Insights into catalysis by gold nanoparticles and their support effects through surface science studies of model catalysts

Abstract

One important aid in understanding catalysis by gold nanoparticles would be to understand the strength with which they bond to different support materials and the strength with which they bond adsorbed intermediates, and how these strengths depend on nanoparticle size. We present here new measurements of adsorption energies by single crystal adsorption calorimetry, and new analyses of other recent measurements by this technique in our lab, which imply that: (1) small nanoparticles of metals like Au bind much more strongly to supports like titania and iron oxide which are generally observed to be effective in making Au nanoparticles active in catalysis than to supports like MgO which are considered less effective, (2) the thermodynamic stability of adsorbed intermediates for catalytic reactions can either increase strongly or decrease strongly with decreasing metal nanoparticle size below 8 nm, depending on the system, and (3) the reaction to insert O2 into the Au-H bond of adsorbed H on the Au(111) surface to make Au-OOH (O2,g + H(ad) --> OOH(ad)) is exothermic by -80 kJ mol(-1). This adsorbed hydroperoxy species is thought to be a key intermediate in selective oxidation reactions over Au nanoparticle catalysts, but its production by this reaction may also provide a route for O2 activation in less demanding reactions (like CO oxidation) as well. Its stability would be even higher on Au nanoparticles below 3 nm in diameter, but even there it is too unstable to be formed by combining adsorbed OH with an O adatom (OH(ad) + O(ad) --> OOH(ad)), which is estimated to be endothermic by 175 kJ mol(-1). The implications of the stability of metal nanoparticles versus particle size on different supports and of the stability and potential reactions of OOH(ad) in Au catalysis will be discussed.

Enhancing the catalytic activity of Ru NPs deposited with carbon species in yolk–shell nanostructures

“…44,[47][48][49][50] The apparent activation energy for toluene hydrogenation is 16.8 kJ mol À1 and 19.9 kJ mol À1 calculated with Arrhenius plots of Ru@C@YSN-500 and Ru@C@YSN-200, respectively (Fig. 55,56 In addition to toluene, Ru@C@YSN-500 could also efficiently catalyze the hydrogenation of aromatic rings with different substituents to afford 100% selectivity to the product with only the aromatic ring hydrogenated (Table 4). This is much lower than the activation energy reported for the transition-metal catalyzed hydrogenation of toluene (E a ¼ 30-50 kJ mol À1 ) that we found in the literature.…”

Tuning the Morphology of Metal Oxides for Catalytic Applications

“…In addition to the size effect of gold particles (Ͻ5 nm to achieve high activity), oxide supports affect their catalytic performance considerably and reducible oxides having highly mobile lattice oxygen are typically superior to the nonreducible ones. 11,[353][354][355] Among these systems, Au/CeO 2 catalysts have gained particular attention because of their exceptionally high activities in low-temperature CO oxidation and WGS reactions. These two probe reactions typically occur on the goldceria interface where the gold particle provides adsorption sites for CO and the ceria support supplies oxygen species generated from either molecular oxygen or water in the feed gases.…”

Tailored Nanoparticles for Clean Technology–Achieving Size and Shape Control

“…Many possible types of active species or crucial structural factors have been advocated-from small metallic particles [106], to ultrasmall nonmetallic particles [107], to ultrasmall anionic clusters [63], to oxidized gold particles [108], to an ensemble of metallic Au atoms and a cationic Au [109] or cationic species [110,111], to a prominent role of the type of support material (i.e., ''active'' and ''inert'' supports) [53,112], presence of specific functional groups or effect of moisture [109,[113][114][115], importance of the particle-support perimeter region [116], and even effects of the specific catalytic testing conditions [117]. Moreover, the thermodynamic stability of adsorbed intermediates depends on the specific catalyst system and could either increase or decrease as a function of particle size below 8 nm [112]. Importantly, the presence of the elusive ultrasmall Au nanoparticles, undetectable by conventional particle X-ray diffraction (PXRD) and transmission electron microscopy (TEM) techniques, in the case of catalysts made using popular supports (SiO 2 -TiO 2 ), has also been reported [118].…”