Dynamic Blocking by CO of Hydrogen Transport across Pd70Au30(110)

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Dynamic Blocking by CO of Hydrogen Transport Across Pd Au (110) Surfaces Shohei Ogura, and Katsuyuki Fukutani J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b10965 • Publication Date (Web): 18 Jan 2017 Downloaded from http://pubs.acs.org on January 24, 2017

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The Journal of Physical Chemistry

Dynamic Blocking by CO of Hydrogen Transport across Pd70Au30(110) Surfaces Shohei Ogura,*,† Katsuyuki Fukutani † †

Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo

153-8505, Japan

ABSTRACT: CO adsorption affects hydrogen transport across surfaces of hydrogen-absorbing materials. On Pd70Au30(110), CO was found to block the desorption sites for absorbed hydrogen. However, the detailed CO adsorption site and hence the blocking mechanism have not been clarified yet. In this study, we investigated the CO adsorption structure on Pd70Au30(110) by using reflection absorption infrared spectroscopy (RAIRS). We demonstrate that the CO adsorption structure depends on the CO coverage and sample temperature. We also performed thermal desorption spectroscopy (TDS) simulations on the basis of the RAIRS results and clarified the dynamical mechanism of the CO blocking where the CO site transfer from the Pd on-top to the Pd-Pd bridge sites enhances the blocking efficiency. These discoveries would lead to understanding and controlling the hydrogen transport across the Pd-Au alloy and Pd-related surfaces.

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1. INTRODUCTION Pd alloyed with Au reveals fascinating properties such as enhancement of hydrogen solubility compared to pure Pd,1-3 catalytic activities for reactions such as vinyl acetate synthesis,4 partial hydrogenation of unsaturated hydrocarbons,5 decomposition of formic acid,6 and H-D exchange of butene.7 Since hydrogen plays important roles in these phenomena, it is required to understand the behavior of hydrogen at surfaces in order to control the hydrogen absorption and catalytic reactions. When the behavior of the hydrogen on surfaces is discussed, CO adsorption has been recognized to affect the absorption and desorption processes of hydrogen by blocking the active site. CO adsorption results in a shift of the hydrogen desorption temperature in the thermal desorption spectra to a higher temperature8,9 and displacement of surface hydrogen into subsurface sites.10 To understand the CO adsorption effects is crucial to control the hydrogen transport across the surfaces of hydrogen-absorbing materials. We have recently shown that hydrogen can be efficiently absorbed in Pd70Au30(110) through a surface Pd area, and demonstrated that a small amount of CO adsorption significantly changes the hydrogen absorption and desorption behavior by blocking the entrance and exit site for hydrogen.11 The desorption peak of the absorbed hydrogen shifts from 240 to 360 K with CO adsorption (Supporting Information, Figure S1). This means that CO can be used as a “molecular cap” for hydrogen transport across the surface. With this effect, hydrogen can be stored in the sample up to room temperature, which is very important for hydrogen storage because there is no need to cool the sample to keep the hydrogen inside of the material. Although it was found that CO molecules block the hydrogen desorption, the CO adsorption site at the atomic level and the CO species responsible for the blocking effect were not clear. Furthermore, our previous study


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showed that the two H2 desorption peaks at 240 and 360 K appear in the same spectrum at a lower CO coverage (Figure S1).11 The presence of the double H2 desorption peaks cannot be explained by the simple CO blocking alone, because hydrogen is expected to diffuse to uncovered Pd sites and desorb as one desorption peak when the blocking is incomplete. The motivation behind the present study is to elucidate the microscopic structure of the CO and the mechanism of the CO blocking. To this end, we have investigated the CO adsorption on a singlecrystal Pd70Au30(110) alloy surface by using reflection absorption infrared spectroscopy (RAIRS). We also performed thermal desorption spectroscopy (TDS) simulations to investigate the mechanism of the CO blocking. We demonstrate that CO molecules dynamically change their adsorption sites depending on the CO coverage and sample temperature, and that the CO site transfer from the Pd on-top to the Pd-Pd bridge sites plays an important role in the CO blocking enhancing the blocking efficiency.

