The partial hydrogenation of butadiene over Al13Fe4: A surface-science study of reaction and deactivation mechanisms

Journal of Catalysis 332 (2015) 112–118

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The partial hydrogenation of butadiene over Al13Fe4: A surface-science study of reaction and deactivation mechanisms Laurent Piccolo a,⇑, Lidiya Kibis b,c a Institut de recherches sur la catalyse et l’environnement de Lyon (IRCELYON), UMR 5256 CNRS & Université Claude Bernard – Lyon 1, 2 avenue Albert Einstein, 69626 Villeurbanne, France b Boreskov Institute of Catalysis SB RAS, Lavrentieva 5, Novosibirsk 630090, Russia c Novosibirsk State University, Pirogova St. 2, Novosibirsk 630090, Russia

a r t i c l e

i n f o

Article history: Received 21 August 2015 Revised 23 September 2015 Accepted 28 September 2015

Keywords: Intermetallic compounds Al13Fe4 Non-noble metals Hydrogenation of butadiene Surface science

a b s t r a c t Non-noble intermetallic compounds have shown promising properties as inexpensive catalyst alternatives to Pt-group metals for alkyne and alkene hydrogenation. In this work, the gas-phase hydrogenation of 1,3-butadiene over the Al13Fe4(0 1 0) surface was investigated in the 0.2–2 kPa total pressure range at 20–200 °C in a batch-type reactor coupled with an ultrahigh-vacuum setup allowing for Auger electron spectroscopy (AES) and low energy electron diffraction (LEED). The results were compared with those previously obtained on Pd(1 0 0) in the same conditions. It is confirmed that Al–Fe is initially as active as Pd and 100% selective to butenes, including at room temperature (RT), with sequential conversions of butadiene to butenes, and butenes to butane. The main difference with Pd comes from the butenes distribution, with a cis/trans 2-butene ratio larger than unity for Al–Fe while it is near zero for Pd. The results are discussed in terms of (i) steric constraints upon p-allylic precursors to 2-butenes and (ii) involvement of adsorbed butyl intermediates allowing for hydro-isomerization of butenes competing with their hydrogenation to butane. A mechanistic reaction scheme is proposed accordingly. The sensitivity of the Al–Fe surface to oxygen-containing impurities leads to gradual deactivation under reaction conditions, which is the main issue for practical use of non-noble metal catalysts. The deactivation and oxidation processes were investigated by combining post-reaction AES measurements with several thermal/chemical treatments. Depending on the pressure conditions, the Al13Fe4 surface chemisorbs oxygen-containing species or forms an Al oxide layer. The RT activity of the surface decreases as the oxygen-containing phase coverage increases. However, this phase can be removed through hightemperature annealing. Ó 2015 Elsevier Inc. All rights reserved.

1. Introduction The foreseen shortage of noble metals in the near future leads researchers in catalysis to explore the potential of alternative catalytic materials based on abundant and cheap sources. One of the most promising strategies relies on non-noble metal combinations. For example, through the choice of relevant descriptors of the catalytic activity and computational screening, Studt et al. have predicted and confirmed the efficiency of bimetallic alloys such as Ni–Zn for partial hydrogenation of acetylene [1], Cu–Ni for CO hydrogenation to methanol [2], and Ga–Ni for CO2 reduction to methanol [3]. The basic idea in such cases consists in tuning the reactant adsorption properties through electronic effects provided by the close interaction between two metals. ⇑ Corresponding author. E-mail address: [email protected] (L. Piccolo). http://dx.doi.org/10.1016/j.jcat.2015.09.018 0021-9517/Ó 2015 Elsevier Inc. All rights reserved.

