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Temperature dependence of the 2-butene hydrogenation over supported Pd nanoparticles and Pd(1 1 1)
Journal of Molecular Catalysis A: Chemical 377 (2013) 137–142
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Journal of Molecular Catalysis A: Chemical journal homepage: www.elsevier.com/locate/molcata
Temperature dependence of the 2-butene hydrogenation over supported Pd nanoparticles and Pd(1 1 1) Aditya Savara, Wiebke Ludwig, Karl-Heinz Dostert, Swetlana Schauermann ∗ Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany
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Article history: Received 17 July 2012 Received in revised form 16 April 2013 Accepted 17 April 2013 Available online 13 May 2013 Keywords: Hydrogenation Model catalysts Subsurface hydrogen Olefin Molecular beams
a b s t r a c t The activity and induction times for 2-butene hydrogenation have been investigated over a Pd(1 1 1) single crystal surface and model Pd nanoparticles supported on Fe3 O4 /Pt(1 1 1) by isothermal pulsed molecular beam experiments, in the temperature range of 220–340 K. C-modification of supported Pd particles induced persistent hydrogenation activity at low temperatures (220–260 K). C-modification of the Pd(1 1 1) surface, in contrast, did not result in significant reactivity changes. At low temperatures (220–260 K), hydrogenation activity was only maintained over the C-modified Pd particles, while at temperatures (≥280 K) persistent hydrogenation was observed over all Pd catalysts at comparable rates. Two principal reaction mechanisms are discussed that could be responsible for the observed hydrogenation activity at different Pd surfaces. We show that on Pd nanoparticles, the reaction mechanism involving subsurface hydrogen species plays an important role under all investigated conditions. This subsurface-related reaction pathway relies on an effective replenishment of the subsurface hydrogen reservoir, which is affected by the presence of strongly adsorbed hydrocarbon species that are formed in the induction period. We discuss the correlation between the induction times and the hydrogenation activity of different Pd surfaces. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The conversion of alkenes by reaction with hydrogen over transition metal catalysts, such as cis-trans isomerization and hydrogenation, is relevant for numerous chemical processes including fine chemical and pharmaceutical synthesis, petrochemical hydrotreatment, and food processing; and is thus an important reaction for industrial applications and catalysis research [1–5]. The chemistry of alkenes has been extensively studied by conventional catalytic studies [2,6]. More recently, investigations by modern surface science techniques have been conducted for single crystal surfaces [1,3,7] and supported model catalysts [8,9]. Surface science studies provided much insight into the mechanistic details, such as key reaction steps. Despite the recent progress, the microscopic picture of the processes controlling the activity and selectivity in alkene conversions with hydrogen as a function of temperature remains incomplete. Over Pd catalysts, alkene conversions with hydrogen proceed via the generally accepted Horiuti–Polanyi mechanism [10]. A reaction scheme for the mechanism when cis-2-butene and D2 are used in the feed has been previously published [11]. In this
∗ Corresponding author. E-mail address: [email protected] (S. Schauermann). 1381-1169/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molcata.2013.04.015
mechanism, a surface-bound 2-butyl-d1 species is formed in a first half-hydrogenation step, and this butyl species is a common intermediate for both the cis-trans isomerization pathway and the hydrogenation pathway. Once the surface-bound butyl species is formed, rotation around the C-C bond followed by a -hydride elimination yields the cis-trans isomerization product, trans-2butene-d1 , with one H atom exchanged for a D atom [12]. Insertion of a second D atom into the Pd-butyl bond leads to the formation of the hydrogenation product, butane-d2 [11]. Recently, it was demonstrated that for the hydrogenation reaction at 260 K the presence of subsurface absorbed H(D) species are required, particularly for the second half-hydrogenation step, while cis-trans isomerization (and thus the formation of the 2butyl-d1 species) can effectively proceed even when only surface adsorbed H(D) species are available [13]. The dynamics of subsurface H(D) replenishment are thus of great interest for selectivity toward the hydrogenation pathway. Small amounts of C located at atomically flexible particle sites were demonstrated to induce sustained hydrogenation activity at low temperatures [11,13], which has been attributed – based on theoretical [14] and experimental [15] evidences – to C-assisted facilitation of H(D) diffusion to the subsurface under reaction conditions. Here, we investigate the hydrogenation activity of a Pd(1 1 1) single crystal surface and a supported Pd/Fe3 O4 /Pt(1 1 1) model catalyst under well-defined UHV conditions in the temperature
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range between 220 and 340 K using isothermal pulsed molecular beam (MB) methods to further clarify the underlying microscopic mechanism active for hydrogenation over Pd catalysts at different temperatures. To investigate the effect of C modification of lowcoordinated particle sites, measurements were also conducted over a C-modified Pd(1 1 1) surface and C-modified Pd nanoparticles. Additionally, for the first time, we systematically investigated the temperature dependence of the induction time over well-defined catalysts. 2. Experimental All molecular beam (MB) experiments were performed at the Fritz-Haber-Institute, Berlin, in a UHV apparatus which has been described in detail previously [16]. In brief, two molecular beams supplying cis-2-butene and D2 , respectively were simultaneously crossed on the sample surface. D2 (3.2 × 1015 molecules cm−2 s−1 ) was provided through a doubly-differentially pumped multichannel array source modulated by remote-controlled valves and shutters. The source was operated at room temperature and the beam diameter was chosen to exceed the sample size. A supersonic beam generated by a triply-differentially pumped source modulated by a remote controlled solenoid valve and shutters was used to dose cis-2-butene (5.6 × 1012 molecules cm−2 s−1 ), with the beam diameter chosen to be smaller than the sample. A quadruple mass spectrometer (QMS) system (ABB Extrel) was used to simultaneously monitor the evolution of the partial pressures of the reactants and products in the gas phase (cis-2-butene was monitored by the 41 amu C3 H5 + fragment, trans-2-butene-d1 by the 42 amu fragment C3 H4 D+ , and butane-d2 by the 45 amu C3 H5 D2 + fragment) as a function of time and the obtained signals were corrected to account for the natural abundance of 13 C (see [10] for details). The supported Pd/Fe3 O4 /Pt(1 1 1) model catalyst was prepared ˚ following an established procedure [17,18]. In brief, a thin (100 A) Fe3 O4 film was grown onto a Pt(1 1 1) single crystal substrate by repeated cycles of Fe (>99.99%, Goodfellow) physical vapor deposition and subsequent oxidation. The quality of the oxide film was checked with LEED and IRAS [19,20]. The Pd particles were grown onto the Fe3 O4 film by Pd (>99.9%, Goodfellow) physical vapor deposition (flux calibrated by a quartz microbalance, final Pd coverage: ∼2.7 × 1015 atoms cm−2 ) while keeping the substrate temperature at ∼115 K and applying a voltage of +800 V to avoid sputtering during evaporation. After stabilization by repeated oxidation–reduction cycles at 500 K, the Pd particles exhibit a well-ordered hexagonal shape exposing a majority of (1 1 1)-facets (∼80%) and a smaller amount of (1 0 0)-facets. The average particle diameter is ∼7 nm with a particle density of 8.3 × 1011 cm−2 (see [17,18] for details). The Pd(1 1 1) single crystal surface was cleaned prior to use by repeated cycles of Ar+ ion bombardment, annealing and oxidation. The Pd(1 1 1) and Pt(1 1 1) single crystals used were ∼10 mm × 10 mm × 1 mm. For C-modification Pd model catalysts were precovered with deuterium (280 L D2 at 100 K) then exposed to 0.85 L cis-2-butene at 100 K and subsequently heated in vacuum to 485 K (see [11] for details). 3. Results Conversion of cis-2-butene with D2 was studied over a Pd(1 1 1) single crystal surface and a Pd/Fe3 O4 /Pt(1 1 1) model catalyst in the temperature range from 220 to 340 K. Pulsed MB experiments were conducted under isothermal conditions over both C-free and C-modified surfaces. Previously, it was shown that carbon is not uniformly distributed over Pd nanoparticles but
Fig. 1. Averaged steady state hydrogenation rates over the C-free Pd(1 1 1) single crystal surface and C-free Pd particles obtained from the 30 last long pulses of the MB experiments. For a direct comparison, the rates are normalized to the number of Pd surface atoms. For temperatures ≤ 260 K, no sustained hydrogenation activity is observed for both C-free Pd surfaces. Only for temperatures ≥ 280 K, hydrogenation activity is maintained at comparable rates over the C-free Pd particles and the C-free Pd(1 1 1) surface. The pulses show a rectangular profile on Pd(1 1 1) and a decreasing profile over Pd nanoparticles, pointing to possible differences in the reaction mechanisms.
