Insights into Hydrogen Gas Environment-Promoted Nanostructural

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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials

Insights into Hydrogen Gas Environment Promoted Nanostructural Changes in Stressed and Relaxed Palladium by Environmental Transmission Electron Microscopy and Variable Energy Positron Annihilation Spectroscopy Vladimir Roddatis, Marian David Bongers, Richard Vink, Vladimir Burlaka, Jakub Cizek, and Astrid Pundt J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b02363 • Publication Date (Web): 28 Aug 2018 Downloaded from http://pubs.acs.org on August 30, 2018

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The Journal of Physical Chemistry Letters

Insights into Hydrogen Gas Environment Promoted Nanostructural Changes in Stressed and Relaxed Palladium by Environmental Transmission Electron Microscopy and Variable Energy Positron Annihilation Spectroscopy Vladimir Roddatis, *, a Marian D. Bongers,a Richard Vink,a Vladimir Burlaka,a Jakub Čížek,b and Astrid Pundt a, c a

Institute of Materials Physics, University of Goettingen, Friedrich-Hund-Platz 1, Goettingen D-37077,

(Germany) b

Department of Low-Temperature Physics Charles University, Prague V Holešovičkách 2, CZ-18000

Praha 8, (Czech Republic) c

Institute for Applied Materials, Karlsruhe Institute of Technology, Engelbert-Arnold-Strasse 4,

Karlsruhe D-76131, (Germany)

Corresponding Author * E-Mail: [email protected] 1 Environment ACS Paragon Plus

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ABSTRACT: Environmental transmission electron microscopy (ETEM) and variable energy positron annihilation spectroscopy (VEPAS) are used to observe hydrogen-induced microstructural changes in stress-free Palladium (Pd) foils and stressed Pd thin films grown on rutile TiO2 substrates. The microstructural changes in Pd strongly depend on the hydrogen pressure and on the stress state. At room temperature, enhanced Pd surface atom mobility and surface reconstruction is seen by ETEM already at low hydrogen pressures (pH < 10 Pa). The observations are consistent with molecular dynamics simulations. A strong increase of the vacancy density was found and so-called superabundant vacancies were identified by VEPAS. At higher pressures, migration and vanishing of intrinsic defects is observed in Pd free-standing foils. The Pd thin films demonstrate an increased density of dislocations with increase of the H2 pressure. The comparison of two studied systems demonstrates the influence of the mechanical stress on structural evolution of Pd catalysts.

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Palladium (Pd) is widely used as a model and industrial catalyst in hydrogenation reactions (1, 2). There is ample evidence that the presence of hydrogen (H2) as a promoter on the surface and in the subsurface region of thin Pd films or particles can strongly modify their catalytic behavior (2-5). Some chemical reactions on the Pd surface can proceed through different competitive routes even at room temperature (RT) and low pressures (< 100 Pa) (6). Recent experimental and theoretical studies have demonstrated that the presence of subsurface hydrogen significantly influences the reactivity and selectivity in the hydrogenation processes (3, 4, 7-11). These changes of catalytic activity are attributed to atomic surface reconstructions (12-14) and associated changes of electronic states (15, 16). During chemical reactions at the catalyst surface hydrogen can enter the catalyst interior (35, 17) and therefrom even participate in the reaction (18). It is well-known that H2 dissociatively chemisorbs on Pd and atomic H occupies interstitial lattice sites forming a solid solution (αphase) (19, 20). If the concentration of hydrogen dissolved in the Pd matrix achieves the solubility limit of the α-phase, Pd-H forms a hydride phase (α’- phase) (21). The solubility of H depends on the hydrogen chemical potential (22), on the mechanical stress (23, 24), on the Pd morphology (25, 26) and on the defect density (27, 28). Hydrogen is efficiently solvable at ‘open-volume’ defects and, vice versa, the defect density can be modified by the presence of hydrogen. The defect formation energy is reduced by the presence of hydrogen (27, 29); on the other hand, defects can get mobile (30). The Pd films grown on substrates initially demonstrate a compressive stress increase of up to -18 GPa/𝑐H , which arises upon hydrogen incorporation, and depends on the hydrogen concentration 𝑐H (H/Pd), the adhesion energy and the sample geometry (31, 32). The highest stress values can be reached for films of a few nanometers in thickness at high H-concentrations