2. EXPERIMENTAL METHODS We performed RAIRS measurements on the Pd70Au30(110) surface prepared by Ar ion sputtering and annealing at TA = 600 and 800 K for 30 s in an ultra-high vacuum. This treatment leads to surface segregation of Au and surface reconstruction: The surface Au concentration is in the range of 45-58% at TA = 600 K and 85-94% at TA = 800 K, and the surface shows a (1×2) reconstruction at TA = 800 K while (1×1) at TA = 600 K.11 After the sample surface was exposed to a certain amount of CO at 100 K, RAIRS spectra were taken at the same temperature. To measure the dependence of the RAIRS spectrum on the sample temperature, the sample surface was first exposed to CO at 100 K and then flashed to a certain temperature TF. The sample was cooled after the flashing and then the RAIRS spectrum was taken at 100 K. In the RAIRS


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measurement, infrared light was focused on the sample through a CaF2 window at an incident angle of 85° from the surface normal, and the reflected light was again focused through a CaF2 window and detected by a mercury cadmium telluride detector. The optical path from the infrared source to the detector was purged with dry nitrogen gas to reduce the absorption by CO2 and H2O. RAIRS spectra were taken with a resolution of 4 cm-1 and averaged over 100 scans. The RAIRS spectra are shown as the transmittance spectra given by the ratios of spectra taken before and after CO exposure. The H2O peaks from the gas phase in the optical path were subtracted using a background spectrum with H2O peaks. CO exposure was performed by backfilling the chamber through a leak valve. The exposure was not corrected for the ion gauge sensitivity and is given in the Langmuir (L) unit (1 L = 1.3×10-4 Pa s).

3. RESULTS AND DISCUSSION 3.1. RAIRS and CO Adsorption Site Figure 1 shows the dependence of the RAIRS spectrum on the CO exposure measured for Pd70Au30(110) annealed at (a) TA = 600 K and (b) 800 K. As shown in Figure 1a, a CO stretching peak at 2069 cm-1 and a small shoulder around 2085 cm-1 appeared at 0.2 L CO. At 0.4 L, another peak appeared at 2110 cm-1 and the peak at 2069 cm-1 slightly shifted to 2071 cm-1. At 0.7 L, an additional peak appeared at 1969 cm-1, while the peak at 2071 cm-1 and the shoulder around 2085 cm-1 decreased in intensity. At 1 L, the peak at 2110 cm-1 shifted to 2114 cm-1 and the intensities of the peaks at 2114 and 1969 cm-1 increased at the expense of the peaks at 2071 and 2085 cm-1. Additionally a shoulder was observed around 2105 cm-1 at 1 L. From the peak symmetry, this shoulder would also exist at 0.7 L.


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Figure 1. RAIRS spectra from Pd70Au30(110) after CO exposure at 100 K. The surface was prepared by annealing at (a) 600 K and (b) 800 K.

In the RAIRS spectra taken from the sample annealed at 800 K, two peaks at 2071 and 2112 cm-1 and a shoulder around 2085 cm-1 appeared at 0.2 L as shown in Figure 1b. These wavenumbers are slightly higher than those observed for the sample annealed at 600 K. Since the peak at 2110 cm-1, which was observed at 0.4 L for the sample annealed at 600 K after the completion of the peak at 2069-2085 cm-1, already appeared at 0.2 L, the number of the adsorption site corresponding to the peak at 2069-2085 cm-1 will be smaller on the sample annealed at 800 K. At 0.4 L, the intensity of the peak at 2112 cm-1 increased further and its peak position slightly shifted to 2110 cm-1, while the peak at 2069-2085 cm-1 almost disappeared. With increasing CO exposure from 0.4 to 1L, the peak at 2110 cm-1 shifted to 2114 cm-1 and its intensity further increased. Similarly to Figure 1a, a shoulder appeared around 2100 cm-1 above 0.4 L. The intensity of the peak at 2114 cm-1 is much higher than that observed for the sample


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annealed at 600 K. In the magnified spectrum at 1 L, a small peak was observed around 1953 cm1

, which will correspond to the peak at 1969 cm-1 observed in Figure 1a. Figure 2 shows the RAIRS spectra taken from Pd70Au30(110) annealed at (a) TA = 600 K and

