Probing the Limits of d-Band Center Theory: Electronic and Electrocatalytic Properties of Pd-Shell–Pt-Core Nanoparticles

Abstract

Theoretical DFT calculations suggest that chemisorption energy, activation barrier, and energy of dissociation of small molecules on metal surface can be correlated to the d-band center of gravity of that metal. This holds true for many systems and reactions, but there are also reports where significant discrepancies were found. Here we present the critical assessment of applicability of the d-band center theory to nonuniform catalytic systems, such as core−shell nanoparticles. For Pt-core−Pd-shell nanoparticles we found a significant enhancement of catalytic activity toward formic acid oxidation, which was assigned to observed changes of density of states close to the Fermi level, in general in agreement with d-band center theory. However, at the same time the changes in dband center for Pt-core−Pd-shell nanoparticles were contrary to those predicted by theory due to incorporation of Pt valence electrons to the overall band structure, which shifted the d-band center in the direction opposite to that predicted by the theory. Our data stress the role of experimental determination of electronic properties of catalytic systems when explaining the observed catalytic activity, as real systems can be more complicated than the one used for theoretical calculations. As a result the changes in d-band center energy must be used with care for explanation of the observed changes in catalytic activity. We show that for nonuniform systems density of states close to the Fermi level is a better predictor of chemisorption strength and catalytic activity than the d-band center. ■ INTRODUCTIONEstablishing the relationship between surface electronic structure and its catalytic activity would revolutionize many branches of science and technology. Among many fundamental studies contributing to better understanding how the electronic properties influence the catalytic activity, one of the most spectacular is the d-band center theory, developed by Norskov and co-workers, correlating the energy of the d-band center of gravity (ε d ) of the metal catalyst to the adsorption energy, activation energy, and dissociation energy of small molecules. 1−3 According to that theory the electron density of states (DOS) close to the Fermi level is correlated to the adsorbate− substrate adsorption energy due to interactions between electrons occupying d-type orbitals of the metal (d-band) and those of the adsorbate. As a result modifying the d-band DOS (d-DOS) should influence the adsorption strength and catalytic activity of the metal surface. To reflect the electronic properties of the d-band (d-DOS) ε d was proposed. According to d-band center theory, ε d can be altered, i.e., as a result of lattice strain. Changing the lattice parameters influences the degree of overlapping between d orbitals forming a d-band in metal and as a result causing narrowing or widening of the d-band and shifting ε d toward or away from the Fermi level. 1−3 There have been significant efforts to verify the d-band center theory. The effect of lattice strain on the d-ba...

Methanol Oxidation Reaction Performance on Graphene‐Supported PtAg Alloy Nanocatalyst: Contrastive Study of Electronic and Geometric Effects Induced from Ag Doping

“…It has been reported that the Pt core‐level (CL) binding energy is highly sensitive to d ‐band structure . and the Pt 4f 7/2 CL binding energy shift is a good indicator (“fringerprint”) for the shift of the occupied d ‐band center .…”

“…It has been reported that the Pt core-level (CL) binding energy is highly sensitive to d-band structure. [12] and the Pt 4f 7/2 CL binding energy shift is a good indicator ("fringerprint") for the shift of the occupied d-band center. [37] So X-ray photoelectron spectroscopy (XPS) was used to investigate the Pt electronic state in PtAg/graphene, which greatly influenced its catalytic performance.…”

“…In PtM alloy, the geometric effect comes from the orbital overlap changes . When metallic Pt is alloyed with metals with smaller lattice constants, it gives rise to contraction strain, and then the d ‐orbital overlaps increase, resulting in a downshift of d ‐band center . The electronic effect originates from the electron transfer between Pt and secondary metal in PtM alloy.…”

“…[11] When metallic Pt is alloyed with metals with smaller lattice constants, it gives rise to contraction strain, and then the dorbital overlaps increase, resulting in a downshift of d-band center. [12] The electronic effect originates from the electron transfer between Pt and secondary metal in PtM alloy. If the secondary metal has a much lower electronegativity, the electron transfer from M to Pt takes place, and it also results in a downshift of d-band center.…”

Core–Shell-Structured Low-Platinum Electrocatalysts for Fuel Cell Applications

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