A derived (although conceptually different) approach consists in isolating the active sites within a non-noble-metal matrix, as proposed by Armbrüster and coworkers for Ga–Pd intermetallic compounds [4,5]. These materials have proved their efficiency as catalysts for the partial hydrogenation of acetylene, a reaction industrially applied to polyethylene production and usually catalyzed by precious metals (Pd and Pd–Ag). The same research group has recently shown the ability of Al13Fe4 and Al13Co4 complex intermetallic compounds to catalyze this reaction [6]. The Al13M4 phase, which is orthorhombic and contains 102 atoms in its unit cell, is considered as a periodic approximant to decagonal Al–M and Al–Ni–M quasicrystals [7,8]. Using scanning tunneling microscopy (STM), low energy electron diffraction (LEED) and density functional theory (DFT) calculations, Ledieu et al. have revealed the complex structure of the Al13Fe4(0 1 0) surface [9,10]. The authors have proposed isolated Fe atoms protruding above pentagonal Al motifs as the catalytically active sites for

L. Piccolo, L. Kibis / Journal of Catalysis 332 (2015) 112–118

acetylene conversion. From DFT calculations, Hafner and Krajcˇí have identified the most favorable acetylene adsorption sites over Al13Co4(1 0 0) (equivalent to Al13Fe4(1 0 0)) as Al2Co trimers [11]. Noticeably, in spite of the different bulk structures, the surface site geometries are very similar for Al13Co4(1 0 0) and GaPd(2 1 0). In both cases, the strong covalent binding of the simple metal (Al, Ga) to the transition metal (Fe, Co, Pd) would increase the reactivity of the former as compared to pure Al or Ga. Guided by the high catalytic performance of Al13Fe4 in the partial hydrogenation of acetylene, we have recently investigated the partial hydrogenation of 1,3-butadiene, the simplest conjugated diene, over the Al13Fe4(0 1 0) surface [12]. This reaction is used industrially for the purification of C4 cuts and the production of butenes before polymerization or alkylation [13]. We have shown that Al13Fe4 is highly active and selective to butenes, even at room temperature (RT), whereas the parent metals are inactive. However, this type of material is much less stable under reaction conditions than the reference metal, palladium, due to adsorption of contaminants and formation of an oxide layer. In this first study, the range of conditions explored was limited and the butenes isomers could not be discriminated. In the present work, we report a detailed investigation of the reaction kinetics in relation with the products distribution, using Pd(1 0 0) as reference model catalyst. Furthermore, with the help of Auger electron spectroscopy (AES), we analyze the causes of deactivation and the possibilities to regenerate the surface.

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molecular pump, and the reaction/evacuation cycle could be performed additional times. 2.3. Products analysis During the reaction, the gases were continuously sampled through a leak valve and analyzed by a mass spectrometer (MS) evacuated by an oil diffusion pump capped with a LN2 trap (base pressure 2
1010 Torr, analysis pressure 2
108 Torr). MS intensities for m/z = 2, 40, 54, 56, and 58 were recorded for hydrogen, argon, butadiene (C4H6), butenes (C4H8), and butane (C4H10), respectively. The hydrogen and hydrocarbon partial pressures could be obtained from the MS signal by taking into account product-dependent sensitivities, ion fragmentation patterns, and gaseous matter conservation [16]. In some specific cases, online gas chromatography (GC) was employed for determining the full distribution of 1,3-butadiene hydrogenation products, i.e. the three butene isomers (1-butene, trans-2-butene and cis-2-butene) and butane. To this aim, an automatic gas sampling device connected to an Agilent 6850 GC–FID was implemented on the UHV-reactor system [15], and was used in addition to MS. With an Agilent HP-AL/KCl column (50 m
0.53
15 lm) maintained at 80 °C, a chromatogram was recorded every 10 min. While noble metal surfaces are essentially unaffected by the use of this inherently non-UHV GC device, the latter had to be employed scarcely and carefully with the Al–Fe sample due to the high sensitivity of this material to contaminants (see Section 3.2).

2. Experimental 3. Results and discussion 2.1. Material preparation The Al13Fe4(0 1 0) sample was purchased from Mateck GmbH (Germany). An ultrapure crystal was grown following the Czochralski method [14] and cut perpendicular to its [0 1 0] direction into a small disk (10 mm
1 mm). The final roughness and disorientation of the as-furnished Al13Fe4(0 1 0) surface were smaller than 0.03 lm and 0.1°, respectively. The surface was further cleaned by repeated cycles of Ar+ sputtering and annealing at ca. 750 °C under ultrahigh vacuum (base pressure 5
1010 Torr). The cleaned surface was contaminant-free, as attested by Auger electron spectroscopy (Fig. 3a, bottom spectrum), and exhibited a sharp low energy electron diffraction (LEED) pattern [12]. The LEED pattern corresponds to a (1
1) structure with an oblique unit mesh, in agreement with previous findings [9].

