Origins of the deactivation process in the conversion of methylbutynol on zinc oxide monitored by operando DRIFTS

Catalysis Today 205 (2013) 67–75

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Origins of the deactivation process in the conversion of methylbutynol on zinc oxide monitored by operando DRIFTS Charlotte Drouilly a,b , Jean-Marc Krafft a,b , Frédéric Averseng a,b , Hélène Lauron-Pernot a,b , Delphine Bazer-Bachi c , Céline Chizallet c , Vincent Lecocq c , Guylène Costentin a,b,∗ a

UPMC, University Paris 06, UMR 7197, Laboratoire Réactivité de Surface, F-75005 Paris, France CNRS, UMR 7197, Laboratoire Réactivité de Surface, F-75005 Paris, France c IFP Energies Nouvelles, Rond-Point de l’échangeur de Solaize, BP3, 69360 Solaize, France b

a r t i c l e

i n f o

Article history: Received 14 June 2012 Received in revised form 31 July 2012 Accepted 1 August 2012 Available online 19 September 2012 Keywords: ZnO Deactivation MBOH Ethanol EPR Operando DRIFT Pre-treatment Oxygen

a b s t r a c t The catalytic behavior of zinc oxide samples was studied thanks to a model methylbutynol (MBOH) conversion reaction. The formation of acetone and acetylene is indicative of the basic properties of the zinc oxide surface. This basic catalyst, whose conversion level depends on the nature of the pre-treatment, was found to deactivate versus time on stream. Poisoning the basic sites by CO2 pre-adsorption only affects the active sites working at the beginning of the reaction, those still working at steady state being not impacted. This is consistent with the modification of the strength and population of the active sites during the catalytic reaction. Indeed, from operando DRIFTS experiments, at the beginning of the reaction, the strong acid base pairs generated upon inert treatment at 773 K promote the self aldol condensation of acetone, leading to oligomers. This reaction also produces water, which dissociates on the surface, filling up oxygen vacancies. This contributes to the lowering of the strength of the active sites, resulting in the quenching of polymerization of acetone at a certain stage, at the benefit of the increase of the amount of diacetone alcohol. Consistently, pre-adsorption of water was shown to lead to a decrease of the conversion level, resulting in a conversion profile similar to that obtained after 10 min of reaction in the absence of pre-adsorbed water. The poisoning mechanism proposed is based on the evolution of the IR bands versus time and is in line with the evolution of the deactivation profile versus time. It is concluded that both the control of the conversion level and the dependence of the reaction toward deactivation are associated to the formation of oxygen vacancies during the pre-treatment step but also on their stability in the conditions on the implemented catalytic reaction. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Due to their large field of applications, notably related to their optoelectronic properties, zinc oxide model systems have been extensively studied as single crystals, thin films or even as nanopowders involving particles with various morphologies [1]. Zinc oxide is also well known as basic oxide, notably used for its basic properties as sulfur adsorbent [2] or as heterogeneous catalyst [3–6]. In particular, in the context of post oil solutions investigations, and of development of biomass exploitation processes (zinc oxide is involved in the composition of catalysts active for biofuels

∗ Corresponding author. Tel.: +33 1 44 27 36 32; fax: +33 1 44 27 60 33. E-mail addresses: [email protected] (C. Drouilly), [email protected] (J.-M. Krafft), [email protected] (F. Averseng), [email protected] (H. Lauron-Pernot), [email protected] (D. Bazer-Bachi), [email protected] (C. Chizallet), [email protected] (V. Lecocq), [email protected] (G. Costentin). 0920-5861/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cattod.2012.08.011

productions [3–5]), there is a renewed interest for the characterization of real zinc oxide powders, easy to produce in quite large amount. This is typically the case of commercial kadox sample and of ex-carbonate zinc oxide that is simply obtained from thermal decomposition of zinc carbonate, that are classically used as real catalyst [7,8]. Recently, we have shown that their basic reactivity toward a model reaction, the methylbutynol (MBOH) conversion or for a reaction of practical interest such as the conversion of bio ethanol into ethylene and acetaldehyde is governed by the concentration of oxygen vacancies formed or filled up during the pre-treatment performed under inert or oxygenated atmospheres, respectively (Eq. (1)). O2− − Zn2+ − O2−   − Zn2+ − O2− + 2e + 21 O2

(1)

Note that the two release electrons associated to oxygen vacancy formation could be formally trapped in the related oxygen vacancy (Vo• and Vo˚ , respectively, for one or two electrons), contribute to zinc reduction (interstitial Zn+ formation), or even be delocalized on