(b) 800 K after 10 L CO exposure at 100 K followed by flashing to a sample temperature TF. After 10 L CO exposure on the sample annealed at TA = 600 K, the CO stretching peaks appeared at 2116 and 1973 cm-1 with a shoulder around 2105 cm-1 as shown in Figure 2a. These wavenumbers are slightly higher compared to those observed at 1 L shown in Figure 1a. The CO coverage is expected to reach saturation at this exposure on the basis of our TDS measurement.11 On warming the sample to TF = 200 K, a broad peak appeared around 2080 cm-1 at the expense of the peaks at 2116 and 1973 cm-1 and the shoulder around 2105 cm-1. The peak at 2116 cm-1 shifted to 2121 cm-1 and the peak at 1973 cm-1 and the shoulder around 2105 cm-1 disappeared. At TF = 260 K, the peak at 2121 cm-1 disappeared, while the broad peak at 2080 cm-1 evolved into two peaks at 2087 and 2077 cm-1. At TF = 340 K, the peak at 2087 cm-1 disappeared and the peak at 2077 cm-1 decreased in intensity and shifted to 2073 cm-1. Furthermore, new peaks appeared around 1975, 1936, and 1902 cm-1. These peaks around 1936 and 1902 cm-1 were not observed in Figure 1, indicating that occupation of these sites is a thermally activated process. At TF = 400 K, the peak at 2073 cm-1 disappeared and the intensities of the peaks around 1975 and 1936 cm-1 decreased while the intensity of the peak around 1902 cm-1 increased. All peaks disappeared on warming the sample to TF = 500 K.


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Figure 2. RAIRS spectra from Pd70Au30(110) after 10 L CO exposure at 100 K followed by flashing the sample to denoted temperatures TF. The surface was prepared by annealing at (a) 600 K and (b) 800 K.

For the sample with a higher surface Au concentration, only the intensity of the peak at 21162121 cm-1 increased while the intensities of other peaks decreased as shown in Figure 2b. The overall behavior was the same as Figure 2a. Almost the same series of RAIRS spectra as Figure 2 was observed on the sample with absorbed hydrogen populated by exposing the sample to 100 L H2 at 140 K. Figure 3 shows the result for the sample annealed at 800 K. Our previous study clarified that hydrogen tends to accumulate in the near-surface region by this H2 exposure.11 Therefore, this result shows that hydrogen absorption in the near-surface region hardly affects the CO adsorption structure.


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Figure 3. RAIRS spectra from Pd70Au30(110) after 100 L H2 exposure at 140 K followed by 10 L CO exposure at 100 K and flashing the sample to a temperature TF. The surface was prepared by annealing at 800 K. The RAIRS spectra taken without H2 exposure in Figure 2b are also displayed as dotted lines for comparison.

In the following, we discuss the microscopic CO adsorption structures. On Pd(110), the CO stretching peaks at 2099 and 1902-2003 cm-1 were observed and assigned to CO adsorption on the on-top and bridge sites, respectively.12 On Au(110), on the other hand, the CO stretching peak at 2108-2118 cm-1 was observed and assigned to CO adsorption on the on-top site.13 On the Pd-Au alloy (111) and (100) surfaces, three peaks were observed in the ranges of 2106-2112, 2074-2089, and 1911-1972 cm-1 and assigned to CO adsorption on the Au on-top, Pd on-top, and Pd-Pd bridge sites, respectively.14-19 The wavenumbers of 2100-2121 cm-1 observed in the present study are close to the values on the Au on-top site, and hence this peak can be assigned to CO adsorption on the Au on-top sites. This peak has two components at 2110-2121 and 2100-


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2105 cm-1, indicating the presence of two or more types of Au on-top sites with different configurations. The peak with the higher wavenumber of 2110-2121 cm-1 would be assigned to CO adsorption on the Au on-top sites surrounded by Au atoms, because only the intensity of this peak increased with increasing surface Au concentration. The shoulder with the lower wavenumber of around 2100-2105 cm-1, on the other hand, would be assigned to CO adsorption on the Au on-top sites surrounded by one or more neighboring Pd atoms next to or below the Au atom. The wavenumbers of 2069-2087 cm-1 are close to the values on the Pd on-top site, and hence this peak can be assigned to CO adsorption on the Pd on-top sites. This peak also has two components at 2069-2077 and 2085-2087 cm-1. Figure 1 shows that the lower wavenumber peak at 2069-2071 cm-1 appeared first, indicating that this peak corresponds to the most stable adsorption site. Therefore, we assign the peak at 2069-2077 cm-1 to CO adsorption on the Pd ontop sites with Pd atoms in the neighboring sites, because these sites are expected to have a higher adsorption energy compared to the Pd on-top site surrounded by Au atoms. We assign the peak at 2087 cm-1 in Figure 2 and the shoulder around 2085 cm-1 in Figure 1 to CO adsorption on the Pd on-top site without Pd atoms in the neighboring sites. On the other hand, it is reasonable to assign the CO stretching peaks between 2000 and 1900 cm-1 to CO adsorption on the bridge sites and peaks below 1900 cm-1 to CO adsorption on the three-hold hollow sites.20 Therefore, the peaks around 1975, 1936 and 1902 cm-1 that appeared at TF = 340 K in Figure 2 can be assigned to CO adsorption on the Pd-Pd bridge sites with different configurations. However, we assign the peak at 1953-1973 cm-1 observed at saturation coverage at 100 K to CO adsorption on the Pd-Au bridge sites as follows. Although CO molecules prefer to adsorb on the Pd-Pd bridge site rather than on the Pd on-top site on Pd(110),12 Pd atoms prefer to form Pd monomers rather than Pd dimers and trimers on the surface of Pd-Au alloys.21-23 CO


