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Mechanism of Olefin Hydrogenation Catalysis Driven by PalladiumDissolved Hydrogen Satoshi Ohno,*,† Markus Wilde,*,† Kozo Mukai,‡ Jun Yoshinobu,‡ and Katsuyuki Fukutani† †

Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, 153-8505 Tokyo, Japan Institute for Solid State Physics, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, 270-8581 Chiba, Japan

‡

S Supporting Information *

ABSTRACT: The Pd-catalyzed hydrogenation of CC double bonds is one of the most important synthetic routes in organic chemistry. This catalytic surface reaction is known to require hydrogen in the interior of the Pd catalyst, but the mechanistic role of the Pd-dissolved H has remained elusive. To shed new light into this fundamental problem, we visualized the H distribution near a Pd single crystal surface charged with absorbed hydrogen during a typical catalytic conversion of butene (C4H8) to butane (C4H10), using H depth profiling via nuclear reaction analysis. This has revealed that the catalytic butene hydrogenation (1) occurs between 160 and 250 K on a H-saturated Pd surface, (2) is triggered by the emergence of Pd bulk-dissolved hydrogen onto this surface, but (3) does not necessarily require large stationary H concentrations in subsurface sites. Even deeply bulk-absorbed hydrogen proves to be reactive, suggesting that Pd-dissolved hydrogen chiefly acts by directly providing reactive H species to the surface after bulk diffusion rather than by indirectly activating surface H through modifying the surface electronic structure. The chemisorbed surface hydrogen is found to promote hydrogenation reactivity by weakening the butene-Pd interaction and by significantly reducing the decomposition of the olefin.

I. INTRODUCTION The highly selective hydrogen (H) addition to olefinic CC double bonds catalyzed by palladium (Pd) is extensively used in petroleum refining and in organic synthesis of fine chemicals and pharmaceuticals.1 Although ample evidence shows that H dissolved in the interior of the active metal phase is an essential ingredient in heterogeneous olefin hydrogenation on Pd catalysts,2−12 the actual role of the absorbed hydrogen in the catalytic mechanism is still not well understood at the microscopic level. Ceyer and co-workers established that hydrogen emerging from subsurface sites of Ni(111) may transiently carry special reactivity13−16 toward the hydrogenation of adsorbed hydrocarbons because the subsurface hydrogen in Ni is a strongly endothermic state that required highly sophisticated experimental means for its production17,18 and is significantly (∼0.7 eV) less stable than surfacechemisorbed hydrogen. Therefore, the subsurface hydrogen is energetically in much closer proximity to the transition state for the hydrogenation reaction19,20 as compared to stably chemisorbed surface hydrogen, which experimentally does not exhibit any hydrogenation reactivity at all in ultrahigh vacuum.13−16 In the following, we refer to a scenario, in which subsurface hydrogen emerges onto the surface to produce reactive surface H species, as a direct mechanism. Such a direct scenario might also explain the particular reactivity that Pd-dissolved hydrogen (Habs) has shown in many Pd-catalyzed olefin hydrogenation reactions.2−11 Pd readily absorbs large amounts of H,21−24 so that subsurface H states © XXXX American Chemical Society

become populated even at low H2 pressures and temperatures, especially in nanoparticles.25 Pd bulk-absorbed H is substantially (∼0.3−0.4 eV) less stable than chemisorbed H on Pd surfaces (Hsurf, adsorption energy ∼ −0.5 eV/H),26−28 yet it is still exothermically bound by about −0.1 eV/H relative to 1/2 H2 in the gas phase. Hydrogen in the first layer subsurface sites is only slightly more stable (−0.19 eV/H).26−28 Although no energy becomes available for the hydrogenation reaction when H resurfaces from Pd subsurface sites, we expect such emerging H to behave more reactively than surface chemisorbed H, because energetically, the initial potential energy difference between subsurface H and chemisorbed H places the former significantly closer to the transition state for the hydrogenation reaction. Owing to the great facility of hydrogen absorption in Pd, it has been suggested alternatively that a significant population of subsurface sites by Habs may indirectly activate otherwise nonreactive chemisorbed reactants (i.e., surface hydrogen and/or the olefin) by altering the electronic structure of the Pd surface. Experimental evidence for a possible indirect mechanism was first proposed for ethylene hydrogenation on Pd(111), where subsurface hydrogen appeared to change the adsorption state of the hydrocarbon at a low temperature.29,30 With respect to surface hydrogen, a recent theoretical study Received: January 29, 2016 Revised: May 8, 2016

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presence of surface carbon. On Pd(110), one monolayer (ML) corresponds to a surface atom density of 9.35 × 1014 cm−2. Ultrapure (7 N) H2 gas was produced by a Pd−Ag membrane permeation purifier. A fully H-covered Pd(110) was prepared with an exposure of 1 L (=1.33 × 10−4 Pa·s) H2 at 115 K. cis-2-Butene (99%) was used as obtained from SigmaAldrich, Japan. An exposure of 1.25 L butene at 115 K was sufficient to saturate Pd(110) while avoiding multilayer condensation. NRA hydrogen depth profiling was performed with an energy-analyzed (ΔE = 3 keV) 15N2+ ion beam near the resonance energy (Eres = 6.385 MeV) of the 1H(15N, αγ)12C nuclear reaction.31 Calibrated detection of the characteristic 4.43 MeV γ-rays emitted in this reaction allows for quantification of the H density in the target at a probing depth z, which is determined by the incident 15N ion beam energy Ei as z = (Ei − Eres)/S, where S is the stopping power of 6.4 MeV 15N in Pd (S = 3.9 keV/nm). At surface-normal ion beam incidence, the near-surface depth resolution corresponds to ∼2 nm, which is limited mainly by Doppler-broadening due to H zero-point vibration.31 TPD was recorded at a linear heating rate of 2 K/s with a shielded and differentially pumped quadrupole mass spectrometer, which simultaneously monitored the surface temperature and the desorbing gas particles of masses m/e = 2, 56, and 58, corresponding to H2, butene, and butane, respectively. After each TPD run, the sample was briefly flashed to 1000 K and annealed at 750 K in 5.0 × 10−5 Pa O2 to eliminate carbonaceous deposits that resulted from butene decomposition. HREELS measurements were performed in a different UHV chamber of the Institute for Solid State Physics, University of Tokyo, using an LK Technologies spectrometer (ELS 5000). The HREEL spectra were recorded at 90 K with an energy resolution of 4.5 meV in a specular-detection geometry at an incidence angle of 60° and a primary electron energy of 7 eV.

