Kinetic Evaluation of Highly Active Supported Gold Catalysts Prepared from Monolayer-Protected Clusters: An Experimental Michaelis−Menten Approach for Determining the Oxygen Binding Constant during CO Oxidation Catalysis

Abstract

Thiol monolayer-protected Au clusters (MPCs) were prepared using dendrimer templates, deposited onto a high-surface-area titania, and then the thiol stabilizers were removed under H2/N2. The resulting Au catalysts were characterized with transmission electron microscopy, X-ray photoelectron spectroscopy, and infrared spectroscopy of adsorbed CO. The Au catalysts prepared via this route displayed minimal particle agglomeration during the deposition and activation steps. Structural data obtained from the physical characterization of the Au catalysts were comparable to features exhibited from a traditionally prepared standard Au catalyst obtained from the World Gold Council (WGC). A differential kinetic study of CO oxidation catalysis by the MPC-prepared Au and the standard WGC catalyst showed that these two catalyst systems have essentially the same reaction order and Arrhenius apparent activation energies (28 kJ/mol). However, the MPC-prepared Au catalyst shows 50% greater activity for CO oxidation. Using a Michaelis−Menten approach, the oxygen binding constants for the two catalyst systems were determined and found to be essentially the same within experimental error. To our knowledge, this kinetic evaluation is the first experimental determination of oxygen binding by supported Au nanoparticle catalysts under working conditions. The values for the oxygen binding equilibrium constant obtained from the Michaelis−Menten treatment (ca. 29−39) are consistent with ultra-high-vacuum measurements on model catalyst systems and support density functional theory calculations for oxygen binding at corner or edge atoms on Au nanoparticles and clusters.

Enhanced Oxygen Activation over Supported Bimetallic Au−Ni Catalysts

“…By using this treatment, the MPC derived Au catalyst was shown to have essentially the same K R value as a traditionally prepared Au/TiO 2 catalyst; the only difference between the two catalysts was in a slightly larger ν max value for the MPC catalyst, which was concluded to have about 40% more active sites. 33 The double reciprocal plots predicted by eq 1 and shown in Figure 3C are linear, indicating that the kinetic data are well described by this treatment. The extracted kinetic parameters are plotted as a function of Ni content in Figure 4B (tabulated values are in the Supporting Information).…”

“…These conditions have been shown to maintain the catalysts under kinetic control. 33 These data are presented in Figure 3. The changes in the slopes of the lines fitted to the data in Figure 3 all show the NP Ni content has a dramatic effect on the kinetic parameters.…”

Adsorption of CO on Supported Gold Nanoparticle Catalysts: A Comparative Study

“…We recently reported an in-depth study of the preparation of the MPC catalyst as well as CO oxidation kinetics for both the WGC and MPC catalysts. 27 XPS and elemental analysis showed no residual sulfur on the MPC a Data based on infrared spectroscopy or temperature-programmed desorption (TPD) measurements. C-C is a Clausius-Clapeyeron treatment of infrared adsorption data.…”

“…Thus, the total number of CO adsorption sites is likely to be only a fraction of the low-coordinated corner and edge atoms. 18 Recent theoretical work suggests that the primary determinant for catalytic activity on Au nanoparticle catalysts is the availability of low-coordinated gold atoms, which are also likely to be important for oxygen binding and activation 27,45,46 The weak binding and activation of oxygen have long been considered keys to the high catalytic activity of supported Au catalysts, and there is a large body of experimental and theoretical literature suggesting that O 2 adsorption likely occurs on nanoparticle corner or edge sites. 2 This means that O 2 , which binds to Au much more weakly than CO, has to compete for similar binding sites.…”

“…27 A complete structural and kinetic characterization of the MPC and WGC catalysts (transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), kinetics of CO oxidation catalysis) has been reported previously. 27 Importantly, the reduction treatment showed no observable changes in particle size for the WGC catalyst. Changes in Au particle size for the MPC catalyst were small and within the error of the TEM measurements; thus, this pretreatment does not appear to induce sintering for Au/TiO 2 catalysts.…”