Role of the nanoscale in catalytic CO oxidation by supported Au and Pt nanostructures

Experiments have found that the catalytic activity of Au increases sharply for supported nanoparticles smaller than $5\mathrm{nm}$, while Pt exhibits the opposite behavior. Several authors, seeking to explain the nanoscale Au activity, reached conflicting conclusions, attributing it to different nanoscale features or to a particular bilayer structure that is independent of size. Here, we report an extensive theoretical study of a large ensemble of $\mathrm{Ti}{\mathrm{O}}_2$-supported Au and Pt nanoparticles and show that several nanoscale features collectively result in the observed contrasting behavior. Low coordination is accompanied by bond weakening in Au and strengthening in Pt, while perimeter sites are active only for Au. Though there are orbital-occupancy differences for catalytically active and inactive configurations, there is insignificant variation in physical-charge transfer. Finally, we report atomically resolved $Z$-contrast images that confirm bond weakening in catalytically active $\mathrm{Ti}{\mathrm{O}}_2$-supported Au nanoparticles.

Figure 1

(Color online) Adsorption and reaction of ${\mathrm{O}}_2$ and CO molecules on $\mathrm{Ti}{\mathrm{O}}_2$-supported [(a)–(c)] Au and [(d)–(f)] Pt nanoparticles. Ti is shown in light gray, O in red, Au in yellow, Pt in blue, and C in dark gray. (a) The relaxed configuration of 11-atom supported Au nanoparticle before (upper panel) and after (lower panel) adsorption. Three Au–Au bonds are now so stretched that they are no longer shown in the schematic, facilitating the rotations and bending needed for the reaction. The O-O distance increases to $1.39\mathrm{\AA }$ here. (b) The reaction energy profiles and schematics of the nanoparticle at the transition state (arrowed). (c) The desorption energy $E_d$ of an ${\mathrm{O}}_2$ molecule and the reaction barrier $E_r$ as a function of the average coordination number $n$ of the two Au atoms to which the ${\mathrm{O}}_2$ molecule is attached. Points correspond to different adsorption sites and/or different nanoparticles located over oxygen vacancies in rutile (110) or anatase (101) surfaces. The average curves of $E_d$ and $E_r$ (bold solid lines) are shown. The lines $E_{d,p}$ and $E_{r,p}$ correspond to the desorption energy and the reaction barrier for the bridge-bond perimeter sites of Au clusters. [(d)–(f)] The corresponding figures for Pt nanoparticles. The O-O distance is $1.48\mathrm{\AA }$. However, no Pt–Pt bonds are eliminated by the adsorbed molecules resulting in a higher reaction barrier.

Figure 2

(Color online) Quasi-one-dimensional Au or Pt structures on $\mathrm{Ti}{\mathrm{O}}_2$. (a) ${\mathrm{O}}_2$ molecules preferentially form bridge bonds between perimeter Au sites and the substrate. The O–O bond in this configuration is stretched to $1.45\mathrm{\AA }$, which lowers the oxidation reaction barrier. (b) For a similar Pt structure, the ${\mathrm{O}}_2$ molecule survives without splitting on the top of the particle only (and splits at the perimeter sites).

Figure 3

(Color online) (a) The integrated radial charge around one of the O atoms in ${\mathrm{O}}_2$ molecules attached to Au (black, threefold coordinated; red, fivefold coordinated) and Pt atoms (green, fourfold coordinated). The horizontal line corresponds to the number of valence electrons per O atom and the vertical line to the atomic radius of an O atom (taken as $0.73\mathrm{\AA }$). (b) The ${\mathrm{O}}_2$-projected partial density of states (DOS) in a sphere of radius $0.73\mathrm{\AA }$ and the integrated DOS (number of states) per O atom.

Figure 4

(Color online) The probability of having a three-, four-, or five-coordinated Au atom on a nanoparticle of a particular diameter, obtained from our ensemble of supported nanoparticles. The solid curves are fitted exponentials.

Figure 5

(Color online) (a) Theoretical simulation of catalytic activity (in arbitrary units) for $\mathrm{Au}∕\mathrm{Ti}{\mathrm{O}}_2$ at $273\mathrm{K}$ (black) and for $\mathrm{Pt}∕\mathrm{Ti}{\mathrm{O}}_2$ at $437\mathrm{K}$ (red). (b) The two contributions (and their sum) to the total reaction rate for $\mathrm{Au}∕\mathrm{Ti}{\mathrm{O}}_2$ at $273\mathrm{K}$. The rates for the perimeter mechanism (blue) and for the low-coordination sites (green) show a crossover at $3\mathrm{nm}$. Below $2\mathrm{nm}$, the rate is dominated by low-coordination sites (inset). (c) Comparison of experimental size dependence (points) from Ref. 33 with the predicted dependence (curve). The theoretical rate curve is scaled arbitrarily.

Figure 6

(Color online) Bilayer Au structures on $\mathrm{Ti}{\mathrm{O}}_2$ and on a single $\mathrm{Ti}{\mathrm{O}}_x$ layer. (a) A Au bilayer structure on the rutile (110) surface with rows of oxygen vacancies. All Au atoms are sixfold and sevenfold coordinated. (b) The top monatomic Au chain in the bilayer structure is unstable against reconstruction, as shown, resulting in itinerant low-coordination sites. (c) Regular bilayer Au structure on a single $\mathrm{Ti}{\mathrm{O}}_x$ $(x=1)$ layer on Mo[112] structure (Mo is shown in turquoise) with compressed Au-Au distances as proposed by Chen and Goodman (Ref. 23). (d) The Au chains buckle when O atoms are introduced under the chains, and low-coordination Au sites (indicated by blue arrows) are created.

Figure 7

(Color online) High-magnification $Z$-contrast micrographs showing $10\mathrm{wt}\%$ loaded Au on anatase (from Ref. 39) after two stages of preparation. (a) In the oxidized precursor state following deposition-precipitation of Au, many individual Au atoms are sharply resolved. (b) In the most active form, after mild reduction in 12% ${\mathrm{H}}_2$ at $423\mathrm{K}$, individual neutral Au atoms are not resolved, consistent with structural fluxionality. (c) Histogram of the distribution of 103 Au-particle diameters in the reduced sample. Single atoms and particles over $10\mathrm{nm}$ were not included. Mean particle size was $1.1\pm 0.5\mathrm{nm}$. (d) Histogram of the approximate distribution of 32 selected Au-particle thicknesses in the reduced sample determined from relative intensities of $Z$-contrast images. Most particles are only 1 or 2 layers thick.