demonstrated that chemisorbed H on Pd(111) becomes energetically destabilized when the first-layer subsurface sites are occupied by Habs to more than 50%, which affects a notable reduction of the activation barrier for the hydrogenation of an ethyl intermediate by surface hydrogen.12 Such a potentially large population of subsurface sites by Habs is a valid consideration for industrial Pd catalysts, where any absorbed hydrogen inside finely dispersed Pd nanoparticles necessarily remains confined in close vicinity to the surface. Therefore, in order to resolve the long-standing dispute whether Pd-dissolved hydrogen affects olefin hydrogenation catalysis in a direct (i.e., as an emerging reactant) or in an indirect (i.e., as an electronic structure modifier) mechanism with respect to chemisorbed surface hydrogen, we investigate here in a unique experimental approach how the surface vicinity of Habs influences its reaction behavior on Pd catalysts. We determine the catalytic reactivity in a prototypical conversion of cis-2-butene (C4H8) to butane (C4H10) through temperatureprogrammed desorption (TPD) measurements of H2 and C4H8 coadsorbed on a Pd(110) surface that has been precharged with Habs through H2 exposure at low temperature and characterize the hydrogen distribution near the Pd surface7 under reaction conditions with H depth profiling via 1H(15N,αγ)12C nuclear reaction analysis (NRA).31 We used a Pd single crystal as a particularly suitable “model catalyst” for this investigation because the initial distribution of Habs underneath the surface is controllable with large variations of depth through the exposure temperature (Te) during H2 charging.32,33 Such control cannot be realized at nanoparticle catalysts due to the lack of an extended bulk region. We furthermore address questions on the mechanistic roles of surface hydrogen as well as of the coadsorbed butene in the catalytic hydrogenation reaction through C4H8 TPD experiments with deuterium (D) isotope-labeled surface hydrogen and high resolution electron energy loss spectroscopy (HREELS). We shall first characterize the hydrogen and butene coadsorption system on Pd(110) through TPD, HREELS, and NRA measurements before discussing the individual roles of Pd-dissolved hydrogen, of surfacechemisorbed hydrogen, and of the butene adsorbate in the catalytic hydrogenation reaction.

III. RESULTS We first characterize the butene and hydrogen coadsorption system on Pd(110). To elucidate the influence of the individual H species (Hsurf, Habs) on the reactive behavior of the butene adsorbate, we performed C4H8 TPD measurements after dosing 1.25 L C4H8 at 115 K onto Pd(110) under three different conditions with respect to the abundance of hydrogen: clean, saturated with chemisorbed surface-hydrogen (Hsurf), and with various amounts of Pd-absorbed hydrogen (Habs) in addition to Hsurf. Figure 1 compares butene TPD spectra from clean and Hsurfsaturated Pd(110). The C4H8 TPD spectrum from clean

II. EXPERIMENTAL METHODS The TPD and NRA experiments were performed in an ultrahigh vacuum (UHV) system with a base pressure below 1 × 10−8 Pa connected to the MALT 5 MV van de Graaf tandem accelerator (The University of Tokyo).31 In UHV, the Pd(110) single crystal could be cooled to 80 K with liquid nitrogen and heated to above 1300 K by radiation and electron bombardment. The sample temperature was measured with a type K thermocouple spot-welded to the crystal edge. Surface cleaning consisted of repeated cycles of sputtering with 800 eV Ar+ ions, annealing to 1000 K, annealing at 750 K in 5.0 × 10−5 Pa O2, reduction at room temperature in 5.0 × 10−5 Pa H2, and a final flash to 600 K in UHV until a clear (1 × 1) pattern was obtained in low energy electron diffraction. Since carbon contamination is common but difficult to detect with Auger electron spectroscopy on Pd, we verified the chemical cleanliness of the surface through the LEED observation of the characteristic H-induced (2 × 1) to (1 × 2) pairing-row phase transition34 of Pd(110) and by a concomitant sharp desorption peak at 200 K in the resulting H2 TPD spectrum.32,35 Both features would be suppressed in the

Figure 1. Butene TPD spectra (m/e = 56) recorded from the clean and Hsurf-saturated Pd(110) surface. B

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Information, S-2). Spectra (a) and (b) show a blue shift of the CC vibrational energy loss peak37 from 187 meV on clean Pd(110) to 192 meV on the surface precovered with chemisorbed H. This shift indicates a strengthening of the C C bond and thereby suggests a Hsurf-induced weakening of the butene interaction with the Pd surface. Similar blue shifts of the CC vibrational frequency and a weakening of the alkene-Pd π-bond in the presence of surface hydrogen have been reported for ethylene adsorbed on clean versus H-covered Pd(110).39,40 This weakened Pd-butene interaction may serve as one tentative explanation for the fact that Hsurf partially suppresses the C4H8 decomposition (Figure 1). A second, alternative rationalization for this effect will be discussed in section IV. Since the HREEL spectra (b) and (c) in Figure 2 are almost identical, it appears that Pd-absorbed H in addition to Hsurf does not further influence the Pd−C4H8 interaction. Moreover, the energy loss peak at 120 meV due to the Pd−H vibration41 visible in spectra (b) and (c) indicates that chemisorbed Hsurf continues to exist on the surface after coadsorption of butene, even when Habs is absent, that is, subsurface sites are vacant. Since the relative surface coverages and a potential competition for adsorption sites between hydrogen and butene may be important in the catalytic hydrogenation, this observation is quite significant and interesting to compare, e.g., with carbon monoxide (CO) coadsorption on H-covered Pd(110), where CO pushes prechemisorbed surface H atoms into the subsurface.42 Apparently, the butene−Hsurf interaction is less repulsive and the C4H8−Pd interaction weaker so that Hsurf can stay on the surface even after C4H8 coadsorption. Assessing now the effect of Habs on the thermal desorption behavior of butene and the reactivity in the catalytic butene hydrogenation, we show in Figure 3 C4H8 desorption traces