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molecules cannot adsorb on the Pd-Pd bridge site on the Pd-Au alloy surfaces with lower surface Pd concentrations, because there are only Pd monomers on such surfaces. It is reported that CO adsorption on the Pd-Pd bridge site has never been observed at the surface Pd concentration below 70%.16 This indicates that the Pd atoms are isolated or that CO adsorption on the Pd-Pd bridge sites is an activated process on the Pd-Au alloy surfaces up to the surface Pd concentrations around 70%. Therefore, CO molecules are not expected to adsorb on the Pd-Pd bridge site at 100 K in the present study, because the surface Pd concentration is about 50% even on the surface annealed at TA = 600 K. If the peak at 1953-1973 cm-1 were assigned to CO adsorption on the Pd-Pd sites, furthermore, this peak would appear first or at latest at the same time as other peaks, as observed on other Pd-Au alloy surfaces with higher surface Pd concentrations.14-19 However, Figure 1 shows that the peak at 2069-2071 cm-1 appeared earlier than the peak at 1953-1973 cm-1. Therefore, we assign the peak at 1953-1973 cm-1 to CO adsorption on the Pd-Au bridge site rather than the Pd-Pd bridge site. The RAIRS result showed that the occupation of the sites requires higher CO coverage. On Pd(110), peaks at 2068, 1992, 1951, 1902, 1864 cm-1 were observed in the (4×2) phase after exposure of 0.6 ML (monolayer) CO at 300 K, which were assigned to CO adsorption on the reconstructed Pd(110) surface.12 The peaks around 1975, 1936 and 1902 cm-1 observed in the present study would be related to such surface reconstruction. On Pd-Au alloy surfaces, COinduced segregation of Pd atoms to the surface was reported experimentally14,15 and theoretically.24,25 Although such surface Pd segregation requires higher CO pressure than that in the present study,14,15 Pd aggregation, for example dimer or trimer formation within the surface, would occur at lower CO pressure because the activation energy for the Pd aggregation is expected to be smaller compared to that for the surface Pd segregation. On the basis of the


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RAIRS data, we suppose that the two Pd monomers form a Pd dimer around TF = 340 K providing a Pd-Pd bridge site, which is a more favorable site for CO molecules. Upon the Pd dimer formation, CO molecules are transferred to the Pd-Pd bridge sites from the Pd on-top site, resulting in the peaks around 1975, 1936, and 1902 cm-1. Note that such Pd dimer formation is operative only with CO adsorption because the Pd dimer is energetically unfavorable in the PdAu alloy surfaces without CO adsorption. Therefore, the Pd dimer is expected to return to the Pd monomer after CO desorption from the Pd-Pd bridge site. It is not clear whether the peak around 1975 cm-1 that appeared at TF = 340 K is the same as the peak at 1953-1973 cm-1 at 100 K. However, these peaks would be different because the peak around 1975 cm-1 exists up to 400 K even after the disappearance of the peak at 2069-2085 cm-1 that corresponds to the most stable adsorption site at 100 K in Figure 1. As mentioned above, the occupation of the sites corresponding to the peak at 1953-1973 cm-1 requires higher CO coverage. To explain this behavior, we propose adsorption and desorption sequences as shown in Figure 4. CO molecules first adsorb on the Pd on-top site with Pd atoms in the neighboring subsurface sites, producing the peak at 2069-2071 cm-1 as shown in Figure 4a. And then CO molecules adsorb on the Pd on-top sites without Pd atoms in the neighboring sites, producing the shoulder around 2085 cm-1. After the saturation of the Pd on-top sites, CO molecules start to occupy the Au on-top sites, producing the peak at 2110-2116 cm-1. Since the CO adsorption on the Au on-top sites next to the surface Pd atoms will be energetically unfavorable due to the repulsive interaction with the CO molecules already adsorbed on the Pd on-top sites, CO molecules will first occupy the Au on-top sites far from the surface Pd atoms. At nearly saturation coverage, however, CO molecules start to occupy the Au on-top site next to the surface Pd atoms, resulting in the peak at 2100-2105 cm-1. Due to the repulsive interaction,


