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Desorption Temperature Control of Palladium-Dissolved Hydrogen through Surface Structural Manipulation Satoshi Ohno, Markus Wilde, and Katsuyuki Fukutani J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b02365 • Publication Date (Web): 15 Apr 2015 Downloaded from http://pubs.acs.org on May 3, 2015

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The Journal of Physical Chemistry

Desorption Temperature Control of PalladiumDissolved Hydrogen through Surface Structural Manipulation Satoshi Ohno*, Markus Wilde, and Katsuyuki Fukutani Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, 153-8505 Tokyo, Japan

ABSTRACT

To assess the possibility to control the desorption temperature of palladium-absorbed hydrogen (Habs) through surface structural manipulation, we investigated co-adsorption systems of H and CO on Habs-charged Pd(110) surfaces through temperature-programmed desorption (TPD), low energy electron diffraction (LEED), and H-depth profiling by nuclear reaction analysis (NRA). A CO coverage of 0.5 ML lifts the H-induced (1×2) pairing-row (PR) reconstruction on Habs precharged Pd(110), and, as on clean Pd(110), heating Pd(110) (1×1) holding 0.3 - 1.0 ML CO gives rise to a missing-row (MR) structure. Whereas Habs desorbs through surface defects of clean, PR-reconstructed Pd(110) at 160 K, CO co-adsorption onto Habs-loaded Pd(110) gives rise to three new high-temperature shifted Habs desorption modes at 200, 270, and 375 K that are

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assigned to different exit sites for resurfacing Habs atoms at regular terraces of the individual Pd(110) structures, i.e., the (1×2) PR, the bulk-terminated (1×1), and the (1×2) MR reconstruction, respectively. Our results thus manifest the ability to control the Habs desorption temperature through surface restructuring in well-defined CO coverage regimes. The longspeculated transfer of chemisorbed H into the Pd interior upon CO co-adsorption is furthermore confirmed directly by NRA, revealing also that all Habs diffuses into the Pd bulk at 200 K.

INTRODUCTION A long-standing objective in surface physical chemistry is to control the rate of surface elementary processes through tailoring surface structures. Here, we focus on the transportation of hydrogen between the gas phase and the interior of palladium, which is of great importance for H storage, purification, and Pd-catalyzed hydrogenation of unsaturated hydrocarbons, because this catalysis requires bulk-absorbed H (Habs).1-6 Early on in surface science, hydrogen absorption at metal surfaces has been recognized as a structure sensitive process that is enhanced by surface roughness and corrugation.7 Gdowski et al. reported a fourfold-increased rate of H absorption on sputter-roughened Pd(111).8 Corners and edges of Pd nanocrystals decorated with subsurface carbon atoms were found to accelerate H penetration by providing an entrance/exit pathway of reduced activation barrier height.2, 9 These studies suggest that the H absorption kinetics are dominated by surface defects, which, however, are difficult to control and to understand systematically. In fact, at the densely packed Pd(111) and Pd(100) surfaces, H absorption has experimentally been observed to proceed most effectively through defect mediation.10-11 Theory predicts the possibility of absorption at regular terraces of Pd(100)12 and Pd(111)13 for the case


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that the surface is locally supersaturated with excess hydrogen atoms, but such a state could not yet be confirmed experimentally. On the more open Pd(110) surface, on the other hand, recent experiments demonstrated that H penetration does in fact take place rather efficiently at regular terrace sites.14 This terrace-related H2 absorption pathway was tentatively ascribed to the particular H-induced (1×2) pairing-row (PR) reconstruction of the Pd(110) surface (Figure 1), which features an atomic step-like structure and, due to the lateral displacement of the paired Pd rows, contains unilaterally widened threefold hollow interstitial channels into the subsurface that are considered to promote H penetration.

Defect

Figure 1. Ball model of structural phases

PR

of the Pd(110) surface and related 160 K, α1

(1×1)

200 K, γ1

MR

desorption modes and peak temperatures of Pd-absorbed hydrogen. White, gray and dark balls denote surface, first and

270 K, γ2

375 K, γ3

second subsurface Pd atoms, respectively. PR:

(1×2)

pairing-row;

MR:

(1×2)

missing-row.