mainly occupies low-coordinated surface sites, such as edges and corners, while leaving the majority of the regular (1 1 1) terraces on Pd nanoparticles C-free [11]. In all measurements, the catalyst was initially exposed to 280 L D2 to saturate both the surface and subsurface regions of the Pd surface with atomic D. During experiments, D2 was continuously supplied at a large excess (3.2 × 1015 molecules cm−2 s−1 , corresponding to 570 D2 molecules per cis-2-butene molecule). Simultaneously, a sequence of cis-2-butene pulses consisting of 50 short (4 s on, 4 s off) and subsequently 30 long (20 s on, 10 s off) pulses with a small 2-butene flux (5.6 × 1012 molecules cm−2 s−1 ) were dosed onto the surface. The temporal evolutions of reactants and products in the gas phase were monitored by mass spectrometry. Production of the primary gas phase products begins only after an induction period during which strongly bound hydrocarbon species, such as 2-butene and/or partly dehydrogenated species, accumulate on the surface [11]. After this induction period, the reaction typically occurs at a high rate and finally reaches a steady state regime. Full details on the evolution of the initial hydrogenation activity on D-saturated surfaces can be found elsewhere [11]. In this report, we will mainly focus on the steady state reactivity, which was typically reached after a few tens of seconds. Please note that under the experimental low-temperature (220–340 K) and ultra-high vacuum pressure (10−9 –10−6 mbar) conditions 2butene does not decompose to carbon on Pd surfaces. 2-butene was shown to form only partly dehydrogenated hydrocarbon species on transition metal surfaces in this temperature and pressure regime with the stoichiometry close to C4 H6 [11]. In the temperature range from 220 to 340 K, both the Pd(1 1 1) surface and the Pd particles exhibit persistent cis-trans isomerization activity at similar rates over all C-free and C-modified Pd surfaces (not shown here). Over all of the investigated Pd surfaces, some initial hydrogenation activity is always observed on D-presaturated surfaces in the temperature range of 220–340 K [11,21]. However, the hydrogenation activity under steady state conditions, depends strongly on the reaction temperature as well as on the particular Pd catalyst surface (Figs. 1 and 2). For the C-free surfaces of both the Pd(1 1 1) single crystal and the Pd nanoparticles, the reaction rates under steady state conditions exhibit a qualitatively very similar evolution with temperature (Fig. 1). Two different temperature regimes can be distinguished: (i) for low temperatures (T ≤ 260 K), hydrogenation activity is neither
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Fig. 2. Averaged quasi steady state hydrogenation rates over the C-modified Pd(1 1 1) single crystal surface and the C-modified Pd particles obtained from the 30 last long pulses of the MB experiments. The rates are normalized to the number of Pd surface atoms to allow for a direct comparison. For the C-modified Pd(1 1 1) surface, similar to the C-free Pd catalysts, persistent hydrogenation activity is only observed for temperatures ≥ 280 K. Over the C-modified Pd particles, in contrast hydrogenation activity is maintained over the entire temperature range. The pulses show a decreasing profile almost in the entire temperature range over C-modified Pd nanoparticles, while the profile is rectangular [22] at all temperatures over Cmodified Pd(1 1 1).
sustained over the Pd(1 1 1) surface nor the Pd particles, even though high initial hydrogenation rates (on the surface presaturated with D) are detected for both catalysts at the beginning of the experiment [21]. At higher temperatures (T ≥ 280 K), persistent hydrogenation activity is observed for both the Pd particles and the Pd(1 1 1) surfaces. At 280 and 300 K, the hydrogenation rates are similar for the Pd(1 1 1) surface and the supported Pd particles, while the Pd particles become noticeably more active (factor of ∼2) than the Pd(1 1 1) single crystal at temperatures ≥ 320 K. The time evolution of the hydrogenation rate is, however, different on Pd nanoparticles and Pd(1 1 1). While the profile of the hydrogenation rate shows a rectangular form [22] on Pd(1 1 1), the reaction rate on Pd nanoparticles decreases over the duration of the butene pulse. The pulse-averaged hydrogenation rates obtained after Cmodification of the Pd(1 1 1) single crystal surface and of the Pd particles are shown in Fig. 2. Over the C-modified Pd(1 1 1) surface, there is virtually no change relative to the C-free Pd(1 1 1) surface. In contrast, over the C-modified Pd particles, high sustained hydrogenation activity is already observable at low temperatures (220–260 K). At higher temperatures (≥280 K), the hydrogenation rates of the C-modified particles are comparable with the reaction rates over C-modified Pd(1 1 1). However – similarly to the C-free surfaces shown in Fig. 1 – the averaged pulse profiles on C-modified Pd nanoparticles and Pd(1 1 1) exhibit a different time dependence, with a rectangular profile over Pd(1 1 1) and a decreasing time profile over Pd nanoparticles. Temperature dependence is observed not only for the steady state hydrogenation activity, but also for the induction period, which we define as the time between the beginning of the experiment and the onset of the product evolution in the gas phase [11]. During this inductions period, strongly bound and/or partly dehydrogenated hydrocarbon species accumulate on the surface, for which the H/C stoichiometry depends on the reaction temperature [11]. The induction time is shown as a function of temperature for the C-free surfaces of the Pd(1 1 1) single crystal and the Pd nanoparticles in Fig. 3. Note that the induction times are not significantly affected by C-modification. This fact is in agreement with the spectroscopic observation that only a minor faction of the metal surface is blocked by deposited carbon, [11] and therefore similar amounts of strongly bound and/or partly dehydrogenated hydrocarbons can be accumulated on the metal surface prior to the onset
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Fig. 3. Initial induction times at the beginning of the MB experiments over the Pd(1 1 1) single crystal surface and the Pd particles as a function of temperature for C-free Pd catalysts (there is no significant change with C-modification). For temperatures ≤ 260 K, both Pd(1 1 1) and Pd particles exhibit similar, short induction times. While the induction period over the Pd nanoparticles remains short also for higher temperatures, over the Pd(1 1 1) single crystal surface, the induction time increases for temperatures ≥ 280 K coinciding with the onset of persistent hydrogenation activity. For the Pd particles, only a slight increase in induction time is observed for the highest applied temperature of 340 K.
