Direct Observation of Solvent-Reaction Intermediate Interactions in Heterogeneously Catalyzed Alcohol Coupling.

The relative stability of reactive intermediates and reactants on a surface, which dictates the rate and selectivity of catalytic reactions in both gas and liquid phases, is dependent on numerous factors. One well-established example is secondary interactions, such as van der Waals interactions between the catalyst surface and the pendant group of the intermediate, which can govern reaction selectivity for coupling reactions. Herein, we directly show that interactions between adsorbed reaction intermediates and reactant molecules increase the binding energy and affects the geometrical arrangement of coadsorbed reactant/solvent molecules. Evidence for this effect is demonstrated for the oxidative coupling reaction of methanol on a single crystal gold (Au(110)) surface. The rate-limiting reaction intermediate for methanol self-coupling, methoxy, stabilizes excess adsorbed methanol, which desorbs as a result of beta-hydride decomposition of the adsorbed methoxy. Direct molecular-scale imaging by scanning tunneling microscopy, supplemented by density functional theory, revealed interactive structures formed by methoxy and coadsorbed methanol. Interactions between the methoxy intermediate and coadsorbed methanol stabilizes a hydrogen-bonded network comprising methoxy and methanol by a minimum of 0.13 eV per methanol molecule. Inclusion of such interactions between reaction intermediates and coadsorbed reactants and solvents in kinetic models is important for microkinetic analysis of the rates and selectivities of catalytic reactions in both the gas and liquid phases whenever appreciable coverages of species from the ambient phase exist.

Toward benchmarking theoretical computations of elementary rate constants on catalytic surfaces: formate decomposition on Au and Cu.

With the emergence of methods for computing rate constants for elementary reaction steps of catalytic reactions, benchmarking their accuracy becomes important. The unimolecular dehydrogenation of adsorbed formate on metal surfaces serves as a prototype for comparing experiment and theory. Previously measured pre-exponential factors for CO2 formation from formate on metal surfaces, including Cu(110), are substantially higher than expected from the often used value of k B T/h, or ∼6 × 1012 s-1, suggesting that the entropy of the transition state is higher than that of the adsorbed formate. Herein, the rate constant parameters for formate decomposition on Au(110) and Cu(110) are addressed quantitatively by both experiment and theory and compared. A pre-exponential factor of 2.3 × 1014 s-1 was obtained experimentally on Au(110). DFT calculations revealed the most stable configuration of formate on both surfaces to be bidentate and the transition states to be less rigidly bound to the surface compared to the reactant state, resulting in a higher entropy of activation and a pre-exponential factor exceeding k B T/h. Though reasonable agreement is obtained between experiment and theory for the pre-exponential factors, the activation energies determined experimentally remain consistently higher than those computed by DFT using the GGA-PBE functional. This difference was largely erased when the metaGGA-SCAN functional was applied. This study provides insight into the underlying factors that result in the relatively high pre-exponential factors for unimolecular decomposition on metal surfaces generally, highlights the importance of mobility for the transition state, and offers vital information related to the direct use of DFT to predict rate constants for elementary reaction steps on metal surfaces.

Facilitating hydrogen atom migration via a dense phase on palladium islands to a surrounding silver surface.

The migration of species across interfaces can crucially affect the performance of heterogeneous catalysts. A key concept in using bimetallic catalysts for hydrogenation is that the active metal supplies hydrogen atoms to the host metal, where selective hydrogenation can then occur. Herein, we demonstrate that, following dihydrogen dissociation on palladium islands, hydrogen atoms migrate from palladium to silver, to which they are generally less strongly bound. This migration is driven by the population of weakly bound states on the palladium at high hydrogen atom coverages which are nearly isoenergetic with binding sites on the silver. The rate of hydrogen atom migration depends on the palladium-silver interface length, with smaller palladium islands more efficiently supplying hydrogen atoms to the silver. This study demonstrates that hydrogen atoms can migrate from a more strongly binding metal to a more weakly binding surface under special conditions, such as high dihydrogen pressure.

Quantitative measurement of biotin-binding immunoglobulin G in human sera using F(ab')2 anti-human IgG-coated multi-well plates.

Biotin-binding human immunoglobulin G (B-IgG) was quantitatively measured using an F(ab')2anti-human IgG-coated multi-well microplate for the first time. B-IgG was caught by F(ab')2anti-human IgG and was detected by the following detecting reagents: Peroxidase-labeled streptavidin, avidin and peroxidase-biotin, or avidin-biotinylated peroxidase complex with method A, B, or C, respectively. Commercially available B-IgG was detected by all these three methods. However, method A and B could not detect B-IgG in the human sera used in this study, but we quantitatively measured the B-IgG level using method C. The result is probably due to the fact that the sensitivity of method C was higher than that of methods A or B. Properties of B-IgG detected by method C are discussed in the text.