Final Technical Report: First Principles Investigations for the Ensemble Effects of PdAu and PtAu Bimetallic Nanocatalysts Page: 2 of 10
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Project Results and Implications
Our results were summarized in 14 publications in international journals, as listed
below. In addition, we presented these results in APS meetings, international
conferences and other institutions.
Formation of Pd second neighborhoods on PdAu surfaces and clusters
Although the presence of different ensembles can be inspected using the
scanning tunneling microscope (STM), very few atomic resolution images have
been reported. More often, the existence of different ensembles are probed with
ethylene, CO and H, using various surface sensitive techniques such as infrared
reflection absorption spectroscopy (IRAS) and temperature programmed
desorption (TPD). It is known that Au tends to segregate toward bimetallic
surfaces with transition metals even at moderate temperatures due to differences
in their surface energies. However, the distribution morphology of PdAu surfaces
appears to be rather complex. For example, contradicting results were reported
for PdAu(111), including the ordered surface alloy phase, the Pd overlayer on
Au(111), and random Pd-Au(111) alloy surfaces with varying Pd ensembles.
From STM images and infrared data, Behm and coworkers concluded that the
critical ensemble for CO adsorption on PdAu(111) is Pd monomer. This
conclusion was supported by the extensive work of Goodman's groupError!
Bookmark not defined.. they found only one feature at 2087 cm-' in the IRAS for
CO/PdAu(111) when the sample is annealed up to 800 K.
To attain a clear understanding of ensemble formation on the PdAu(111) and
PdAu(001) surfaces, we performed systematic density functional calculations.
The thermal stability of different Pd configurations on the surface is characterized
by the formation energy as
AEPd =-[EdAu - EAslb + NPd (Eubuk - EPd,,A)]/NPd (1)
Here, EPdAU, EAU-Surf, EAu-bulk and Epd-atom represent the total energies of the PdAu
surface, clean Au slab, bulk gold (per atom), and isolated Pd atom, respectively.
NPd is the number of surface Pd atoms in the unit cell. As seen in Fig. 1 for a few
selected surface ensembles on PdAu(111), our results explained the absence of
nearest Pd neighborhoods since they produce lower formation energies. The Pd-
Au bond is found to be slightly ionic, and is stronger than Au-Au and even Pd-Pd
bonds. As a consequence, Pd atoms tend to take positions where the number of
Pd-Au bonds maximizes. Although the disturbance of Pd substituent is effectively
screened by the Au lattice, not much beyond its first neighborhoods in plots of
charge density and density of states, the formation of second neighbor Pd
ensembles is slightly preferential compare to isolated Pd monomers.
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Wu, Ruqian. Final Technical Report: First Principles Investigations for the Ensemble Effects of PdAu and PtAu Bimetallic Nanocatalysts, report, May 18, 2012; United States. (https://digital.library.unt.edu/ark:/67531/metadc835709/m1/2/: accessed April 19, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.