2.2. Catalytic testing

The hydrogenation of butadiene over clean Al13Fe4(0 1 0) was investigated in our surface-science ‘‘batch” reactor from RT (23– 25 °C) to 200 °C for initial hydrogen/butadiene ratios comprised between 3 and 30 at total reactant pressures in the range 2–15 Torr (0.2–2 kPa).2 3.1. Products distribution and reaction kinetics Fig. 1 shows the time evolution of hydrocarbon concentrations under our two different initial conditions, standard (0.5 Torr C4H6, 5 Torr H2, RT) or with higher H2 pressure (15 Torr). The resulting selectivity vs. conversion curves (Fig. 1c and d), and 1-butene/2butenes and cis-2-butene/trans-2-butene selectivity ratios (Fig. 1e and f) are also shown. In Fig. 1c and e, the data obtained for Pd(1 0 0) [15] in standard reaction conditions are reported for comparison.

The butadiene hydrogenation reaction was carried out in a dedicated static catalytic reaction cell (volume ca. 120 cm3) coupled to the surface preparation/analysis chamber [15]. Similar experiments were performed on Al13Fe4(0 1 0) and Pd(1 0 0), the latter serving here as a reference. In the cell, the Al–Fe sample could be heated on the backside through a porthole using an infrared laser beam and the surface temperature was measured by an infrared pyrometer (surface emissivity set to 0.3). In a typical experiment, a mixture of butadiene (N26 purity), hydrogen (N55) and argon (N56) was prepared in a separate chamber before injection into the reactor. All the gases were purchased from Air Liquide. The standard initial conditions were 0.5 Torr1 butadiene, 5 Torr H2, and 1 Torr Ar (argon was used for internal calibration). At the end of the reaction run, the products were evacuated using a turbo-

3.1.1. First vs. second hydrogenation As for Pd(1 0 0), it is seen in Fig. 1a and b that the hydrogenation process is sequential on Al13Fe4(0 1 0), butane being mostly produced after butenes, i.e. butenes formation is selective. This suggests that, in both cases, the adsorption competition between butadiene and butenes is highly favorable to the former hydrocarbon [17]. However, the second hydrogenation (butenes-to-butane conversion) is about 2.5 times faster than the first hydrogenation (butadiene-to-butenes conversion) on Al13Fe4, while it is 3–4 times slower on Pd, consistently with our previous report [12]. This implies that Al13Fe4 is a more efficient catalyst than Pd for mono-alkene hydrogenation at RT. According to a classical hypothesis [13,18], this can be related to a stronger adsorption of butenes on Al13Fe4 than on Pd.

1 1 Torr = 133 Pa. This unit is the most widely employed in surface and vacuum sciences.

2 The supplementary data file contains all the catalysis raw data not shown in the main article.

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(a)

(b) 5 Torr H2

Hydrocarbon concentration (mol %)

Hydrocarbon concentration (mol %)

100

butadiene 1-butene trans-2-butene cis-2-butene butane

80

60

40

20

0 0

50

100

150

200

250

300

15 Torr H2

100

butadiene 1-butene trans-2-butene cis-2-butene butane

80

60

40

20

0

350

0

50

Time (min)

(c) 100

100

60

40

1-butene trans-2-butene cis-2-butene butane

60

40

20

20

0

0 0

20

40

60

80

100

0

20

4

(f)

5 Torr H2

60

4 Pd

80

100

15 Torr H2

1b/2b cis-2b/trans-2b

1b/2b cis-2b/trans-2b

3

Selectivity ratios

3

Selectivity ratios

40

Butadiene conversion (%)

Butadiene conversion (%)

(e)

150

15 Torr H2

80

Pd

Selectivity (mol %)

80

Selectivity (mol %)

(d)

