Comparison of the Catalytic Activity of Au₃, Au₄⁺, Au₅, and Au₅^- in the Gas-Phase Reaction of H₂ and O₂ to Form Hydrogen Peroxide:  A Density Functional Theory Investigation

Abstract

We report a detailed density functional theory (B3LYP) analysis of the gas-phase H2O2 formation from H2 and O2 on Au3, Au4+, Au5, and Au5-. We find that H2, which interacts only weakly with the Au clusters, is dissociatively added across the Au-O bond, upon interaction with AunO2. One H atom is captured by the adsorbed O2 to form the hydroperoxy intermediate (OOH), while the other H atom is captured by the Au atom. Once formed, the hydroperoxy intermediate acts as a precursor for the closed-loop catalytic cycle. An important common feature of all the pathways is that the rate-determining step of the catalytic cycle is the second H2 addition to form H2O2. The H2O2 desorption is followed by O2 addition to AunH2 to form the hydroperoxy intermediate, thus leading to the closure of the cycle. On the basis of the Gibbs free energy of activation, out of these four clusters, Au4+ is most active for the formation of the H2O2. The 0 K electronic energy of activation and the DeltaGact at the standard conditions are 12.6 and 16.6 kcal/mol respectively. The natural bond orbital charge analysis suggests that the Au clusters remain positively charged (oxidic) in almost all the stages of the cycle. This is interesting in the context of the recent experimental evidence for the activity of cationic Au in CO oxidation and water-gas shift catalysts. We have also found preliminary evidence for a correlation between the activation barrier for the first H2 addition and the O2 binding energy on the Au cluster. It suggests that the minimum activation barrier for the first H2 addition is expected for the Au clusters with 7.0-9.0 kcal/mol O2 binding energy, i.e., in the midrange of the expected interaction energy. This represents a balance between more favorable H2 dissociation when the Aun-O2 interaction is weaker and high O2 coverage when the interaction is stronger. On the basis of this work, we predict that the hydroperoxy intermediate formation can be both thermodynamically and kinetically viable only in a narrow range of the O2 binding energy (10.0-12.0 kcal/mol)-a useful estimate for computationally screening Au-cluster-based catalysts. We also show that a competitive channel for the OOH desorption exists. Thus, in propylene epoxidation both OOH radicals and H2O2 can attack the active Ti in/on the Au/TS-1 and generate the Ti-OOH sites, which can convert propylene to propylene oxide.

The direct epoxidation of propene over gold–titania catalysts—A study into the kinetic mechanism and deactivation

“…One point on which agreement seems to have been reached, is that the main role of gold is the formation of a peroxide species or hydrogen peroxide. This has been confirmed both by experimental and theoretical studies [8,10,[12][13][14]24]. In the model we propose, this is also an essential step, but we also propose another role for gold, namely the activation of propene, which thereafter reacts with titania surface groups [5,19,22] and only in a second step reacts with a peroxide species.…”

H2 adsorption and H/D exchange on Au/TS-1 and Au/S-1 catalysts

“…Naito and Tanimoto [74] have experimentally shown that the hydrogen-deuterium equilibration at 423 K on a 5 wt% Au/SiO 2 catalyst is poisoned by O 2 , but its rate is increased 10 to 30 times on 0.05 wt% Au/SiO 2 catalyst when O 2 is present. The DFT calculations on gas-phase Au 3 clusters are consistent with this observation of facilitated H 2 dissociation in the presence of O 2 [45,46].…”

“…We have demonstrated the utility of DFT calculations in our previous work on small unsupported (gas-phase) Au clusters [44][45][46][47][48]. We showed that H 2 and O 2 can react on small Au clusters to form H 2 O 2 [45,46], which may carry out propylene epoxidation on Ti sites in TS-1 [49][50][51][52].…”

Nitrogen oxides and SO2 adsorption on Au/MOR catalyst: Adsorption sites, thermodynamic and vibrational frequencies. ONIOM study

“…The higher charge transfer is related with a lower ionization potential. It is well known that for Au clusters, the ionization potential values decrease as the number of Au atoms increase in the cluster [57]. According to our results the interaction, measured by the E or H values, is stronger for Au/T than for Au 3 /T even though there is a higher charge transfer for the latter than for the former.…”