate and the produced olefin. Regarding ethane oxidative dehydrogenation reaction, ethane CAH bond energy is equal to 419.5 kJ/mol, ethylene CAH bond energy is equal to 444 kJ/mol, and C@C bond equal to 720 kJ/mol [26]. As can be easily observed in literature data for vanadium based catalysts, ethylene selectivity significantly decreases with increasing ethane conversion at constant reaction temperature [14], as opposed to nickel-oxide based systems, over which selectivity decrease is less pronounced. Taking into account the difference in bond energies between CAH and C@C ethylene bonds, the fact that over NiO-based catalysts the rate determining step appears to be the activation of carbon–carbon double bond could explain the different selectivity-conversion plots of those systems compared to other catalysts. At the same time, pure NiO hosts a high surface concentration of excess electrophilic oxygen species as thoroughly reported in literature [2,5,10,29]. Based on their reactivity, the presence of these O species leads to oxidative C@C bond breaking [28]. Taking into account

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that the rate determining step for the activation of ethylene is most probably C@C bond activation it is expected that C2H4 oxidation reaction would be favored over pure NiO compared to Ni0.85Nb0.15Ox mixed oxide. In line with the above statement, nickel oxide presents significantly higher ethylene surface consumption rates compared to nickel–niobium mixed oxide, as shown in Fig. 4a and b. Similar values were obtained for the apparent activation energy in ethylene oxidation reaction, equal to 93 kJ/mol for Ni0.85Nb0.15Ox and 98 kJ/mol for NiO. Therefore, the difference in the reaction rates can be attributed mainly to a difference in pre-exponential factors.

3.2. Methanol sorption/desorption experiments After identifying the rate determining step for the activation of ethane and ethylene, as well as confirming the participation of catalyst oxygen in all reactions occurring during ethane oxidative dehydrogenation in a previous study [23], the final step into comprehending the reaction mechanism is the identification of the nature of catalyst active site. The most commonly applied techniques for this purpose include in situ or operando spectroscopic techniques (indicatively [30,31]). However, the application of these widely used techniques for the mechanistic assessment of oxidative dehydrogenation is limited, as ODH reactions take place at temperatures 400 °C, where the surface and gas phase concentrations of intermediate products are rather low [32]. Moreover, in contrast to metal-based catalysts, the identification and quantification of active sites on oxide catalysts is much more difficult, due to the

complexity of the surface and the weak interaction of alkanes and oxygen with oxidic surfaces [33]. Taking the above limitations into consideration, methanol was employed as probe molecule in order to investigate the catalytic surface of pure and Nb-doped catalyst. Methanol is a suitable probe molecule since selectivity toward the various possible products depends strongly on the nature of the catalyst: redox centers lead to formaldehyde formation, acid centers catalytically active for dehydration reaction lead to DME production, and finally, CO2 is produced over basic centers [34]. In order to investigate the interaction of pure NiO and Ni0.85Nb0.15Ox with methanol, methanol adsorption/desorption was studied over both catalysts via thermogravimetric analysis (TGA), in situ DRIFTS spectroscopy, and temperature-programmed desorption (TPD).

3.2.1. Methanol sorption/desorption followed by TGA During the MeOH adsorption step, the samples were exposed to a MeOH-saturated nitrogen flow at a constant temperature of 100 °C. Both samples underwent a weight increase almost immediately after the introduction of MeOH/N2 flow, indicating adsorption of methanol on the catalytic surface. The system was then flushed for 1 h at 100 °C in order to remove any physisorbed species. During this step, significant weight loss was observed over both samples providing a first indication that the majority of adsorbed species are physisorbed. The amount of chemisorbed species was found equal to 0.11 lmol CH3OH/m2 for NiO and 0.91 lmol CH3OH/m2 for Ni0.85Nb0.15Ox. These species can be either molecularly or dissociatively chemisorbed on the catalytic surface depending on the catalyst (pure or Nb-doped NiO). As will be elucidated by

