Pd (core)-Au (shell) nanoparticles catalyzed conversion of NADH to NAD+ by UV-vis spectroscopy--a kinetic analysis

Kinetics of Pd (core)-Au (shell) nanoparticles (NPs) catalyzed transformation of dihydronicotinamide adenine dinucleotide (NADH) to NAD(+) was monitored by UV-vis spectroscopy. Pd (core)-Au (shell) NPs were prepared by microwave irradiation method. High resolution transmission electron microscopy image reveals the core-shell morphology. X-ray diffraction pattern shows the presence of distinct crystalline domains for Pd and Au. The changes in absorbances at 340 nm were followed for various time intervals. Rates of conversion of NADH to NAD(+) were determined for different conditions. The conversion of NADH to NAD(+) was to be first order with respect to NADH at lower concentrations (upto 0.04 mM) and pseudo-first-order beyond 0.04 mM. Rate constants for the Pd (core) Au-(shell) NPs catalyzed transformation of NADH to NAD(+) were deduced.

Chlorine interaction with defects on the Au(111) surface: a first-principles theoretical investigation

Chlorine is an important element in promoting oxidation on noble metal surfaces. Here, we report a comprehensive theoretical study of chlorine interaction with defects on the Au(111) surface, using density functional theory calculations and periodic slabs to model the surface. We find that chlorine binds preferentially on steps, vacancies, and gold adatoms. The increase in binding energy per chlorine atom, compared to binding on the flat, defect-free surface, is 0.29 eV when the chlorine atom is on top of a gold adatom, 0.38 eV when it is at the edge of a step, and 0.19 eV when it is next to a single surface vacancy. An extensive study of chlorine interaction with different numbers of surface gold vacancies revealed that chlorine interacts the strongest with three vacancies.

Sustainability in catalytic oxidation: an alternative approach or a structural evolution?

This Review documents some examples of recent innovations in the field of catalytic selective oxidation from an industrial point of view. The use of alkanes as building blocks for the synthesis of bulk chemicals and intermediates is discussed, along with the main properties that catalysts should possess in order to efficiently catalyse the selective oxidation of these hydrocarbons. The currently developed processes for propene oxide and new processes under investigation for the synthesis of adipic acid are also described, highlighting innovative aspects for a better sustainability of the chemical industry.

Catalysis by gold dispersed on supports: the importance of cationic gold

There are many examples of catalysis in solution by cationic complexes of gold, and recent results, reviewed here in this critical review, demonstrate that cationic gold species on oxide and zeolite supports are also catalytically active, for reactions including ethylene hydrogenation and CO oxidation. The catalytically active gold species on supports are evidently not restricted to isolated mononuclear gold complexes, but include gold clusters, which for at least some reactions are more active than the mononuclear complexes and for some reactions less active. Fundamental questions remain about the nature of cationic gold in supported catalysts, such as the nature of the cationic gold clusters and the nature of gold atoms at metal-support interfaces (88 references).

Density functional study of the interaction between small Au clusters, Aun (n=1–7) and the rutile TiO2 surface. I. Adsorption on the stoichiometric surface

This is the first paper in a series of four dealing with the adsorption site, electronic structure, and chemistry of small Au clusters, Au(n) (n=1-7), supported on stoichiometric, partially reduced, or partially hydroxylated rutile TiO(2)(110) surfaces. Analysis of the electronic structure reveals that the main contribution to the binding energy is the overlap between the highest occupied molecular orbitals of Au clusters and the Kohn-Sham orbitals localized on the bridging and the in-plane oxygen of the rutile TiO(2)(110) surface. The structure of adsorbed Au(n) differs from that in the gas phase mostly because the cluster wants to maximize this orbital overlap and to increase the number of Au-O bonds. For example, the equilibrium structures of Au(5) and Au(7) are planar in the gas phase, while the adsorbed Au(5) has a distorted two-dimensional structure and the adsorbed Au(7) is three-dimensional. The dissociation of an adsorbed cluster into two adsorbed fragments is endothermic, for all clusters, by at least 0.8 eV. This does not mean that the gas-phase clusters hitting the surface with kinetic energy greater than 0.8 eV will fragment. To place enough energy in the reaction coordinate for fragmentation, the impact kinetic energy needs to be substantially higher than 0.8 eV. We have also calculated the interaction energy between all pairs of Au clusters. These interactions are small except when a Au monomer is coadsorbed with a Au(n) with odd n. In this case the interaction energy is of the order of 0.7 eV and the two clusters interact through the support even when they are fairly far apart. This happens because the adsorption of a Au(n) cluster places electrons in the states of the bottom of the conduction band and these electrons help the Au monomer to bind to the five-coordinated Ti atoms on the surface.