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Fig. 1. Activation (A) and deactivation of Bvac site depending on the induced modification of electronic density related to oxygen vacancy formation or filling up, respectively or to direct poisoning of the acid base pair by adsorbed reactants (B, C) or products (D). The Bweak site, present on the kadox sample only, is not poisoned by any of the species in the experimental conditions investigated.

the whole semi-conductor material resulting in an empty Vo•• (or ) oxygen vacancy. In fact in the conditions of a catalytic reaction, the electronic density associated to the formation or filling up of O vacancies modulates the basicity of active sites, hereafter referred to as to Bvac sites, that are classically built from acid–base pairs, which oxide anion’s electron density has thus be enriched Zn2+− O2− +␦− (Fig. 1A). An additional type of active basic sites weaker than those sensitive to oxygen vacancies formation was proposed to be present only on the well-defined hexagonal particles of the kadox sample, and called Bweak (Fig. 1A), resulting in a less drastic poisoning impact of CO2 adsorption observed for this sample and in a predicted residual basic activity in absence of any oxygen vacancy [9]. Moreover, it was observed that the conversion levels of the two studied reactions do not vary similarly upon time: though, fortunately, it is very stable for the ethanol transformation reaction, the ZnO catalysts were found to deactivate over time in the model MBOH conversion reaction. A model reaction is expected to provide important understanding data on the catalytic behavior of a system for a class of reactions [10]: besides identification of the type of reactivity, basic one in the case of zinc oxide, and classification of the amount and strength of the related active sites, in this study we aim at studying the origins of the deactivation process of zinc oxide toward MBOH conversion. These data, discussed in relation with the stability observed for ethanol transformation, should help us to go deeper in the knowledge of the parameters governing the catalytic properties of zinc oxide toward alcohol transformation. The influence of natural surface adsorbates, i.e. OH− and CO3 2− or even adsorbed reactants or products might impact the evolution of conversion versus time. To clarify the various processes possibly involved in the deactivation process toward MBOH reaction,

two types of experiments were performed: (1) after classical pre-treatment implemented at 773 K under nitrogen or O2 flow, adsorption of CO2 , H2 O or ethanol, taken as a model of alcohol reactant, was carried out just prior to implementation of the catalytic reaction. (2) An operando DRIFTS study of the catalytic reaction was performed to investigate the nature and the related amount variation of the different adsorbates formed on the surface during the reaction in relation with the deactivation profile. 2. Experimental 2.1. Samples A commercial kadox ZnO sample (Kadox 911, Horsehead Co., 99.995%), 9 m2 g−1 , involving well defined hexagonal shaped particles (50–300 nm) was studied. Its behavior was compared to that of an ex-carbonate sample prepared by thermal decomposition of zinc carbonate (FLUKA, 98.8%) at 773 K in air for 2 h resulting in smaller (30–60 nm) and less defined shaped particles (22 m2 g−1 ). 2.2. Pre-treatment: activation and adsorption procedures The catalysts were first activated under nitrogen or oxygen flow (20 mL min−1 ) as follows: the sample was heated (5 K min−1 ) up to 773 K, maintained at this temperature for 2 h, then the temperature was progressively cooled down to the MBOH reaction temperature (403 K). Then, the reaction was directly performed in the operando setup, or, in a classical U tube reactor. In this latter case, some experiments were done to evaluate the impact of the adsorption of CO2 , H2 O or ethanol on catalytic performances prior to the implementation of the reaction: just after the classical activation at 773 K under

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2.3.1. Classical procedure MBOH conversion experiments were performed in an automated differential flowing micro-reactor. The catalyst (50 mg) was put on a porous quartz disk, in the center of a U shape quartz tube of 10 mm internal diameter. The temperature of the catalyst bed was controlled by a thermocouple located close to the wall of the quartz tube. The feed consisted of MBOH diluted by bubbling nitrogen (50 mL min−1 ) in liquid MBOH (Fluka, 99.9%) at 293 K leading to a MBOH partial pressure of 1.73 kPa [11]. It was checked that in these conditions, there was no diffusional limitation. Reaction products were analyzed every 2 min using a Varian micro gas chromatograph equipped with a catharometric detector and a CP WAX 52 CB column. Samples were pre-treated in situ following the procedures described above. In any cases, the final temperature reached was 403 K which corresponds to the reaction temperature. In most of the cases, the reactant feed was then allowed to pass through the catalyst. In few cases, an intermediate adsorption step (described above) was performed just before the implementation of the reaction. Acetone and acetylene were the only products detected with ZnO as catalyst. The partial pressure of each product Pi was calculated from chromatographic area measurements, using the appropriate response coefficient and the value of the initial partial pressure of MBOH in the feed PMBOH . Conversion  MBOH (in %) is given by