(33). This hydrogen-induced mechanical stress often results in defect generation allowing for stress release (30, 34). These defects can also affect the catalytic performance of the metal (3, 35-37). The chemical potential of hydrogen with pressures

𝑝H

below 1000 Pa, only allows for the

formation of the Pd-H solid solution (21, 30). The hydrogen content can be approximated by use of the Sievert’s law (22),

𝑐H = 𝐾S (𝑇) ∙ √𝑝H

. This law is valid for bulk material at low concentrations,

and relates the hydrogen concentration 𝑐H in the sample to the applied pressure 𝑝H . For bulk Pd, the Sievert’s constant 𝐾S,Pd (300 K) = 1.43∙10−4 H/Pd /√Pa, using ∆𝐻0 = −9.6 𝑘𝐽⁄𝑚𝑜𝑙𝐻 and ∆𝑆0 = −58.2 𝐽⁄(𝑚𝑜𝑙𝐻𝐾) (38). For a pressure of 100 Pa this gives a hydrogen concentration of 1.43 ∙10-3 H/Pd, in bulk Pd. This value is slightly different for thin films. However, also here the hydrogen concentration can be regarded as low within the α-phase. Thus, the related hydrogen-induced compressive elastic stress is about 25 MPa, for a Pd film on r-TiO2 (39). There is no hydrogen-related stress in case of the Pd-foil as it can expand freely. The diffusion kinetics of H in Pd is fast at RT resulting in diffusion length of a micron within 30 ms diffusion time (40). Transmission electron microscope (TEM) has become a prominent tool for defects investigations at the atomic scale in heterogeneous catalysis (41). Recent developments in environmental TEM (ETEM) and of gaseous and of liquid specimen holders allow studying not only the initial and final state of the catalyst, for example (42), but also visualize its transformations directly during the reaction (43-46). The transformations during activation and operation of the catalyst influence the amount and nature of active states, which govern catalytic reactions (47). ETEM also provides a distinct advantage to compliment surface sensitive techniques because it allows observing the subsurface regions by using cross-section specimens and recording movies during the experiment with high spatial resolution (48). However, the


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impact of the electron beam on the specimens must be minimized by using appropriate imaging conditions. In addition, the observed transformations have to be verified by parallel “blank” experiments (41). Complementarily, variable energy positron annihilation spectroscopy (VEPAS) allows characterizing defect depth profiles of samples (49, 50) in hydrogen environments. Positrons allow detecting electrically neutral or negatively charged open-volume defects while positively charged defects repel positrons and cannot be observed by VEPAS. In Pd, defects like vacancies, dislocations or planar defects locally possess regions with openvolume that can be detected by positrons (50). This method is sensitive in the low concentration range, starting at about 1011 defects/cm-3 up to 1015 defects/cm-3 (51). In this paper, we address morphological and structural changes of Pd films and foils at different hydrogen gas pressures and at two different mechanical stress states. While hydrogeninduced stress arises in Pd films grown on r-TiO2, it is absent in the free-standing Pd foils. We use ETEM and VEPAS to analyze in situ the surface and subsurface microstructural changes at 𝑝H =10-4 Pa to 500 (1000) Pa at RT. The 200 nm Pd films were grown on rutile TiO2 (110) single crystal substrates (r-TiO2) at 1023 K by magnetron sputtering (sputter rate ≈ 11.8 nm/min). Texture analyses on the Pd films show two preferential orientation relations, suggesting local epitaxy, with an [2-1-1](111)||[110](110)r-TiO2 orientation relationship of Pd. The mean intrinsic stress of the prepared films is in the range of 700 MPa, as derived via the Pd (111) peak shift in x-ray diffraction measurements (see SI). Note, that this includes the assumption of an isotropic in-plane stress state. Domain sizes in these Pd films are in the nanometer range. In contrast, the Pd foils are stress-free and polycrystalline with grain sizes in the micron range.