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in situ DRIFTS and MeOH-TPD followed by MS, the molecular adsorption of methanol is favored over Ni0.85Nb0.15Ox, whereas dissociative adsorption is favored over NiO. Finally, temperature was increased to 450 °C at a constant rate of 10 °C/min under N2 flow, in order to allow desorption of chemisorbed species. During temperature-programmed desorption, both samples underwent higher weight loss than the initial gain during the methanol sorption step. Although TGA alone cannot provide safe speculation about the nature of the desorbed species, this observation indicates that desorption of chemisorbed surface species involves incorporation of catalyst oxygen into desorption products and catalyst reduction, as a result of the interaction of methanol with the catalytic surface [33]. This additional weight loss during the desorption step corresponds to catalyst oxygen consumption of 15.3 lmol O/m2 for pure NiO compared to 6.2 lmol O/m2 for Ni0.85Nb0.15Ox. These values highlight that either more methanol is reacted and/or that MeOH is more deeply oxidized over pure nickel oxide. It should be noted that the above numbers contain uncertainties, due to the complexity of the phenomena that take place and experimental errors, and therefore should not be considered as absolute values. For example, these numbers do not take into account the possible formation and subsequent gasification of water molecules during methanol sorption at 100 °C. Nevertheless, the results of the present work are in good compliance with a former study by the group of Wachs [33] dealing with the quantification of active sites over oxide catalysts, including NiO. Thus, it can be safely concluded that methanol oxidation is more favored over NiO than over Nb-doped NiO, pointing to higher surface concentration and/or activity of active sites on the former. This conclusion is further supported by methanol sorption/desorption experiments followed by in situ DRIFTS and MS as discussed in the following paragraphs. 3.2.2. Methanol sorption/desorption followed by in situ DRIFTS An in situ DRIFTS setup was employed for investigating surface species during methanol sorption over pure and Nb-doped NiO catalysts. DRIFT spectra of NiO and Ni0.85Nb0.15Ox powder samples under MeOH-saturated helium flow at temperatures 20–250 °C are presented in Figs. 5a and 5b, respectively. The spectra resulted from subtraction of reference spectra under helium flow at each temperature, thus corresponding purely to the surface species formed on the catalytic surface.

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250oC

(CH3O-)surf

The double band at 2920/2940 cm1 observed in the spectrum of NiO (Fig. 5a) can be assigned to CAH asymmetric bond stretching of either molecularly adsorbed MeOH [35,36] or CH3OA species [37] resulting from the dissociation of methanol on the catalytic surface. At these frequencies, the distinction between molecularly and dissociatively adsorbed methanol is proved rather difficult [38]. According to the prevailing aspect, the higher frequency bands at 2960 and 2860 cm3 are assigned to molecularly adsorbed methanol [36,39], while the more intense bands at 2945 and 2840 cm3 to dissociatively adsorbed methanol [37,39,40]. According to some authors, these last bands are assigned to asymmetric and symmetric CAH bond stretching, respectively [37,38], however, according to most studies they correspond to Fermi resonance between symmetric vibration mode (ms) and overtone bend (2ds) of CH3 entities of adsorbed CH3OA species [39,41,42]. An increase in the sorption temperature from room temperature (20 °C) to 100 °C leads to a decrease in the intensity of these bands. Moreover, bands associated with the formation of formic species via methanol oxidation on the catalytic surface are evident already at room temperature, as shown from the band at 1350 cm1 [37,43] and the band at 1570 cm3 assigned to OACAO asymmetric stretching vibrations [37,43]. The oxidation of adsorbed methoxy species also leads to the formation of CO2 (2360, 2320 cm1 [37]). At 250 °C, the absence of intense bands associated with adsorbed methoxy or methanol species points to complete conversion of these species toward oxidation products [46], mainly CO2. As shown in Fig. 5b, bands corresponding to molecularly adsorbed methanol are present in the low-temperature spectra of Ni0.85Nb0.15Ox as evident by the intense m(OH) band at 3100– 3500 cm1 [37,40]. Moreover, two bands at 2950–2840 cm1 are also present at room temperature. At higher temperatures, the high frequency band disappears while at the same time a new one at 2925 cm1 appears and the 2840 cm1 is shifted to lower frequencies, indicating the contribution of methoxy groups replacing molecularly adsorbed species. The presence of such groups is further indicated by the rocking q(CH3) [38] mode at 1140 cm1, present in the NiO spectra as well. Moreover, the absorption peaks corresponding to formate species are present only at temperatures higher than 100 °C, while peaks associated with CO2 appear only at 250 °C. According to Collins et al. [38], both molecularly adsorbed

v(O-C-O)s v(O-C-O)as

CO2

Intensity, a.u.