This apparent structure sensitivity of the H2 absorption at Pd surfaces prompted us to explore the possibility to deliberately control the H transport between the gas phase and the metal interior, because Pd(110) is prone to structural changes upon adsorption of gas particles such as carbon monoxide (CO) molecules. CO adsorption onto the PR phase of Pd(110) first lifts this H-


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induced reconstruction, restoring the bulk-terminated (1×1) phase.7, 15 Furthermore, upon moderate heating to 310 - 340 K at CO coverages above 0.3 ML, Pd(110)(1×1) undergoes a missing-row (MR) reconstruction, which in turn is lifted again by additional heating.16 These CO-induced reconstructions are expected to influence the hydrogen sorption behavior of Pd(110). Behm et al. reported a pronounced (210 K) high-temperature shift in the TPD spectrum of Pd-absorbed hydrogen from CO-saturated Pd(110) that they discussed in relation to the COinduced lifting of the PR reconstruction.7 At the time of their study, however, the existence of the MR reconstruction taking place during the TPD heating ramp had not yet been discovered. Therefore, the desorption behavior of Habs through CO-co-adsorbed Pd(110) surfaces deserves a thorough re-evaluation in light of the full information on the CO-induced structural modifications of and on the hydrogen penetration process at Pd(110) that is available today. In addition to restructuring the surface, CO also exerts direct influence on co-adsorbed H. In general, the interaction between CO and H on Pd surfaces is repulsive.17-18 On a Pd-covered SiO2/Si device, CO-post-adsorption causes pre-chemisorbed H (Hsurf) to desorb into the vacuum.19 CO-induced migration of Hsurf into the subsurface region of Pd single crystals has also been suggested in several studies.15, 18, 20-26 This latter effect of post-adsorbed CO, however, could not yet be confirmed directly due to the experimental difficulty to detect H beneath the surface. The adsorbed CO was furthermore found to prevent desorption of absorbed hydrogen by site blocking of exit locations, leading to H trapping until the blocking CO thermally desorbed.2328

Thus, CO site blocking renders the Habs release selectable, i.e., to take place at the desorption

temperature of the clean surface or at an elevated temperature where the capping CO is removed. This possibility to choose the temperature at which Habs becomes available at the surface is potentially useful for H storage and for hydrogenation catalysis.1-3, 5-6 In such applications, a


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wider range of temperature options than provided by the CO site blocking effect may be desirable. Therefore, in order to explore the possibility to control the release kinetics of Pdabsorbed hydrogen through CO-induced surface structural manipulation, we here report a detailed co-adsorption study of CO on Habs-loaded Pd(110) using TPD, LEED, and nuclear reaction analysis (NRA). Combined TPD and LEED experiments clarify that the H release from Pd-absorbed states is modified in three distinct steps related to the stages of CO-induced surface (de-)reconstruction. Hydrogen depth profiling with NRA directly demonstrates the alleged COinduced transition of Hsurf into the Pd interior and further reveals for the first time that all absorbed H atoms, including those in first layer subsurface sites, diffuse into the Pd bulk at 200 K.

EXPERIMENTAL METHODS The experiments were performed in an ultra-high vacuum (UHV) chamber with a base pressure below 1×10-8 Pa attached to the MALT 5 MV van de Graaf tandem accelerator at the University of Tokyo.29 A Pd single crystal rod was Laue-oriented to the (110) direction, cut with a precision wire saw within 0.5° precision and mechanically polished with a suspension powder of Al2O3 (0.05 µm grain size). The Pd(110) sample was cleaned in UHV by repeated cycles of sputtering with 800 eV Ar+ ions, annealing to 1000 K, oxidation at 750 K in 5.0×10-5 Pa O2, reduction at RT in 5.0×10-5 Pa H2, and final flashing to 600 K in UHV until a clear LEED (1×1) pattern was observed and no impurities were detectable by Auger electron spectroscopy (AES). Gases were introduced to the UHV chamber through variable leak valves. The ion gauge pressure reading was corrected by sensitivity factors of 0.5 for H2 and 1.0 for CO. For the co-adsorption


