Hydrogen Atom Absorption in Pd(110): A density functional theory study

• Main Author: Padama, Allan Abraham B.
• Format: text
• Language: English
• Published: Animo Repository 2013
• Online Access: https://animorepository.dlsu.edu.ph/etd_doctoral/362
• Institution: De La Salle University

The absorption of H atom from the surface to the subsurface region of Pd(110) is investigated by employing density functional theory based calculations. The effect of the motion of the Pd atoms or the relaxation of the substrate atoms upon H absorption is identified by performing frozen lattice calculations and climbing image-nudged elastic band method. Frozen lattice calculation method calculates the total energies of the H / Pd(110) system for different positions of H, in which, the coordinates of the Pd atoms are kept fixed. The results enable the construction of potential energy surface wherein the energetically stable states of H, transition state and minimum energy paths are determined. The climbing image-nudged elastic band method, on the other hand, employs intermediate images from the initially determined stable states of H to determine the reaction path. In this method, the three-topmost layers of the Pd(110) and the H atom are allowed to move until the calculation converges to the minimum energy path. The effect of the motion of the Pd atoms to the absorption of H is determined by comparing the activation barriers and the binding energies of H. Vibrational frequencies of the absorbed and adsorbed H atom are calculated to obtain zero-point energy corrections. This energy correction arises from the contribution of the vibration of H in the presence of the Pd atoms as derived from harmonic approximation. In the surface, H is strongly adsorbed in the pseudo-threefold and long bridge sites with binding energies of -0.462 and -0.444 eV, respectively, where the more v negative values imply stronger binding energy. In the subsurface regions, the region below the topmost layer of the Pd(110), H is energetically more stable in the octahedral site than in the tetrahedral site. The calculated binding energies of H residing in the octahedral site of 1st subsurface and 2nd subsurface layers are -0.182 and -0.141 eV, respectively. On the other hand, the binding energy of H in the tetrahedral sites of 1st subsurface and 2nd subsurface layers are -0.106 and +0.021 eV, respectively. It was identified that the Pd atoms in the topmost layer strongly interact with H atom yielding a higher binding energy of H in the 1st subsurface layer than in 2nd subsurface layer. The results also verified that the absorption process of H on Pd is the rate-limiting process in the formation of Pd hydride. It was identified from previous works that the dissociation of H2 is accompanied by negligible activation barrier (Okuyama et al., 1995 Ledentu et al., 1998). However, as found in this study, the absorption of H is activated and therefore limits the formation of Pd metal hydrides. Nonetheless, the absorption of H in Pd(110) offers lower activation barrier compared to more compact low index surface facets, the (111) and (100) surface facets. The relaxation of substrate atoms gives stronger binding energy and lower activation barrier in comparison with the frozen lattice calculation case. In particular, comparable binding energies of H in the octahedral and tetrahedral sites in the 1st subsurface layer are obtained by considering the relaxation of the surface atoms. The absorption process from the surface to the subsurface region is accompanied by large activation barrier, whereas, a lower activation barrier is involved for the diffusion of H in vi a plane parallel to the surface plane. This phenomenon is attributed to the presence of vacuum which provides the strong tendency for the Pd atoms to move in the direction perpendicular to the surface as H diffuses. For the absorption process, conversely, Pd atoms tend to move in the direction parallel to the surface plane but are hindered by the presence of other Pd atoms. With zero-point energy correction, the binding energies were modified. The correction specifically identified that the octahedral site is the more energetically preferred position of H in agreement to the identified behavior of H in the bulk structure. The absorption path is qualitatively consistent with the analysis made for H absorption with surface relaxation, in which, high activation barriers are obtained for H absorption process while lower activation barriers are calculated for H diffusion along the plane parallel to the surface plane. From the obtained configurations of the absorbed H in Pd(110), it was identified that the absorption in the tetrahedral site can facilitate reconstruction of Pd(110). The reconstruction involves the migration of surface Pd atoms to other sites in the presence of adsorbate. The energetics of H atom residing in the tetrahedral and octahedral sites of the 1st subsurface layer with migrated Pd atom to other surface sites are compared. The tetrahedral site configuration is energetically more preferred and agrees with the experimentally proposed model of previous study (Yoshinobu et al., 1995).