Cross-section specimens for the ETEM experiments were prepared by mechanical polishing followed by low-angle Ar+ ion milling until perforation. At the final stage of ion milling the acceleration voltage was gradually decreased to prevent radiation damages and the formation of amorphous layers (52). Plasma-cleaned TEM specimens were studied with a FEI Titan 80-300 ETEM operated at 300 kV, and equipped with a Cs-image corrector. During the measurements, hydrogen pressures ranging from 5∙10−5 Pa to 500 Pa were applied. Within 1 h hydrogen gas exposure at each pressure step, it is expected that hydrogen has spread over all available sites within the Pd TEM lamellas and that its distribution has reached equilibrium conditions because of its fast diffusion. Most of the time the electron beam was blanked to minimize its influence on the samples. In addition, control experiments were performed in vacuum at different electron fluxes to estimate the beam influence (see SI). VEPAS studies were carried out on 200 nm Pd films on r-TiO2 using a magnetically guided slow positron beam with W foil moderator. The energy of incident positrons was varied in the range from 80 eV up 35 keV. The Doppler’s broadening of the annihilation photo-peak was measured by the HPGe detector with an energy resolution of 1.09 keV at 511 keV. The shape of the Doppler broadened annihilation photo-peak was characterized using the S (sharpness) parameter (51). The chosen waiting time (about 1 hour) was similar to that of the ETEM experiments, and is, again, sufficient for hydrogen equilibration in the whole sample volume. However, because of the different sample geometry the hydrogen-induced stress state is slightly different. A variety of microstructural changes is observed in the high-resolution TEM (HRTEM) images during the stepwise increase of the H2 pressure (Figure 1). The microstructure and the surface of the sample look unchanged up to 𝑝H =10-2 Pa (Figure 1a). The surface and some defects


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are marked with the blue dotted line, the red and the yellow arrows, respectively. Then, at 𝑝H =10-1 Pa (Figure 1b) first changes happen at the Pd surface. The rounded surface humps (Figure 1a) become faceted and parallel to Pd(111) lattice planes (Figure 1b). The enlarged images are shown in Figure 2a, focusing on one prominent surface hump, marked with the rectangle in Figure 1a.

Figure 1. HRTEM images taken on the Pd foil at the same position but at different hydrogen gas pressures pH ranging from 10-2 Pa to 500 Pa. A few defects are marked with the yellow arrows. The red arrows mark a position where one stacking fault vanishes after hydrogen treatment of 100 Pa. The blue line indicates the surface contour of the initial state of the sample surface. The total electron dose during the experiment was estimated to be 6∙105 e-/Å2.



The further increase of

𝑝H

to 1 Pa (Figure 1c), and then to 10 Pa (Figure 1d) results in a

decrease of the hump height (see also Figure 2b and 2c). This leads to a reduction of the surface border length, a commonly observed effect (53). The orange dashed lines in Figure 2 are drawn to estimate the hump height change throughout the experiment. With the help of these lines the different amount of Pd (111) lattice planes (atomic layers) can be distinguished: 9-10 layers are counted in the initial state at 𝑝H =10-2 Pa (Figure 2a), 8 layers are seen at 𝑝H =10-1 Pa (Figure 2b) and

𝑝H

=10 Pa (Figure 2c). Note, that the morphology of the hump has already significantly

changed with the increase of

𝑝H

from 10-2 Pa to 10-1 Pa. At the maximal 𝑝H =500 Pa (Figure 2d)

the surface is faceted and the height of the hump is 7 atomic planes.

Figure 2. Enlarged HRTEM images of a selected area in Figure 1a focusing on a prominent hump. (a) The hump has a rounded shape with the height of 9-10 atomic layers; (b); the first changes become visible, the height is 8 atomic layers; (c) the hump becomes faceted; (d) the height of the hump is 7 atomic planes; the surface is faceted at pH = 500 Pa. The total decrease in the hump height and its geometry with respect to the orange dashed line is visible.


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With the further increase of

𝑝H

up to 100 Pa, some of the intrinsic defects disappear (one of

those is marked with the orange arrow, Figure 1e. This suggests that the edge dislocations and stacking faults become mobile at

𝑝H

= 100 Pa. We also note that the defects located in thicker

parts of the specimen are less mobile than those which are close to the surface of the Pd foil. Thus, the pressure of