of reaction. Two temperature regimes can be distinguished for Pd(1 1 1): At low temperatures (T ≤ 260 K) a short induction period is observed, while at temperatures above 280 K, which coincides with the onset of sustained hydrogenation activity over Pd(1 1 1), the induction time increases. Over Pd particles, the induction time remains nearly constant over the entire temperature range, with a slight increase observed only for the highest temperature investigated (340 K). 4. Discussion The hydrogenation activity at low temperatures (220–280 K) was previously found to depend strongly on the presence of subsurface absorbed H(D) species [13,23]. For all investigated Pd surfaces, the onset of hydrogenation activity was observed after an initial induction period, during which strongly bound hydrocarbon species of various H/C stoichiometries are formed [11]. After this induction time, hydrogenation was found to proceed at high reaction rates over all investigated catalysts, both C-free and C-modified. However, this initial high hydrogenation activity vanished in steady state at low temperatures (below 280 K) on all surfaces except for C-modified Pd nanoparticles and this effect was previously ascribed to inhibited H(D) diffusion to subsurface sites under reaction conditions, presumably due to the presence of coadsorbed hydrocarbon species [21]. In turn, based on experimental [15] and theoretical [14] evidence the sustained hydrogenation observed over the C-modified Pd particles in this temperature range (220–280 K) was attributed to facilitated H(D) diffusion through C-modified low-coordinated sites such as edges and corners into particles’ volume. On Pd nanoclusters, high atomic flexibility of these low-coordinated sites modified with carbon was theoretically shown to enable formation of wide channels for barrierless H subsurface diffusion [14], which allows fast replenishment of subsurface H(D) species under steady state conditions. The onset of persistent hydrogenation activity over the C-free Pd(1 1 1) single crystal and the C-free Pd particles for temperatures ≥ 280 K clearly shows that the second half-hydrogenation step proceeds in this high temperature regime. This fact may indicate that the diffusion of H(D) to subsurface sites becomes sufficient at these higher temperatures – either due to an increased diffusion rate constant or/and due to reduced poisoning of the surface by co-adsorbed hydrocarbon species. Alternatively, at ≥280 K the mechanism of the second half-hydrogenation step can change and become dominated by insertion of different type of D species into the metal-butyl bond, namely a surface D species in absence of
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Fig. 4. Comparison of the time evolution of 2-butene hydrogenation rate at 320 K on different surfaces upon modulation of the butene beam (the hydrogenation rates are normalized for easier comparison). On both Pd(1 1 1) catalysts, the pulse profiles show a rectangular form [22], while on Pd nanoparticles the hydrogenation rates decrease over the butene pulse. The latter type of kinetic behavior arises from the depletion of subsurface D species in coarse of the reaction [13,24] and indicates that the subsurface-related mechanism plays an important role on Pd nanoparticles at this temperature (for detailed explanation see text).
subsurface D. This scenario implies that the intrinsic rate constant for the insertion of a surface D into the Pd-butyl bond (in absence of subsurface D species) may become sufficiently large at these high temperatures resulting in measurable hydrogenation rates on the surface containing only surface D species. The judgment about feasibility of this hypothesis is difficult based on the available experimental data, but kinetic analysis of the pulse profiles [24] may give some indications on the rate limiting reaction steps and possible reaction mechanisms. For the Pd particles, a decreasing pulse profile is detected for the hydrogenation product in the temperature range between 280 and 320 K (Figs. 1 and 4). This observation was previously assigned to a decaying concentration of subsurface D species over the length of the 2-butene pulse [13,24] for the low-temperature regime (≤280 K). In contrast, a rectangular pulse shape [22] is observed for the isomerization product at these temperatures, indicating that the concentration of the 2-butyl species and the concentration of the surface D species remain essentially constant over the 2-butene pulse length (data not shown, see reference [24]). A decreasing pulse profile for the hydrogenation product therefore suggests that even at high temperatures there is a pathway for hydrogenation which requires the presence of a second D species (different from
regular surface D), which was previously identified to be related to subsurface D [13]. As the subsurface D reservoir depletes over the duration of the pulse, the overall hydrogenation rate decreases over the pulse length. It should be noted that the pulse shape observed for the hydrogenation product over the Pd(1 1 1) single crystal surface exhibits a basically rectangular form, similar to that found for the isomerization product (see Fig. 4 for comparison of the pulse shapes over Pd(1 1 1) and Pd nanoparticles at 320 K). This observation indicates that the reaction on Pd(1 1 1) most likely does not involve the particular D species whose concentration strongly decreases during the duration of the butene pulse. Such behavior may be consistent with a second microscopic mechanism where only surface D species unrelated to subsurfaces D species are involved in both half-hydrogenation steps and whose concentration does not significantly change over the length of the butene exposure (as also indicated by the observed rectangular pulse shape of the cis-trans isomerized product [24], not shown). The combination of all of the experimental observations at different temperatures suggests that two hydrogenation mechanisms can operate in parallel: (1) a mechanism involving only surface D(H) species (in absence of subsurface D(H)) with a high activation
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barrier and (2) a mechanism involving subsurface D(H) species in the second half-hydrogenation step with a much lower activation barrier. The second mechanism most likely dominates (i) at surfaces pre-saturated with D(H) at all reaction temperatures as indicated by a very high initial hydrogenation activity observed for all investigated catalysts in the whole studied temperature regime (see also [21,24]) and (ii) under steady state conditions at low temperatures on surfaces, at which fast replenishment of subsurface H(D) species is possible. The only example observed in our studies of a surface capable of high hydrogenation activity at low temperatures was the catalyst consisting of C-modified Pd nanoparticles, where C-modification at the low-coordinated sites decreases the activation barrier for subsurface hydrogen diffusion [14] and with this enables fast replenishment of the subsurface hydrogen reservoir [15]. At higher temperatures (>280 K), the first hydrogenation mechanism – with a high apparent activation barrier and involving only surface D(H) species – may gain in importance due to an increase in the corresponding reaction rate constant. This hypothesis can explain the observed hydrogenation activity of Pd(1 1 1) at elevated temperatures, which is not accompanied by any prominent decrease of the hydrogenation rate over the duration of the butene pulse. In contrast, on Pd nanoparticles the first mechanism (involving only surface D species) alone cannot account for the hydrogenation behavior observed at >280 K, since a decrease of reaction rate is observed during the reaction pulse. At >280 K, both mechanisms most likely operate simultaneously on the Pd nanoparticles, resulting in overall higher reactivity per surface