Energetics of Cu Adsorption and Adhesion onto Reduced CeO₂(111) Surfaces by Calorimetry

Abstract

The morphology and interfacial energetics of vapor-deposited Cu on slightly reduced CeO 2 (111) surfaces at 300 K have been studied using single crystal adsorption calorimetry (SCAC), He + low-energy ion scattering spectroscopy (ISS), Xray photoelectron spectroscopy (XPS), and low energy electron diffraction (LEED). Copper grows as three-dimensional nanoparticles with a density of ∼10 13 particles/cm 2 on CeO 2−x (111) (x = 0.05, 0.1, and 0.2). The initial heat of adsorption of Cu decreased with the extent of reduction, showing that stoichiometric ceria adsorbs Cu more strongly than oxygen vacancies. On CeO 1.95 (111), the heat dropped quickly with coverage in the first 0.1 ML, attributed to nucleation of Cu clusters on stoichiometric steps, followed by the Cu particles spreading onto less favorable sites (step vacancies and terraces). Above ∼0.1 ML (>0.8 nm in diameter), the Cu adsorption energies showed no variation with extent of ceria reduction: the heat of adsorption increased slowly with coverage (particle size) due to the formation of more Cu−Cu bonds per adatom as the size grows, finally approaching the heat of sublimation of bulk Cu by 3.5 ML (2.5 nm). The adhesion energy of Cu(solid) to CeO 1.95 (111) was found to be 3.52 J/m 2 for 2.2 nm diameter particles, decreasing slightly with the extent of reduction. The Ce 3d XPS line shape showed an increase in the Ce 3+ /Ce 4+ ratio with Cu coverage, corresponding to donation of at most ∼0.17 and 0.06 electrons per Cu atom to CeO 1.95 (111) and CeO 1.8 (111), respectively. INTRODUCTIONHeterogeneous catalysts are generally composed of late transition metal nanoparticles dispersed over high surface area oxide supports. The interaction of the supported metal and underlying oxide can greatly influence catalytic properties such as long-term sinter resistance, activity, and selectivity. To improve our understanding of how the choice of metal and support can influence catalytic properties, detailed studies of model systems, where metal atoms are vapor deposited onto single crystal oxide supports, are often employed. With these model systems, the structure of the support surface, the size of the metal particles, and surface cleanliness can be better controlled. 1−5 Studies of this type provide the basic understanding necessary for the intelligent design of new, more efficient, and greener catalysts. Here we apply that approach to study model Cu/CeO 2 catalysts consisting of Cu nanoparticles grown by vapor deposition on CeO 2 (111) surfaces with controlled extents of reduction.We study the energies of the Cu atoms in this system using single crystal adsorption calorimetry (SCAC). This method directly measures the adsorption energy of the incoming metal atoms as they bind to the oxide surface, and to metal nanoparticles on that surface as they grow in size. 6−11 The Cu nanoparticle morphology is characterized using ion scattering spectroscopy (ISS) and X-ray photoelectron spectroscopy (XPS). Adsorption energies measured using SCAC along with detailed adsorbate structura...

Structure of the catalytically active copper–ceria interfacial perimeter

“…The strong copper-ceria interaction for constructing the metal-support interface proceeds via a unique pattern, which largely differs from the case of precious metals on the same substrate, ceria. Unlike Au and Pt nanoparticles that form well-defined crystalline structures on ceria [27][28][29][30] , copper locates mainly at the step edges, instead of the terraces, on reduced ceria surfaces 21 , forming irregular, small clusters that are very difficult to be imaged by electron microscopy because of the much lower degree of crystallinity and the rather poor contrast between copper and cerium 5 .…”

“…Theoretical investigations on the growth of copper atoms/clusters on reduced ceria surfaces have verified that the Cu-O-Ce interfacial interaction is energetically comparable to the Cu-Cu intracluster interaction; the competition between them determines the morphology of Cu clusters19,20 . The strong binding between copper and the reduced ceria surface, especially on the defect sites, was addressed both experimentally and theoretically with regards to the reduction degree of the ceria surface and the copper coverage21,22,43 . Onone hand, the Cu-Cu intracluster interaction, i.e.…”

Ni Nanoparticles on CeO2 (111): Energetics, Electron Transfer and Structure by Ni Adsorption Calorimetry, Spectroscopies and DFT

“…As seen, the chemical potential generally decreases with increasing particle size, as has been reported for many related systems, and is largely related to the increasing number of metal-metal bonds per atom with increasing size. [26][27][28]60 The chemical potential initially increases with increasing particle size for CeO1.95 (111) at 300 K, due to the initial population of more stable sites at step edges, which saturate quickly as coverage increases.…”

“…The percentage of Ce 3+ in the Ce 3d XPS probe depth (~1 nm) was determined from lineshape fitting of the XPS Ce 3d peak as described previously. 26,61 The Ce 3+ percentage is plotted with respect to Ni coverage in Figure 4.…”

“…Using the number of Ce atoms per unit area in this probe depth (2.5 × 10 15 Ce atoms per cm 2 ), the data point in Figure 4 at each Ni coverage can then be converted to the average number of electrons donated per Ni atom, as done previously for Cu on this same surface. 26 If we further assume that Ni can only be in the form of neutral Ni or Ni 2+ , this average number of electrons donated per Ni atom can be converted to the fraction of total Ni that is oxidized to Ni 2+ . Figure 5a shows the resulting number of electrons donated per Ni atom and fraction of Ni 2+ , calculated based on data in Figure 4, plotted versus Ni coverage.…”