 MBOH %

i= / MBOH

˛Pi

o PMBOH PMBOH



1/2Pi

i= / MBOH

N2 regenerationN2 O2

35 30 25 20 15 10 5 0 0

10

20

30

40

50

60

70

80

reaction time (min.)

b

80 N2 O2

70 60 50 40 30 20 10 0 0

10

20

30

40

50

60

70

80

reaction time (min.) Fig. 2. MBOH conversion versus time on stream of (a) kadox sample, after N2 , O2 pre-treatments and second conversion test after regeneration performed under nitrogen flow up to 773 K (N2 procedure). (b) Ex-carbonate sample after N2 and O2 pre-treatments.

3. Results and discussion 3.1. Evolution of the MBOH conversion versus time

with ˛ = 1/2 for acetone and acetylene.

Selectivities in acetone and acetylene are by: Si% =

45 40

MBOH conversion (%)

2.3. MBOH reaction tests

a

MBOH conversion (%)

nitrogen flow, samples have been swept at 403 K by CO2 /N2 (5:100), H2 O/N2 (4:100) or EtOH/N2 (5:100) gaseous mixtures (total flow of 20 mL min−1 ) for 10 min. Finally, to evaluate the reversibility of the influence of water adsorption on the catalytic properties, a second heating step up to 773 K under nitrogen flow was added just after the water adsorption step. The pre-treatments described above will be hereafter referred to as N2 , O2 , N2 CO2 /N2 , O2 CO2 /N2 , N2 H2 O/N2 , N2 EtOH/N2 or N2 H2 O/N2 N2 .

69

˛Pi

.

2.3.2. Operando DRIFTS experiments Diffuse reflectance infrared spectra were recorded under operando conditions in the 4000–1200 cm−1 range (resolution 4 cm−1 , 256 scans/spectrum, MCT detector) using a Brüker IFS 66 V spectrometer. As already described elsewhere [12], the kadox sample was placed inside a heated crucible located in a Thermo Spectra-Tech high temperature cell equipped with two ZnSe windows and with appropriate gas inlet and outlet connections as to pass the gas flow through the catalytic bed and to ensure homogenous contact between the sample and the flowing gas. The various pre-treatments of the kadox ZnO and the catalytic transformation of MBOH were conducted in the conditions of temperature and contact time described above, the N2 flow being adjusted to 35 mL min−1 due to the limited volume of the cell that implied a smaller sample weight (35 mg) than for classical MBOH test experiments. During the operando experiments, DRIFT spectra were recorded every 2 min together with simultaneous microGC gas phase analysis at the exit of the DRIFT cell in conditions as above. The DRIFT spectra are reported in absorbance after subtraction of a reference spectrum of KBr (Fluka, purity > 99.5%) registered at the same temperature and subsequent conversion into Kubelka–Munk units.

As reported earlier [9], the initial MBOH conversion level depends on the nature of the pre-treatment atmosphere, being greatly enhanced after N2 pre-treatment compared to O2 atmosphere. This enhancement was shown to be related to the promoting effect of the formation of oxygen vacancies during the pre-treatment under inert atmosphere (Fig. 1A). As expected from MBOH reactivity on basic surfaces [13], acetone and acetylene are the lone carbonated decomposition products detected in the gas phase. However, as reported in Fig. 2a and b, both ZnO catalysts deactivate versus time on stream: after a first rapid deactivation step within the first 8 min, the deactivation goes on but more slowly reaching the steady state only after 60 min of reaction. Globally, the same features are observed whatever the atmosphere of pretreatments, N2 or O2 , and final conversions are ordered as the initial ones. However, the variation between the initial and steady state conversions is a bit leveled down for O2 pre-treatment, the first rapid deactivation step being more intense in the case of the inert treatment. This is likely due to higher initial conversion levels, which are related to the higher concentration of oxygen vacancies resulted from the N2 treatment. As illustrated in Fig. 2a, a second catalytic cycle was performed on the kadox sample after an attempt to regenerate the system through a second N2 treatment at 773 K: though the initial conversion measured for the second cycle (25%) is higher than that reached at steady state for the first one (∼10%), it remains lower than the initial conversion level measured after the first N2 activation procedure (41%). The second N2 treatment at 773 K did not

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MBOH conversion (%)

a

45 N2 O2 N2 -CO2 /N2 O2 -CO2 /N2

40 35 30 25 20 15 10 5 0

0

10

20

30

40

50

60

70

80

reaction time (min.)