Pd atom relative to Pd(1 1 1) (Figs. 1 and 2). Importantly, the pulse profiles observed on the particles clearly demonstrate decreasing hydrogenation rates over the length of the pulse (Fig. 4), which attests to the involvement of subsurface D species in the second half-hydrogenation step (for more detailed discussion of the pulse profiles see [24]). Thus, we believe that a mechanism involving subsurface hydrogen species plays an important role in determining the overall hydrogenation activity of Pd nanoparticle even in the high temperature region, where the concentration of subsurface hydrogen is believed to be low. The contribution of the subsurface-related pathway would also be expected to provide a significant contribution to the overall hydrogenation activity over Pd particles at ≥1 bar pressure conditions. The temperature dependence of the induction period was investigated for both C-free and C-containing Pd catalysts. As discussed in detail earlier [13], during the induction period, the catalyst surface builds up a critical coverage of various strongly bound and/or partly dehydrogenated hydrocarbon species prior to production of the primary gas phase products. For the Pd(1 1 1) single crystal surface, a lengthening of the induction time is observed at >280 K, which coincides with the onset of sustained hydrogenation activity (Fig. 3). The increase in the induction time at higher temperatures clearly demonstrates that the accumulation process of strongly bound hydrocarbon species becomes slower, presumably due to faster 2-butene desorption or due to the altered formation/decomposition of the hydrocarbon species at higher temperatures. Considering that co-adsorbed hydrocarbons may inhibit both the dissociative H2 (D2 ) adsorption and the H(D) diffusion to subsurface sites [11,24], changes in the co-adsorbed hydrocarbon species and/or their coverage are expected to affect the H(D) availability on and in Pd and thereby also the steady state hydrogenation activity. For the Pd particles, the induction time remains basically constant over the entire investigated temperature range, demonstrating that over the Pd particles a critical coverage of hydrocarbon species is quickly reached even at higher temperatures. This observation may be due to more effective formation of strongly bound and/or partly dehydrogenated hydrocarbon species on the
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low-coordinated particle specific sites compared to regular (1 1 1)facets. It should be noted that activated H(D) diffusion into Pd is known to be a structure sensitive process, which is faster for more open sites than for close-packed (1 1 1)-facets [25–29]. Suppression of steady state hydrogenation as observed at low temperatures for all investigated catalysts (except for C-modified Pd nanoparticles) was previously attributed to inhibition of H(D) diffusion into the subsurface due to blocking of surface sites by strongly bound and/or partly dehydrogenated hydrocarbons accumulated in the induction period [11,24]. With increasing temperature, the surface sites might be partly unblocked either due to faster desorption or even stronger decomposition of surface hydrocarbon species, leaving larger areas of the metal surface available for hydrogen subsurface diffusion. These effects can result in a more facile replenishment of the subsurface H reservoir at higher temperatures and, consequently, in the persistent hydrogenation activity. In agreement with the experimentally observed pulse profiles, this replenishment is expected to be more pronounced for Pd particles exhibiting low-coordinated sites compared to the close-packed Pd(1 1 1) surface, despite a faster build-up of strongly-bound hydrocarbon species.
5. Conclusions We have studied the temperature dependence of the induction time and steady state rates for hydrogenation of cis-2-butene with D2 over a Pd(1 1 1) single crystal surface and over Pd nanoparticles in the temperature range from 220 to 340 K. Measurements were conducted over C-free as well as C-modified Pd catalysts. At low temperatures (220–260 K), hydrogenation activity is only sustained over the C-modified Pd particles, which is attributed to fast H(D) diffusion to subsurface sites through the C-modified low-coordinated surface sites (e.g. edges and corners). In contrast, over both the C-free Pd particles and Pd(1 1 1) single crystals, at 220–260 K the hydrogenation pathway becomes deactivated after a short period of initial high activity on the D-saturated surface. This deactivation is interpreted as a result of inhibited subsurface H(D) diffusion due to the presence of co-adsorbed hydrocarbon species. For higher temperatures (≥280 K), persistent hydrogenation activity is observed over all investigated Pd catalysts. Analysis of the time dependence of the hydrogenation rate recorded after modulation of the 2-butene beam – in combination with previous experimental observations – suggests that two principal mechanisms can operate in parallel on Pd surfaces: (1) a mechanism involving only surface D(H) species (in absence of subsurface D(H)) with high activation barrier and (2) a mechanism involving subsurface D(H) species in the second half-hydrogenation step with much lower activation barrier. While the first mechanism might be responsible for the high-temperature reactivity observed for Pd(1 1 1) surface, a large portion of the hydrogenation over Pd nanoparticles occurs via the subsurface-hydrogen related mechanism (mechanism 2) even at high temperatures, i.e. over the whole temperature region investigated. At low temperatures (<280 K), the subsurface-related pathway appears to be the major pathway over all surfaces, and relies on the fast replenishment of the subsurface H(D) reservoir. At <280 K, this requirement could be achieved under steady state conditions only on Pd clusters with C-modified low-coordinated surface sites (edges and corners). In the high temperature regime (>280 K), the subsurface-related pathway still remains important over Pd particles, but a fraction of the product likely originates from the first mechanism, involving only surface H(D) species. With these results, we demonstrate that the subsurface-related mechanism plays an important role in determining the hydrogenation activity of Pd nanoparticles in the whole
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temperature range investigated. Under ≥ 1 bar pressure conditions, the subsurface-related pathway would be expected to provide a significant contribution to the overall hydrogenation activity. A lengthening of the induction period over the Pd(1 1 1) single crystal surface at temperatures > 280 K was found to coincide with the onset of sustained hydrogenation. The lengthening of the induction time can be most likely referred to faster desorption and stronger decomposition of the hydrocarbon reactant. Both effects can also lead to partial unblocking of the metal surface, resulting in easier formation of both surface and subsurface D(H) species, and consequently an increase in the overall hydrogenation activity at >280 K. Acknowledgments The authors thank H.-J. Freund, F. Zaera, M. Wilde, K. Fukutani, K. Neyman, R.J. Madix. S.S. acknowledges support from Fonds der chemischen Industrie. References [1] G.C. Bond, Metal-Catalysed Reactions of Hydrocarbons, Springer Science, New York, 2005. [2] T.I. Taylor (Ed.), Catalysis, 5, Reinhold, New York, 1957, p. 257. [3] G.A. Somorjai, Introduction to Surface Science Chemistry and Catalysis, John Wiley and Sons, New York, 1994. [4] V. Ponec, G.C. Bond, Catalysis by Metals and Alloys, Elsevier, Amsterdam, 1995. [5] F. Zaera, Prog. Surf. Sci. 69 (2001) 1. [6] C. Kemball (Ed.), Advances in Catalysis., vol. 11, Academic Press, New York, 1959, p. 223. [7] D. Teschner, J. Borsodi, A. Wootsch, Z. Revay, M. Havecker, A. Knop-Gericke, S.D. Jackson, R. Schlogl, Science 320 (2008) 86.