CO adsorption on pure and binary-alloy gold clusters: a quantum chemical study

We performed density-functional theory analysis of nondissociative CO adsorption on 22 binary Au-alloy (Au(n)M(m)) clusters: n=0-3, m=0-3, and m+n=2 (dimers) or 3 (trimers), M=Cu/Ag/Pd/Pt. We report basis-set superposition error corrections to adsorption energies and include both internal energy of adsorption (DeltaU(ads)) and Gibbs free energy of adsorption (DeltaG(ads)) at standard conditions (298.15 K and 1 atm). We found onefold (atop) CO binding on all the clusters except Pd2 (twofold/bridged), Pt2 (twofold/bridged), and Pd3 (threefold). In agreement with the experimental results, we found that CO adsorption is thermodynamically favorable on pure Au/Cu clusters but not on pure Ag clusters and also observed the following adsorption affinity trend: Pd>Pt>Au>Cu>Ag. For alloy dimers we found the following patterns: Au2>M Au>M2 (M=Ag/Cu) and M2>M Au>Au2 (M=Pd/Pt). Alloying Ag/Cu dimers with (more reactive) Au enhanced adsorption and the opposite effect was observed for PdPt dimers. The Ag-Au, Cu-Au, and Pd-Au trimers followed the trends observed on dimers: Au3>M Au2>M2Au>M3 (M=Ag/Cu) and Pd3>Pd2Au>PdAu2>Au3. Interestingly, Pt-Au trimers reacted differently and alloying with Au systematically increased the adsorption affinity: PtAu2>Pt2Au>Pt3>Au3. A strikingly different behavior of Pt is also manifested by the triplet spin state and onefold (atop) binding in Pt3-CO which is in contradiction with the singlet spin state and threefold binding in Pd3-CO. We found a linear correlation between CO binding energy (BE) and elongation of the CO bond. For Ag-Au and Cu-Au clusters, the increase in CO BE (and elongation of the C-O bond which is probably due to the back donation) is accompanied by the decrease in the cluster-CO distance suggesting that the donation (from 5sigma highest occupied molecular orbital in CO to cluster lowest unoccupied molecular orbital) mechanism also contributes to the BE. For Pd-Au clusters, the cluster-CO distance (and CO bond length) increases with increase in the BE, suggesting that the donation mechanism may not be important for those clusters. No clear trend was observed for Pt-Au clusters.

Analysis of O2 Adsorption on Binary−Alloy Clusters of Gold: Energetics and Correlations