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experiments, clean Pd(110) was exposed to first H2 and then to CO. Therefore, CO always encounters a H-saturated Pd(110) surface with a Hsurf coverage of 1.5 ML.14, 30 The H2-predosage was carried out either at the sample temperature below 120 K or at 160 K. The former condition leads to near-surface Pd hydride nucleation within a few nanometers below the surface, whereas the latter produces a more dilute Habs layer that extends several tens or hundreds of nanometers into the Pd bulk.14 TPD was recorded with a shielded and differentially pumped quadrupole mass spectrometer at a linear heating rate of 2 K/s by radiation from a tungsten filament behind the sample. NRA hydrogen depth profiling was performed with an energyanalyzed (∆E = 3 keV) 15N2+ ion beam via the 1H(15N,αγ)12C nuclear reaction.29 Measuring the characteristic 4.43 MeV γ-rays emitted in this narrow width (Γ = 1.8 keV) nuclear reaction resonance at Eres = 6.385 MeV allows for depth-resolved H detection. Incident 15N ions at the energy Ei > Eres probe H situated at a depth z = (Ei – Eres)/S, where S is the stopping power of 6.4 MeV 15N in Pd (S = 3.9 keV/nm). For surface-normal incidence, the near-surface depth resolution corresponds to ~2 nm, which is mainly limited by Doppler-broadening due to H zeropoint vibration.

RESULTS AND DISCUSSION We first present CO-TPD and LEED data that characterize the CO-induced surface reconstructions of H-pre-exposed Pd(110). Figure 2 (a) displays TPD traces of CO obtained after adsorbing increasing amounts of CO on Pd(110) that has been pre-exposed to 1000 L H2 at 160 K, which produces approximately 3 ML of Habs in the bulk. At small CO post-doses, first a slightly asymmetric feature at 500 K (β2) appears that saturates after 0.6 L. Then a low-


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temperature shoulder grows around 400 K (β1) and saturates at 1.36 L. Subsequently, a sharp peak at 340 K (α3) emerges. Further CO exposure gives rise to two low-temperature features at 280 K (α2) and 100 K (α1). The final 3 L CO-TPD spectrum in Figure 2 (a) is identical to that obtained from clean Pd(110) after 2 L CO. The integrated desorption intensity of simultaneously recorded H2 is preserved after the CO post-dosages, excluding the possibility to eliminate adsorbed H species into the vacuum upon CO adsorption, which has been reported to occur on a Pd-covered SiO2/Si device at temperatures above 223 K.19 The CO-TPD spectra thus appear to be insensitive to the co-existence of hydrogen. Figure 2 (b) shows the integrated CO-TPD area obtained from H-pre-exposed Pd(110) as a function of the CO exposure. The data are given in ML after normalizing with the known CO saturation coverage (1 ML) on clean Pd(110).31 The graph demonstrates that the CO coverage grows linearly up to saturation at 2 L with a constant CO sticking probability of 0.5. The two data sets in Figure 2 (b) also indicate that the CO adsorption behavior is insensitive to the initial depth distribution of Habs, which differs strongly for the two temperatures of H2-pre-exposure (Te = 120 K: surface-near Habs; Te = 160 K: bulkdissolved Habs).14


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Figure 2. (a) Series of CO-TPD spectra

QMS ion current [10

-10

A]

from the co-adsorption system of Hsurf and 25 3 1.66 1.36 1.2 0.8 0.6 0.3 0.1 (L)

20 15 10 5

β1

CO on Pd(110) pre-charged with 3 ML Habs by sequential exposure to first 1000 L

β2

H2 and then to various doses of CO at 160

α2

α1

0 100

(a)

α3

K. Numbers in the graph represent the CO 200

300 400 500 Temperature [K]

1.0 CO amount [ML]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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600

exposure for each trace from the start to (b)

0.8

the end point of the arrow. (b) Integrated CO desorption intensities from Pd(110)

0.6 0.4

pre-exposed to 1000 L H2 at Te = 120 K

0.2

(filled symbols) or Te = 160 K (empty

0.0 0

2

4 6 CO exposure [L]

8

symbols), as a function of the CO dosage at the same exposure temperature (Te).

We next address the correlation between the CO-TPD features and the CO-induced MR reconstruction. Previous studies on clean Pd(110) demonstrated that the sharp α3 TPD peak is associated with the CO-induced MR reconstruction, which causes a sudden drop of the CO saturation coverage to ~0.7 ML and releases the excess CO by desorption in a narrow temperature range.16, 31 The origin of α3 is expected to be the same in the present study, since we found the CO-TPD spectra of our co-adsorption system with hydrogen to be identical to those of clean Pd(110). The α3 peak temperature thus signals the temperature onset of the MR reconstruction. However, the α3 feature appears only when the initial CO coverage exceeds 0.7 ML at the onset temperature of the MR reconstruction, whereas the minimum CO coverage