200oC

100oC

50oC

20oC

3950

3450

2950

2450

1950

1450

950

Wavenumber, cm-1 Fig. 5a. DRIFT spectra of MeOH sorption over pure NiO at various temperatures. (Spectra resulting after subtraction of reference spectra under He flow.)

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(CH3OH)surf

CO2

250oC

Intensity, a.u.

v(O-C-O)s v(O-C-O)as

(CH3O-)surf

200oC

100oC

50oC

20oC

3450

2950

2450

1950

1450

950

Wavenumber, cm-1 Fig. 5b. DRIFT spectra of MeOH sorption over Ni0.85Nb0.15Ox at various temperatures. (Spectra resulting after subtraction of reference spectra under He flow.)

CH3OHsurf species (1030 cm1) as well as CH3OA methoxy groups (1070 cm1) contribute to the band at 1070 cm1. As the sorption temperature increases, the shift of the CAO vibration area toward higher frequencies implies increased contribution from the latter species. In contrast to pure NiO, bands corresponding to CH3OA species remain in the spectrum of Ni0.85Nb0.15Ox even at 250 °C. Gas phase methanol is known to chemically adsorb on oxide surfaces at low temperature (<150 °C) and form stable methoxy groups (CH3OA) [40,44], while molecular adsorption is also possible on metal cations presenting Lewis acidity [39]. Moreover, according to Sanders et al. [45], methanol adsorbs dissociatively over non-stoichiometric oxygen species O, and molecularly over Ni2+AO2 centers. Our results confirm the above observations, as methanol adsorbs molecularly over Ni0.85Nb0.15Ox at significant extent, indicative of the increased Lewis acidity of the mixed oxide compared to pure NiO [2]. It can be therefore deduced that (CH3OH)surf species are associated with Nb5+ surface cations and/or Ni2+AO2 centers on Ni0.85Nb0.15Ox surface. Over pure NiO, methanol is mainly adsorbed dissociatively, forming (CH3OA)surf and OHAsurface species. The above results comply with a higher concentration of active species over pure NiO compared to Ni0.85Nb0.15Ox mixed oxide, capable of abstracting methanol hydrogen atoms, and leading to the formation of methoxy groups. Moreover, the fact that over Ni0.85Nb0.15Ox the dissociative adsorption of methanol takes place only above 100 °C indicates lower activity of these centers compared to pure NiO. According to the widely accepted reaction scheme for methanol dissociative adsorption, the reaction of methoxy groups with a second surface oxygen leads to the formation of a second hydroxyl and formaldehyde [46]. Formaldehyde production requires weak Lewis acid sites to prevent too strong adsorption and a relatively low amount of active oxygen to prevent fast over-oxidation to formic acid, methyl formate, and CO2 [47]. As already discussed, Ni0.85Nb0.15Ox mixed oxide complies with the above requirements. However, over pure NiO, the existence of highly electrophilic O species leads to further oxidation reactions even at the lowest temperatures studied. The above results are further verified by temperature-programmed MeOH desorption experiments, discussed in the next paragraph. 3.2.3. Methanol sorption/desorption followed by MS Temperature-programmed methanol desorption experiments were conducted in order to identify the various gaseous products

formed upon methanol sorption over NiO and Ni0.85Nb0.15Ox. Methanol was initially adsorbed at 100 °C, and the samples were consequently purged with He until room temperature (approximately for 1 h). The sample temperature was then increased under helium flow, and the reactor exit was monitored by on-line mass spectrometry. Desorption profiles for NiO and Ni0.85Nb0.15Ox are depicted in Fig. 6a and b, respectively. In all cases, CH3OH, CH2O, H2O, CH4, and CO2 were detected in the TPD reactor effluent. No DME was observed, indicating the absence of strong Lewis/Brönsted acid sites capable of methanol dehydration [34]. Methanol evolution can result from either direct desorption of physisorbed CH3OH and/or recombination of methoxy- and hydroxy-groups in the gas phase [48]. Dehydrogenation of remaining (CH3OA)surf species leads to the formation of formaldehyde and/or CO2 [48]. Formaldehyde is probably produced via the following reactions:

CH3 OH þ NiðsurfÞ þ OðsurfÞ ! CH3 ONiðsurfÞ þ OHðsurfÞ CH3 ONiðsurfÞ þ OðsurfÞ ! HCHO þ OHðsurfÞ þ NiðsurfÞ Its further oxidation can lead to the production of CO2 as follows:

HCHO þ 2OðsurfÞ ! CO2 þ H2 O Moreover, methane is produced from the following reaction:

CH3 ONiðsurfÞ þ OHðsurfÞ ! CH4 þ 2OðsurfÞ þ NiðsurfÞ Overall, the interaction of MeOH with the surface of nickel oxide and nickel–niobium mixed oxide probably follows a pathway of sequential oxidation reactions [34], with no important dehydration steps taking place, as indicated by the absence of DME and methyl formate in the desorption profiles. It is therefore verified that the active catalytic centers over both materials constitute redox sites capable of catalyzing the several dehydrogenation steps taking place also in the oxidative dehydrogenation of ethane. Integration of the desorption peaks corresponding to unreacted CH3OH and all dehydrogenation/oxidation products shows that over Nb-doped NiO the ratio of unreacted to reacted adsorbed methanol is one order of magnitude higher than over pure NiO (5.1% compared to 0.6%). These calculations can explain the results of methanol sorption followed by TGA that showed higher amount of chemisorbed methanol over Ni0.85Nb0.15Ox: the chemisorption

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3.3. Investigation of redox pair via O2-TPD and catalytic activity studies

H2O

CH2O

CH3OH

m/z:46=0 m/z:60=0

CH4 20

70

120

170

220

270

320

370

420

Temperature, oC

a

MS signal/m2

CO2

H2O

CH4 CH3OH 20

70

120

170

m/z:46=0 m/z:60=0

CH2O 220

270

320

370

420

Temperature, oC

b Fig. 6. Evolution of products in as a function of temperature following methanol adsorption for (a) NiO and (b) Ni0.85Nb0.15Ox.

mechanism differs between pure and Nb-doped NiO. The TPD-MS studies confirmed that a significantly higher amount of methanol molecules adsorb dissociatively and further react on pure nickel oxide than on Nb-doped NiO, pointing to higher surface concentration of active sites on the former, as already stated. Moreover, oxidation of methoxy species occurs at significantly lower temperatures than over Ni0.85Nb0.15Ox. This relates to the existence of more active oxygen species on NiO surface, in accordance with the findings of in situ DRIFTS CH3OH adsorption results. The integration of the desorption peaks corresponding to formaldehyde and CO2 leads to a HCHO/CO2 production ratio of 3.6% for NiO and 12.3% for Ni0.85Nb0.15Ox, pointing out to a greater contribution of total oxidation reaction products over pure nickel oxide. Summarizing, methanol interaction with NiO and Ni0.85Nb0.15Ox surfaces leads to the same redox-originating products, indicating that both catalysts share the same type of redox surface sites, the concentration and activity of which appears to be higher over pure NiO compared to Ni0.85Nb0.15Ox mixed oxide. Overall, the fact that similar trends were recorded for the activity of the two catalysts in MeOH oxidation and ethane ODH reactions supports the suitability of MeOH as a probe molecule for the characterization of redox active surface species over both oxides. Similar conclusions have been driven in the case of ethanol and propane oxydehydrogenation reactions for the investigation of redox active species over V-based catalysts [27].

According to the results of methanol sorption/desorption experiments presented above, the two catalysts mainly host active redox type species, that are presumably involved in ethane oxidative dehydrogenation reaction. In order to obtain a picture of the oxidation state of the catalysts in ethane ODH conditions, O2-TPD experiments were conducted over used NiO samples. The used samples were acquired from ODH experiments of same duration with different ethane conversion levels, equal to 9% for sample NiO-used_1 and 18% NiO-used_2 (45% and 90% O2 conversion, respectively) at 400 °C. The surface area of the samples was measured again after reaction. As shown in Fig. 7, a significantly lower amount of oxygen is desorbed from the used samples compared to the fresh catalyst. Moreover, the used sample from higher conversion level experiment demonstrates lower surface concentration of non-stoichiometric oxygen species. This result further verifies the participation of excess oxygen species in ethane oxidative dehydrogenation reaction. In a former study conducted by our group, temperature-programmed experiments performed over Ni0.85Nb0.15Ox with ethane in the absence of gas phase oxygen showed that at 460 °C ethane undergoes rapid consumption leading to oxidation products (CO2 and H2O) and subsequent catalyst reduction accompanied by the formation of H2, CO, CO2, and coke; no measurable quantities of ethylene were detected at these temperatures [49]. Similar results were reported also by Chen et al. [50], who observed the selective formation of C2H4 over NiO during C2H6 pulses in the absence of gas phase oxygen only at temperatures lower than 450 °C. It was, however, found that at 450 °C ethane rapidly consumes the available, selective for the production of C2H4, species and subsequently reduces NiO to Ni0. Aika and Lunsford have also observed the consumption of O species within a few seconds during ethane adsorption on MgO at 25 °C, concluding that non-stoichiometric oxygen species and not O2 lattice oxygen species are the active sites for the formation of ethylene [51]. Thus, NiO yield in ethane oxidative dehydrogenation was associated with the ease of Ni(2+d)+ANi2+, (0 < d < 1) transition [50]. The above perspective was restated by Shuurmann et al., according to whom relation of the ODHE reaction activity of nickel oxides with the ease of the Ni2+ANi0 transition can be ruled out [52]. Having excluded O2 species, Shuurmann et al. proposed that O species could be possibly considered as the active site [52], since they are more