MBOH conversion (%)

b

80

N2 O2

70

N2-C O22/N22

60 50 40 30 20 10 0

0

10

20

30

40

50

60

70

80

reaction time (min.) Fig. 3. Influence of CO2 pre-adsorption on the MBOH conversion profile for (a) kadox sample, after N2 , O2 , N2 CO2 /N2 and O2 CO2 /N2 pre-treatments. (b) Ex-carbonate sample after N2 and O2 , N2 CO2 /N2 pre-treatments.

succeed to fully regenerate the state of the catalyst issued from the first treatment. It was checked that the two successive thermal treatments do not impact the specific surface area. The modifications of the surface occurring during deactivation process are thus partially irreversible. To investigate the origin of such deactivation, and locate the corresponding active sites, we performed selective poisoning of surface sites by adsorbates (CO2 , water and ethanol, Section 3.2) and operando DRIFTS experiments (Section 3.3). 3.2. Plausible origins of the deactivation: selective poisoning of the surface by adsorbates 3.2.1. CO2 adsorption For both samples, whatever the N2 or O2 atmosphere applied during the pre-treatment step, subsequent CO2 /N2 adsorption performed just before the reaction implementation (N2 CO2 /N2 and O2 CO2 /N2 pre-treatments) results in a decrease of the MBOH conversion levels, indicating that CO2 poisons active sites for both samples (Fig. 3), but not necessary all sites. Indeed, in the case of the kadox sample (Fig. 3a), CO2 seems to inhibit the active sites implied in the initial activity only since at steady state, the conversions resulting from N2 CO2 /N2 and O2 CO2 /N2 procedures are very similar to that obtained after N2 or O2 treatments, respectively. Thus, the active sites poisoning resulting from CO2 adsorption appears quite similar to that occurring at the very beginning of the catalytic reaction in the absence of preadsorbed CO2 . Such behavior is compatible with the presence of two types of sites on the kadox samples, the strongest basic ones are poisoned upon CO2 adsorption, whereas those still working at steady state are finally less sensitive to CO2 adsorption (Fig. 1B).

Considering the two active sites of the kadox sample (see Section 1 and ref. [9]), we can propose that most of the strongest sites (Bvac ), linked to the presence of oxygen vacancies, are poisoned by CO2 , whereas the weaker sites (Bweak ) only present on the kadox sample are not (Fig. 1B). This suggests that in the absence of pre-adsorbed CO2 , Bvac sites are poisoned in the course of the reaction, whereas Bweak are not–nor by CO2 , nor in the course of the reaction. On the opposite, in the case of the ex-carbonate sample (Fig. 3b), compared to the conversion profile obtained after N2 treatment, CO2 adsorption (N2 CO2 /N2 pre-treatment) leads to a decrease of the conversion, both initially and at steady state, resulting in conversion level close to that measured after O2 treatment that was shown to have filled up native oxygen vacancies [9]. This behavior is consistent with a more uniform nature of active sites on this sample, that was ascribed to the absence of any clear faceting [9]. This confirms that Bvac sites are the lone active sites present on this sample, that they are responsible for both initial and steady state activities, and that they are poisoned both in the course of the reaction, and by CO2 (Fig. 1A and B). What could be the molecular origin of the poisoning of Bvac by CO2 ? Carbonation of the surface can be easily evidenced by infra red spectroscopy (spectra not shown), indicating that CO2 adsorption directly poisons the O2− basic active sites, which basicity was shown to be impacted by an additional electronic density generated upon oxygen vacancies formation in zinc oxide [9]. The possible additional indirect influence of CO2 on the strength of the related basic sites via its interaction with these oxygen vacancies is also questionable. Indeed, from DFT studies CO2 could dissociate on electron rich oxygen vacancies VO  (two electrons trapped in an oxygen vacancy) resulting (at 0 K) in the adsorption of CO molecule [14]. In this latter case, the decrease of catalytic activity observed upon CO2 adsorption would be associated to the loss of a promoting influence on strong basic sites (Bvac ) of the electron density associated to oxygen vacancy rather than to direct poisoning of these basic sites. Unfortunately, the occurrence of such process cannot be checked experimentally, since (1) there is no characterization technique able to directly follow these VO  oxygen vacancies (EPR only specifically probes paramagnetic Vo• ones), (2) taking into account that CO2 adsorption was performed at 403 K, the expected generated adsorbed CO is not stable at this temperature, explaining why it cannot be detected by infra red. The question that remains to be answered is the origin of the poisoning of Bvac sites in the course of the reaction. This will be discussed further, on the basis of operando DRIFTS experiments. 3.2.2. Water and ethanol adsorption Compared to N2 treatment, adsorption of water or ethanol, N2 H2 O/N2 and N2 C2 H5 OH/N2 treatments respectively, similarly lead to a decrease of MBOH conversion for both ZnO samples, even resulting for the kadox sample in a conversion level as low as that obtained after O2 treatment (Fig. 4). It should be noted that this preadsorption of water results in a conversion profile similar to that obtained after ∼10 min of reaction in the experiment without water pre-adsorption. The similar conversion levels obtained upon water or ethanol adsorption are consistent with Nagao et al. results [7] concluding to the same nature of adsorption sites on ZnO for water and ethanol. The impact of water on the surface could be related to oxidation of the surface by filling up of VO •• (empty oxygen vacancy), oxygen vacancies, leading either to the formation of two equivalent surface hydroxyls (Eq. (2)). The occurrence of such process is supported by the uniqueness of the OH formed upon water dissociation on ZnO that was evidenced on single crystals studies [15,16]—or to the formation of dihydrogen (Eq. (3)). Note that corresponding oxidation of ZnO surface with ethanol would lead to the formation of ethylene and dihydrogen (Eq. (4)) [17]. Considering such process, the

