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

Abstract

The evolution of the Au 30 Pd 70 (110) surface was studied by coupling grazing incidence X-ray diffraction and mass spectrometry under oxygen-rich conditions at moderate temperatures (300 to 470 K). This allows us to correlate the depth profile of its structure to its catalytic properties for carbon monoxide (CO) oxidation. Under increasing pressure from ultrahigh vacuum up to 100 mbar, both oxygen and CO induce Pd segregation, even at room temperature. However, in pure oxygen the surface is reorganized with a (1 × 2) missing row reconstruction, whereas in pure CO it is strongly roughened. When oxygen pressure is increased a phase corresponding to the initial step of the oxidation with oxygen dissolution in the subsurface region appears at first. Then, from about 400 K onward, an oxidized thin Pd layer (≤1 nm) is formed growing in the [100] PdO direction. This PdO phase is strained and does not coincide with the P4 2 /mmc structure usually observed for this oxide under ambient conditions. It is more probably consistent with the high pressure I4/mmm PdO structure strained by epitaxy on the underneath alloy. For higher oxidizing conditions and layer thickness, the oxide will then relax to the usual PdO structure. This strained oxide is easily reduced by CO and exhibits a very high activity for CO oxidation. Its catalytic performance at 470 K is comparable to the one found on surfaces of pure palladium at higher temperatures. Furthermore, on the clean Au 30 Pd 70 (110) surface, surface oxidation is hindered up to 470 K if CO is introduced prior to oxygen. This indicates that when Pd is alloyed with gold, its binding with CO is stronger than with oxygen. The weakening of the Pd−O binding by surrounding gold atoms is the key of the formation of a well-ordered and very active thin PdO film on Au 30 Pd 70 (110).

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 Pd70Au30(110) (from SPL) is an unreconstructed Au-rich (~ 85-90%) surface under stable conditions after preparation cycles (ion bombardment followed by annealing). We have shown that upon exposure to CO, Pd segregates to the surface and the surface becomes rough (corrugation ~0.05 nm) [4]. In presence of oxygen, the surface reconstructs exhibiting a (nx2) structure ((1x2) with vacancies) and, as for CO, Pd segregates to the surface (Fig.…”

“…In the case of P(O2)/P(CO)>>10, SXRD shows that an oxide strained layer (< 10 atomic layers) is formed (Fig. 3) that is the active surface in such conditions [4]. Surface oxidation and its role in the CO oxidation strongly depend on the reaction conditions and for a same system surface oxidation can be either noxious or the best option to the reaction.…”

“…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.…”

Recent Progress on Rational Design of Bimetallic Pd Based Catalysts and Their Advanced Catalysis

“…However, there is a different reaction mechanism undergoes in CO oxidation during oxygen-rich conditions. Saint-Lager et al 149 reported that the surface segregation of CO-induced Pd clusters is dominant in the case of low O 2 /CO ratio; however, in the case of high O 2 /CO ratio, O 2 penetrated into the lattice to form the segregated PdO thin layer on the surface of AuPd alloys. Compared with the usual PdO phase, the AuPd@PdO thin layer showed similar lattice constants to the high-pressure tetragonal phase, which contracted in volume and thus led to the compressive strain.…”

Oxidative Removal of Volatile Organic Compounds over the Supported Bimetallic Catalysts

“…There are also many explorations on the reasons for the activity improvement of bimetallic catalysts. Saint-Lager et al [26] studied the changes of AuPd particles during the reaction process. The evolution of the Au30Pd70(110) surface was examined by means of coupling grazing incidence X-ray diffraction and mass spectrometry under the oxygen-rich conditions at moderate temperatures (27 to 197 o C), so that the depth profile of the structure could correlate to its catalytic properties for carbon monoxide oxidation (Figure 6).…”