5 Torr H2 1-butene trans-2-butene cis-2-butene butane

100

Time (min)

2

1

2

1

Pd

0

0 0

20

40

60

80

100

Butadiene conversion (%)

0

20

40

60

80

100

Butadiene conversion (%)

Fig. 1. Hydrogenation of butadiene on Al13Fe4(0 1 0) at RT for initial hydrogen pressures of 5 Torr (left panel) and 15 Torr (right panel). (a, b) Hydrocarbon concentrations. (c, d) Product selectivities. (e, f) Selectivity ratios. The data for Pd(1 0 0) at 5 Torr H2 are represented with open symbols. The initial butadiene pressure was 0.5 Torr. The data were recorded using GC. Butadiene conversion is defined as 1-pC4H6/pC4H6(t = 0). 1b and 2b stand for 1-butene and 2-butene, respectively.

3.1.2. Effect of reaction conditions on butadiene and butenes hydrogenation activities Table 1 shows the initial reaction rate and the ratio (R = Dt2/ Dt1) between second (butenes-to-butane) and first (butadieneto-butenes) reaction durations, i.e. the ratio between the average rates of first and second hydrogenations, for the range of conditions explored (the reverse ratio is also reported for convenience). As we will discuss in the next section, the values of Dt1 and Dt2 (and the corresponding activities) may vary significantly depend-

ing on the amounts of oxygen-containing contaminants, which may themselves depend on the reactant partial pressures. However, we have established that the R ratio (as well as the initial activity) is essentially contamination-independent, making it a relevant descriptor of the surface reactivity. High values of R, which correspond to slow butane formation as compared to butadiene consumption, are expected to relate to high butene selectivities in conventional flow-fixed-bed reactor conditions. Whatever the temperature (RT or 110 °C), the highest R value (R = 0.83 at RT

L. Piccolo, L. Kibis / Journal of Catalysis 332 (2015) 112–118 Table 1 Reaction conditions and kinetic results. pC4H6 (t = 0)a

pH2 (t = 0)a

r1 (t = 0)b

R = Dt2/Dt1c

1/R = Dt1/Dt2

RT 0.5 0.5 0.5 0.15 1.5

1.5 5 15 5 5

10 15 18 13 11

– 0.30 0.19 0.83 –

– 3.3 5.3 1.2 –

110 °C 0.15 0.5 0.5

5 5 15

– 94 –

1.0 0.71 0.59

1.0 1.4 1.7

200 °C 0.5

5

77

3.8

0.26

a

Initial partial pressures in Torr. Initial butadiene conversion rate in Torr/min. Ratio between the second (butenes-to-butane) and the first (butadiene-tobutenes) hydrogenation time periods. b

c

and 1.0 at 110 °C) is obtained for the lowest initial butadiene pressure (0.15 Torr) with an initial hydrogen pressure of 5 Torr. With similar pressure ratio but higher total pressure (0.5:15 Torr), R decreases (R = 0.19 at RT and 0.59 at 110 °C). For our standard pressure conditions (0.5:5 Torr), R is seen to increase with temperature (from 0.30 at RT to 3.8 at 200 °C) while the initial activity is maximum at the intermediate temperature (110 °C), as already pointed out in our previous paper [12]. At RT and in the investigated pressure range, the butenes formation order (slope of the log(rate) vs. log(pH2) plot) appears slightly positive (0.26 ± 0.05) for hydrogen and about zero for butadiene. For comparison, the reaction order on Pd catalysts is unity or above for H2 and zero or below for butadiene [13,16]. These data at RT are consistent with butadiene hydrogenation kinetics determined by dissociative adsorption of hydrogen on Pd [17] and, to smaller extent, on Al13Fe4. At higher temperature, whereas hydrogen activation becomes faster, desorption of the hydrocarbons may become limiting, explaining the activity maximum at 110 °C for both butadiene and butene hydrogenations (Table 1 and supplementary data). The increase of R with temperature can be explained by the greater destabilization of butene as compared to butadiene upon heating, the former having a lower adsorption energy than the latter. 3.1.3. Selectivity to butenes Under standard conditions, the initial selectivity to butenes is 100% (Fig. 1c) with a 1-butene/2-butenes ratio of 2.4 for Al13Fe4, similar to the value of 2.7 found for Pd (Fig. 1e). In the course of butadiene conversion, the ratio remains around 2.5 while butane concentration increases slowly. On Al13Fe4, cis-2-butene is initially produced in somewhat greater amount than trans-2-butene, and the cis/trans ratio decreases from 1.3 to 0.81 in the course of the first hydrogenation reaction, while it is only 0.1–0.2 for Pd. For a greater amount of hydrogen (initial pressure of 15 Torr), the 1-butene selectivity slightly decreases at the benefit of butane selectivity. Again the main effect is seen on the cis/trans ratio, which is twice superior as under standard conditions (2.1 vs. 1.2) before gradually decreasing down to 0.9 throughout the conversion. Similar experiments, although for different partial pressures of reactants (5 mbar butadiene and 10 mbar hydrogen), performed at 25 °C and 100 °C by Silvestre-Albero et al. on Pd (1 1 1) and (1 1 0) single-crystal surfaces [19] and alumina-supported Pd nanoparticles [20] confirm the small cis/trans ratio (<0.5) during the whole reaction process. A very small fraction of cis isomer was also measured using conventional Pd catalysts, and this fraction is smaller than unity for most other metals [13,21]. Hence, the large value

