Carbon-carbon coupling is an important step in many catalytic reactions and performing sp 3 -sp 3 carbon-carbon coupling heterogeneously is particularly challenging. It has been reported that PdAu single-atom alloy (SAA) model catalytic surfaces are able to selectively couple methyl groups producing ethane from methyl iodide. Herein, we extend this study to NiAu SAAs and find that Ni atoms in Au are active for C-I cleavage and selective sp 3 -sp 3 carbon-carbon coupling to produce ethane. Furthermore, we performed ab initio kinetic Monte Carlo simulations that include the effect of the iodine atom, which was previously considered a bystander species. We find that model NiAu surfaces exhibit a similar chemistry to PdAu, but the reason for the similarity is due to the role the iodine atoms play in terms of blocking the Ni atom active sites. Specifically, on NiAu SAAs the iodine atoms outcompete the methyl groups for occupancy of the Ni sites leaving the Me groups on Au, while on PdAu SAAs, the binding strengths of methyl groups and iodine atoms at the Pd atom active site are more similar. These simulations shed light on the mechanism of this important sp 3 -sp 3 carbon-carbon coupling chemistry on SAAs. Furthermore, we discuss the effect of the iodine atoms on the reaction energetics and make an analogy between the effect of iodine as an active site blocker on this model heterogeneous catalyst and homogeneous catalysts in which ligands must detach in order for the active site to be accessed by the reactants.
The oxidation of carbon monoxide on oxygen-modified Au(111) surfaces is studied using a combination of reflection-absorption infrared spectroscopy (RAIRS) and temperatureprogrammed desorption (TPD). TPD reveals that CO desorbs in two states with the lowtemperature state have a peak temperature between ~130 to 150 K, and the higher-temperature state having a peak temperature that varies from ~175 to ~220 K depending on the initial oxygen and CO coverages. Infrared spectroscopy indicates that the low-temperature CO desorption state is predominantly associated with CO adsorbed on Au δ+ sites, while the higher-temperature states are due to CO on Au 0 sites. No additional vibrational features are detected indicating that CO reacts directly with adsorbed atomic oxygen on gold to form CO 2. Estimates of the activation energy for CO 2 formation suggest that they are in the same range and found for supported gold catalysts at reaction temperature below ~300 K.
The surface chemistry of ethylene is explored on model Au/Pd(100) alloy surfaces using a combination of temperature-programmed desorption and reflection-absorption infrared spectroscopy. The heat of adsorption of ethylene on the model alloy surface is found to increase monotonically with increasing palladium coverage in the alloy, from~33 kJ/mol for a completely gold-covered surface to~80 kJ/mol as the gold coverage decreases to zero. A large change in heat of adsorption is found for palladium coverages between 0 and~0.35 monolayers, where previous studies have shown that the surface comprises exclusively isolated palladium sites. The heat of adsorption changes more slowly for higher palladium coverages, when palladium-palladium bridge sites appear. Vinyl species are identified for palladium coverages above~0.8 ML from a vibrational mode at~1120 cm −1 , which disappears when the sample is heated to~250 K, due to vinyl decomposition.
Oxygen adsorption was studied on
a Au/Pd(100) single crystal as
a model for single-atom-alloy catalysts since the surface contains
isolated palladium atoms surrounded by gold at high gold coverages,
and density functional theory calculations show sharp, atomlike palladium
electron density of states, typical of single-atom alloy systems.
Since O2 does not adsorb dissociatively on these high-gold-coverage
alloys, atomic oxygen was dosed using ozone. The strength of oxygen
adsorption was investigated by temperature-programmed desorption experiments
on substrates with different gold coverages. Kinetic Monte Carlo simulations
were used to determine the desorption activation energies from the
experiments, which correlated well with the results of density functional
theory calculations. These calculations also enabled the adsorbate
energy levels to be identified. It has been proposed that adsorbate
binding and reactivity are dominated by the atomlike states, but no
simple atomlike quantum theory models could successfully describe
the energy-level locations. However, good correlations were found
between the binding energy and location of the d-band center, implying
that the adsorbate interacts with the extended substrate energy bands
rather than with a single localized state of the active component.
Similar calculations were performed for a molecular system comprising
molecular oxygen on the alloy with similar conclusions and where the
O2 binding energy also correlates well with the d-band
center energy.
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