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Journal of Molecular Catalysis A: Chemical journal homepage: www.elsevier.com/locate/molcata
Temperature dependence of the 2-butene hydrogenation over supported Pd nanoparticles and Pd(1 1 1) Aditya Savara, Wiebke Ludwig, Karl-Heinz Dostert, Swetlana Schauermann ∗ Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany
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Article history: Received 17 July 2012 Received in revised form 16 April 2013 Accepted 17 April 2013 Available online 13 May 2013 Keywords: Hydrogenation Model catalysts Subsurface hydrogen Olefin Molecular beams
a b s t r a c t The activity and induction times for 2-butene hydrogenation have been investigated over a Pd(1 1 1) single crystal surface and model Pd nanoparticles supported on Fe3 O4 /Pt(1 1 1) by isothermal pulsed molecular beam experiments, in the temperature range of 220–340 K. C-modification of supported Pd particles induced persistent hydrogenation activity at low temperatures (220–260 K). C-modification of the Pd(1 1 1) surface, in contrast, did not result in significant reactivity changes. At low temperatures (220–260 K), hydrogenation activity was only maintained over the C-modified Pd particles, while at temperatures (≥280 K) persistent hydrogenation was observed over all Pd catalysts at comparable rates. Two principal reaction mechanisms are discussed that could be responsible for the observed hydrogenation activity at different Pd surfaces. We show that on Pd nanoparticles, the reaction mechanism involving subsurface hydrogen species plays an important role under all investigated conditions. This subsurface-related reaction pathway relies on an effective replenishment of the subsurface hydrogen reservoir, which is affected by the presence of strongly adsorbed hydrocarbon species that are formed in the induction period. We discuss the correlation between the induction times and the hydrogenation activity of different Pd surfaces. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The conversion of alkenes by reaction with hydrogen over transition metal catalysts, such as cis-trans isomerization and hydrogenation, is relevant for numerous chemical processes including fine chemical and pharmaceutical synthesis, petrochemical hydrotreatment, and food processing; and is thus an important reaction for industrial applications and catalysis research [1–5]. The chemistry of alkenes has been extensively studied by conventional catalytic studies [2,6]. More recently, investigations by modern surface science techniques have been conducted for single crystal surfaces [1,3,7] and supported model catalysts [8,9]. Surface science studies provided much insight into the mechanistic details, such as key reaction steps. Despite the recent progress, the microscopic picture of the processes controlling the activity and selectivity in alkene conversions with hydrogen as a function of temperature remains incomplete. Over Pd catalysts, alkene conversions with hydrogen proceed via the generally accepted Horiuti–Polanyi mechanism [10]. A reaction scheme for the mechanism when cis-2-butene and D2 are used in the feed has been previously published [11]. In this
∗ Corresponding author. E-mail address: [email protected] (S. Schauermann). 1381-1169/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molcata.2013.04.015
mechanism, a surface-bound 2-butyl-d1 species is formed in a first half-hydrogenation step, and this butyl species is a common intermediate for both the cis-trans isomerization pathway and the hydrogenation pathway. Once the surface-bound butyl species is formed, rotation around the C-C bond followed by a -hydride elimination yields the cis-trans isomerization product, trans-2butene-d1 , with one H atom exchanged for a D atom [12]. Insertion of a second D atom into the Pd-butyl bond leads to the formation of the hydrogenation product, butane-d2 [11]. Recently, it was demonstrated that for the hydrogenation reaction at 260 K the presence of subsurface absorbed H(D) species are required, particularly for the second half-hydrogenation step, while cis-trans isomerization (and thus the formation of the 2butyl-d1 species) can effectively proceed even when only surface adsorbed H(D) species are available [13]. The dynamics of subsurface H(D) replenishment are thus of great interest for selectivity toward the hydrogenation pathway. Small amounts of C located at atomically flexible particle sites were demonstrated to induce sustained hydrogenation activity at low temperatures [11,13], which has been attributed – based on theoretical [14] and experimental [15] evidences – to C-assisted facilitation of H(D) diffusion to the subsurface under reaction conditions. Here, we investigate the hydrogenation activity of a Pd(1 1 1) single crystal surface and a supported Pd/Fe3 O4 /Pt(1 1 1) model catalyst under well-defined UHV conditions in the temperature
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range between 220 and 340 K using isothermal pulsed molecular beam (MB) methods to further clarify the underlying microscopic mechanism active for hydrogenation over Pd catalysts at different temperatures. To investigate the effect of C modification of lowcoordinated particle sites, measurements were also conducted over a C-modified Pd(1 1 1) surface and C-modified Pd nanoparticles. Additionally, for the first time, we systematically investigated the temperature dependence of the induction time over well-defined catalysts. 2. Experimental All molecular beam (MB) experiments were performed at the Fritz-Haber-Institute, Berlin, in a UHV apparatus which has been described in detail previously [16]. In brief, two molecular beams supplying cis-2-butene and D2 , respectively were simultaneously crossed on the sample surface. D2 (3.2 × 1015 molecules cm−2 s−1 ) was provided through a doubly-differentially pumped multichannel array source modulated by remote-controlled valves and shutters. The source was operated at room temperature and the beam diameter was chosen to exceed the sample size. A supersonic beam generated by a triply-differentially pumped source modulated by a remote controlled solenoid valve and shutters was used to dose cis-2-butene (5.6 × 1012 molecules cm−2 s−1 ), with the beam diameter chosen to be smaller than the sample. A quadruple mass spectrometer (QMS) system (ABB Extrel) was used to simultaneously monitor the evolution of the partial pressures of the reactants and products in the gas phase (cis-2-butene was monitored by the 41 amu C3 H5 + fragment, trans-2-butene-d1 by the 42 amu fragment C3 H4 D+ , and butane-d2 by the 45 amu C3 H5 D2 + fragment) as a function of time and the obtained signals were corrected to account for the natural abundance of 13 C (see [10] for details). The supported Pd/Fe3 O4 /Pt(1 1 1) model catalyst was prepared ˚ following an established procedure [17,18]. In brief, a thin (100 A) Fe3 O4 film was grown onto a Pt(1 1 1) single crystal substrate by repeated cycles of Fe (>99.99%, Goodfellow) physical vapor deposition and subsequent oxidation. The quality of the oxide film was checked with LEED and IRAS [19,20]. The Pd particles were grown onto the Fe3 O4 film by Pd (>99.9%, Goodfellow) physical vapor deposition (flux calibrated by a quartz microbalance, final Pd coverage: ∼2.7 × 1015 atoms cm−2 ) while keeping the substrate temperature at ∼115 K and applying a voltage of +800 V to avoid sputtering during evaporation. After stabilization by repeated oxidation–reduction cycles at 500 K, the Pd particles exhibit a well-ordered hexagonal shape exposing a majority of (1 1 1)-facets (∼80%) and a smaller amount of (1 0 0)-facets. The average particle diameter is ∼7 nm with a particle density of 8.3 × 1011 cm−2 (see [17,18] for details). The Pd(1 1 1) single crystal surface was cleaned prior to use by repeated cycles of Ar+ ion bombardment, annealing and oxidation. The Pd(1 1 1) and Pt(1 1 1) single crystals used were ∼10 mm × 10 mm × 1 mm. For C-modification Pd model catalysts were precovered with deuterium (280 L D2 at 100 K) then exposed to 0.85 L cis-2-butene at 100 K and subsequently heated in vacuum to 485 K (see [11] for details). 3. Results Conversion of cis-2-butene with D2 was studied over a Pd(1 1 1) single crystal surface and a Pd/Fe3 O4 /Pt(1 1 1) model catalyst in the temperature range from 220 to 340 K. Pulsed MB experiments were conducted under isothermal conditions over both C-free and C-modified surfaces. Previously, it was shown that carbon is not uniformly distributed over Pd nanoparticles but
Fig. 1. Averaged steady state hydrogenation rates over the C-free Pd(1 1 1) single crystal surface and C-free Pd particles obtained from the 30 last long pulses of the MB experiments. For a direct comparison, the rates are normalized to the number of Pd surface atoms. For temperatures ≤ 260 K, no sustained hydrogenation activity is observed for both C-free Pd surfaces. Only for temperatures ≥ 280 K, hydrogenation activity is maintained at comparable rates over the C-free Pd particles and the C-free Pd(1 1 1) surface. The pulses show a rectangular profile on Pd(1 1 1) and a decreasing profile over Pd nanoparticles, pointing to possible differences in the reaction mechanisms.
