Oxidation of Pd(553): From ultrahigh vacuum to atmospheric pressure

Abstract

The oxidation of a vicinal Pd͑553͒ surface has been studied from ultrahigh vacuum ͑UHV͒ to atmospheric oxygen pressures at elevated sample temperatures. The investigation combines traditional electron based UHV techniques such as high resolution core level spectroscopy, low-energy electron diffraction, scanning tunneling microscopy with in situ surface x-ray diffraction, and ab initio simulations. In this way, we show that the O atoms preferentially adsorb at the step edges at oxygen pressures below 10 −6 mbar and that the ͑553͒ surface is preserved. In the pressure range between 10 −6 and 1 mbar and at a sample temperature of 300-400°C, a surface oxide forms and rearranges the ͑553͒ surface facets and forming ͑332͒ facets. Most of the surface oxide can be described as a PdO͑101͒ plane, similar to what has been found previously on other Pd surfaces. However, in the present case, the surface oxide is reconstructed along the step edges, and the stability of this structure is discussed. In addition, the ͑ ͱ 6 ϫ ͱ 6͒ Pd 5 O 4 surface oxide can be observed on ͑111͒ terraces larger than those of the ͑332͒ terraces. Increasing the O pressure above 1 mbar results in the disappearance of the ͑332͒ facets and the formation of PdO bulk oxide.

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

“…After the rows have been shifted, the oxygen can be added in a similar way as for the c(2 × 4) structure on Pd(110). 8,21 In fact, the STM appearance is very similar to that of the c(2 × 4) on Pd(110). An oxygen induced shifted row has also been observed for the Pt 50 Rh 50 (100) surface.…”

“…At this point, we need to add the oxygen atoms, and here the uncertainty is high. A straightforward explanation for the twofold periodicity along the step would be oxygen atoms arranged in a zig-zag pattern below and above the Pd step atoms as observed on other vicinal surfaces at low oxygen coverage, 8,19 shown in Fig. 4(e) bottom left.…”

“…As expected, there is no signal from the O 1s level. 23 The decomposition of the clean surface Pd 3d spectrum reveals the presence of two components: the one on the higher binding-energy side is attributed to the bulk Pd atoms, while the peak on the lower binging-energy side arises from the undercoordinated step atoms, 8 exhibiting a shift (−0.37 eV) similar to that from the Pd(100) surface (−0.35 eV). 24 The Pd atoms at the terrace generate only a small surface core-level shift similar to that from Pd(111) 25 and therefore the corresponding signal is hidden under the bulk component.…”

“…As a step back in complexity as compared to nanoparticles, vicinal surfaces allow the investigation of step edges with well-defined adsorption geometry present in a high density on the surface. [7][8][9] The present study focuses on the interaction of oxygen with the Pd(112) surface to gain more insight into the stability of the (112) step geometry under oxygen exposure. In situ surface x-ray-diffraction (SXRD) measurements were performed at various oxygen pressures ranging from ultrahigh vacuum (UHV) to 5 mbars and temperatures between 523 and 673 K. Additional information was obtained by low-energy electron diffraction (LEED), scanning tunneling microscopy (STM) measurements, and high-resolution corelevel spectroscopy (HRCLS).…”

The ReactorSTM: Atomically resolved scanning tunneling microscopy under high-pressure, high-temperature catalytic reaction conditions

“…insights that go beyond the mere extrapolation of the lowpressure results. [3][4][5][6] This difference is often referred to as the "pressure gap." 7 Recently, several surface analysis techniques have been adapted to more realistic conditions.…”