Pd(110) shows three roughly equally intense peaks at 135, 165, and 225 K. In the presence of Hsurf, a new large desorption feature appears at 190 K in addition to these peaks. In both cases, butene desorption ceases at 250 K. This high temperature limit of C4H8 desorption is presumably caused by dehydrogenation of C4H8 to C4H6 (butadiene), which completes at ∼250 K and is the first step of butene decomposition on Pd(110).36 Above 250 K, the remaining C 4 H 6 disintegrates further into carbonaceous deposits (Supporting Information, S-1). Quantification of the C4H8 coverage with NRA at 90 and 250 K reveals that although the initial butene coverage on clean and Hsurf-saturated Pd(110) is the same (0.47 ± 0.06 ML), the C4H8 desorption yield increases from 0.18 to 0.28 ML, and the amount of decomposing C4H8 decreases from 0.29 to 0.14 ML in the presence of Hsurf. These findings indicate that the additional butene desorption intensity in the 190 K TPD feature from the Hsurf-saturated surface stems from C4H8 that would have decomposed on bare Pd(110). It can thus be stated that Hsurf partially steers the butene reaction behavior away from decomposition but toward desorption. Figure 2 compares the HREEL spectra of butene adsorbed on clean versus Hsurf-saturated and additionally Habs-loaded

Figure 2. HREEL spectra recorded at 90 K of cis-2-butene adsorbed on (a) clean, (b) Hsurf-saturated, and (c) Hsurf-saturated Pd(110) loaded with several monolayers of Habs at 115 K.

Pd(110). Table 1 summarizes the assignments of energy loss peaks in each spectrum. The full range EEL spectrum is shown in Supporting Information, S-2. It is remarkable that each spectrum shows two peaks corresponding to CC stretching vibrations at around 190 and 205 meV.37,38 The latter peak disappears after brief annealing to 140 K and quenching to 90 K, while all other peaks remain discernible (Supporting

Figure 3. Butene TPD spectra (m/e = 56) after coadsorbing 1.25 L C4H8 at 115 K onto H-covered Pd(110) precharged with different quantities of Habs at 115 K.

recorded after dosing 1.25 L C4H8 at 115 K onto the Pd(110) surface that had been precharged with different amounts of Habs at 115 K. With increasing quantities of Habs from 0 to 3.7 ML, the C4H8 TPD spectra are seen to develop a significant depletion around 165 K, whereas all other peaks remain more or less unchanged (the concomitant slight decrease in the neighboring features at 135 and 190 K may be due to a certain overlap with the 165 K feature). Simultaneously with the series of butene TPD spectra from the Habs-loaded Pd(110) catalyst, we recorded the TPD traces of C4H10, the C4H8 hydrogenation product, that are shown in Figure 4. For a small H2 predosage of 1 L that only saturates the Hsurf states32,35 the TPD trace does not show any significant C4H10 desorption. On the other hand, after large H2 preexposures that produce 0.8, 1.7, and 3.7 ML Habs beneath a Hsaturated Pd(110) surface, sizable C4H10 desorption signals are

Table 1. Observed Energy Loss Peaks for Butene-Adsorbed Pd(110) for Different H2 Pre-Exposure Conditionsa H−Pdb CH3 rockc CH bend (i)c CH3 s-deformc CH3 d-deformd CC stretchc,e

clean

Hsurf-saturated

Habs-loaded

124, 140 150 169 177 187, 205

120 125, 139 151 170 177 192, 205

120 126, 138 152 171 177 192, 205

a

The vibrational energies are expressed in meV. bRef 41. cRef 38. dRef 37. eRef 36; s-, symmetric; d-, degenerate. C

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place above 200 K (Figure 4). The strong suppression of Habs desorption below ∼220 K is not caused by Habs consumption in the hydrogenation reaction because the butane reaction yield is very small compared to the amount of precharged Habs. The smaller H2-TPD peaks at 340, 400, and 500 K seen after coadsorption of C4H8 derive from butene (butadiene) decomposition (cf. Supporting Information, S-1). To investigate now the influence of the depth location of Pdabsorbed hydrogen on the catalytic hydrogenation reactivity, we first demonstrate our ability to prepare the Pd(110) catalyst with different initial depth distributions of Habs. Figure 6 shows

Figure 4. Series of C4H10 TPD spectra (m/e = 58) recorded from the H and C4H8 coadsorption system on Pd(110) precharged at 115 K with various amounts of absorbed hydrogen (in ML).

recognized between 160 and 250 K. Consistent with earlier reports, these data demonstrate that Pd-absorbed H is indispensable for the catalytic hydrogenation of the olefin, whereas chemisorbed surface H alone does not support this reaction, neither on Pd single crystal surfaces nor on Pd nanoparticles.2−11 The butane formation rate is seen to maximize at a reaction temperature (TR) of around 218 K in Figure 4. As demonstrated in Supporting Information, S-3, desorption of molecularly adsorbed C4H10 from clean (and from H- and C4H8-preadsorbed) Pd(110) peaks already at 180 K. Hence, the C4H10 desorption in Figure 4 reflects the rate of C4H10 production in the catalytic butene hydrogenation. This reaction appears to cease abruptly at 250 K, presumably also due to the aforementioned dehydrogenation of surfaceadsorbed butene into butadiene, which completes at 250 K.36 The influence of the butene coadsorbate on the thermal desorption behavior of Pd-absorbed hydrogen is studied in Figure 5, which compares the H2 desorption spectrum from

Figure 6. Near-surface H-depth profiles from Pd(110) exposed to H2 to prepare an equivalent of 10 ML of absorbed hydrogen at Te = 115 (circles), 160 (squares), and 200 K (diamonds). Triangles and a corresponding Gaussian fit (solid line) denote the NRA profile of surface hydrogen at saturation coverage (1.5 ML).