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of the cis/trans ratio (>0.6 whatever the reaction conditions) obtained for Al13Fe4 before complete conversion of butadiene appears typical of this material. After full butadiene conversion, for both pressure conditions, the 1:cis-2:trans-2 butene mixture rapidly switches from the 69:14:17% composition to the nearequilibrium composition of 5:24:71% through isomerization up to full butene conversion into butane. This behavior is very similar to the one observed for Pd [15]. 3.1.4. Mechanistic implications The non-zero initial selectivity toward 2-butenes suggests the primary nature of all butenes. On Pd, while 1-butene is produced from 1,2 addition of hydrogen to butadiene, the formation of 2-butenes has been proposed to proceed through 1,4 addition of hydrogen to the diene via the formation of adsorbed p-allylic intermediates [17,22]. Our data on Al13Fe4 are in line with this mechanism if one additionally permits the trans–cis interconversion of 2-butene precursors. As a matter of fact, since the trans-1,3-butadiene is by far the most stable isomer of butadiene, the previous hypothesis is necessary to explain the initial large fraction of cis isomer. The sensitivity of the initial butenes distribution to initial hydrogen pressure and the formation of butane throughout butenes interconversion suggest the occurrence of an additional route to 2-butenes, namely hydro-isomerization of 1-butene via an alkyl reversal mechanism [17]. This step involves the protonation of 1-butene to an adsorbed C4H9 intermediate (first step of the classical Horiuti–Polanyi hydrogenation mechanism), which readily deprotonates to 2-butene. Further half-hydrogenation of the C4H9 intermediate leads to butane. Scheme 1 summarizes the abovementioned steps, including the semi-hydrogenation of butadiene into the syn-1-methyl-p-allyl intermediate (step 1), which can itself be semi-hydrogenated into trans-2-butene (4) or isomerized to the anti-1-methyl-p-allyl intermediate (2) leading to cis-2-butene (5).3 From steric considerations it is likely that, for the initially high coverage of butadiene, the syn p-allyl isomer is destabilized at the benefit of the anti one, which leads to cis-2-butene. The additional steric constraint brought about by the higher H2 pressure may increase the initial fraction of cis isomer (Fig. 1f), but it is more likely due to a fast onset of the hydroisomerization steps (6–8). It should be noted that the isolated nature and specific structure of the active sites of the Al13Fe4 surface may also stabilize such species as the anti-1-methyl-p-allyl intermediate, which is of minor importance on Pd. As the butadiene pressure decreases, the cis/trans ratio slowly tends to its equilibrium value via steps 50 + 20 + 4 or 8 + 70 (step n0 is the reverse of step n). When butadiene is fully consumed, 1-butene readily adsorbs on the surface and is hydrogenated to butane either directly (steps 6 + 9) or after previous isomerization to 2-butenes (steps 6 + 70 and 6 + 80 ). At this stage, the cis-2-butene can easily convert to trans-2-butene due to the lower coverage of (less strongly bounded) butenes as compared to butadiene during the first hydrogenation reaction. 3.2. Deactivation and regeneration 3.2.1. Nature of the contaminant(s) As already pointed out in our previous work [12], depending on the sample history, carbon was either not detected or detected in very small amount on Al13Fe4(0 1 0) by AES. Conversely, the surface was contaminated with oxygen after catalytic tests, and even after long periods in the preparation chamber. Oxygen contamination 3 It should be noted that adsorbed p-allylic intermediates have also been proposed to be involved in 1-butene/2-butene isomerization on Pd [23]. However, along with direct interconversion between the three butene isomers, these additional species and steps are not needed to better rationalize our observations and thus not explicitly considered in Scheme 1.