1.00 0.90

Desorbed Ο2, mg/m2

MS signal/m2

CO2

0.80 0.70 0.60 0.50 0.40 0.30 0.20 0.10 0.00

NiO fresh

NiO-used_1

NiO-used_2

(9% C2H6 conv.)

(18% C2H6 conv.)

Fig. 7. Excess oxygen determined from O2-TPD experiments for fresh (NiO-fresh) and used nickel oxide catalysts. NiO-used_1 acquired from ODH experiment with 9% C2H6 conversion, NiO-used_2 acquired from ODH experiment with 18% C2H6 conversion (400 °C, 10%C2H6/5%O2/He).

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C2H6, consumptio n rate, μmol/m2.s

stable than O 2 and have already been proposed as active sites for oxidative dehydrogenation of ethane over typical OCM catalysts [53,52,54]. The participation of O species in ethane activation and ethylene formation has also been proposed by Kaddouri et al. NiMoO4 catalysts [55]. We also previously observed [5,10] that, over NiO-based catalysts, the surface concentration of nonstoichiometric electrophilic oxygen O species is directly related to the ethane activation rate and inversely correlated to ethylene selectivity, in general agreement with the concept of site isolation [56,57]. Combined with the results presented in the previous sections, it can be safely deduced that over NiO and Ni0.85Nb0.15Ox, the active sites for ethane oxidative dehydrogenation are the monovalent electrophilic surface oxygen species. It seems that depending on the conversion level, the catalyst under reaction conditions is at semi-reduced state defined by the relative rates of consumption/replenishment of non-stoichiometric catalyst oxygen, simply described as the NiO1+x M NiO transition. At constant ethane conversion, the equilibrium point is determined by the reaction temperature and ethane/oxygen partial pressures that determine the relative rates of catalyst reduction to re-oxidation. Having stated the above, the next emerging question concerns the activity of a catalytic sample that has zero concentration of non-stoichiometric oxygen species. In order to answer the above question, fresh NiO and Ni0.85Nb0.15Ox samples were subjected to temperature-programmed oxygen desorption, involving temperature increase under He flow until 800 °C, in order to remove excess oxygen to the furthest possible extent. After the O2-TPD experiment, both samples underwent a color change, from dark gray (indicative of non-stoichiometric nickel oxide [58]) to light green. The resulting catalytic samples (NiO-after TPD and Ni0.85Nb0.15Oxafter TPD) were characterized as to their surface areas and were tested in ethane oxidative dehydrogenation reaction. The results are shown in Fig. 8, where the results for fresh NiO and Ni0.85Nb0.15Ox catalysts calcined at 450 °C under air flow are also depicted for comparison. NiO-after TPD and Ni0.85Nb0.15Ox-after TPD present significant activity, comparable to that of the original materials. These results indicate that gas phase oxygen replenishes the surface active oxygen species. Desorption of oxygen during TPD-O2 experiments results in the formation of anionic vacancies on the catalytic surface. According to Mc Farland and Metiu [59], these vacancies act as bases (electron acceptors) and under reaction conditions interact with gas phase oxygen that acts as an acid. This interaction leads to catalyst re-oxidation and subsequent ‘‘regeneration’’ of part of the electrophilic O species. The capability of NiO to recover its excess oxygen has been previously been reported by Chen et al. [50], who observed identical O2-TPD curves after