NRA hydrogen depth profiles in the near-surface region of Pd(110) after H2 exposures at Te = 115, 160, and 200 K that each produced an equivalent of 10 ML Habs as confirmed by TPD. The data reveal that Habs accumulates within a few nanometers below the surface for Te = 115 K, whereas it dissipates into deeper bulk regions for Te ≥ 160 K. In the latter case, the absorbed H distributions appear nearly flat and the H concentrations measured at a depth below the surface peak are in the order of one percent of the Pd atomic density. Based on this information, we estimate that the H has diffused into depths of about 100 nm.32 As a reference, the NRA signal of a saturated layer of surface-chemisorbed H (Hsurf, 1.5 ML coverage) is also shown, which appears as a ∼1 nm broad (fwhm) Gaussian profile due to convolution with the NRA instrumental function.31 We next determined the actual near-surface H depth distribution in the catalyst during the hydrogenation reaction by briefly annealing the H and C4H8 coadsorption system on Habs-charged Pd(110) to TR = 218 K, where the C4H10 production rate maximizes (Figure 4). The NRA H profiles shown in Figure 7 were subsequently obtained after rapidly cooling the sample to 90 K. Again, 10 ML Habs were precharged into Pd(110) at several Te’s before coadsorbing C4H8. The H profiles of the 218 K annealed coadsorption system (Figure 7) evidence that irrespective of initially profoundly different hydrogen distributions (Figure 6) all H profiles approach the NRA signature of the saturated surface-H chemisorption layer, that is, only a very small Habs concentration remains directly underneath the Pd surface at TR (∼0.5 at. %, evaluated in 5 nm depth, where the γ-yield contribution of surface H is negligible). The H2 TPD spectrum from the butene coadsorbed surface in Figure 5 indicates that during annealing to 218 K only a small amount of Habs desorbs, hence the disappearance of the potentially large initial H-density (Figure 6, Te = 115 K)

Figure 5. Hydrogen (H2) TPD recorded from Pd(110) sequentially exposed to 1000 L H2 at 115 K and then to 1.25 L butene (solid line). The spectrum for an identical Habs-loading without butene postdosage is shown by the dotted line.

Pd(110) exposed to 1000 L H2 at 115 K (producing a Hsaturated surface and 3.1 ML of Habs) to the TPD trace from the same preparation after coadsorption of 1.25 L butene. From clean Pd(110), Habs desorbs between 160 and 200 K, whereas the higher temperature features (250−400 K) are due to surface-adsorbed H states.32 Figure 5 shows that coadsorbed butene strongly suppresses Habs desorption in the original temperature region and instead induces a new dominant H2 desorption feature at 280 K. This peak grows with the amount of precharged Habs, proving its origin in Pd-absorbed H. Obviously, the desorption of Habs is delayed by at least 80 K due to butene coadsorption relative to clean Pd(110). This high-temperature shifted desorption enables the participation of Habs in the catalytic butene hydrogenation reaction that takes D

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Figure 7. Hydrogen depth profiles of the H and C4H8 coadsorption system taken at 90 K after briefly heating to the hydrogenation reaction temperature (218 K). The NRA H signal of coadsorbed C4H8 was subtracted from the data. Initially, 10 ML Habs were prepared by exposing Pd(110) to H2 at Te = 115 (circles), 160 (squares), and 200 K (diamonds). Triangles and a corresponding Gaussian fit (solid line) denote the NRA profile of surface hydrogen at saturation coverage (1.5 ML).

Figure 8. (a) Butane reaction yield as a function of the amount of Habs prepared at Te = 115 (circles), 160 (rectangles), and 200 K (diamonds), evidencing higher reactivity of initially surface-near absorbed H. The straight lines are guides to the eye. (b) Butane reaction yield as a function of the amount of H2 that desorbs in TPD from bare Pd(110) below 250 K, i.e., the quantity of Habs supplied from the Pd bulk to the surface up to the reaction temperature. The symbols indicate the temperature of the Habs preparation (Te) as in panel (a). The solid line is a guide to the eye.

from the near-surface region must be caused by H diffusion into the deeper Pd bulk. We demonstrated in ref 42 that nearsurface Pd-absorbed hydrogen prepared by low-temperature H2 exposure is highly unstable against diffusion into the Pd bulk at temperatures above ∼160 K, so that for a sufficiently long heating already at 200 K no significant Habs concentration remains underneath the surface, notably not even in the firstlayer subsurface sites that bind H slightly more strongly (∼0.1 eV) than the interstitial sites in the Pd bulk.26,27 As discussed previously,31,43 the thermodynamic driving force for this indiffusion is configuration entropy, because the three-dimensional Pd bulk offers many more possibilities to distribute the absorbed H atoms than the two-dimensional surface and subsurface layers. The Gaussian-shaped NRA profiles in Figure 7 may, in principle, be assigned to H in surface chemisorption or to H in first-layer subsurface sites. These two H locations cannot be distinguished with NRA because the width of the Dopplerbroadened surface resonance peak limits the depth resolution at the surface.31 Since chemisorbed hydrogen, however, is about 0.3 eV more stable on Pd surfaces than H absorbed in first-layer subsurface sites,26,27 only a negligible population of the latter is expected at 90 K, so that we assign the 1.5 ML of H represented by the integral area of the Gaussian peak profile in Figure 7 to the saturated H chemisorption layer on the Pd(110) surface. The HREEL spectrum (c) in Figure 2, which reveals surface Pd−H vibrations after butene coadsorption, corroborates this assignment. For the Habs preparation at 115 K, two data points at zero and ∼1.5 nm depth in Figure 7 suggest by the surplus γ-yield in addition to that of surface hydrogen (Gaussian profile) that in this case some Habs has remained in the Pd subsurface region even after annealing to 218 K. Since in this experiment the Pd(110) crystal was heated up rapidly just to reach 218 K, we consider that this brief annealing may not have been sufficient to completely decompose the near surface hydride phase32 that had formed upon H2 absorption at 115 K and to redistribute all the absorbed H from the surface-vicinal region into the Pd bulk by diffusion. Evaluating finally the influence of the initial Habs depth distribution (Figure 6) on the hydrogenation reactivity, Figure 8a shows the C4H10 reaction yield (determined from the butane TPD integral) as a function of the total amount of Habs prepared at the three different Te values, that is, at 115, 160, and 200 K. These data first indicate that at each Te the C4H10