C. Drouilly et al. / Catalysis Today 205 (2013) 67–75

MBOH conversion (%)

a

45

reconstruction could lead to a more stable surface with lower ability to desorb oxygen upon N2 treatment. This would be in line with the dependence of zinc oxide catalytic reactivity on morphology [9] or surface topology. To conclude, CO2 , water or ethanol adsorptions prior to the catalytic activity measurement result in the poisoning of specific sites, mainly Bvac sites, sometimes similarly to the self-poisoning occurring in the course of the reaction in the absence of pre-adsorbed CO2 , water or ethanol.

N2 O2 N 2-EtOH/N2

40 35

N 2-H -H 2O/N2 N 2-H 2O/N2-N 2

30 25 20 15 10 5 0

3.3. Operando DRIFTS study: identification of the IR bands 0

10

20

30

40

50

60

70

80

reaction time (min.)

MBOH conversion (%)

b

71

80 70 60 50 40 30 20 10 0

reaction time (min.)

Fig. 4. Influence of Ethanol or water pre-adsorption on the MBOH conversion profile for (a) kadox sample, after N2 , O2 , N2 EtOH/N2 and N2 H2 O/N2 and N2 H2 O/N2 N2 pre-treatments. (b) Ex-carbonate sample after N2 and O2 , N2 EtOH/N2 and N2 H2 O/N2 pre-treatments.

poisoning effect of ethanol can be related to the filling of vacancies. In all cases, Bvac sites are to be invoked to explain the sensitivity of the catalytic activity to water or ethanol pre-adsorptions (Fig. 1C).

(2 and 3)

(4) To go deeper in the understanding of the process occurring upon water adsorption, its reversibility was investigated measuring the basic reactivity after the N2 H2 O/N2 N2 treatment (Fig. 4). The initial conversion measured after N2 treatment (41%) is not recovered after a second N2 treatment subsequent to water adsorption, and the resulting conversion profile obtained is very similar to that measured after the N2 H2 O/N2 treatment (19% of initial conversion). Such irreversible modification occurring upon water adsorption could account for an additional effect, the lone filling up of oxygen vacancies process being expected to be reversible upon N2 treatment (known to form oxygen vacancies). Besides this process, it could be proposed that a surface reconstruction occurs upon water adsorption that would also contribute to the decrease of the amount or the strength of the basic sites. If a decrease of surface specific area is effectively observed upon water adsorption, the effect remains quite tiny (from 9 to 7 m2 g−1 with no detectable impact on the TEM pictures). It could be proposed that the related surface