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0.5

clean surface pre-oxygenated surface

butane butenes

Partial pressures (Torr)

0.4

0.3

1

0.2

0.1

2 3

Scheme 1. Mechanistic scheme of the hydrogenation of butadiene on Al13Fe4(0 1 0). The adsorbed intermediate species are depicted in blue (p-allyl) or red (butyl). In the p-allyl group, syn- and anti- refer to the position of the middle hydrogen with respect to the methyl group [31]. In the adsorbed butyl species, half-hydrogenation is represented by a half-dashed double-bond. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

was ascribed to water traces inherently present in reactive gases (and in UHV chambers as well). Indeed, water is the main impurity in research-grade gases such as ‘‘ultrapure” H2 and Ar (2 ppm). O2, CO2 or CO may also be present and adsorb on reactive surfaces, although none of these molecules could be detected in the reactants using MS. In addition, commercial butadiene contains 4-tert-butylcatechol (PTBC, concentration < 200 ppm), a polymerization inhibitor which might adsorb on the surface as well. The adsorption of oxygen-containing species was held responsible for the gradual deactivation of the surface in reactions conditions, as the one observed in the example of Fig. 2 showing three successive reaction runs. Similarly, the powdered Al13Fe4 single-crystal used by Armbrüster and coworkers to catalyze the hydrogenation of acetylene was prone to deactivation [6]. With the aim of gaining insight into the deactivation process and its relationship with the nature of the oxygen-containing phase, an AES investigation was undertaken. Fig. 3a and b shows Auger spectra recorded for the clean surface (spectrum I-i), or for the sample subjected to several treatments. Fig. 3c reports the corresponding Al67/Fe701 and O507/Fe701 peak-to-peak intensity ratios (simply noted Al/Fe and O/Fe later on). Due to the small escape depth of electrons in the 30–70 eV energy domain (ca. 2 Å) and the fact that valence energy levels are involved in the emission process, these low-energy Auger peaks are very sensitive to the metal oxidation state [24]. For metallic Al0, a single Auger peak (LVV levels) is expected at 64–69 eV, while for aluminum oxide (mostly Al2O3) a doublet of peaks at 32–36 and 50–54 eV is generally detected [25–28]. In the case of Fe, the single MVV peak at 47– 48 eV for metallic Fe0 splits into a doublet of peaks at 43–46 and 51–52 eV upon oxidation [24,29], making it difficult to detect FeOx in the presence of AlOx. 3.2.2. Oxide formation and deactivation In a first series of experiments (Fig. 3, black color), the sample freshly prepared by annealing at ca. 750 °C under UHV was allowed to cool down to RT for 3 h and, prior to the catalytic tests reported in Fig. 2, an Auger spectrum (I-ii) was recorded. After this cooling period, it is seen (Fig. 3a and b) that oxygen is already present at the surface. It is not surprising if we consider that, for a total pressure of 1
109 Torr, a vacuum composed of 1/3 oxygencontaining species and a sticking coefficient of 1, roughly 1 monolayer of these molecules adsorbs on the surface in 3 h [30]. This contamination significantly affects the low-energy structure