yield is approximately linearly proportional to the total amount of Habs. Furthermore, the slope of these linear relationships becomes steeper at lower Te, implying that for a given quantity of Habs the reaction yield is larger when Habs is initially located closer to the surface (cf. Figure 6). The initial Habs distribution determines the fraction of Habs atoms that can arrive at the surface by diffusion during the TPD heating ramp (the remainder of Habs diffuses into the deeper Pd bulk), and this H fraction reaching the surface is obviously larger at lower Te values. Reference H2 TPD data (cf. Figure 5) support this interpretation. In Figure 8b, the butane yield data are plotted against the amount of Habs that desorbs as H2 from clean Pd(110) below 250 K, where the hydrogenation reaction ceases (Figure 4). All C4H10 yield data in Figure 8b fall into close vicinity of the same straight line independent of the individual Te, demonstrating that the butane reaction yield scales linearly with the number of Pd-absorbed H atoms that reach the surface up to the reaction temperature after diffusion from the Pd bulk.

IV. DISCUSSION A. Role of Pd-Absorbed H in the Hydrogenation Mechanism. The evidently linear correlation between the number of Pd-absorbed H atoms that arrive at the surface after diffusion from the bulk and the hydrogenation reactivity (Figure 8b) strongly supports the direct reaction scenario, that is, the hypothesis that upon resurfacing Habs provides a source for especially reactive H species that apparently cannot be generated by chemisorbed surface hydrogen alone (Figure 4). Having thus identified that resurfacing of Pd-absorbed H atoms triggers the hydrogenation reaction, we deliberate on their special reactivity and on the plausibility of the direct reaction mechanism. Theory has shown repeatedly that in a competing reaction pathway to hydrogenating an alkyl adsorbate, resurfaced Habs atoms tend to quickly stabilize in vacant chemisorption wells12,19,20,44 if such empty sites are available. Our NRA H profiles in Figure 7, however, demonstrate clearly that the Pd surface in fact remains saturated with chemisorbed Hsurf at the reaction temperature. Thus, the deactivation channel through settling into chemisorption wells is inaccessible when Habs atoms resurface onto the H-saturated Pd surface. We therefore regard the saturated E

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amount of absorbed hydrogen (on the order of 0.5 ML) that remains directly underneath the surface at the reaction temperature (TR = 218 K). This residual Habs in subsurface sites may suggest that the high butane reaction yield from Habs precharged at 115 K (Figure 8a) results in part from an indirect hydrogenation mechanism. On the other hand, this high reaction yield can be explained equally well by the direct mechanism, because during the TPD temperature ramp the initially near-surface absorbed Habs migrates only into a shallow bulk region within the thermal diffusion length from the surface. Here, it can no longer influence the surface electronic structure but still retains a high probability to diffuse back and to cause direct hydrogenation events by resurfacing. Since the direct mechanism clearly proceeds even without any measurable subsurface site occupation (Figure 7, Habs preparations at 160 and 200 K), we consider it more fundamental. Moreover, we do not regard sizable subsurface site occupations realistic under steady-state conditions at TR for extended Pd crystals, because we found previously that upon sufficiently long annealing entropy completely drives subsurface H into the Pd bulk by indiffusion already at 200 K.42 Still, we do not deny the possibility that under conditions where high subsurface site occupancies can be realized, such as in Pd nanoparticle catalysts under elevated H2 pressures,7,25 the theoretically predicted indirect mechanism of surface H destabilization12 may be operative in addition to direct hydrogenation triggered by resurfacing of Pddissolved H atoms. One obvious additional question one may ask in the framework of the direct hydrogenation mechanism is whether the hydrocarbon is actually attacked by the resurfaced Habs atoms or by originally chemisorbed hydrogen. We believe that this fundamental distinction cannot be made conclusively (such as by an experiment with isotope-labeled surface and subsurface hydrogen) because there is a high probability that chemisorbed surface and emerging, formerly Pd-dissolved H atoms become interchanged, either in the surface penetration step32 or upon surface diffusion of the resulting excess H atoms. Theory has shown that excess H atoms can diffuse on the H-covered Pd surface in an exchange-like manner with chemisorbed hydrogen.46 In this process an excess H atom replaces a surface H at the chemisorption site, while the originally chemisorbed H atom is pushed into the excess atom state. In further such exchange-diffusion events, the role of the excess H atom may be passed on to other chemisorbed H atoms until the excess H atom finally encounters an empty chemisorption site to accommodate in (or a hydrocarbon molecule to react with). B. Role of Surface Hydrogen. As described in the previous Section, we consider that the saturated layer of surface hydrogen plays an important role in the hydrogenation mechanism by preventing the deactivation of resurfaced Habs atoms in empty chemisorption sites. Furthermore, we have seen in section III that Hsurf exerts a strong influence on the reaction behavior of the coadsorbed butene, that is, it changes the branching ratio between desorption and decomposition of the hydrocarbon. As demonstrated by Figure 1, Hsurf markedly increases the butene desorption yield and substantially suppresses the decomposition of C4H8 relative to the clean Pd(110) surface. Surface-chemisorbed H thus clearly enhances the catalyst’s selectivity for the hydrogenation reaction. A similar suppression of olefin decomposition by coadsorbed hydrogen has been reported on Ni surfaces.50,51 Besides weakening the olefin−Pd interaction, as proposed in section III, it is conceivable that such a reduction of the butene