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the CO molecules adsorbed on the Pd on-top sites are forced to move to the Pd-Au bridge sites. As a result, the peaks at 2116, 2100-2105, and 1953-1973 cm-1 were observed at the saturation coverage and the peak at 2069-2087 cm-1 disappeared due to the CO site transfer from the Pd ontop to the Pd-Au bridge sites. The desorption process is the reverse process of the adsorption as shown in Figure 4b. CO molecules adsorbed on the Au on-top sites next to the Pd atoms desorb first around 200 K, resulting in the decrease of the peak intensity around 2100-2105 cm-1. Induced by the desorption of this CO species, CO molecules forced to adsorbed on the Pd-Au bridge sites move back to the Pd on-top sites, resulting in the increase of the peak intensity at 2069-2087 cm-1 and the decrease of the peak intensity at 1953-1973 cm-1. At 260 K, CO molecules on the Au on-top sites completely desorb and CO molecules occupy only the Pd ontop sites, resulting in the disappearance of the peak at 2110-2121 cm-1 and the recovery of the peak at 2069-2087 cm-1. CO molecules on the Pd on-top sites desorb around 340 K. A part of the CO molecules on the Pd on-top sites moves to the Pd-Pd bridge sites formed by the CO-induced Pd dimer formation that occurs at TF = 340 K, producing peaks around 1975, 1936, and 1902 cm1

. CO molecules corresponding to the peaks around 1975 and 1936 cm-1 desorb around 400 K,

and the CO molecules corresponding to the peak around 1902 cm-1 desorb around 500 K.


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Figure 4. Schematic sequences of (a) adsorption and (b) desorption of CO. Tdes denotes the peak temperature in the TDS spectra.

Another explanation for the compensation behavior among the peaks at 2100-2121, 1953-1973 and 2069-2087 cm-1 is the intensity transfer between CO molecules on different adsorption sites where the intensity of a lower wavenumber component is transferred to a higher one on the basis of dynamic dipole-dipole coupling, while the two components both exist on the surface.26 In the present case, the lower and higher wavenumber components correspond to CO molecules on the Pd on-top and Au on-top sites, respectively. With this effect, the intensity of the peak at 20692087 cm-1 is transferred to that of the peak at 2100-2121 cm-1. However, if this compensation behavior were due to the intensity transfer, CO molecules on the Pd on-top and the Pd-Au bridge sites would coexist. Since these two CO molecules are too close to each other, such an adsorption structure would be energetically unfavorable. Furthermore, there is no satisfactory explanation for the growth of the peak at 1953-1973 cm-1 together with the disappearance of the peak at 2069-2087 cm-1. Thus, the site transfer model shown in Figure 4 will be more reasonable.


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Although the model shown in Figure 4 surely explains the observed behavior, such complicated behavior has not been observed on other Pd-Au alloy surfaces. This behavior would be specific to the (110) surface of the Pd-Au alloy and would be determined by the competition among the CO adsorption energy, repulsion between CO molecules, and the mobility of CO molecules and substrate atoms. From the temperature dependence of the RAIRS data shown in Figure 2, each RAIRS peak can be assigned to each desorption peak in the TDS spectra (Figure S1) as shown in Figure 4b. Although the peaks at 2069-2087 cm-1 show a different behavior from the other peaks, the decrease of the RAIRS peak intensities can be related to the desorption peaks in the TDS data. The peak at 2100-2121 cm-1 corresponds to the desorption peak at 170 K. The peak at 2121 cm-1 corresponds to the desorption peak around 225 K. The peak at 2069-2087 cm-1 corresponds to the desorption peak around 300 K. The peaks around 1975 and 1936 cm-1 correspond to the desorption peak around 360 K and the peak around 1902 cm-1 corresponds to the desorption peak around 420 K. It should be noted that the composition in the subsurface region would also affect the CO stretching frequency and desorption temperature. A lower wavenumber and a higher desorption temperature are expected on the site with more Pd atoms in the subsurface region. On the (1×2) surface, furthermore, the second and third layers are also available for CO adsorption, and thus the wavenumber and binding energy also depend on whether CO adsorbs on the first, second, or third layers. The broad peaks observed in the TDS spectra (Figure S1) would be a reflection of such a wide distribution of site configurations.