We report a B3LYP density-functional theory (DFT) analysis of O(2) adsorption on 27 Au(n)M(m) (m, n = 0-3 and m + n = 2 or 3; M = Cu, Ag, Pd, Pt, and Na) clusters. The LANL2DZ pseudopotential and corresponding double-zeta basis set was used for heavy atoms, while a 6-311+G(3df) basis set was used for Na and O. We employed basis-set superposition error (BSSE) corrections in the electronic adsorption energies at 0 K (deltaE(ads)) and also calculated adsorption thermodynamics at standard conditions (298.15 K and 1 atm), i.e., internal energy of adsorption (deltaU(ads)) and Gibbs free energy of adsorption (deltaG(ads)). Natural Bond Orbital (NBO) analysis showed that all the clusters donated electron density to adsorbed O(2) and we successfully predicted intuitive linear correlations between the NBO charge on adsorbed O(2), O-O bond length, and O-O stretching frequency. Although there was no clear trend in the O(2) binding energy (BE = -deltaE(ads)) on pure and alloy dimers, we found the following interesting trend for trimers: BE (MAu(2)) < BE (M(3)) < or = BE (M(2)Au). The alloy trimers containing only one Au atom are most reactive toward O(2) while those with two Au atoms are least reactive. These trends are discussed in the context of the ensemble effect and coulomb interactions. We found an approximate linear correlation between the O(2) BE and charge transfer to O(2) for all 27 clusters. The clusters having strongly electropositive Na atoms (e.g., Na(3) and Na(2)Au) donated almost one full electron to adsorbed O(2), and the BE is maximum on these clusters. Although O(2) dissociation is likely in such cases, we have restricted this study to trends in the adsorption of molecular O(2) only. We also found an approximate linear correlation between the charge transfer and BE versus energy difference between the bare-cluster HOMO and O(2) LUMOs, which we speculate to be a fundamental descriptor of the reactivity of small clusters toward O(2). Part of the scatter in these correlations is attributed to the differences in the O(2) binding orientations on different clusters (geometric effect). Relatively higher bare-cluster HOMO energy eases the charge transfer to adsorbed O(2) and enhances the reactivity toward O(2). The Frontier Orbital Picture (FOP) is not always useful in predicting the most favorable O(2) binding site on clusters. It successfully predicted the cluster-O(2) ground-state configurations for 10 clusters, but failed for the others. Finally, the energetics of fragmentation suggest that the bare and O(2)-covered clusters reported here are stable.

A Quantum Chemical Study of Comparison of Various Propylene Epoxidation Mechanisms Using H2O2and TS-1 Catalyst

We present a comparison of the following prominent propylene epoxidation mechanisms using H2O2/TS-1 at a consistent density functional theory (DFT) method: (1) the Sinclair and Catlow mechanism on tripodal site through Ti-OOH species, (2) the Vayssilov and van Santen mechanism on tetrapodal site without Ti-OOH formation, (3) the Munakata et al. mechanism involving peroxy (Ti-O-O-Si) species, (4) the defect site mechanism with a partial silanol nest, and (5) the defect site mechanism with a full silanol nest. We have reproduced the previously published (ethylene epoxidation) pathways (1-3) for propylene epoxidation using larger and SiH3-terminated cluster models of the T-6 crystallographic site of TS-1. Mechanism 5 is a new mechanism reported here for the first time. The use of a consistent level of theory for all the pathways allows for the first time a meaningful comparison of the energetics representing the aforementioned pathways. We have rigorously identified the important reaction intermediates and transition states and carried out a detailed thermochemical analysis at 298.15 K and 1 atm. On the basis of the Gibbs free energy of activation, the Sinclair and Catlow mechanism (Delta G(act) = 7.9 kcal/mol) is the energetically most favorable mechanism, which is, however, likely to operate on the external surface of TS-1 due to the tripodal nature of the Ti site in their model. The newly reported defect site mechanism (with a full silanol nest) is a competitive propylene epoxidation mechanism. There are two main steps: (1) hydroperoxy formation (Delta G(act) = 8.9 kcal/mol) and (2) propylene epoxidation (Delta G(act) = 4.6 kcal/mol). This mechanism is likely to represent the chemistry occurring inside the TS-1 pores in the liquid-phase epoxidation (H2O2/TS-1) process and could operate in direct gas-phase epoxidation (H2/O2/Au/TS-1) as well. If only the propylene epoxidation step is considered, then the Munakata peroxo intermediate (Si-O-O-Ti) is the most reactive intermediate, which can epoxidize propylene with a negligible activation barrier. However, formation of the Munakata intermediate is a very activated step (Delta G(act) = 19.8 kcal/mol). We also explain the trends in the activation barriers in different mechanisms using geometric and electronic features such as orientation of adsorbed H2O2 and propylene, hydrogen bonding, O1-Ti bond distance in the Ti-O1-O2-H intermediate, and O1-O2 stretching in the transition state. Implications of different Ti site models are also discussed in light of the nature of external/internal and nondefect/defect sites of TS-1.

