Enhanced Oxygen Activation over Supported Bimetallic Au−Ni Catalysts

Abstract

New bimetallic Ni-Au supported nanoparticle catalysts were prepared by using dendrimer templated nanoparticles. Amine-terminated generation 5 polyamidoamine (PAMAM) dendrimers were anchored to a commercial silica with a siloxane linked anhydride. The dendrimer was then alkylated and used to template Ni-Au nanoparticles, which were subsequently extracted into organic solution as thiol monolayer protected clusters (MPCs). Transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS) indicated bimetallic nanoparticles of about 2 nm in size. Nanoparticles were deposited onto P-25 TiO 2 , and the capping thiol ligands were removed under flowing H 2 . DRIFTS infrared spectra of adsorbed CO showed only Au on the catalyst surface; no bands attributable to Ni or NiO were observed. Density functional theory (DFT) calculations showed that Au is substantially more stable than Ni on the surface of model slabs. DFT calculations also indicated that the incorporation of Ni into Au slabs results in stronger adsorption of O and CO on Au surfaces. Catalysts were evaluated with low-temperature CO oxidation. Kinetics studies indicated a substantial modification of Au catalysis through Ni incorporation. Apparent activation energies decreased by more than 50% and O 2 reaction orders increased from 0.2 to 0.9. These results are placed in the context of the available literature regarding support effects for Au catalysts. The observed changes to Au chemistry in the current work are substantially larger than previous reports have attributed to support effects. A Michaelis-Menten (enzyme) treatment of the kinetics data indicated that the O 2 reactivity constant increased by a factor of 40 for catalysts with high Ni content. This was in good qualitative agreement with the DFT calculations. At the same time, the introduction of Ni reduced the relative number of catalytically active sites.

Understanding the synergistic effects of gold bimetallic catalysts

“…Here we just give some examples highly relevant to catalysis by gold [25,40,76]. Dai and co-workers synthesized highly dispersed and uniform Au-Cu alloy particles by reduction of a Cu precursor on preformed Au/SiO 2 in an organic solvent in the presence of oleylamine and oleic acid [25].…”

“…They also used a colloid-based method to synthesize small and highly dispersed Au-Ni NPs supported on silica [40]. By using a dendrimer-templated-nanoparticle method, Chandler and coworkers synthesized silica-supported Au-Ni bimetallic nanoparticles with an average size of only 2 nm [76]. In all of these cases, calcination and/or reduction pretreatment is required to activate the catalysts, which exclusively results in phase segregation from Au-BM alloy to Au-BMO x heterostructure, corroborating our proposed models.…”

CO oxidation over Au/TiO2 catalyst: Pretreatment effects, catalyst deactivation, and carbonates production

“…Since the pretreatments appeared to induce few if any changes to the Au particles, we next consider potential changes to the catalytic active site using a Michelis-Menten (M-M) kinetic treatment that we have previously developed and employed [48,49,59]. In this treatment, activity data as a function of oxygen pressure can be evaluated with double reciprocal plots (Fig.…”

“…Similarly, m max depends both on the intrinsic reaction barrier and the number of active sites. This kinetic treatment has been previously published and has been shown to well describe kinetic data for CO oxidation over several Au [48,49] and bimetallic NiAu [59] catalysts, including a series of intentionally poisoned Au catalysts [48]. One of the primary utilities of the M-M kinetic treatment is that it provides measures of both the intrinsic activity of the active sites (K R ) and the total number of active sites (proportional to m max ).…”

Theoretical insights on the effect of reactive gas on the chemical ordering of gold-based alloys

“…In heterogeneous catalysis, bimetallic gold nanoparticles have shown high reactivity in a number of catalytic reactions including the direct synthesis of hydrogen peroxide from H 2 and O 2 [5,6], synthesis of vinyl acetate [7], selective hydrogenation of butadiene [8] and so forth. In the context of CO reforming, recent experiments on gold nanoparticles with different Ni contents show an improvement of the CO oxidation rate [9]. Palladium [10] and platinum [11] also emerge as good candidates for the enhancement of such reaction.…”