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3.2. TDS Simulation The problem to be solved is how the CO molecules affect the hydrogen transport across the Pd-Au surface. It can be concluded that the CO molecules adsorbed on the Pd on-top site block the hydrogen desorption, because this is the only species existing around 300 K where hydrogen desorption occurs without CO adsorption. However, it is not clear whether the CO molecules adsorbed on the Pd-Pd and Pd-Au bridge sites block the hydrogen desorption or not. Our previous study showed that the desorption peak of absorbed hydrogen shifts from 240 to 360 K with CO adsorption (Figure S1 and Figure 6a), and that these two peaks appear at the same time at a lower CO coverage.11 The double H2 desorption peaks at 240 and 360 K cannot be explained by a simple CO blocking alone, because hydrogen is expected to diffuse to uncovered Pd sites and desorb as one desorption peak at a lower CO coverage. In order to solve these problems, we performed a TDS simulation on the basis of the RAIRS results. In the TDS simulation, the hydrogen diffusion in the bulk and the CO blocking effect are taken into account. The CO site transfer was also considered in the TDS simulation. Figure 5a shows the TDS simulation model. To include hydrogen diffusion in the bulk, the sample was divided into layers with a thickness of dLz in the depth direction. Then, the number concentration Ci of H atoms in the layer i was calculated as a function of time by considering one-dimensional H diffusion perpendicular to the surface. i = 0 denotes the gas phase and i ≥ 1 denotes the inside of the sample. The rate equation of Ci is given in the following by considering H flux between neighboring layers dC i Ddif (C i +1 − 2C i + C i −1 ) = 2 dt dL z

(i > 1) ,

dC1 C Pd Ddes (C 0 − C1 ) Ddif (C 2 − C1 ) , = + 2 2 dt dL z dL z

(1)

(2)


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where Ddif and Ddes are the diffusion coefficients for diffusion and desorption, respectively, and CPd is the surface Pd concentration corresponding to the concentration of the hydrogen desorption site. These diffusion coefficients are expressed as

Ddes

 E  a 2ν des exp − des  , = 2  k BT 

Ddif =

a 2ν dif 2

 E dif exp −  k BT

  , 

(3)

(4)

where Edes and Edif are the activation barriers for desorption and diffusion, respectively, νdes and

νdif are the respective attempt frequencies, and a is the jump distance in the [110] direction. The Ddif was assumed to be the same in the entire bulk. Since the gas phase is ultrahigh vacuum, C0 was set to 0. CL of the maximum layer L was treated as an absorbing boundary so that the diffusion from the layer L to the L-1 layer was not allowed. The desorption rate of H2 molecules per unit area was assumed to be proportional to the concentration gradient between the top layer and gas phase as

dN des C Pd Ddes (C1 − C 0 ) = . dt dLz

(5)

This assumption implies that H atoms coming to the surface through the surface Pd area desorb immediately forming H2 molecules without any additional barrier.


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Figure 5. (a) TDS simulation model including the hydrogen diffusion in Pd70Au30(110). (b) The Pd dimer formation and CO site transfer model.

The site transfer and blocking effect of CO were included as shown in Figure 5b. At 200 K where H2 desorption starts (Figure S1), CO molecules occupy the Au and the Pd on-top sites as shown in Figure 2. Since the hydrogen desorption only occurs through the surface Pd sites,11 we only consider the CO molecules adsorbed on the Pd on-top site in the initial condition of the TDS simulation. And then, we assumed that a Pd dimer is formed from one Pd monomer with CO on it and another without CO and that the CO molecule immediately moves to the formed Pd-Pd bridge site. Since two Pd monomers without CO have no driving force and two Pd monomers both with CO have to leave one CO upon the Pd dimer formation, the Pd dimer formation from these configurations were assumed not to occur. On this assumption, the rate equation for the Pd dimer concentration Cdi is expressed as

 E   E   E  dC di = θ ot (C Pd − θ ot )ν di exp − di  − θ b1ν b1 exp − b1  − θ b2ν b2 exp − b2  . dt  k BT   k BT   k BT 


(6)

