Construction and Reactivity of Pt-Based Bi-component Catalytic Systems

The rapid consumptions of coal, oil, and other fossil energies have resulted in globally environmental pollution and climate warming issues.

The work is mainly carried out in ultrahigh vacuum (UHV) systems and fixed-bed microreactor. The UHV systems include an Omicron Multi-NanoProbe System, a DUV-PEEM/LEEM System, and a Leybold XPS System. The fixed-bed microreactor for reactivity test of supported nanoparticles was designed and installed in laboratory.

We construct and study the catalytic properties of various Ni–Pt(111) model surfaces. We find the sandwich-like structure which consists of surface NiO_1−X and subsurface Ni performs best CO oxidation reactivity. The surface NiO_1−X provides active sites for O₂ dissociative adsorption. The subsurface Ni lowers the barrier for CO + O elemental reaction. As thus, the synergetic effect of surface NiO_1−X and subsurface Ni promotes CO oxidation on Pt. The effect of reduction temperature on the surface structure and reactivity of catalysts has often been overlooked. In our studies, we show the surface structure of Pt–Ni/CB nanoparticles can be simply modulated by reduction temperatures. The XANES investigations combined with ICP measurements indicate that the high reduction in temperature can induce more Ni diffuse inward. Upon the reduction at 523 K, the sandwich-like structure with half Ni on the surface and another half Ni inside the nanoparticle can be formed, which shows high CO oxidation reactivity.

We show that the cycling oxidative and reductive treatments at variable temperatures can reversibly alternate the surface structure and reactivity of Pt–Ni bicomponent catalysts. Low-temperature (~423 K) oxidation of Pt-skin structure (Pt/Ni/Pt(111)) induces part of Ni diffuse outward and form NiO on surface. After further oxidation at a higher temperature of 623 K, the catalysts are completely encapsulated by NiO. When the Pt@NiO core–shell structure is reduced at low temperature (~423 K), part of Ni starts to diffuse inward. Upon the reduction at a high temperature of 623 K, the formation of Pt-skin surface is observed. The catalysts pretreated at low temperatures show high CO oxidation reactivity due to the formation of the sandwich-like structure with surface and subsurface Ni species.

In this chapter, we compare the reactivity and stability of Pt–Fe and Pt–Ni bicomponent catalysts. The interfacial confinement effect results in the formation of monolayer-thick FeO_1−X and NiO_1−X nanoislands on Pt(111). The edge structures of the FeO_1−X and NiO_1−X nanoislands provide the active sites for O₂ dissociative adsorption, and thus promote CO oxidation reaction. But the stabilities of FeO_1−X/Pt(111) and NiO_1−X/Pt(111) systems are different after the oxidation at 473 K with a O₂ partial pressure of 1.3 × 10⁻⁶mbar. FeO_1−X nanoisland is oxidized to O–Fe–O trilayer structure after oxidation, while the chemical state of NiO_1−X is unchanged after same oxidative treatment. The result of model catalytic systems is well consistent with the observation of supported Pt–Fe/CB and Pt–Ni/CB catalysts. In situ XANES investigations show the chemical state of Fe is 2+ under CO oxidation with excess H₂, whereas the Fe is further oxidized in O₂-rich atmosphere. In contrast, the chemical state of Ni is constant under H₂-rich and O₂-rich CO oxidation conditions. Therefore, the CO conversion over Pt–Ni/CB catalyst increases when the concentration of O₂ is increasing.

By employing real-time microscopic techniques, the confinement effect of single-layer graphene on the chemistry of CO/Pt(111) system is studied. We find that the CO molecules can easily intercalate between graphene and Pt(111) even under UHV condition at room temperature. Interestingly, CO desorb from the interfacial space between graphene and Pt(111) around room temperature. In contrast, the desorption temperature of CO from bare Pt(111) is ~420 K. DFT calculations show the adsorption energy of CO on Pt(111) is decreased by the confinement effect of graphene on top. Furthermore, the dynamic process of CO oxidation is also studied. We find that the graphene wrinkles act as the reaction channels for CO desorption and CO oxidation processes.

This thesis focuses on the correlation between the structure and CO oxidation reactivity of Pt–Ni bicomponent catalysts from model catalytic systems to supported nanoparticle catalysts.