𝑝H

=100 Pa is required to mobilize pre-existing defects in the Pd foil and

therewith activate a sample ‘healing’. New hydrogen-induced dislocations were not detected in the free-standing Pd foil, see Figure 1a – 1f. To further investigate the extent to which atomic hydrogen (H) can induce structural changes in the Pd host lattice, molecular dynamics (MD) simulations were performed at T=300K (see SI for details). The initial state of the simulation is a pure fcc Pd “lamella” without hydrogen featuring a protrusion similar in shape and size as that of Figure 2a. Upon thermalization, the simulations reveal that this initial state, i.e., without hydrogen, remains stable over many millions of MD steps, corresponding to several tens of ns of real time at least. A typical thermalized configuration is shown in Figure 3a which confirms that the initial fcc structure has remained intact. Figure 3b shows the corresponding HRTEM image, which was obtained from the MD coordinates using the QSTEM software (54). Upon insertion of hydrogen, we observe a very strong tendency for H atoms to first collect in octahedral sites near the exposed surfaces of the lamella, before gradually occupying bulk regions. Near the surface, the hydrogen content thus becomes very high, resembling a “hydride” phase, while bulk regions initially remain essentially void of H atoms. These qualitative observations are in agreement with earlier simulations (55, 56). Introducing the overall hydrogen content as XH=NH/NPd, with NH (NPd) the number of H (Pd) atoms in the lamella, we observe that a threshold value XH~0.7 is needed before pronounced changes in the protrusion shape start



to become visible, see Figure 3c, 3d for the MD snapshot and corresponding HRTEM image. We note a broadening of the protrusion at its base, a slight reduction of the protrusion height, as well as surface re-arrangements in the vicinity of the protrusion, where an extra layer of atoms has now appeared directly to the right side of the protrusion. The process depicted suggests a “spreading out” of the protrusion onto the underlying substrate, consistent with the experimental data of Figure 2. For smaller values of XH, shape changes also occur in the simulations, but they are less pronounced (see SI). The MD simulations thus imply that, in order for pronounced shape changes to occur, the hydrogen content in the lamella must be very high, i.e., close to that of a hydride. In bulk samples, at T~300K, the correspondingly required pressure exceeds 1 kPa (55). In contrast, the experimental data of Figure 2 reveal shape changes already at much lower pressure. The most likely explanation is that, in nano-sized samples, the hydride already appears at a much reduced pressure, a fact that is well-established by experiment, theory, and simulation (see e.g. the size-dependent Pd-H pressure-composition isotherms of Ref. 55). The MD simulations were designed to capture the experimental lamella as closely as possible. That is, protrusion thickness and height were chosen to match those of the experiment, as well as the orientation of the Pd (111) surface, whose normal in the snapshots of Figure 3 points in the vertical direction. For these conditions, the revealed shape change process is representative. That is, repeating the simulation for the same conditions but using different random numbers for the H insertion process, snapshots were obtained that are qualitatively similar, i.e. all featuring the "spreading out" of the protrusion through faceting. We thus believe this to be the preferred shape change process, the underlying physical mechanism being surface energy minimization (53, 57). As future extension, it would be interesting to perform


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experiments and simulations at different surface orientation and film thickness, in order to investigate the sensitivity of the observed shape changes to lamella geometry and orientation.

Figure 3. MD snapshots (left) and the corresponding simulated HRTEM images (right) of a pure Pd system (no hydrogen) with protrusion (hump) (a,b) and the final hydrogenated system (c,d) at T=300K. The hydrogen content of the hydrogenated state XH=0.7. Only Pd atoms are shown. Upon hydrogenation, pronounced structural changes are induced, indicating a “spreading out” of the protrusion onto the underlying Pd. The response of Pd thin films on r-TiO2 to the H2 loading is significantly different. At the beginning (𝑝H < 100𝑃𝑎), the enhanced Pd-surface atom mobility, surface flattening and defect healing was also observed in HRTEM images upon hydrogen gas exposure, similar to the Pd foil (see SI for details). But at higher pressures, the appearance of new defects is detected. Figure 4a, 4c and 4e show high resolution scanning TEM (HRSTEM) images of the middle part of a 200 nm Pd film, collected at three different 𝑝H , using annular dark field (ADF) detector.



Figure 4. a) - f) HRSTEM-ADF images taken in the middle of a cross sectional TEM lamella of a 200 nm Pd film on a r-TiO2 substrate, in the initial state (10-4 Pa background pressure) and at different pH (10 Pa and 5∙102 Pa). g) Number of dislocations N obtained at five different positions (all in the middle of the Pd film) and at different hydrogen gas pressures. The number of dislocations stays about constant up to 100 Pa, and increases by a factor of approximately 6 between 100 Pa and 500 Pa hydrogen gas pressure. The investigated area of each HRSTEM image was 58∙58 nm2. This corresponds to dislocation densities of approximately 1.5 ∙1015 - 1016 m-2.