The IR spectrum recorded at the end of the pre-treatment step (not shown), performed under N2 flow (20 mL min−1 ) evidences the presence of residual surface OH groups with bands in the 3670–3560 cm−1 range, and of very weak carbonate bands (∼1470 cm−1 ). From the difference infra red spectra reported for the kadox sample in Fig. 5 after 2 min of reaction, the introduction of the reactant flow on the sample performed after the same N2 pretreatment immediately results in (1) the perturbation of these pre existing ␯OH and ␯CO IR bands, explaining the negative (3668, 3619 and 3558, 1470 cm−1 ) and positive contributions (bands around 3640–3500 cm−1 and 1450 cm−1 ) and (2) in the appearance of new set of bands. The latter can be assigned from Table 1, gathering the main IR bands reported from the literature from MBOH, acetylene, acetone or its condensation products adsorbed on various systems. Among them, few are unambiguously assigned to adsorbed MBOH [12,32] with (i) the stretching ␯OH mode of its alcohol function [12,18,19,21], (ii) the stretching ␯C H mode of methyl groups (bands at 2979, 2934 and 2868 cm−1 ) [12,18,19]. In addition, the two ␯≡C H contributions (3325 and 3308 cm−1 ) were also assigned to the acetylenic function of MBOH adsorbed by its reactive alcohol function [12,18,21]. Indeed, possible involvement of adsorbed acetylene in the ␯≡C H contributions was considered as quite unlikely, on the basis of the well known ability of acetylene formed on basic surface to immediately desorb in gas phase in dynamic studies [12]. Note that, contrary to what was observed in the case of basic zeolites [12], no spectator adsorption of MBOH by its acetylenic function is observed. The ıC(CH3 )2 bending mode of the C(CH3 )2 groups (1379 and 1362 cm−1 ) are also ascribed to MBOH [12,18,19,21] rather than to acetone [18,20,28,29]. Indeed, as all the new bands already mentioned, their relative intensity is constant versus time, which is not the case for the bands related to ␯CO of adsorbed acetone (1737 and 1362 cm−1 ) [18,20,21,29]. Besides these expected bands related to adsorbed MBOH and acetone, additional bands are detected, that evidence the formation of heavier products issued from the self condensation of acetone, expected to occur on basic surfaces [21,33] and which polymerization scheme is reported in Fig. 6. Indeed, poorly intense ␯CO bands at 1585 and 1550 cm−1 were ascribed to the formation of an enolate intermediate [12,18,20,28,29], and the formation of adsorbed diacetone alcohol is evidenced by the ıCH2 bands (1437 and 1419 cm−1 ) [18–21]. The ␯CO bands at 1670 and 1643 cm−1 are also possibly assignable to diacetone alcohol or even to heavier condensation products such as isophorone [20,21,28–31]. The first important result is the splitting of the bands related to adsorbed MBOH (␯≡C H and ıC(CH3 )2 ), but also the splitting of all the bands related to adsorbed acetone, enolate intermediate or even condensation products. This is an additional confirmation of the involvement of two different types of active sites (Bvac and Bweak ) present on the kadox sample as proposed in our previous study [9]. Second, it should be underlined that the occurrence of polymerization of acetone evidenced by diacetone alcohol or isophorone formation is also expected to lead to water formation (Fig. 6).

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Fig. 5. Difference DRIFT spectrum obtained by subtracting the spectrum registered at the end of the N2 pre-treatment of the kadox sample to that registered in running conditions after 2 min of reaction.

In the absence of any increase of water formation in gas phase (detectable by the microGC analysis), nor of band related to molecularly adsorbed water, the new ␯OH band at 3528 cm−1 could be assigned to the hydroxyl groups obtained by dissociation of water on the ZnO surface. Such assignment is supported by previous studies dealing with dissociation of water on ZnO single crystal, leading to the formation of a unique band due to the filling up of an oxygen vacancy (Eq. (2)) [15]. 3.4. Unraveling the deactivation mechanism: evolution of the IR bands versus deactivation profiles It was checked that the operando study performed for the kadox sample leads to a deactivation profile identical to those described in Fig. 2 from classical MBOH studies. In the absence of any variation

of the bands related to MBOH adsorption versus time, and of any spectator adsorption mode by its acetylenic function [12] there is no evidence for any poisoning of active sites by strong molecular adsorption of the reactant. On the opposite, the intensities of the bands related to the detected primary (acetone) and secondary (diacetone alcohol, isophorone, etc.) products vary differently, which can help us to rationalize the deactivation profile (Fig. 7). First, as proposed from the correlation between the initial conversion level and the evolution of the concentration of oxygen vacancies [9], it is expected that the higher the concentration of oxygen vacancies, the higher the amount of electron released associated to these oxygen vacancies formation, the higher the strength of the acid base pair involved, and the higher its basic reactivity. Thus, just after nitrogen pre-treatment, the surface exhibits very reactive acid base pairs (Fig. 1A), explaining the high level

Fig. 6. schematic of the mechanism involved in the self polymerization of acetone.

