Surface Reactivity of Pd Nanoparticles Supported on Polycrystalline Substrates As Compared to Thin Film Model Catalysts:  Infrared Study of CO Adsorption

Abstract

To identify the nature and the local structure of the surface of supported catalyst nanoparticles, we have performed a detailed comparative study of CO adsorption on two categories of oxide-supported Palladium catalysts: (1) polycrystalline MgO and γ-Al2O3 supported Pd metal catalysts prepared by impregnation techniques and characterized by different degrees of regularity and perfection and (2) single-crystal based Pd model catalysts prepared under ultrahigh vacuum (UHV) conditions. The assignment of the CO vibrational frequencies to different types of sites on these systems has allowed a detailed structural characterization. For the Pd model catalyst, at low CO coverage, the infrared (IR) reflection absorption spectra closely resemble the expected behavior for terminations by a majority of (111) facets and a minority of (100) facets. The spectral features are indicative of defect sites such as particle steps and edges. Occupation of the defect sites can be affected by surface contaminations such as atomic carbon. Thus the CO spectra at high coverage can be used as both a structural and chemical probe under reaction conditions, provided that complementary information on the particle morphology is available. For the MgO and γ-Al2O3 supported Pd systems, two distinct narrow bands (ν ≅ 2070 and ≅ 1970 cm-1) have been assigned to linearly bonded and bridge-bonded CO species, on Pd (100)/(111) edges or facets, in agreement with the previous results obtained on model catalysts. The broad character of the 2070 cm-1 feature indicates the simultaneous presence of (100) and (111) faces, with edge and corner sites present at their intersection. The high intensity and the small half-width (fwhm) of the band at 1970 cm-1 on a Pd/MgO sample treated at high temperature, assigned to bridge-bonded CO species, suggests that the metal particles expose faces with a high level of regularity. Further spectroscopic features (ν ≅ 1920−1800 cm-1), are ascribed to the presence of different types of 3-fold hollow sites on (111) faces.

Transition‐metal nanoparticles: synthesis, stability and the leaching issue

“…Recent reports focused on the catalytic properties of NPs supported on metal oxides. [182] The metals are from Si, [183] Al, [184] Ti [185] or Zr [186] to Ca, [187] Mg [188] and Zn [189] . The majority of them, and the most extensively used, contain a form of silica.…”

In situ surface characterization of the intermetallic compound PdGa – A highly selective hydrogenation catalyst

“…In the first case, the full isolation of the Pd-CO was not achieved because at low CO partial pressures the bridged Pd 2 -CO species becomes energetically more favorable. However, a significant red shift of ~30 cm -1 was detected for single crystals and nanoparticulate Pd samples [23][24][25][26][27]. Isolating the Pd atoms by alloying Pd with Ag leads to the formation of a disordered Pd-Ag alloy and causes a red shift of the on-top Pd-CO band from 2078 cm -1 for Pd/SiO 2 to 2048 cm -1 and 2034 cm -1 for different PdAg/SiO 2 samples depending on the Pd:Ag ratio.…”

“…Such an isolation of the adsorption centers can lead to a significant red shift of the CO vibrational frequency [23][24][25][26][27][28]. Up to now, the isolation has either been achieved by decreasing the CO partial pressure [23][24][25][26][27] or by alloying Pd with another metal, e.g. Ag [28].…”

Oxygen-induced Restructuring of a Pd/Fe3O4 Model Catalyst

“…The use of CO as a probe molecule allows us to monitor morphological changes of the Pd particles in great detail (compare e.g. [36]). In figure 4, IRAS spectra of adsorbed CO at 125 K after different preparation steps are shown.…”