0.0 0

10

20

30 Time (min)

40

50

Fig. 2. Comparison between reaction kinetics on the initially clean surface (thin lines, 3 successive runs) and the surface intentionally contaminated with oxygen (thick lines). Reaction conditions: same as Fig. 1. The data were recorded using MS. For clarity, butadiene partial pressure is not shown (butane pressure was zero during runs 2 and 3).

of the Auger signal (Fig. 3b), and the Al/Fe ratio considerably decreases (Fig. 3c). The latter suggests preferential adsorption of oxygen-containing molecules on Al atoms. However, Al and Fe fully remain in metallic state, as indicated by the absence of AlOx and FeOx features in spectrum I-ii. After the first reaction run (Fig. 2), spectrum I-iii was recorded, showing further enrichment of the surface in oxygen, further decreased Al/Fe ratio, and the presence of an AlOx phase in addition to Al0 and Fe0. Indeed, due to the particular environment of the Al13Fe4(0 1 0) surface containing both Al and Fe, we infer that the pair of peaks at 39 and 56 eV (instead of 32–36 and 50–54 eV for Al2O3) can be ascribed to AlOx (x 6 3/2), possibly interacting with Fe or FeOx. Finally, after the third reaction run (Fig. 2), spectrum I-iv was recorded, showing unchanged Al/Fe ratio and slightly higher O/Fe ratio. In order to directly relate the presence and nature of adsorbed oxygen or oxide to the butadiene hydrogenation activity, the cleaned surface was kept under UHV for 15 h, then analyzed by AES, and finally tested in standard reaction conditions. The corresponding data are reported in Fig. 2 (thick lines). As a result, the activity of the surface intentionally contaminated with oxygen is similar to that of the surface in the second run of the previous series of experiments, and the Auger spectra for those two surfaces are also similar (Fig. 3b, spectrum I-iii and thick-line spectrum). Hence, the deactivation process is closely related to the adsorption of oxygen-containing species onto the Al13Fe4(0 1 0) surface and the Al oxide formation. 3.2.3. Surface oxidation using O2 In a second series of experiments (Fig. 3, red color), the clean surface was exposed to low-pressure oxygen (5
107 Torr O2 for 200 s), then to elevated-pressure oxygen (6 Torr O2 for 200 s), both at RT. While spectrum II-i, recorded after small oxygen exposure, is similar to spectrum I-ii showing only metallic Al and Fe and Al/Fe  O/Fe  1, the large oxygen exposure leads to the quasi disappearance of Al0 and Fe0 contributions and the appearance of the AlOx signature (double peak in spectrum II-ii, Fig. 3b). This correlates with a twofold increase of the O/Fe ratio to 2.1 (Fig. 3c), and a downshift of the O KLL peak from 508 to 506 eV (Fig. 3b). Such a low energy of the O KLL peak is typical of aluminum oxide, the values measured for pristine Al2O3 being 503–504 eV [26,28]. Furthermore, the quasi absence of Fe-related signal at low energy suggests that a continuous AlOx overlayer has formed.

L. Piccolo, L. Kibis / Journal of Catalysis 332 (2015) 112–118

(a)

III-ii III-i II-ii II-i I-iv I-iii I-ii I-i

Fe

O

C

Fe

Al 100

0

200

300

400

500

600

700

Energy [eV]

(b)

117

sure) or 40 min (after reaction or exposure to elevated O2 pressure).4 In order to check whether the AlOx layer can be stabilized, we carried out a third series of experiments (Fig. 3, blue color), during which the clean surface was heated up to 500 °C in the presence of reactants (total pressure 6 Torr). This led to a fully oxidized surface (almost no Al0 or Fe0, spectrum III–i, Fig. 3b) and an O/Fe ratio of 3.1 (Fig. 3c). With respect to the phase formed on high-pressure O2 exposure at RT (spectrum II-ii), the aluminum oxide layer formed at high temperature appears similar in nature (see the similarity of the two spectra in Fig. 3b). However, in the latter case the oxide film is thicker (larger O/Fe ratio). Furthermore, annealing the sample at ca. 750 °C under UHV for 40 min did not permit to remove oxygen, the O/Fe ratio remaining roughly constant (Fig. 3c). Only the Al/Fe ratio increased (to 0.8), indicating a partial reduction of the AlOx layer.