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The first term in the right hand side denotes the Pd dimer formation rate where θot is the CO coverage on the Pd on-top sites, Edi is the energy barrier and νdi is the attempt frequency for the Pd dimer formation. The second and third terms in the right hind side mean that the Pd dimer turns back to the two Pd monomers upon the first-order desorption of CO from the two types of Pd-Pd bridge sites where θb1 and θb2 are the CO coverages on these Pd-Pd bridge sites, Eb1 and Eb2 are the energy barriers and νb1 and νb2 are the attempt frequencies for CO desorption from these Pd-Pd bridge sites. Since the RAIRS results show the presence of three types of the Pd-Pd bridge sites (peaks around 1975, 1936, 1902 cm-1 in Figure 2), we distinguished these sites in the TDS simulation. Since the peaks around 1975 and 1936 cm-1 behave similarly, we only consider the two species in the simulation. θb1 is the coverage of the CO molecules on the Pd-Pd site corresponding to the peaks at 1975 and 1936 cm-1 in the RAIRS and the peak around 360 K in the CO TDS, and θb2 is the coverage of the CO molecules on the Pd-Pd bridge sites corresponding to the peak at 1902 cm-1 in the RAIRS and the peak around 420 K in the TDS. The rate equation for the Pd monomer density Cmono can be given by

dC mono dC = −2 di . dt dt

(7)

The time evolution of each CO coverage is expressed as

 E   E  dθ ot = −θ otν ot exp − ot  − θ ot (C Pd − θ ot )ν di exp − di  , dt  k BT   k BT 

(8)

 E   E  dθ b1 = −θ b1ν b1 exp − b1  + Pb1θ ot (1 − θ ot )ν di exp − di  , dt  k BT   k BT 

(9)

 E   E  dθ b2 = −θ b2ν b2 exp − b2  + (1 − Pb1 )θ ot (1 − θ ot )ν di exp − di  , dt  k BT   k BT 

(10)


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where the first term in the right hand side in each equation denotes the desorption rate. The second term of equation 8 denotes the decrease of the CO coverage on the Pd on-top site upon the Pd dimer formation accompanied by the CO site transfer from the Pd on-top to the Pd-Pd bridge sites. Pb1 in equations 9 and 10 denotes the fraction of the CO molecules transferred to the Pd-Pd site corresponding to θb1 upon the Pd dimer formation. The rest of the CO molecules are transferred to the Pd-Pd bridge site corresponding to θb2. Pb1 is a parameter to be determined by the simulation. As the CO blocking effect, we assumed that the CO molecule on the Pd on-top site blocks one hydrogen desorption site and the CO molecule on the Pd-Pd bridge site blocks two hydrogen desorption sites. On this assumption, the H2 desorption rate of equation 5 is modified to

dN des D (C − C0 ) = (C Pd −θ ot −2θ b1 − 2θ b 2 ) des 1 . dt dLz

(11)

This means that the hydrogen desorption rate is reduced by the factor of (CPd-θot-2θb1-2θb2) by CO adsorption. The sample temperature is given by T = T0 + βt, where T0 is the initial temperature and β is the heating rate. By solving these equations under an initial H distribution with a set of parameters, a TDS spectrum can be obtained by plotting dNdes/dT against T. The initial H distribution was set based on our previous TDS results and hydrogen depth profiles obtained by nuclear reaction analysis (NRA).11 The H concentration in the bulk was set to 0.17% up to 250 nm. The concentration was set to the value obtained by NRA on the assumption that hydrogen is uniformly distributed up to a certain penetration depth and the penetration depth was set so that the amount of H atoms becomes identical to that obtained from the TDS results.


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Figure 6a shows the simulated H2 TDS spectra at initial CO coverages θot of 0, 0.075, and 0.1 ML under the parameters for hydrogen diffusion and desorption of Edif = 0.28 eV, Edes = 0.275 eV, νdif = 1013 s-1, and νdes = 1012 s-1 and for CO desorption and the Pd dimer formation of Eot = 0.37 eV, Eb1 = 0.42 eV, Eb2 = 0.47 eV, νot = νb1 = νb2 = 105 s-1, Edi = 0.305 eV, νdi = 106 s-1, and Pb1 = 0.65. Other parameters were dLz = 50 nm, L = 2000, β = 2.5 K s-1, CPd = 0.1, and a = 1.4×10-10 m. This CPd corresponds to the sample annealed at TA = 800 K. The diffusion barrier for hydrogen of 0.28 eV is slightly larger than 0.23 eV in pure Pd,27 which is consistent with previous studies.2,3 Without CO adsorption, the H2 TDS can be well reproduced by these parameters. The low attempt frequencies for CO are necessary to reproduce the CO TDS with broad peaks (Figure S2). The peak width cannot be reproduced with usual attempt frequencies of 1012-13 s-1. Such low attempt frequencies will reflect a wide distribution of the CO adsorption sites as described above. The broad CO desorption peaks would be the result of the summation of the CO desorption from many different configurations, each of which will have a usual attempt frequency of 1012-13 s-1. There may be another explanation for the broad desorption peak that a coverage-dependent repulsive interaction between CO molecules causes a continuous peak shift as a function of CO coverage, which was observed on Pt(111).28 Since only the shape of the CO TDS affects the simulation results, however, we used these low attempt frequencies in the simulation for simplicity.