1.20 1.00 0.80

NiO-fresh NiO-after TPD Ni0.85Nb0.15Ox-fresh Ni0.85Nb0.15Ox-after TPD

0.60 0.40 0.20 0.00 320

340

360

380

400

420

Temperature, oC Fig. 8. Surface rates of C2H6 consumption for fresh (full symbols) and after TPD (empty symbols) NiO and Ni0.85Nb0.15Ox samples.

desorption–oxidation–desorption cycles. Analogous indications for the consumption/regeneration process of O active species were recently reported by our group in collaboration with Marcu and co-workers [60]. During in situ electrical conductivity measurements, NiAMeAO (Me = Li, Mg, Al, Ga, Ti, Nb) mixed oxides were subjected to subsequent reduction–oxidation cycles. During the reduction step (ethane or ethane/air flow), a decrease in electrical conductivity was observed, indicative of O consumption. After each reduction cycle, the electrical conductivity was restored under oxidative conditions (air flow) pointing to regeneration of the main charge carriers, O species. The differences in consumption rates between fresh and ‘‘after TPD’’ samples shown in Fig. 8 could be attributed to possible structural changes that the materials undergo due to their treatment at high temperature (800 °C) which could lead to partial re-arrangement of the catalyst surface and of the amount of exposed active sites. In conclusion, the concentration of electrophilic oxygen species cannot be significantly modified by simply subjecting the catalysts to oxygen desorption, because under reaction conditions, gas phase oxygen replenishes these species. Oxygen over-stoichiometry constitutes an intrinsic property of nickel oxide, and as such can be tuned by lattice reconstruction during catalyst synthesis, by adding for example a second higher valence metal, such as Nb. 3.4. Proposed reaction mechanism The extended isotopic and characterization results presented in the previous paragraphs, along with previously published results on ethane ODH over NiO-based catalysts [5,10,23] constitute a solid base for the elucidation of the reaction mechanism. Methanol sorption–desorption experiments showed that both catalysts mainly host redox active centers comprising surface oxygen atoms. The participation of catalyst oxygen species in ethane ODH reaction was verified in the past by our group [23], thus indicating that over NiO and Ni0.85Nb0.15Ox, the reaction proceeds much like a typical Mars-van Krevelen mechanism [61]. The systematic investigation of the effect of Nb loading in the physicochemical and catalytic properties of NiANbAO mixed oxides led to the establishment of the important role of excess oxygen species both for catalytic activity as well as for ethylene selectivity. While pure NiO is a non-stoichiometric oxide with oxygen excess due to cationic vacancies [29,62], the introduction of Nb leads to a decrease in the surface concentration of non-stoichiometric O species [2,5,10]. Moreover, Ni0.85Nb0.15Ox surface oxygen species appear to be less active compared to pure NiO as shown by isotopic oxygen exchange studies [23] and further indicated by in situ DRIFTS MeOH sorption and MeOH-TPD results in the present study. For ethane and propane ODH, alkane adsorption on the catalytic surface is considered reversible and quasi-equilibrated [14,63]. It is proposed that this is also the case for NiO and Ni0.85Nb0.15Ox catalysts, as shown in Scheme 2a. The existence of a strong kinetic isotope effect during isotopic H/D substitution of ethane CAH bonds showed that the rate determining step in ethane activation is CAH bond cleavage, which is a common step for pure and Nbdoped NiO catalysts. The absence of metal centers of strong Lewis/Bronsted acidity most likely points out to a single electron process, that is homolytic CAH bond rupture involving C2H5 and H formation. Moreover, the direct correlation of specific surface activity with the concentration of OA species, indicates that Hatom extraction is catalyzed by these surface oxygen species, as shown in Scheme 2b. The subtracted hydrogen is linked to a surface O anion (Scheme 2b); however, the fate of C2H5 radicals is not as clear. Because of their short lifetime, identification of the nature of the key intermediate species formed between ethyl radicals and the catalyst surface has proved rather controversial. The ethyl radicals

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127

Scheme 2. Proposed reaction mechanism for ethane ODH over NiO-based catalysts.