mainly occupies low-coordinated surface sites, such as edges and corners, while leaving the majority of the regular (1 1 1) terraces on Pd nanoparticles C-free [11]. In all measurements, the catalyst was initially exposed to 280 L D2 to saturate both the surface and subsurface regions of the Pd surface with atomic D. During experiments, D2 was continuously supplied at a large excess (3.2 × 1015 molecules cm−2 s−1 , corresponding to 570 D2 molecules per cis-2-butene molecule). Simultaneously, a sequence of cis-2-butene pulses consisting of 50 short (4 s on, 4 s off) and subsequently 30 long (20 s on, 10 s off) pulses with a small 2-butene flux (5.6 × 1012 molecules cm−2 s−1 ) were dosed onto the surface. The temporal evolutions of reactants and products in the gas phase were monitored by mass spectrometry. Production of the primary gas phase products begins only after an induction period during which strongly bound hydrocarbon species, such as 2-butene and/or partly dehydrogenated species, accumulate on the surface [11]. After this induction period, the reaction typically occurs at a high rate and finally reaches a steady state regime. Full details on the evolution of the initial hydrogenation activity on D-saturated surfaces can be found elsewhere [11]. In this report, we will mainly focus on the steady state reactivity, which was typically reached after a few tens of seconds. Please note that under the experimental low-temperature (220–340 K) and ultra-high vacuum pressure (10−9 –10−6 mbar) conditions 2butene does not decompose to carbon on Pd surfaces. 2-butene was shown to form only partly dehydrogenated hydrocarbon species on transition metal surfaces in this temperature and pressure regime with the stoichiometry close to C4 H6 [11]. In the temperature range from 220 to 340 K, both the Pd(1 1 1) surface and the Pd particles exhibit persistent cis-trans isomerization activity at similar rates over all C-free and C-modified Pd surfaces (not shown here). Over all of the investigated Pd surfaces, some initial hydrogenation activity is always observed on D-presaturated surfaces in the temperature range of 220–340 K [11,21]. However, the hydrogenation activity under steady state conditions, depends strongly on the reaction temperature as well as on the particular Pd catalyst surface (Figs. 1 and 2). For the C-free surfaces of both the Pd(1 1 1) single crystal and the Pd nanoparticles, the reaction rates under steady state conditions exhibit a qualitatively very similar evolution with temperature (Fig. 1). Two different temperature regimes can be distinguished: (i) for low temperatures (T ≤ 260 K), hydrogenation activity is neither
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Fig. 2. Averaged quasi steady state hydrogenation rates over the C-modified Pd(1 1 1) single crystal surface and the C-modified Pd particles obtained from the 30 last long pulses of the MB experiments. The rates are normalized to the number of Pd surface atoms to allow for a direct comparison. For the C-modified Pd(1 1 1) surface, similar to the C-free Pd catalysts, persistent hydrogenation activity is only observed for temperatures ≥ 280 K. Over the C-modified Pd particles, in contrast hydrogenation activity is maintained over the entire temperature range. The pulses show a decreasing profile almost in the entire temperature range over C-modified Pd nanoparticles, while the profile is rectangular [22] at all temperatures over Cmodified Pd(1 1 1).
sustained over the Pd(1 1 1) surface nor the Pd particles, even though high initial hydrogenation rates (on the surface presaturated with D) are detected for both catalysts at the beginning of the experiment [21]. At higher temperatures (T ≥ 280 K), persistent hydrogenation activity is observed for both the Pd particles and the Pd(1 1 1) surfaces. At 280 and 300 K, the hydrogenation rates are similar for the Pd(1 1 1) surface and the supported Pd particles, while the Pd particles become noticeably more active (factor of ∼2) than the Pd(1 1 1) single crystal at temperatures ≥ 320 K. The time evolution of the hydrogenation rate is, however, different on Pd nanoparticles and Pd(1 1 1). While the profile of the hydrogenation rate shows a rectangular form [22] on Pd(1 1 1), the reaction rate on Pd nanoparticles decreases over the duration of the butene pulse. The pulse-averaged hydrogenation rates obtained after Cmodification of the Pd(1 1 1) single crystal surface and of the Pd particles are shown in Fig. 2. Over the C-modified Pd(1 1 1) surface, there is virtually no change relative to the C-free Pd(1 1 1) surface. In contrast, over the C-modified Pd particles, high sustained hydrogenation activity is already observable at low temperatures (220–260 K). At higher temperatures (≥280 K), the hydrogenation rates of the C-modified particles are comparable with the reaction rates over C-modified Pd(1 1 1). However – similarly to the C-free surfaces shown in Fig. 1 – the averaged pulse profiles on C-modified Pd nanoparticles and Pd(1 1 1) exhibit a different time dependence, with a rectangular profile over Pd(1 1 1) and a decreasing time profile over Pd nanoparticles. Temperature dependence is observed not only for the steady state hydrogenation activity, but also for the induction period, which we define as the time between the beginning of the experiment and the onset of the product evolution in the gas phase [11]. During this inductions period, strongly bound and/or partly dehydrogenated hydrocarbon species accumulate on the surface, for which the H/C stoichiometry depends on the reaction temperature [11]. The induction time is shown as a function of temperature for the C-free surfaces of the Pd(1 1 1) single crystal and the Pd nanoparticles in Fig. 3. Note that the induction times are not significantly affected by C-modification. This fact is in agreement with the spectroscopic observation that only a minor faction of the metal surface is blocked by deposited carbon, [11] and therefore similar amounts of strongly bound and/or partly dehydrogenated hydrocarbons can be accumulated on the metal surface prior to the onset
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Fig. 3. Initial induction times at the beginning of the MB experiments over the Pd(1 1 1) single crystal surface and the Pd particles as a function of temperature for C-free Pd catalysts (there is no significant change with C-modification). For temperatures ≤ 260 K, both Pd(1 1 1) and Pd particles exhibit similar, short induction times. While the induction period over the Pd nanoparticles remains short also for higher temperatures, over the Pd(1 1 1) single crystal surface, the induction time increases for temperatures ≥ 280 K coinciding with the onset of persistent hydrogenation activity. For the Pd particles, only a slight increase in induction time is observed for the highest applied temperature of 340 K.