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Figure 6. (a) Comparison of the TDS spectra between the simulation and experimental results. (b) Fraction of the uncovered Pd site as a function of the sample temperature.

The experimentally observed three situations, one peak at 240 K, double peaks at 240 and 360 K, and one peak at 360 K, can be reproduced only by changing the CO initial coverage. A tail above 350 K would be due to the H2 desorption from other parts than the sample such as the sample holder. It should be noted that the double H2 desorption peaks at 240 and 360 K observed at the CO exposure of 0.2 L (Figure 6a and Figure S1) cannot be reproduced without the Pd dimer formation. If the Pd dimer formation was absent, only one H2 desorption peak around 300 K was observed (Figure S3). This is because H atoms can diffuse to find the H2 desorption sites which are not blocked by CO initially. With the Pd dimer formation, however, one CO molecule blocks two H2 desorption sites. In Figure 6b, the concentration of the uncovered Pd sites (CPd-θot2θb1-2θb2) is shown as a function of the sample temperature. At an intermediate initial CO


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coverage of 0.075 ML, H2 can desorb through the initially open Pd sites producing the H2 desorption peak at 240 K. With increasing sample temperature, however, the concentration of the uncovered Pd sites decreases around 290 K due to the increase in θb1 and θb2 caused by the Pd dimer formation. As a result, the H2 desorption rate decreases around 290 K as shown in Figure 6a. When the CO molecules adsorbed on the Pd-Pd bridge sites start to desorb above 300 K, the concentration of the uncovered Pd sites increases and the H2 desorption rate increases again, which results in the H2 desorption peak at 360 K. This is the origin of the double H2 desorption peak at an intermediate CO coverage. At an initial CO coverage of 0.1 ML, on the other hand, the surface Pd sites are fully covered by CO initially and they become available only above 300 K, which results in only one H2 desorption peak at 360 K. We conclude that the Pd dimer formation followed by the CO site transfer and the concomitant increase in the blocking efficiency are the crucial mechanism of the CO blocking. From the RAIRS measurement and TDS simulation, it can be concluded that CO molecules adsorbed on the Pd on-top and the Pd-Pd bridge sites block the hydrogen desorption. On PdAu(111) and PdAg(110) surfaces, it is reported that the most stable site for hydrogen adsorption at a lower surface Pd concentration is the hollow site around the Pd monomer.29,30 Since hydrogen atoms will come to the surface through the hollow site, the CO adsorption on the Pd on-top site is not expected to directly block the hydrogen diffusion to the surface. Therefore, we suppose that the CO molecules on the Pd on-top sites will block the H2 formation site which is necessary for hydrogen to desorb. The CO molecules on the Pd-Pd bridge sites would block two H2 formation sites. This would be the justification of the assumption that one CO molecule on the Pd-Pd bridge sites blocks two desorption sites in the TDS simulation. There may be another possibility that CO adsorption on the surface Pd atoms changes the surface electronic


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property so that the energy barrier for hydrogen diffusion to the surface is increased. However, theoretical calculations would be required to understand the details of the CO blocking effect.

4. CONCLUSIONS We have investigated the CO adsorption structure on Pd70Au30(110) by RAIRS and found that CO molecules dynamically change their adsorption sites depending on the CO coverage and sample temperature. We have also performed TDS simulations and clarified the dynamical mechanism of the CO blocking where the CO site transfer from the Pd on-top to the Pd-Pd bridge sites accompanied by an increase in the blocking efficiency play important roles. These discoveries would lead to understanding and controlling the hydrogen transport across the Pd-Au alloy and Pd-related surfaces.

ASSOCIATED CONTENT

Supporting Information. Details of the CO TDS and TDS simulation results. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION

Corresponding Author *Email: [email protected]

Notes The authors declare no competing financial interests. ACKNOWLEDGMENT


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This work was supported by JSPS KAKENHI Grant Number JP16K04957.

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TOC Graphic


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Figure 1 81x71mm (300 x 300 DPI)


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Dynamic Blocking by CO of Hydrogen Transport across Pd70Au30(110)

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