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The HRSTEM images allow us to estimate the number of dislocation N using the Fourier analysis, and to avoid the influence of contrast changes because of thickness and defocus variations as in the case of bright field STEM imaging. The images were processed automatically and the areas, where at least one dislocation was detected, are marked with the red color in Figures 4b, 4d and 4f. These areas were inspected more carefully by eye. The resulting amount of dislocations in each experimental image is summarized in Table S1 (see SI). The dislocation density strongly increases at 𝑝H > 100 Pa, as shown in Figure 4f. The dislocation density was also determined in areas close to the Pd/r-TiO2 interface and at the surface of the Pd film, also at different 𝑝H . These results show the same trends as the one presented in Figure 4g. The detectable dislocation density stays almost constant before the hydrogen pressure reaches of about 100 Pa. Then, it increases by a factor in the order of 10 at 𝑝H ≈500 Pa. VEPAS was employed to characterize the defect depth profile in the 200 nm Pd/r-TiO2 samples, at different hydrogen gas pressures. The results are shown in Figure 5a.



Figure 5. a) VEPAS depth profiles measured on the Pd film on TiO2 at different pH (from 10-2 Pa to 103 Pa). The position of the Pd/TiO2 interface at 13 keV is marked with the dashed line. The S parameter in the Pd film (E < 13 keV) increases at a hydrogen partial pressure of 1 Pa. b) Vacancies concentrations given per Pd atom derived from the changes in the S(E) curves of a).

At very low incident energy E (E < 1 keV) positrons annihilate almost exclusively on the sample surface. With increasing E positrons penetrate deeper into the Pd film and the fraction of positrons diffusing back to the surface decreases. Further increase of incident energy (E > 13 keV) allows positrons also to penetrate into the r-TiO2 substrate. In the Pd film (E < 13 keV) the S parameter monotonically increases with increasing hydrogen pressure. While the S parameter 14 Environment ACS Paragon Plus

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is low before hydrogen gas exposure (red squares in Figure 5a), it significantly increases upon hydrogen gas exposure to 1 Pa (green triangles in Figure 5a).The increase in the S-parameter testifies that hydrogen loading creates new open-volume defects in the Pd film. These new hydrogen-induced defects can be dislocations and/or vacancies. Vacancies are not directly visible in the HRSTEM studies. But, the ETEM images in Figure 4 demonstrate no noticeable increase in the detected dislocations density for hydrogen pressures ranging up to 100 Pa. This suggests that the observed changes in the S parameter in the Pd-film shown in Figure 5a are due to the generation of vacancies, at least for pressures below 100 Pa. Assuming that the changes in the S parameter are solely due to the generation of hydrogen-induced vacancies in the Pd film, the change in the Pd vacancy concentration ∆𝑐𝑉 can be calculated, relative to the initial (or virgin) state. The results are presented in Figure 5b. The vacancy concentration within the Pd film had increased up to 120 ppm at 1 Pa, and to 165 ppm at 100 Pa. This is a very high vacancy concentration in Pd and resembles the concentration of thermal vacancies close to the melting temperature (58). An enhanced vacancy concentration (so-called ‘superabundant vacancies’, SAV) was also found for metal films at low hydrogen concentrations, but upon hydrogen-loading by electrochemistry (59). Huge increase in the vacancy concentration is due trapping of absorbed hydrogen in vacancies which reduces the vacancy formation energy. As a consequence the equilibrium concentration of vacancies in Pd containing absorbed hydrogen increases by many orders of magnitude (58). SAVs were further observed for hydrogen gas loaded samples at exceptional conditions or for electrochemically hydrogen-loaded samples (60-62). SAVs have been recently detected by positron annihilation spectroscopy (PAS) on electrochemically loaded free-standing Pd foils (63). For higher hydrogen gas pressures ETEM measurements reveal a strong increase of the detected dislocation density (Figure 4g). Thus, any vacancy concentrations



determined above 100 Pa (and ∆𝑐𝑉 above 165 ppm) are not reliable as the obtained concentration values contain dislocation contributions. SAVs can also explain the observed mobilization and healing of intrinsic dislocations as detected in the Pd foil (Figure 1). The ETEM results in Figures 1 illustrate dislocation movement at