of reaction. Two temperature regimes can be distinguished for Pd(1 1 1): At low temperatures (T ≤ 260 K) a short induction period is observed, while at temperatures above 280 K, which coincides with the onset of sustained hydrogenation activity over Pd(1 1 1), the induction time increases. Over Pd particles, the induction time remains nearly constant over the entire temperature range, with a slight increase observed only for the highest temperature investigated (340 K). 4. Discussion The hydrogenation activity at low temperatures (220–280 K) was previously found to depend strongly on the presence of subsurface absorbed H(D) species [13,23]. For all investigated Pd surfaces, the onset of hydrogenation activity was observed after an initial induction period, during which strongly bound hydrocarbon species of various H/C stoichiometries are formed [11]. After this induction time, hydrogenation was found to proceed at high reaction rates over all investigated catalysts, both C-free and C-modified. However, this initial high hydrogenation activity vanished in steady state at low temperatures (below 280 K) on all surfaces except for C-modified Pd nanoparticles and this effect was previously ascribed to inhibited H(D) diffusion to subsurface sites under reaction conditions, presumably due to the presence of coadsorbed hydrocarbon species [21]. In turn, based on experimental [15] and theoretical [14] evidence the sustained hydrogenation observed over the C-modified Pd particles in this temperature range (220–280 K) was attributed to facilitated H(D) diffusion through C-modified low-coordinated sites such as edges and corners into particles’ volume. On Pd nanoclusters, high atomic flexibility of these low-coordinated sites modified with carbon was theoretically shown to enable formation of wide channels for barrierless H subsurface diffusion [14], which allows fast replenishment of subsurface H(D) species under steady state conditions. The onset of persistent hydrogenation activity over the C-free Pd(1 1 1) single crystal and the C-free Pd particles for temperatures ≥ 280 K clearly shows that the second half-hydrogenation step proceeds in this high temperature regime. This fact may indicate that the diffusion of H(D) to subsurface sites becomes sufficient at these higher temperatures – either due to an increased diffusion rate constant or/and due to reduced poisoning of the surface by co-adsorbed hydrocarbon species. Alternatively, at ≥280 K the mechanism of the second half-hydrogenation step can change and become dominated by insertion of different type of D species into the metal-butyl bond, namely a surface D species in absence of
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Fig. 4. Comparison of the time evolution of 2-butene hydrogenation rate at 320 K on different surfaces upon modulation of the butene beam (the hydrogenation rates are normalized for easier comparison). On both Pd(1 1 1) catalysts, the pulse profiles show a rectangular form [22], while on Pd nanoparticles the hydrogenation rates decrease over the butene pulse. The latter type of kinetic behavior arises from the depletion of subsurface D species in coarse of the reaction [13,24] and indicates that the subsurface-related mechanism plays an important role on Pd nanoparticles at this temperature (for detailed explanation see text).
subsurface D. This scenario implies that the intrinsic rate constant for the insertion of a surface D into the Pd-butyl bond (in absence of subsurface D species) may become sufficiently large at these high temperatures resulting in measurable hydrogenation rates on the surface containing only surface D species. The judgment about feasibility of this hypothesis is difficult based on the available experimental data, but kinetic analysis of the pulse profiles [24] may give some indications on the rate limiting reaction steps and possible reaction mechanisms. For the Pd particles, a decreasing pulse profile is detected for the hydrogenation product in the temperature range between 280 and 320 K (Figs. 1 and 4). This observation was previously assigned to a decaying concentration of subsurface D species over the length of the 2-butene pulse [13,24] for the low-temperature regime (≤280 K). In contrast, a rectangular pulse shape [22] is observed for the isomerization product at these temperatures, indicating that the concentration of the 2-butyl species and the concentration of the surface D species remain essentially constant over the 2-butene pulse length (data not shown, see reference [24]). A decreasing pulse profile for the hydrogenation product therefore suggests that even at high temperatures there is a pathway for hydrogenation which requires the presence of a second D species (different from
regular surface D), which was previously identified to be related to subsurface D [13]. As the subsurface D reservoir depletes over the duration of the pulse, the overall hydrogenation rate decreases over the pulse length. It should be noted that the pulse shape observed for the hydrogenation product over the Pd(1 1 1) single crystal surface exhibits a basically rectangular form, similar to that found for the isomerization product (see Fig. 4 for comparison of the pulse shapes over Pd(1 1 1) and Pd nanoparticles at 320 K). This observation indicates that the reaction on Pd(1 1 1) most likely does not involve the particular D species whose concentration strongly decreases during the duration of the butene pulse. Such behavior may be consistent with a second microscopic mechanism where only surface D species unrelated to subsurfaces D species are involved in both half-hydrogenation steps and whose concentration does not significantly change over the length of the butene exposure (as also indicated by the observed rectangular pulse shape of the cis-trans isomerized product [24], not shown). The combination of all of the experimental observations at different temperatures suggests that two hydrogenation mechanisms can operate in parallel: (1) a mechanism involving only surface D(H) species (in absence of subsurface D(H)) with a high activation
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barrier and (2) a mechanism involving subsurface D(H) species in the second half-hydrogenation step with a much lower activation barrier. The second mechanism most likely dominates (i) at surfaces pre-saturated with D(H) at all reaction temperatures as indicated by a very high initial hydrogenation activity observed for all investigated catalysts in the whole studied temperature regime (see also [21,24]) and (ii) under steady state conditions at low temperatures on surfaces, at which fast replenishment of subsurface H(D) species is possible. The only example observed in our studies of a surface capable of high hydrogenation activity at low temperatures was the catalyst consisting of C-modified Pd nanoparticles, where C-modification at the low-coordinated sites decreases the activation barrier for subsurface hydrogen diffusion [14] and with this enables fast replenishment of the subsurface hydrogen reservoir [15]. At higher temperatures (>280 K), the first hydrogenation mechanism – with a high apparent activation barrier and involving only surface D(H) species – may gain in importance due to an increase in the corresponding reaction rate constant. This hypothesis can explain the observed hydrogenation activity of Pd(1 1 1) at elevated temperatures, which is not accompanied by any prominent decrease of the hydrogenation rate over the duration of the butene pulse. In contrast, on Pd nanoparticles the first mechanism (involving only surface D species) alone cannot account for the hydrogenation behavior observed at >280 K, since a decrease of reaction rate is observed during the reaction pulse. At >280 K, both mechanisms most likely operate simultaneously on the Pd nanoparticles, resulting in overall higher reactivity per surface Pd atom relative to Pd(1 