Adsorption of Small Aun (n = 1−5) and Au−Pd Clusters Inside the TS-1 and S-1 Pores

We used a hybrid quantum-mechanics/molecular-mechanics (QM/MM) approach to simulate the adsorption of Au(n)() (n = 1-5), AuPd, and Au(2)Pd(2) clusters inside the TS-1 and S-1 pores. We studied nondefect and metal-vacancy defect sites in TS-1 and S-1 for a total of four different environments around the T6 crystallographic site. We predict stronger binding of all clusters near Ti sites in Ti-substituted framework compared to adsorption near Si sites-consistent with the experimental finding of a direct correlation between the Ti-loading and the Au-loading on the Au/TS-1 catalysts with high Si/Ti ratio. The cluster binding is also stronger near lattice-metal vacancies compared to fully coordinated, nondefect sites. In all the cases, a trend of binding energy (BE) versus Au cluster size (n) shows a peak at around n = 3-4. Our results show that there is enough room for the attack of H(2)O(2) on the Ti-defect site even with Au(1-4) adsorbed-a result that supports the possibility of H(2)O(2) spillover from the Au clusters to the adjacent Ti-defect sites. Mulliken charge analysis indicates that in all the cases there is electron density transfer to adsorbed clusters from the zeolite lattice. In the case of both gas-phase and adsorbed Au-Pd clusters, all the Pd atoms were positively charged, and all the Au atoms were negatively charged due to the higher electron-affinity of Au. We also found a correlation between the BE and the charge transfer to the clusters (the higher the charge transfer to the clusters, the higher the BE), and a universal correlation was found for Au(2-5) when BE and charge transfer were plotted on a per atom basis. A relatively larger charge transfer to the adsorbed clusters was found for the Ti sites versus the Si sites, and for the defect sites versus the nondefect sites. The trends in the BE were corroborated using Gibbs free energy of adsorption (DeltaG(ads)), and the implications of DeltaG(ads) in sintering of Au clusters are also discussed. Our results confirm that electronic factors such as cluster-charging are potentially important support effects for the Au/TS-1 catalyst.

Silica-Supported Au Nanoparticles Decorated by TiO2: Formation, Morphology, and CO Oxidation Activity

Au-TiO(2) interface on silica support was aimed to be produced in a controlled way by use of Au hydrosol. In method A, the Au colloids were modified by hydrolysis of the water-soluble Ti(IV) bis(ammoniumlactato)dihydroxide (TALH) precursor and then adsorbed on Aerosil SiO(2) surface. In method B, Au sol was first deposited onto the SiO(2) surface and then TALH was adsorbed on it. Regular and high-resolution transmission electron microscopy (TEM and HRTEM) and energy dispersive spectrometry (EDS) analysis allowed us to conclude that, in method A, gold particles were able to retain the precursor of TiO(2) at 1.5 wt % TiO(2) loading, but at 4 wt % TiO(2) content the promoter oxide appeared over the silica surface as well. With method B, titania was detected on silica at each TiO(2) concentration. In Au-TiO(2)/SiO(2) samples, the stability of Au particles against sintering was much higher than in Au/TiO(2). The formation of an active Au-TiO(2) perimeter was proven by the greatly increased CO oxidation activity compared to that of the reference Au/SiO(2).

Water Activated by Atomic Oxygen on Au(111) to Oxidize CO at Low Temperatures

The Au(111) surface was populated with atomic oxygen [16O] followed by oxygen-labeled water [H218O] at surface temperatures as low as 77 K. When a CO beam was impinged on this surface, both [C16O16O] and [C16O18O] were produced. The results strongly suggest the direct involvement and promoting role of water in CO oxidation on oxygen covered Au(111) at low temperatures.