layer of chemisorbed surface hydrogen as an additional, possibly vital, condition for the catalytic hydrogenation reaction of subsurface hydrogen in Pd that so far has not been given explicit consideration in the literature.12−16,19,20 Hydrogensaturation of the stable chemisorption sites implies that resurfacing of Habs may supersaturate the Pd surface with excess surface H atoms that we consider possible reactive H species. First-principle calculations45−48 demonstrated that such excess H species can form as a result of H2 dissociation at H monovacancies in the dense H chemisorption layer, where the excess H atoms bind much more weakly at energetically less favorable adsorption sites than the occupied deep chemisorption wells. The observation that low temperature hydrogen absorption at H2-exposed Pd surfaces proceeds with a strikingly small (∼0.1 eV) activation barrier,32,49 while the surface chemisorption sites are H-saturated, suggested the possibility that weakly bound excess H species may play a role of intermediates in the absorption mechanism.32 With regard to microscopic reversibility, we expect that such energetic surface H species should also exist at least transiently when Pdabsorbed hydrogen emerges onto the H-saturated Pd surface during thermal desorption, because the desorption temperature of Habs is substantially lower (160−200 K) than that of chemisorbed hydrogen (∼330 K, Figure 5 and refs 32, 33, and 43). Even on only partially H-covered Pd-surfaces excess H species can exist for appreciable lifetimes on the order of several picoseconds.48 On a fully H-saturated Pd surface (such as under hydrogenation conditions, Figure 7) excess H atom lifetimes will be even longer, thus, giving them time and a chance for reactive encounters with coadsorbed hydrocarbon molecules during their expected surface diffusion.45−47 Further arguments for a direct rather than an indirect mechanism can be drawn from comparing the reaction yield for the different initial depth distributions of Habs. Figure 8 shows that significant catalytic butene hydrogenation results also when Habs is precharged into Pd(110) at 160 and 200 K. In particular, even Pd bulk-dissolved hydrogen prepared at Te = 200 K with very little Habs near the surface (Figure 6) hydrogenates butene efficiently in our subsequent reaction TPD experiments. Since only surface chemisorbed H prevails near the surface under reaction conditions for Habs preparation at 160 or 200 K (Figure 7) this observation demonstrates that the catalytic hydrogenation of C4H8 to C4H10 on Pd(110) evidently does not necessarily require a H-rich layer in the subsurface region. This finding strongly suggests that large quantities of Habs in subsurface sites such as required by the indirect mechanism scenario of surface-hydrogen activation through electronic surface structure modification by subsurface hydrogen is not a generally essential precondition for Pdcatalyzed olefin hydrogenation through absorbed hydrogen. Reference12 found that at least 50% of the first-layer subsurface sites underneath the H-covered Pd(111) surface have to be occupied by absorbed hydrogen in order to achieve a substantial energetic destabilization of surface-chemisorbed hydrogen that causes a significant reduction of the activation energy for catalytic ethyl hydrogenation by surface hydrogen. Such high subsurface site occupation conditions are obviously not fulfilled at the hydrogenation reaction temperature when the Habs is prepared at 160 K or at 200 K (Figure 7). In the case of the 115 K preparation, where the Habs is initially strongly concentrated within an only a few nanometer deep hydride phase underneath the surface (Figure 6 and ref 32), the data in Figure 7 appear compatible with a significant F

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The Journal of Physical Chemistry C decomposition results from a site blocking effect.52 Decomposition of C 4 H 8 is initiated by dehydrogenation to butadiene,36 which requires two empty sites to deposit the emitted H atoms. Therefore, preoccupation of surface sites with chemisorbed Hsurf may inhibit C4H8 decomposition. C. Nature of the Reactive Olefin Adsorption State. Catalytic olefin hydrogenation reactions on Pt group metal surfaces very often proceed conform with the Horiuti-Polanyi mechanism,53 in which the adsorbed olefin is first halfhydrogenated to form a surface alkyl intermediate that is subsequently converted into the alkane product molecule in a second half-hydrogenation step. Although this mechanism suitably describes many catalytic systems, the detection of reaction intermediates and the characterization of the olefin adsorption state that actually engages in the catalytic hydrogenation reaction have still remained challenging topics of current research.54 To elucidate these questions for the present catalytic reaction, we aim to identify in the following which role the individual C4H8 adsorption states corresponding to the desorption features at 135, 165, 190, and 225 K in the complex TPD spectrum of butene from Hsurf-saturated Pd(110) (Figure 1) play in the catalytic hydrogenation. Adsorption of butene on clean, Hsurf-covered, and additionally Habs-loaded Pd(110) at 115 K produces a small CC vibration peak in the HREEL spectra (Figure 2) at 205 meV, which is similar to cis-2-butene in the gas, liquid, or solid (206− 207 meV) phase.38 This HREELS peak disappears after flashannealing to 140 K (Supporting Information, S-2) and thus correlates with the butene TPD feature at 135 K. TPD spectra resulting from coadsorption of C4H8 at 115 K on Pd(110) saturated with surface deuterium instead of hydrogen (Supporting Information, S-4) indicate that the butene desorbing in the 135 K feature contains very little D. Due to its gas-phase like CC vibration frequency, suggestive of a very weak C4H8−Pd interaction, and because of the almost complete lack of H-D exchange with the Pd surface, we tentatively assign the butene adsorption state corresponding to the 135 K TPD feature as a molecule in the second C4H8 adsorbate layer. Since this species is eliminated from the surface well before the onset of the butane formation at 160 K (Figure 4), such a weakly adsorbed second layer butene species is evidently not important for the catalytic hydrogenation reaction. Turning next to the C4H8 state corresponding to the 165 K desorption feature, it appears from Figures 3 and 4 as if this species was either consumed in the hydrogenation reaction or as if it became increasingly suppressed with the supply of Habs. Although a definite assignment of the adsorbed butene state that desorbs at 165 K is not possible at this stage, it seems to be clearly involved in the catalytic hydrogenation reaction. With reference to Supporting Information, S-4, we can further state that this apparently reactive butene species does not engage in significant H-D exchange with surface hydrogen in absence of Habs, implying that it rarely enters the butyl intermediate state of the Horiuti-Polanyi mechanism. However, in the presence of Habs, butane production is observed (Figure 4), suggesting that Habs changes the behavior of this reactive butene state from desorption toward hydrogenation by allowing it to enter the first half-hydrogenation step of the Horiuti-Polanyi mechanism. Finally, the butene species in the desorption features at 190 and 225 K engage in very substantial H-D exchange with the Pd surface and may incorporate up to eight D atoms (full exchange, Supporting Information, S-4). Unless the H-D