C. Drouilly et al. / Catalysis Today 205 (2013) 67–75

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Table 1 Assignment of IR bands upon MBOH, acetylene, acetone or its condensation products on basic systems (MSO = mesityl oxide, DAA = diacetone alcohol and IPH = isophorone). Assignment

MBOH

Acetylene

Acetone

Polymerized products of acetone

Expected wavenumber (cm−1 )

System

Refs.

␯O H

3300–3450

[12,18,19]

␯ C H (MBOH adsorbed by its alcohol function) ␯ C H (MBOH adsorbed by its alkyn function) ␯ CH3

3310–3330

Cs, Ba/MgO basic zeolites, TiO2 , Al2 O3 , SiO2 , ZrO2 Cs, Ba, Y/MgO basic zeolites

3250–3290

Cs, Ba, Y/MgO, zeolites TiO2 , Al2 O3 , SiO2 , ZrO2

[12,18–21]

2990–2865

[12,18,19]

␯C C ␦ C (CH3 )2

2100 (weak) 1368–1370

␯

3150–3210

Cs, Ba, Y/MgO, basic zeolites TiO2 , Al2 O3 , SiO2 , ZrO2 Cs, Ba, Y/MgO Cs, Ba, Y/MgO, zeolites TiO2 , Al2 O3 , SiO2 , ZrO2 , ZSM-5 Faujasite, Basic zeolite Cs, Ba et Y/MgO, SiO2 , TiO2 , CaO, MgO SiO2 , TiO2 , CaO, MgO Basic zeolite Cs, Ba,Y/MgO H-ZSM-5 SiO2 , TiO2 , CaO, MgO Faujasite Y/MgO TiO2 , Al2 O3 , SiO2 , ZrO2 , Y/MgO, zeolites TiO2 , Al2 O3 , SiO2 , ZrO2 ZSM-5, zeolites Cs, Ba, Y/MgO Y/MgO TiO2 , Al2 O3 , SiO2 , ZrO2 ZSM-5 Y/MgO TiO2 , Al2 O3 , SiO2 , ZrO2 zeolites Cs, Ba, Y/MgO Isophorone in solution TiO2 , Al2 O3 , SiO2 , ZrO2 TiO2 , Al2 O3 , SiO2 , ZrO2 ZSM-5, zeolites Cs, Ba, Y/MgO

C H (bonded by H)

␯ H C C. . . ␯H C C H

3250–3300 3240–3250

␯C C

1940–2030

␦ CH2 ␯ C O (enolate)

1450–1380 1560–1610

␯C

1650–1711

O

␦ C (CH3 )2

1370 1368

␯ C H of MSO ␦ CH2 of DDA

3017 1420–1460 1423

␯C

O of IPH

1630–1685

␯ C O or ␯ C C of MSO and DAA

1560–1690

[12,18–20]

[12,18,20,21] [12,18,19,21]

[22–26]

[26] [23,24,27]

[22,26] [20] [20,28,29] [18–21,29]

[18,20,28,29]

[20] [18–21]

[30,31] [18–21,28,29]

Fig. 7. Origins of deactivation observed in the MBOH conversion measured on kadox sample after pre-treatment under nitrogen comparing (a) the deactivation profile of MBOH conversion versus time and (b) the evolution of the IR bands from difference DRIFT spectra obtained by subtracting the spectrum registered at the end of the N2 pre-treatment to those registered in running conditions at increasing reaction times.