can be associated with the catalytic surface either as ethoxides, or as ethyl-nickel entities. The former possibility was proposed by Busca et al. [64] and is in agreement with DFT studies by Witko [65] and Piskorz et al. [66]. The formed ethoxide may either rapidly decompose to form ethylene or be further oxidized to CO2. On the contrary, according to Kung [67], CAH bond rupture leads primarily to the formation of ethyl radicals associated with a metal – and not oxygen – atom, which are further subjected to b-hydrogen elimination to produce ethylene. Alternatively, the formation of CAO bonds between the alkyl radicals and the catalytic surface leads to oxygenated products. Finally, according to Oyama et al. [68], hydrocarbon intermediates bonded to metal centers through oxygen atoms result in selective oxidation products, whereas Mbonded intermediates lead to total oxidation products. It should be noted that the majority of the above studies were conducted over catalysts based on group V and/or VI elements, while none of them referred to NiO-based catalysts. Very few attempts to elucidate the reaction mechanism have been undertaken either experimentally or on a theoretical level for the latter family of catalysts. It was only recently that interesting DFT studies were conducted on ethane activation over NiO-based systems. According to Metiu and coworkers, after initial ethane activation over NiO, the formation of OAC2H5 and OAH surface bonds is equally likely to the formation of NiAC2H5 and OAH [69]. A similar approach has been published by Lin et al., who studied the activation of ethane and the possible intermediate species leading to the final products, ethylene and CO2 [70]. In short, it was found that for the ethyl-nickel entity, the most favorable pathways are (a) bhydrogen elimination by an O atom in close vicinity and (b) isomerization to ethoxide. Moreover, since isomerization of ethyl-nickel to ethoxide was found to be facile thermodynamically favorable,

the possible further reactions of ethoxide were also investigated. It was found that upon ethoxide formation CAC bond cleavage was the most favorable route, while b-hydrogen elimination to produce ethylene was rather unlikely [70]. In the present study, the dissociative adsorption of methanol over NiO and Ni0.85Nb0.15Ox was confirmed by MeOH adsorption– desorption studies. According to literature, the oxidation of methanol over metal oxides takes place on MeAO centers [33] (Me: metal such as Ni), by the abstraction of the hydroxyl H-atom by a surface O and the concurrent formation of a surface methoxy (CH3O) species on a Me atom. In situ DRIFTS results of methanol adsorption showed that the formed methoxy groups remain attached to the surface nickel atoms even at high temperatures. Based on the above, it can be deduced that after H-abstraction, the ethyl radicals associated with the catalytic surface are most probably linked to a Ni atom in close vicinity, as shown in Scheme 2b. The important similarities between NiO and Ni0.85Nb0.15Ox established in the present study regarding ethane activation indicate that the above described elementary reaction steps are common for both materials. The type of further interaction of the ethyl-nickel group with the catalytic surface determines the selective C2H4 or unselective CO2 formation. The selective transformation of the ethyl-nickel group to ethylene involves the elimination of a b-hydrogen as shown in Scheme 2c, which is more likely to occur over a low O concentration surface (such as that of Ni0.85Nb0.15Ox), since it was found that the decrease in non-stoichiometric oxygen species leads to increased ethylene selectivity [5]. In line with the well-known property of nucleophilic O2 species to catalyze the formation of selective partial oxidation products [71], we propose that b-hydrogen elimination takes place by a normal lattice oxygen atom

