Oxygen-Induced Changes of the Au₃₀Pd₇₀(110) Surface Structure and Composition under Increasing O₂ Pressure

Abstract

The structure and composition of the Au30Pd70(110) surface were followed by grazing incidence X-ray diffraction under increasing oxygen pressure from ultrahigh vacuum (UHV) to 500 mbar at moderate temperatures (300–420 K). These measurements were complemented by Auger electron spectroscopy and environmental scanning tunneling microscopy (STM) images with atomic resolution up to 500 mbar. After the cleaning and preparation procedures in UHV, the Au30Pd70(110) surface is (1 × 1) with a quasipure topmost layer of segregated gold. Under oxygen pressure, Pd segregation occurs and progressively enriches the near-surface region. The surface evolves through different states that slightly differ from those of pure Pd(110). First, the (1 × 2) missing-row reconstruction was induced by oxygen adsorption that is stable in a large range of pressures depending on the temperature. Actually, STM images show regular vacancies every two atoms along the dense [11̅0] rows. Then, for higher oxygen pressures, a transition phase appears before the formation of an oxidized pure Pd film growing in the [100]PdO direction.

Carbon Monoxide Oxidation Promoted by a Highly Active Strained PdO Layer at the Surface of Au30Pd70(110)

“…The surface evolved through different phases as identified in our previous work. 19 In UHV, the clean surface is (1 × 1) (see the left inset of Figure 1b) and exhibits an intense surface peak at (0 1 0.05). When 2 mbar of oxygen were introduced, this peak rapidly decreased while a broad peak grew at K = 1.50 ± 0.01.…”

“…In this basis, α = β = γ = 90°, a 1 = a 3 = a 0 / 2, and a 2 = a 0 with a 0 = 3.942 Å (Au 30 Pd 70 lattice constant). 19 Unless otherwise mentioned, the Miller indices H, K, and L, in reciprocal space lattice unit (r.l.u.) of the Au 30 Pd 70 (110) surface frame, will be used.…”

“…In previous work, we studied the (110) surface of a Au 30 Pd 70 alloy and showed that Pd segregation occurred under pure oxygen and also pure CO pressure, even at room temperature. 18,19 Under near ambient pressure of oxygen, for temperatures above 420 K, an oxidized pure Pd layer grew in the [100] PdO direction. This PdO layer was strained and its structure not completely solved.…”

On Surface Oxidation Issues Arising during CO Oxidation on a PdAu(110) Surface. An In Situ Study by Complementary Environmental Methods (STM, SXRD, XPS)

“…The SXRD environmental system was developed by M.C. Saint-Lager et al to be operated in the ESRF BM32 beamline [3,4]. Both systems are capable of operating over 12 decades of gas pressure (10 -9 -10 3 mBar) and variable temperature.…”

Orientation-Dependent Hindrance to the Oxidation of Pd–Au Alloy Surfaces

“…The alloy surfaces are composed of the mixture of Pd and Au atoms, and the Au-rich surfaces are preferred in energetics. − Judged from surface-sensitive XPS measurements, the near-surface Pd fractions of the clean Pd 3 Au single-crystals are estimated with an analyzing depth of about 1 nm to be 52% for the (100) surface, whereas it is 36% for the (111) surface, indicating that these surface Pd/Au compositions are more Au-rich than those of bulk composition (Figure S1). The Pd-rich segregation on the (100) surface is also observed on a Pd–Rh alloy .…”