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of initial activity, but also its tendency to polymerize acetone, explaining the initial rapid deactivation, likely both by polymers themselves (Fig. 1D), and by filling up the vacancies by the water co-product (Fig. 1C). The high strength of the related active sites is supported by the results reported above on poisoning effect of CO2 adsorption (Fig. 1B). All these results are in line with the prominent role of the water by-product of acetone polymerization in the poisoning of the Bvac sites, able to promote such polymerization. Consistently, the decrease of the basicity resulting from water byproduct adsorption is very consistent with loss of initial reactivity observed upon water adsorption performed just before the catalytic reaction (Fig. 4). However, as seen in Fig. 7b, the related ıCH2 (1437 and 1419 cm−1 ) and ␯CO (1670 and 1643 cm−1 ) bands but also ␯OH (3528 cm−1 ) band (figure not shown) are rapidly saturated. In fact, the filling up of some oxygen vacancies by dissociation of the water formed upon polymerization decreases the number or strength of the Bvac active sites, which rapidly limits the extension of polymerization. This may explain why the sharp deactivation slows down at a certain stage. However, besides this drastic initial effect, the conversion measured after water adsorption still slowly deactivates versus time (Fig. 7a), indicating that, beside polymerization effect and water dissociation resulting in the filling up of few oxygen vacancies, there is an additional process responsible for the second deactivation step occurring after the very first minutes of reaction. In fact, consistently with the stop of the extension of polymerization, the bands related to the enolate intermediate that were present from the very beginning progressively disappears at the benefit of the increase of adsorbed acetone bands (Fig. 7b). Such increasing adsorbed acetone provides an explanation for the second slower deactivation step (Fig. 1). Such deactivation due to strongly adsorbed acetone was already reported for basic zeolites [12]. Finally the steady state is reached once the intensity of the adsorbed acetone bands has been stabilized. As a synopsis, all the deactivation processes elucidated in the present work are summarized in Fig. 1. 3.5. Practical and general implications of this work Beside the initial strength of active sites modulated by varying the nature of the atmosphere of pre-treatment, and the nature and amount of adsorbates (adsorbed primary and secondary products), that very classically play a role in the deactivation processes [12], the main original result deals with the influence of the instability of oxygen vacancies under the related reaction conditions (here due to reactivity with the water produced by the secondary reactions) in the deactivation process toward MBOH conversion. Thus, the formation or filling up of oxygen vacancies upon activation procedures not only control the strength of active sites and the related initial conversion level [9], but the evolution of the oxygen vacancies concentration during the reaction is also a determining parameter that should be taken into account to predict the stability of oxygen deficient oxide catalysts system during the catalytic process. Indeed, from previous studies [9], ethanol conversion measured after similar pre-treatment conditions than for the MBOH conversion is very stable versus time. In fact, despite this different behavior, the present results also provide explanation for the stability observed for the ethanol conversion. Indeed, the ethanol conversion is carried out at higher temperature, 623 K against 403 K. Beside making the poisoning of the surface by adsorbates less sensitive, we have shown by a previous EPR study that the higher the temperature, the lower the sensitivity of oxygen vacancy toward filling up process, at least toward O2 molecule [9]. Even if we cannot totally reject that the filling up of oxygen

vacancies could occur following the process described in Eq. (4), its efficiency is expected to be quite limited considering the high reaction temperature. At reverse, such quite high temperature range makes the reductive effect of the hydrogen formed from this process as well as that issued from the acetaldehyde formation much more effective: hydrogen could thus participate to the regeneration of oxygen vacancies, ensuring global stability of the system.

4. Conclusion In order to rationalize the deactivation of the MBOH conversion versus time on stream on zinc oxide, pre-adsorption experiments and operando DRIFTS experiments were carried out. From regeneration experiment, deactivation cannot be explained by loss of specific surface area even if a partially irreversible modification of the surface state takes place. MBOH only adsorbs upon its reactive adsorption mode (alcohol function) leading to the formation of acetylene that rapidly desorbs and to acetone which affinity toward adsorption is much higher. As evidenced by the splitting of the bands relative to adsorbed reactant and products, two different active sites are involved, which is consistent with the CO2 adsorption experiments. In fact, the adsorbed products are implied for the deactivation process, the acetone condensation products being involved in the blocking of the active sites at the very beginning of the reaction, that were shown to be the strongest one from CO2 adsorption experiments. However, this effect remains limited since no extension of the polymerization is observed, which is explained by the progressive decrease of the strength of the active sites upon the influence of the water formed upon polymerization. Consistently, pre-adsorption of water was shown to greatly impact the conversion level, leading to a conversion profile similar with that obtained after 10 min of reaction in the classical experiment. After this first rapid deactivation step, a second slow deactivation step could be ascribed to the increase amount of adsorbed acetone (no more consumed through its self condensation process) and steady state is reached once no more modification of the IR bands is observed. Water does not directly interact with the strong active sites that are made of acid base pair which basicity is controlled by the electronic density generated by the oxygen vacancies. Nevertheless, water actively contributes to the decrease of their strength, via the filling up of oxygen vacancies resulting in a single infra red band at 3528 cm−1 . Thus, it can be underlined that oxygen vacancies concentration is a crucial parameter determining the catalytic behavior of zinc oxide: increasing their relative amount upon convenient pretreatment conditions allows enhancing the conversion level, but also influences the occurrence of an eventual deactivation process in relation with their reactivity in the conditions of the reaction with the reactant or products. In the present case water is implied in the deactivation process, whereas in the case of ethanol transformation, possible filling up of oxygen vacancies by the reactant itself, could be compensated by the hydrogen formed upon ethanol dehydrogenation into acetaldehyde, ensuring the whole stability of catalytic system.

Acknowledgements The authors are grateful to the organizers of the Operando IV Congress for giving them the opportunity to present their work and to the participants for fruitful discussion.

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