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(Scheme 2c). Ethylene C@C bond is then formed and C2H4 is subsequently desorbed in the gas phase (Scheme 2d). The two hydroxyl groups that are formed are subsequently recombined and produce water. Both over pure NiO as well as over Ni0.85Nb0.15Ox, part of ethane is primarily unselectively converted to carbon dioxide [2,5]. According to the aforementioned DFT study by Lin et al. [70], the most favorable pathway for the production of CO2 during ethane activation over nickel oxide is the isomerization of the ethyl-nickel group toward the formation of a surface ethoxide. These findings agree with our experimental results: over NiO surface, hosting a high concentration of O species, ethane total oxidation to carbon dioxide is the prevailing route. In that sense, the existence of a large electrophilic oxygen population in close vicinity to the formed ethyl-nickel group increases the possibility of isomerization to ethoxide (Scheme 2e) and subsequent CO2 formation (Scheme 2f). Carbon dioxide is probably formed via CAC bond scission and subsequent dehydrogenation steps, however, a-hydrogen elimination pathway is also possible, although not as thermodynamically favored [70]. Water is also formed, leading to catalyst reduction, as shown in steps g–h in Scheme 2. Regarding ethylene oxidation reaction, temperature-programmed surface reaction experiments with C2D4 isotopic feed revealed the absence of kinetic isotope effect (KIE = 1), and therefore indicated that, unlike ethane activation, the CAH bond cleavage is not a kinetically-relevant step in the activation of ethylene over NiO and Ni0.85Nb0.15Ox. The rate determining step is most probably the activation of ethylene C@C bond, catalyzed by O and thus favored over non-stoichiometric pure NiO. The reaction sequence after the C@C bond rupture is likely to follow similar pathways as the primary ethane oxidation route, leading to CO2 and water formation. Oxygenated reaction products (CO2 and H2O) are formed via the consumption of catalyst oxygen, confirmed in the past by 18O2 studies [23]. The redox cycle is terminated by catalyst reoxidation via the interaction of gas phase oxygen with formed anionic vacancies (Scheme 2i). In the present study, it was established that under reaction conditions, the catalyst functions at a semi-reduced state. Depending on ethane conversion levels and reaction conditions, the catalyst initial oxygen over-stoichiometry reaches an equilibrium determined by the relative rates of the reduction/oxidation steps of a Mars-van Krevelen type mechanism. Finally, it should be noted that our experimental evidence are limited to macroscopic observations. Therefore, excluding the definite character of the isotopic and TPD experimental results, the proposed reaction scheme should be considered as a tentative mechanism, a working assumption to be further verified. Consequently, we cannot utterly exclude all other possible surface reaction pathways following initial CAH cleavage. Considering the complexity of surface elementary reactions and the multiplicity of possible intermediate surface species, detailed DFT studies need to be conducted, in order to further verify the above proposed mechanism in terms of thermodynamic ab initio calculations of the different transition states. Such efforts have already been undertaken to a certain extent [69,70]; however, none of the studies published so far takes into consideration the importance of non-stoichiometric oxygen species pointed out by our work.

4. Conclusions The present study constitutes an in-depth attempt to elucidate the mechanism of ethane oxidative dehydrogenation over NiObased catalysts, highlighting the similarities and differences between pure and Nb-doped nickel oxide. The comparison of alkane consumption rates during oxidative dehydrogenation of

unlabeled and deuterated labeled ethane proved the existence of strong KIE for ethane activation over NiO and Ni0.85Nb0.15Ox, indicating that over both catalysts CAH bond scission is the rate determining step. On the contrary, ethylene C@C bond activation seems to be the rate determining step for ethylene activation. The similar KIE values obtained for pure NiO and Ni0.85Nb0.15Ox mixed oxide demonstrate that both catalysts share similar active sites for ethane activation. Methanol adsorption/desorption followed by TGA, MS, and in situ DRIFTS was successfully employed for the investigation of the nature of active sites present on the surface of pure and Nb-doped nickel oxide. The results showed that both catalysts primarily host the same redox active sites that differ in terms of abundance (i.e. surface concentration) and activity. In combination with previous studies that verified the importance of non-stoichiometric oxygen species, it is proposed that the active sites for ethane ODH over NiO-based catalysts are O surface species. Thus, ethane ODH reaction takes place via a Mars-van Krevelen type of mechanism. The redox transition can be described by the NiO1+x M NiO equilibrium; O2-TPD studies of used catalysts combined with activity testing of catalyst samples after excess oxygen desorption verified that nonstoichiometric oxygen species are constantly consumed and replenished under reaction conditions. At constant ethane conversion, the equilibrium point is determined by the reaction temperature and ethane/oxygen partial pressures that determine the relative rates of catalyst reduction to re-oxidation. Based on the above conclusions, a reaction mechanism is proposed. Ethane activation over the catalytic surface involves the subtraction of a hydrogen atom by a monovalent electrophilic oxygen atom. The surface concentration of non-stoichiometric oxygen determines further reaction of the formed ethyl radical. In case of low O concentration, the b-hydrogen elimination leads to ethylene production; in the opposite case, ethyl radicals are prone to total oxidation and CO2 formation. The importance of the conclusions reported in this paper lies in the implications for further catalyst improvement, based on the proposed reaction mechanism. The target could be summarized as the minimization of ethane primary and ethene secondary oxidation to CO2 via further eliminating the surface concentration of monovalent oxygen species over a high surface area catalyst: lower non-stoichiometric oxygen concentration could lead to selectivity increase; however, a high surface area is needed to obtain the necessary population of O active sites for satisfactory ethane conversion levels.

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