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necessary to induce this reconstruction is only 0.3 ML.31 It follows that for CO coverages smaller than 0.7, the MR reconstruction may exist although no α3 feature emerges in the CO TPD spectrum. On the other hand, the CO sticking probability of 0.5 (Figure 2 (b)) and the minimum CO coverage (0.3 ML) required for the MR phase correlate this reconstruction more closely with the β1 TPD feature, which appears above 0.6 L CO (Figure 2 (a)). This notion is supported by our LEED observation that heating CO-saturated Pd(110) to 360 K - 450 K converts the LEED (4×2) structure into a (1×1) pattern, which signals lifting of the MR reconstruction16 in the temperature region of the β1 desorption feature. Apparently, MR reconstruction lifting renders a certain amount of CO to desorb in the β1 feature. We therefore regard the appearance of the β1 feature at CO dosages above 0.6 L as an indication for the MR reconstruction. We monitored the LEED pattern development of Pd(110) pre-exposed to 1000 L H2 at 160 K under ambient CO pressure (4×10-6 Pa) at 90 K. The original (1×2) pattern characteristic for the H-induced PR reconstruction32-33 is stable up to 0.25 L CO, but then becomes faint. A clear (1×1) pattern is retrieved at 1 L CO. Further CO exposure up to 3 L gradually gives rise to (2×1) spots. The disappearance of the H-induced (1×2) superspots upon CO post-adsorption has been ascribed to CO-induced lifting of the PR reconstruction.7, 15 The well-developed (1×1) pattern at 1.0 L indicates PR reconstruction lifting in the entire Pd(110) surface area already at half a ML of CO. This suggests complete dispersion of the CO adsorbate over the surface, compatible with the proposed formation of a mixed adsorbed phase of H and CO on Pd(110).15 The final (2×1) LEED pattern is again in common with CO adsorption on clean Pd(110), where the CO adsorbates at short bridge sites of unreconstructed Pd(110) (1×1) form an ordered layer with their molecular axes tilted alternately towards both sides of the Pd atomic chains.31, 34-35 We postulate that a similar CO-ordering takes place in our experiment on H-pre-exposed Pd(110).


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In the case of Pd(111), pre-adsorbed H notably allows for CO adsorption only above 125 K, whereas it completely blocks CO adsorption at lower temperatures.25-26 This temperaturedependent blocking effect of Hsurf on Pd(111) was ascribed to the existence of an activation barrier for the CO-induced transition of surface-adsorbed H into the first Pd subsurface layer, which apparently requires temperatures above 125 K. It is thus important to recall that our LEED observations of CO post-adsorption were performed at 90 K. Also on H-pre-saturated Pd(100) CO adsorption is reported to occur at 80 K.22-23 These observations suggest that the migration of Hsurf into the subsurface at Pd(110) and at Pd(100) is easier than at the close-packed Pd(111) surface, highlighting the possible structure sensitivity of the H penetration process. Hydrogen subsurface penetration due to repulsive co-adsorbate interactions has also been reproduced by DFT calculations for Pd(100)36 and Pd(111)18. We next applied NRA H-depth profiling to reveal the temperature evolution of the nearsurface H depth distribution that precedes desorption of Habs. Thereby the CO-induced transition of Hsurf into the Pd interior speculated above is directly visualized. Initially, Pd(110) was sequentially exposed to 8000 L H2 and then to 10 L CO at 115 K to produce an equivalent of 10 ML Habs beneath a H- and CO-saturated surface. This co-adsorption system of H and CO was then flash-annealed and quenched to 100 K before taking the near-surface NRA H-depth profiles displayed in Figure 3. At 115 K (as exposed condition), the H-depth profile shows large intensity in the surface vicinal region which smoothly decays into Pd bulk, revealing accumulation of Habs in a shallow (~10 nm) subsurface region, as reported previously.14 The H-concentration in 5 nm depth, where overlap with the Gaussian-shaped profile of Hsurf (FWHM ~1.5 nm) is negligible,29 corresponds to 8.0 at. %. After flash-annealing to 180 K, the H-depth profile shows fluctuations, indicating redistribution of Habs by diffusion. Finally at 200 K, almost all H is lost from the