exchange proceeded through an initial dehydrogenation step of the adsorbed olefin (which is highly unlikely on a H or Dsaturated Pd surface), this efficient isotope exchange with the Pd surface suggests that these butene species should have passed several times (possibly reversibly) through the first halfhydrogenation step of the Horiuti-Polanyi mechanism and formed a butyl adsorbate, which is the reaction intermediate that enables both hydrogenation (if Habs is available) and isomerization of butene. Since the C4H8 desorption features at 190 and 225 K are present even in absence of Habs and because they engage in intense H/D exchange with the surface, Hsurf seems to be sufficient to produce the butyl intermediate from these C4H8 states. It is therefore quite surprising that the strong butene desorption states at 190 and 225 K show nonetheless much lower reactivity in the hydrogenation reaction relative to the 165 K species, because their intensities do not change significantly with the supply of Habs (Figure 3). It thus appears as if the catalytic hydrogenation reaction reactivity and the H/D exchange with surface hydrogen exhibited an opposite trend in the stability of the butene adsorption state. The nature of the C4H8 adsorption states that give rise to their different desorption temperatures and characteristic hydrogenation reactivity is presently poorly understood and deserves further investigation. HREELS measurements after heating the butene-covered Pd(110) surface in steps to successively deplete the lower desorption temperature states did not produce characteristic differences in the vibrational loss spectra that would have allowed for the identification of individual species. The four butene TPD states appear successively in the order of decreasing desorption temperature as the coverage is increased, suggesting that the adsorbed C4H8 molecules become destabilized through repulsive interactions at higher coverages. This may serve as one tentative explanation for the fact that the most weakly bound (and, hence, most reactive) 165 K state seems to react preferably upon supply of Habs when the TPD experiment is performed with an initially butene-saturated catalyst surface, as in Figure 3. To summarize this subsection, we have identified at least four C4H8 states that desorb from Hsurf-saturated Pd(110) at 135, 165, 190, and 225 K. These states differ in their thermal stability on and in their degree of H-D exchange with the Pd surface, as well as in their catalytic hydrogenation reactivity. Whereas the 135 K feature appears to be too weakly bound to even exchange H with the surface and simply desorbs before the onset of the catalytic reaction, the species at 165 K appears to play the most active role in the catalytic hydrogenation when Habs is supplied. More stably adsorbed C4H8 species desorbing at 190 and 225 K engage in rather efficient H-D exchange with the Pd surface but much less in hydrogenation.

V. CONCLUSIONS The present TPD, HREELS, and NRA investigation of C4H8 hydrogenation on a H-loaded Pd(110) model catalyst clarifies several aspects in the long-standing debate on the role of Pdabsorbed H (Habs) in catalytic olefin hydrogenation reactions on Pd surfaces: 1. The catalytic hydrogenation reaction is triggered by “resurfacing” of Pd bulk-dissolved hydrogen atoms, generating reactive H species on the H-saturated and olefin coadsorbed Pd surface, reminiscent of the “direct” reaction scenario originally proposed for nascent H atoms emerging from subsurface sites onto Ni surfaces. G