𝑝H

> 100 Pa. Dislocations can move via a slip and/or a glide mechanism. While

dislocation slip is a conservative mechanism, dislocation climb requires the presence and movement of vacancies. Therefore, when SAV are present, dislocation climb and defect ‘healing’ is more likely. The presence of SAV in hydrogen-gas loaded free-standing foils is, thereby, indirectly indicated. Recently, the mobilization of intrinsic defects was also reported for low hydrogen concentrations in Pd films on Si- and on Al2O3-substrates, as determined by mechanical stress and acoustic emission measurements (30). The defect mobilization was related to stress release events within the overall linear elastic stress-strain regime, and named ‘Discrete stress relaxation’ (DSR) events. Thus, the ETEM and VEPAS studies revealed a sequence of microstructural changes of the Pd foil and the Pd film on r-TiO2 in hydrogen environment already happening at RT. The changes depend on the applied hydrogen pressure and include: a) an enhanced mobility of Pd surface-atoms and defects. By both processes the surface is reconstructed and flat facets are formed (pH ≥ 10-1 Pa); b) a movement and vanishing of dislocations (starting from pH = 102 Pa). The vicinity of surfaces favors the healing of defects. c) SAVs occur in the Pd film at about 1 Pa H2 as detected by VEPAS experiments in combination with ETEM studies.


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d) ETEM experiments show an increase of dislocation density in Pd films grown on r-TiO2 substrates at pressures above 100 Pa. The mechanical stress arising between the film and the substrate is responsible for this difference in the emergence of dislocations within the α–phase of the Pd-H systems; Furthermore, dislocations and vacancies act as traps for hydrogen atoms (64-66). For vacancies in Pd, first principle calculations suggest a high local content of about 6 Hatoms/vacancy (67). These hydrogen-filled vacancies and dislocations can provide a high number of atomic hydrogen and, therefore, might serve as additional source for atomic hydrogen required for catalytic processes. The near-surface microstructure thereby influences the density of catalytically active sites and, also, might offer the atomic species required for the catalytic reaction. We suggest that similar changes can also occur in other hydrogen-absorbing catalysts in the presence of hydrogen even at low hydrogen gas pressures and at room temperature. ORCID Vladimir Roddatis: 0000-0002-9584-0808 NOTES The authors declare no competing financial interests. ACKNOWLEDGMENT This research was supported by the Deutsche Forschungsgemeinschaft (DFG) via the Sonderforschungsbereich 1073 (TP C06, TP Z02, TP A01), by the Heisenberg grant PU131/9-2, and by the Czech Science Agency (project P108/12/G043).



SUPPORTING INFORMATION Experimental methods including sample preparation, X-ray diffraction, HRTEM of Pd foils in vacuum, HRTEM of Pd/r-TiO2 in hydrogen environment, and HR-STEM of 200 nm Pd/ r-TiO2 in different hydrogen environment investigations. Additional figures including evaluation of dislocations density from HR-STEM images, Molecular Dynamics and HRTEM simulations.

REFERENCES (1) Ertl, G.; Knözinger, H.; Weitkamp, J. Handbook of Heterogeneous Catalysis; VCH Verlagsgesellschaft mbH: Weinheim, Germany; 1997. (2) McCue, A. J.; Anderson, J. A. Recent Advances in Selective Acetylene Hydrogenation Using Palladium Containing Catalysts. Front. Chem. Sci. Eng., 2015, 9, 142-153. (3) Teschner, D.; Borsodi, J.; Wootsch, A.; Révay, Z.; Hävecker, M.; Knop-Gericke, A.; Jackson, S. D.; Schlögl, R. The Roles of Subsurface Carbon and Hydrogen in PalladiumCatalyzed Alkyne Hydrogenation, Science. 2008, 320, 86-89. (4) 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, 49, 13371-13375. (5) Aleksandrov, H. A.; Viñes, F.; Ludwig, W.; Schauermann, S.; Neyman, K. M. Tuning the Surface Chemistry of Pd by Atomic C and H: A Microscopic Picture. Chemistry – A European Journal, 2013, 19, 1335-1345. (6) Kaichev, V. V.; Miller, A. V.; Prosvirin, I. P. Bukhtiyarov, V. I. In situ XPS and MS Study of Methanol Decomposition and Oxidation on Pd (111) under Millibar Pressure Range. Sur. Sci., 2012, 606, 420-425.


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