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profiled region and the H-concentration in 5 nm depth below the surface decreases to 0.7 at. %. As we demonstrate below (Figure 4 (c)), the saturated layer of post-adsorbed CO effectively prevents H2 desorption below 300 K. Therefore, the nearly complete disappearance of H from the near-surface region at 200 K indicates H dissipation into bulk regions deeper than those profiled in Figure 3. Most notably, Figure 3 shows further that also Hsurf vanishes after heating to 200 K, i.e., that Hsurf is dissipated into the Pd bulk. These NRA data unambiguously demonstrate the CO-induced migration of Hsurf into the Pd interior and thereby support previous discussions, where this behavior was postulated but could not be substantiated by direct evidence.15, 18-26 The NRA data in addition provide insight into the behavior of Habs prior to its desorption in the TPD experiments: The absorbed hydrogen does not rigidly reside underneath the surface but diffuses inside the Pd bulk so that it only occasionally arrives at the surface to desorb. It is further worth emphasizing that not even first-layer subsurface sites appear to stabilize Habs against diffusion into the Pd bulk at 200 K, although these are known to bind Habs slightly stronger than bulk interstitial sites.37-38


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Figure 3. Temperature evolution of the NRA Hdepth profile recorded from the co-adsorption

0

2

Depth [nm] 4

system of an equivalent of 10 ML Habs prepared 6

250 γ-yield [counts/µC]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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8

at 115 K and post-dosed CO at saturation

200

coverage

150 100

at

Pd(110).

The

flash-annealing

temperatures are 115 K (circles), 180 K

50

(squares), and 200 K (triangles). The profile of

0 6.390 6.400 6.410 15 N ion energy [MeV]

1.5 ML Hsurf on Pd(110) (diamonds) and a corresponding Gaussian fit are also shown for comparison.

Whereas the CO desorption spectrum is unchanged by co-existing H, the desorption behavior of H2 is sensitive even to the smallest amount of CO (~0.05 ML) and becomes modified in three distinct steps with increasing CO coverage, as shown in Figure 4. Figure 4 (a) displays desorption traces of H2 after exposing Pd(110) to 1000 L H2 and subsequently to small doses (0.6 L) to induce MR surface reconstruction during the TPD heating ramp at 340 K (Figure 1). Thus γ3 originates in Habs desorption from the MR phase of Pd(110). MR-reconstructed Pd(110) inherently has vacant surface sites owing to the reduced maximum CO coverage of ~0.7 ML.31, 41-42 We therefore tentatively postulate that vacant sites in the MR reconstruction allow for desorption of Habs. We have thus found as many as four modes of Habs desorption at 160, 200, 270 and 375 K, and their relative intensity depends sensitively on the CO coverage. The final desorption mode, γ3, apparently takes place just after the MR structure is formed at 340 K. It is possible that the H2 desorption temperature of 375 K is determined by the onset of this reconstruction and thus is not inherent to the MR phase. We addressed this problem through the following experiment: First, the MR structure was prepared by heating Pd(110) to 375 K after it had been exposed to first 1 L H2 and then to 10 L CO at 120 K. This MR Pd(110) was then exposed to 1000 L H2 at 200 K. In the subsequent TPD spectrum, we observed a total of 2.4 ML H2 in a single peak at 375 K, identical to γ3. This result suggests that the γ3 desorption temperature is not determined by the onset of the MR phase formation. Since non-reconstructed Pd(110) shows H2 desorption at a lower temperature (270 K, γ2), the MR surface appears to suppress Habs desorption, in contrast to the promotion effect of the PR structure. This result is at first sight counterintuitive since the MR structure of Pd(110), consisting of two intersecting planes of three atoms wide (111) microfacets (Figure 1), exposes periodically aligned ledges that resemble edges of Pd nanocrystals, which have been recognized as the entrance/exit location for H absorption.2, 9 The ledges on MRreconstructed Pd(110), however, are also known to be preferentially occupied by adsorbed CO.31, 41-42

Repulsive forces from such CO may suppress the ledge-related desorption pathway and Habs


