Oxygen interaction with the Pd(112) surface: From chemisorption to bulk oxide formation

We investigated the interaction of oxygen with the Pd(112) surface from ultrahigh vacuum up to 5 mbars oxygen partial pressure in a temperature range from 523 to 673 K. We combined in situ surface x-ray diffraction with scanning tunneling microscopy, high-resolution core-level spectroscopy, and low-energy electron diffraction. A structural model of the clean Pd(112) is proposed based on the x-ray-diffraction data. The morphology of the Pd(112) surface is strongly influenced by the oxidation conditions: at 673 K, upon exposure to oxygen at pressures from $2\times {10}^{-8}$ to $5\times {10}^{-5}$ mbar, the (112) surface undergoes a massive rearrangement and (113)- and (335)-type facets are formed. Further increase of the O${}_2$ partial pressure leads to a new rearrangement into (111)- and (113)-type facets. This is in contrast to the previous observation that (112) facets are stabilized on MgO supported Pd nanoparticles under oxygen exposure [P. Nolte, A. Stierle, N. Kasper, N. Y. Jin-Phillipp, N. Jeutter, and H. Dosch, Nano Lett. 11, 4697 (2011)]. Based on the core-level spectroscopy and scanning tunneling microscopy measurements, the transition from chemisorbed oxygen to surface oxide formation was identified to take place at pressures of 10${}^{-3}$ mbar O${}_2$ and 623 K. Kinetic barriers for the formation of the PdO bulk oxide are observed to be reduced compared to low index Pd surfaces.

Figure 1

(a) Schematic top and side view of the Pd(112) surface. The black rectangle marks the (112) surface unit cell used for indexing the x-ray data. The step atoms are represented by the light (blue) spheres. (b) ($1\times 1$) LEED pattern from the clean Pd(112) surface measured at an incident electron energy of 120 eV. (c) Experimental map of the diffracted intensity in the ($H,0,L$) plane measured for the clean Pd(112) surface at 673 K in UHV. (d) $H$ scan at $L=1.2$ and $K=0$ from the clean Pd(112) surface. The signal at $H\approx -2.4$ stems from some residual (111) facets.

Figure 2

Magnitude of the CTR structure factors of the clean Pd(112) surface recorded at 673 K (open circles, together with error bars). The solid (red) line is the best-fit structure factor, as described in the text. The structure factors calculated for an ideal Pd(112) surface are shown for comparison by the dashed (blue) line.

Figure 3

(a) Schematic top and side view of the Pd(113) and (335) surface. (b) Zipperlike periodic arrangement of (113) and (335) surfaces resulting in a (112) surface with every second atomic row missing along the [010] direction. The (113) and (335) surface unit cells are indicated by solid lines. The cross marks the position of the missing atoms. (c) Realistic model of the defective Pd(112) surface compatible with the x-ray-diffraction data. The local (113), (335), and (112) geometries are indicated by solid lines.

Figure 4

(a) $H$ scan at ($K=0$, $L=1.2$) measured at 673 K and $2\times {10}^{-8}$ mbar oxygen pressure. Half-order satellites indicate a doubling of the periodicity across the steps with long-range order. (b) ($2\times 2$) LEED pattern measured at 120 eV after oxidation at 623 K and $1\times {10}^{-8}$ mbar oxygen. (c) Room temperature STM image ($8\times 4$ nm${}^2$, 0.3 nA, 2 V) after oxidation at 623 K and $1\times {10}^{-8}$ mbar oxygen, where the (111) terraces were made flat (top), and the same image after applying a high-pass filter (bottom). (d) Line profile along the white line marked in (c). (e) The perfect Pd(112) surface at room temperature (top left), oxygen induced step bunching and shifting of rows (see text) into Pd(113) and Pd(335) steps (top right), and possible oxygen adsorption scenarios leading to a ($2\times 2$) reconstruction (see text) (bottom left and right). Oxygen atoms are represented by the small (red) spheres.

Figure 5

(a) $H$ scan at $K=0$, $L=1.2$ measured at 673 K and $1\times {10}^{-7}$ mbar oxygen. (b) Corresponding LEED pattern showing a ($1\times 9$) reconstruction.

Figure 6

(a) Experimental map of the diffracted intensity in the ($H,0,L$) plane measured at 673 K and $6\times {10}^{-7}$ mbar O${}_2$. (b) Schematic representation of the (113) and (335) types of facets (side view). (c) LEED image from the Pd(112) surface after exposure to $6\times {10}^{-7}$ mbar of oxygen at 623 K. The periodicity of the spots both along the $H$ and $K$ directions has changed due to faceting. (d) STM image ($V=0.5$ V, $I=0.1$ nA) illustrating the presence of the (113) and (335) facets ($40\times 40$ nm). (e) STM image ($V=0.5$ V, $I=0.1$ nA) from the (113) facets. (f) Model of the surface structure on the (113) facets, indicating the surface unit cell as well as the rectangular shape observed in the STM image in (e).

Figure 7

HRCL spectra from the Pd $3d^{5/2}$ and O $1s$ levels recorded from clean Pd(112) (bottom) and after exposure at 623 K and different oxygen pressures: $1\times {10}^{-7}$ mbar (middle) and $1\times {10}^{-3}$ mbar (top).

Figure 8

(a) Experimental reciprocal space maps in the ($H,0,L$) plane measured at 673 K and $1\times {10}^{-4}$ mbar O${}_2$. (b) Schematic representation of the (113) and (111) types of facets (side view). (c) LEED image from the Pd(112) surface after exposure to ${10}^{-3}$ mbar of oxygen at 623 K. (d) STM image ($V=0.5$ V, $I=0.3$ nA) illustrating the presence of (113) and (111) facets and the formation of the surface oxide ($50\times 50$ nm).

Figure 9

(a) Experimental map of the in-plane diffracted intensity in the ($H,K,0.1$) plane measured at 673 K and 1 mbar O${}_2$. Reflections arising from two main orientations of PdO were observed, i.e., (100) (black open squares) and (001) (black open circles), which are indexed in terms of the PdO bulk reciprocal lattice. (b) $H$ scan at $K=0$, $L=1.2$ measured at 523 K and different oxygen partial pressures from near UHV up to 5 mbars (open triangles, clean surface; solid lines, temporal evolution at 5 mbars). No new peaks appear until the pressure reaches 5 mbars, when the peak evolving at $H=-1.58$ marks the formation of PdO bulk oxide. In addition, a signal from (113) and (335) facets is observed.