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(2) Doyle, A. M.; Shaikhutdinov, S. K.; Jackson, S. D.; Freund, H. J. Hydrogenation on Metal Surfaces: Why Are Nanoparticles More Active Than Single Crystals? Angew. Chem., Int. Ed. 2003, 42, 5240− 5243. (3) Morkel, M.; Rupprechter, G.; Freund, H. J. Ultrahigh Vacuum and High-Pressure Coadsorption of CO and H2 on Pd(111): A Combined SFG, TDS, and LEED Study. J. Chem. Phys. 2003, 119, 10853−10866. (4) Rupprechter, G.; Morkel, M.; Freund, H. J.; Hirschl, R. Sum Frequency Generation and Density Functional Studies of CO-H Interaction and Hydrogen Bulk Dissolution on Pd(111). Surf. Sci. 2004, 554, 43−59. (5) Morkel, M.; Rupprechter, G.; Freund, H. J. Finite Size Effects on Supported Pd Nanoparticles: Interaction of Hydrogen with CO and C2H4. Surf. Sci. 2005, 588, L209−L219. (6) Doyle, A. M.; Shaikhutdinov, S. K.; Freund, H. J. Alkene Chemistry on the Palladium Surface: Nanoparticles vs Single Crystals. J. Catal. 2004, 223, 444−453. (7) Wilde, M.; Fukutani, K.; Ludwig, W.; Brandt, B.; Fischer, J. H.; Schauermann, S.; Freund, H. J. Influence of Carbon Deposition on the Hydrogen Distribution in Pd Nanoparticles and Their Reactivity in Olefin Hydrogenation. Angew. Chem., Int. Ed. 2008, 47, 9289−9293. (8) Ludwig, W.; Savara, A.; Schauermann, S.; Freund, H.-J. Role of Low-Coordinated Surface Sites in Olefin Hydrogenation: A Molecular Beam Study on Pd Nanoparticles and Pd(111). ChemPhysChem 2010, 11, 2319−2322. (9) Ludwig, W.; Savara, A.; Dostert, K. H.; Schauermann, S. Olefin Hydrogenation on Pd Model Supported Catalysts: New Mechanistic Insights. J. Catal. 2011, 284, 148−156. (10) Nilius, N.; Risse, T.; Schauermann, S.; Shaikhutdinov, S.; Sterrer, M.; Freund, H. J. Model Studies in Catalysis. Top. Catal. 2011, 54, 4−12. (11) Schauermann, S.; Nilius, N.; Shaikhutdinov, S.; Freund, H. J. Nanoparticles for Heterogeneous Catalysis: New Mechanistic Insights. Acc. Chem. Res. 2013, 46, 1673−1681. (12) Aleksandrov, H. A.; Kozlov, S. M.; Schauermann, S.; Vayssilov, G. N.; Neyman, K. M. How Absorbed Hydrogen Affects the Catalytic Activity of Transition Metals. Angew. Chem., Int. Ed. 2014, 53, 13371− 13375. (13) Johnson, A. D.; Daley, S. P.; Lutz, A. L.; Ceyer, S. T. The Chemistry of Bulk Hydrogen: Reaction of Hydrogen Embedded in Nickel with Adsorbed CH3. Science 1992, 257, 223−225. (14) Daley, S. P.; Utz, A. L.; Trautman, T. R.; Ceyer, S. T. Ethylene Hydrogenation on Ni(111) by Bulk Hydrogen. J. Am. Chem. Soc. 1994, 116, 6001−6002. (15) Haug, K. L.; Burgi, T.; Gostein, M.; Trautman, T. R.; Ceyer, S. T. Catalytic Hydrogenation of Acetylene on Ni(111) by SurfaceBound H and Bulk H. J. Phys. Chem. B 2001, 105, 11480−11492. (16) Ceyer, S. T. The Unique Chemistry of Hydrogen beneath the Surface: Catalytic Hydrogenation of Hydrocarbons. Acc. Chem. Res. 2001, 34, 737−744. (17) Maynard, K. J.; Johnson, A. D.; Daley, S. P.; Ceyer, S. T. A New Mechanism for Absorption: Collision-Induced Absorption. Faraday Discuss. Chem. Soc. 1991, 91, 437−449. (18) Johnson, A. D.; Maynard, K. J.; Daley, S. P.; Yang, Q. Y.; Ceyer, S. T. Hydrogen Embedded in Ni: Production by Incident Atomic Hydrogen and Detection by High-Resolution Electron Energy Loss. Phys. Rev. Lett. 1991, 67, 927−930. (19) Michaelides, A.; Hu, P.; Alavi, A. Physical Origin of the High Reactivity of Subsurface Hydrogen in Catalytic Hydrogenation. J. Chem. Phys. 1999, 111, 1343−1345. (20) Ledentu, V.; Dong, W.; Sautet, P. Heterogeneous Catalysis through Subsurface Sites. J. Am. Chem. Soc. 2000, 122, 1796−1801. (21) Alefeld, G.; Völkl, J. E. Hydrogen in Metals I; Springer: Berlin; Heidelberg, 1978; Vol. 28. (22) Alefeld, G.; Völkl, J. E. Hydrogen in Metals II; Springer: Berlin; Heidelberg, 1978; Vol. 29. (23) Mueller, W. M.; Blackledge, J. P.; Lipowitz, G. G. Metal Hydrides; Academic Press: New York, 1968.

2. The catalytic hydrogenation reaction does not necessarily require large stationary H densities in subsurface sites. Habs dissolved deeply in the Pd bulk is equally reactive as long as it reaches the surface by diffusion at the reaction temperature (160−250 K). 3. The catalytic hydrogenation proceeds on a Pd surface fully saturated with chemisorbed hydrogen. This surface hydrogen supports the catalysis by reducing olefin decomposition (presumably due to weakening the butene-Pd interaction or blocking adsorption sites for emitted H atoms) and by preventing deactivation of resurfaced Habs atoms in empty chemisorption sites. 4. The butene species that appears to behave most actively in the catalytic hydrogenation reaction desorbs from Hsurf-saturated Pd(110) at 165 K but reacts toward butane in the presence of Habs. This butene species exchanges very little with surface hydrogen. More strongly adsorbed C4H8 species desorbing at 190 and 225 K engage in rather efficient H-D exchange with the Pd surface but do not appear to undergo hydrogenation. 5. Coadsorption of butene on the Habs-loaded Pd(110) surface markedly increases the desorption temperature of Habs from 160 to 200 K on the clean surface to 280 K and thereby enables the participation of Habs in the catalytic hydrogenation reaction.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b00987. Contains additional information regarding hydrogen (H2) desorption upon butene decomposition on clean and Hsurf-saturated Pd(110), the full range HREEL spectrum, the reaction-limited desorption of butane, and the isotope-exchange between C4H8 and chemisorbed surface deuterium on Pd(110) (PDF).

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research is supported by Grants-in-Aid for Scientific Research (Grant Numbers 24246013 and 26108705) of the Japan Society for the Promotion of Science (JSPS). S.O. thanks JSPS for a Research Fellowship for Young Scientists. M.W. acknowledges support from a Grant-in-Aid for Scientific Research in Innovative Areas “Material Design through Computics: Complex Correlation and Nonequilibrium Dynamics” from the Ministry of Education, Culture, Sports, Science, and Technology of Japan. We are grateful to H. Matsuzaki and C. Nakano at the University of Tokyo for assistance in the MALT accelerator operation. The HREELS measurements were performed using facilities of the Institute for Solid State Physics, the University of Tokyo.

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