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may instead use vacant sites near the troughs of the MR surface, where the Pd atoms are closepacked in the (111) arrangement (Figure 1). It is interesting to note that the γ3 peak at 375 K from MR-reconstructed Pd(110) coincides exactly with the desorption of Habs from a COcovered Pd(111) surface.25-26 Hence, the CO-induced MR reconstruction in a sense converts Pd(110) into a Pd(111)-like surface. To discuss the origin of the high-temperature shifts in the three novel CO-induced modes of Habs desorption, we carried out leading edge analyses of the individual TPD features by plotting the initial 3 % of the desorption signals rdes = νθn exp(-E*/kBT) on a logarithmic scale against 1/T (E*: activation energy, νθn: pre-exponential term, kB: Boltzmann constant, T: temperature). Table 1 summarizes the obtained values of E* and νθn for the four Habs features as well as their peak temperature, the optimum CO coverage to induce the respective surface structures, and the assumed exit sites for Habs desorption. Most notably, the activation energy E* remains almost unchanged during the first two stages of CO-induced modification (the experimental uncertainty on a 2σ confidence level is 0.02 eV), but increases nearly two-fold upon the third stage from (1×1)-γ2 to MR-γ3, concomitant with the largest upshift of the Habs desorption temperature by 105 K. This result may reflect an increased activation barrier for H resurfacing through the more densely packed (111)-like arrangement of surface Pd atoms in the MR structure troughs relative to the more open lattices of the PR and (1×1) phases of Pd(110) (Figure 1). Indirectly, this interpretation is supported by DFT calculations according to which the H penetration barrier in the opposite direction (surface to subsurface) at the close-packed Pd(111) surface is larger (0.4 eV)38 than at Pd(110) (0.27 eV).37 Stronger H repulsion by the dense CO overlayer coverage in the MR structure may be an additional contribution to the increased desorption barrier height of the γ3 feature. In the first modification stage from defect-related α1 to terrace-related γ1


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desorption in the PR-structure, on the other hand, the pre-exponential term νθn drops markedly by more than one order of magnitude (Table 1). This result is entirely consistent with the 10-fold dilution of the near-surface Habs concentration due to in-diffusion of Habs into the Pd bulk (Figure 3), which effectively reduces θ and thereby decelerates Habs desorption. Moreover, the unchanged value of E* is reminiscent of our previous finding that defect- and terrace-mediated H2 uptake pathways at PR-reconstructed Pd(110) have nearly identical activation energies.14 The second modification step from PR-γ1 to (1×1)-γ2 appears to involve a combined effect of E* increase and νθn decrease (Table 1), in line with the aforementioned possible mechanisms of increasing CO-repulsion, PR-reconstruction lifting that removes the widened Pd(110) penetration channels, and further Habs dilution by bulk diffusion at the higher peak temperature. These results highlight that - reinforcing the trapping effect the CO-modified surface - bulk dissipation of Habs by thermal diffusion may additionally retard desorption of Pd-absorbed hydrogen. This latter effect will be absent in Pd nanoparticles employed as industrial catalysts due to the lack of extended bulk regions.


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Table 1. Leading edge analysis results for the four desorption modes (α1, γ1, γ2, and γ3) of Pd-absorbed hydrogen (Habs) and corresponding peak temperatures, CO coverages and Pd(110) surface structures.

Habs TPD feature

α1

γ1

γ2

γ3

E* [eV]

0.24

0.22

0.26

0.49

log(νθn)

13.0

11.4

10.7

12.5

Peak temperature [K]

160

200

270

375

CO coverage [ML]

0

0.2

0.5

1

Surface structure

PR

PR

(1×1)

MR

Exit site

Defect

Terrace

Terrace

Terrace

CONCLUSIONS Through combined LEED and TPD measurements of CO co-adsorbed on Habs-loaded Pd(110), which exposes several CO-induced surface phases, we demonstrated the possibility to manipulate the desorption kinetics of Pd-absorbed hydrogen via surface structural modification. CO-induced surface (de-)reconstruction leads to evolution of novel Habs desorption modes, where the peak temperature is shifted in three distinct steps from 160 K for defect-mediated desorption


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from the pairing-row-reconstructed Pd(110) (1×2) surface to 200, 270, and 375 K. These COinduced H2 TPD features are assigned to Habs desorption from regular terraces of the (1×2) pairing-row, the (1×1) bulk-terminated, and the (1×2) missing-row reconstructed Pd(110) surface, respectively. These surface structures and hence the relative intensity of the individual Habs desorption modes are controllable by the CO coverage, which hence works as a molecular switch. Through NRA H-depth profiling we furthermore directly visualized the migration of Hsurf into the Pd interior upon co-adsorption of CO at 90 K and demonstrated complete dissipation of Habs from the near-surface region by diffusion into the Pd bulk at 200 K.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT 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,


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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.

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Desorption Temperature Control of Palladium ... - ACS Publications

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