1 1) (Figs. 1 and 2). Importantly, the pulse profiles observed on the particles clearly demonstrate decreasing hydrogenation rates over the length of the pulse (Fig. 4), which attests to the involvement of subsurface D species in the second half-hydrogenation step (for more detailed discussion of the pulse profiles see [24]). Thus, we believe that a mechanism involving subsurface hydrogen species plays an important role in determining the overall hydrogenation activity of Pd nanoparticle even in the high temperature region, where the concentration of subsurface hydrogen is believed to be low. The contribution of the subsurface-related pathway would also be expected to provide a significant contribution to the overall hydrogenation activity over Pd particles at ≥1 bar pressure conditions. The temperature dependence of the induction period was investigated for both C-free and C-containing Pd catalysts. As discussed in detail earlier [13], during the induction period, the catalyst surface builds up a critical coverage of various strongly bound and/or partly dehydrogenated hydrocarbon species prior to production of the primary gas phase products. For the Pd(1 1 1) single crystal surface, a lengthening of the induction time is observed at >280 K, which coincides with the onset of sustained hydrogenation activity (Fig. 3). The increase in the induction time at higher temperatures clearly demonstrates that the accumulation process of strongly bound hydrocarbon species becomes slower, presumably due to faster 2-butene desorption or due to the altered formation/decomposition of the hydrocarbon species at higher temperatures. Considering that co-adsorbed hydrocarbons may inhibit both the dissociative H2 (D2 ) adsorption and the H(D) diffusion to subsurface sites [11,24], changes in the co-adsorbed hydrocarbon species and/or their coverage are expected to affect the H(D) availability on and in Pd and thereby also the steady state hydrogenation activity. For the Pd particles, the induction time remains basically constant over the entire investigated temperature range, demonstrating that over the Pd particles a critical coverage of hydrocarbon species is quickly reached even at higher temperatures. This observation may be due to more effective formation of strongly bound and/or partly dehydrogenated hydrocarbon species on the
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low-coordinated particle specific sites compared to regular (1 1 1)facets. It should be noted that activated H(D) diffusion into Pd is known to be a structure sensitive process, which is faster for more open sites than for close-packed (1 1 1)-facets [25–29]. Suppression of steady state hydrogenation as observed at low temperatures for all investigated catalysts (except for C-modified Pd nanoparticles) was previously attributed to inhibition of H(D) diffusion into the subsurface due to blocking of surface sites by strongly bound and/or partly dehydrogenated hydrocarbons accumulated in the induction period [11,24]. With increasing temperature, the surface sites might be partly unblocked either due to faster desorption or even stronger decomposition of surface hydrocarbon species, leaving larger areas of the metal surface available for hydrogen subsurface diffusion. These effects can result in a more facile replenishment of the subsurface H reservoir at higher temperatures and, consequently, in the persistent hydrogenation activity. In agreement with the experimentally observed pulse profiles, this replenishment is expected to be more pronounced for Pd particles exhibiting low-coordinated sites compared to the close-packed Pd(1 1 1) surface, despite a faster build-up of strongly-bound hydrocarbon species.
5. Conclusions We have studied the temperature dependence of the induction time and steady state rates for hydrogenation of cis-2-butene with D2 over a Pd(1 1 1) single crystal surface and over Pd nanoparticles in the temperature range from 220 to 340 K. Measurements were conducted over C-free as well as C-modified Pd catalysts. At low temperatures (220–260 K), hydrogenation activity is only sustained over the C-modified Pd particles, which is attributed to fast H(D) diffusion to subsurface sites through the C-modified low-coordinated surface sites (e.g. edges and corners). In contrast, over both the C-free Pd particles and Pd(1 1 1) single crystals, at 220–260 K the hydrogenation pathway becomes deactivated after a short period of initial high activity on the D-saturated surface. This deactivation is interpreted as a result of inhibited subsurface H(D) diffusion due to the presence of co-adsorbed hydrocarbon species. For higher temperatures (≥280 K), persistent hydrogenation activity is observed over all investigated Pd catalysts. Analysis of the time dependence of the hydrogenation rate recorded after modulation of the 2-butene beam – in combination with previous experimental observations – suggests that two principal mechanisms can operate in parallel on Pd surfaces: (1) a mechanism involving only surface D(H) species (in absence of subsurface D(H)) with high activation barrier and (2) a mechanism involving subsurface D(H) species in the second half-hydrogenation step with much lower activation barrier. While the first mechanism might be responsible for the high-temperature reactivity observed for Pd(1 1 1) surface, a large portion of the hydrogenation over Pd nanoparticles occurs via the subsurface-hydrogen related mechanism (mechanism 2) even at high temperatures, i.e. over the whole temperature region investigated. At low temperatures (<280 K), the subsurface-related pathway appears to be the major pathway over all surfaces, and relies on the fast replenishment of the subsurface H(D) reservoir. At <280 K, this requirement could be achieved under steady state conditions only on Pd clusters with C-modified low-coordinated surface sites (edges and corners). In the high temperature regime (>280 K), the subsurface-related pathway still remains important over Pd particles, but a fraction of the product likely originates from the first mechanism, involving only surface H(D) species. With these results, we demonstrate that the subsurface-related mechanism plays an important role in determining the hydrogenation activity of Pd nanoparticles in the whole
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temperature range investigated. Under ≥ 1 bar pressure conditions, the subsurface-related pathway would be expected to provide a significant contribution to the overall hydrogenation activity. A lengthening of the induction period over the Pd(1 1 1) single crystal surface at temperatures > 280 K was found to coincide with the onset of sustained hydrogenation. The lengthening of the induction time can be most likely referred to faster desorption and stronger decomposition of the hydrocarbon reactant. Both effects can also lead to partial unblocking of the metal surface, resulting in easier formation of both surface and subsurface D(H) species, and consequently an increase in the overall hydrogenation activity at >280 K. Acknowledgments The authors thank H.-J. Freund, F. Zaera, M. Wilde, K. Fukutani, K. Neyman, R.J. Madix. S.S. acknowledges support from Fonds der chemischen Industrie. References [1] G.C. Bond, Metal-Catalysed Reactions of Hydrocarbons, Springer Science, New York, 2005. [2] T.I. Taylor (Ed.), Catalysis, 5, Reinhold, New York, 1957, p. 257. [3] G.A. Somorjai, Introduction to Surface Science Chemistry and Catalysis, John Wiley and Sons, New York, 1994. [4] V. Ponec, G.C. Bond, Catalysis by Metals and Alloys, Elsevier, Amsterdam, 1995. [5] F. Zaera, Prog. Surf. Sci. 69 (2001) 1. [6] C. Kemball (Ed.), Advances in Catalysis., vol. 11, Academic Press, New York, 1959, p. 223. [7] D. Teschner, J. Borsodi, A. Wootsch, Z. Revay, M. Havecker, A. Knop-Gericke, S.D. Jackson, R. Schlogl, Science 320 (2008) 86.
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