AlOx

4. Conclusion and outlook

III-ii III-i II-ii II-i I-iv I-iii I-ii I-i O

Fe Al 20

40

60

80

480

500

520

Energy [eV]

(c) Al67/Fe701

3

O507/Fe701

2

1

0

I-i

I-ii

I-iii

I-iv

II-i

II-ii

III-i

III-ii

Experiment number Fig. 3. (a) Auger electron spectra recorded after various treatments. (b) Close view of the 20–90 eV and 470–530 eV ranges (see text for thick-line spectrum). (c) Ratios of the peak-to-peak intensities of Al peak at 67 eV over Fe peak at 701 eV (Al67/ Fe701), and O peak at 506–508 eV over Fe peak at 701 eV (O507/Fe701). Legend of the experiment numbers: 1st series (black color): I-i immediately after UHV cleaning; I-ii after prolonged UHV exposure; I-iii after reaction run 1 (Fig. 2) at RT; I-iv after reaction run 3 (Fig. 2) at RT. 2nd series (red color): II-i after UHV cleaning and lowpressure O2 exposure; II-ii after elevated-pressure O2 exposure. 3rd series (blue color): III-i after UHV cleaning and reactions at up to 500 °C; III-ii after prolonged UHV exposure and UHV annealing at ca. 750 °C for 40 min. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

3.2.4. Thermal stability of the surface oxide Noticeably, in any of the above cases, oxygen could be removed from the surface and the model catalyst could be fully regenerated by annealing the sample at ca. 750 °C under UHV for 20 min (after long period in UHV or exposure to low O2 pres-

The kinetics of butadiene hydrogenation over the highly active and selective Al13Fe4(0 1 0) surface was investigated, with a close focus on the distribution of butene isomers, which are all primary products of butadiene partial hydrogenation. As for Pd(1 0 0), 1-butene is the main product, with a 1-butene/2butenes ratio of 2–3 depending on the reaction conditions. The main specificity of Al13Fe4 lies in the cis-2-butene/trans-2butene ratio which is in the 1–2 range, against 0.1–0.2 for Pd and a variety of other metals. Based on the literature, this led us to propose adsorbed anti-1-methyl-p-allyl as a critical precursor to cis-2-butene. This species would be favored over its syn counterpart due to (i) steric effects at high reactant coverage and (ii) the isolated nature of the Fe-centered active sites of Al13Fe4(0 1 0). Other interesting characteristics of this catalytic surface are the sensitivity of the butenes distribution to the initial hydrogen pressure and the slow formation of butane in the course of butadiene conversion. These results suggest the possibility of hydro-isomerization of butenes via a butyl intermediate, itself precursor of butane. Accordingly, we proposed a mechanistic scheme, which would be valuably assessed in the future through microkinetic simulations and/or first-principles calculations. The second part of this paper addressed the question of the sensitivity of the Al13Fe4(0 1 0) surface to oxygen-containing contaminants, such as those inherently present in reactive gases (PTBC in butadiene, water in all cases). Using pre/post-reaction AES complemented with pure oxygen exposures, we demonstrated that the gradual deactivation of the model catalyst in reaction conditions is due to surface chemisorption of atomic oxygen or oxygen-containing species (low pressures) and/or formation of an AlOx adlayer (elevated pressures). This oxide layer is most probably sub-stoichiometric with respect to alumina and interacts with iron atoms. The surface coverage of the oxygenated phase is directly related to the hydrogenation activity. When formed at RT, this phase could be easily removed by sample annealing at ca. 750 °C under UHV. When formed at high temperature (ca. 500 °C), the AlOx film is thicker and relatively stable against UHV annealing. Possible strategies to solve the deactivation issue, which is inherent to non-noble metals, include (i) addition of a promoter, (ii) replacement of Al with another post-transition metal, and (iii) dispersion of the Al–Fe phase on a suitable support. In all cases, the idea is to provide unfavorable interaction of the resulting combination with oxygen through electronic effects. 4 However, low-pressure H2 annealing at ca. 600 °C did not restore the clean surface.

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