(560iu) Understanding the Promotional Effects of Acids and Halides on Direct Synthesis of H2O2 and Their Effect on the Nature of Catalytically Active Pd

Direct synthesis (H₂ + O₂ → H₂O₂) of H₂O₂ over supported Pd nanoparticles offers a promising alternative for H₂O₂ productionto replace chlorinated oxidants in industrial processes instead of the current cost- and energy- intensive technology, the anthraquinone autoxiation (AO) process. However, direct synthesis suffers from low H₂O₂ selectivities (< 60%) on standard Pd catalysts [1]. Acids (e.g. HCl, H₂SO₄, H₃PO₄) and halides (KCl, KBr, NaBr) are often added to reaction mixture to improve the H₂O₂ selectivities, yet presence of acids can lead to significant rates of metal dissolution [2]. The dissolved Pd is oxidized to homogeneous Pd complexes which may catalyze H₂O₂ formation. Additionally, the fundamental role of these complex promoters in influencing catalysis is still not clear despite the vast body of literature reporting their effects on catalyst performance. Here, we combine kinetic and spectroscopic measurements to probe the relationships between rates, selectivities, and populations of homogeneous and heterogeneous Pd species to elucidate the nature of active species in direct synthesis. Furthur, we aim to understand the intrinsic manner in which these promoters improve the rates and selectivities in direct synthesis of H₂O₂ by conducting steady state rate measurements as a function of promoter concentrations (NaBr and H₃PO₄), H₂ and O₂ pressures and temperature.

To understand the nature of active sites, kinetic measurements were conducted in a semi batch reactor equipped with operando UV-Vis capabilities. Pd atoms primarily exist as suspended Pd⁰ nanoparticles at steady state (>75% of total Pd) in absence of HCl regardless of the form of Pd introduced (PdCl₂, PdSO₄, Pd(NO₃)₂, Pd-SiO₂). Injection of HCl after reaction has achieved steady state increases the Pd²⁺ concentration due to oxidation of the Pd⁰ nanoparticles to PdCl_x(H₂O)_(4-x)^(x-2). When water is used as solvent, the H₂O₂ selectivities increase after the first HCl addition on Pd-SiO₂ and Pd(NO₃)₂ and decrease following a 2^nd HCl injection. Yet for PdCl₂, the selectivity decreases after both HCl additions. H₂ and O₂ consumption rates decrease after each HCl addition which alludes to a loss of active sites. These observations are inconsistent with soluble Pd²⁺ species acting as the active catalyst for H₂O₂ formation. Rather, the suspended Pd nanoparticles are responsible for H₂O₂ formation. Chloride ions (in case of PdCl₂, the counter anion of the salt) can strongly bind to suspended Pd⁰ nanoparticles and inhibit the cleavage of the critical O-O bond, hence reducing primary H₂O formation and H₂O₂ hydrogenation rates. Similar trends in H₂O₂ rates and selectivities were observed when methanol was used as solvent instead of water indicating that the form of the active species is independent of the choice of solvent.

Steady-state H₂O₂ and H₂O formation rates were measured within a fixed bed, plug flow reactor as functions of H₂ and O₂ pressure (10-400 kPa), temperature (275-295 K), and NaBr concentration (0 – 10^-3 M) in presence and absence of H₃PO₄ (10^-3 M) on silica-supported ~7 nm Pd nanoparticles to probe the role of promoters in enhancing selectivities. H₂O₂ selectivities increase with increasing [Br^-] (15% in absence of Br^- to 40% at 10^-4 M Br^-). However, increasing [Br^-] further to 10^-3 M decreased the selectivity to 35% indicating that there is an optimum [Br^-] to achieve high selectivity. Furthermore, rates of H₂O₂ formation increase with increasing [Br^-] up to 10^-4 M Br^-, but decline with further addition of Br^-. These observations suggest that bromide ions adsorb on the Pd clusters and reduce the number of active sites available for O₂ adsorption leading to a decrease in H₂O₂ formation rates and selectivities beyond 10^-4 M [Br^-]. Addition of 10^-3 M H₃PO₄ further improved the H₂O₂ selectivity to 77% at 10^-4 M NaBr and to ~95% at 10^-3 M NaBr. Addition of H₃PO₄ in presence of 10^-4 M NaBr raises barriers for H₂O formation (ΔH(H₂O)) (9±1 and 50±6 kJ mol^-1 in absence and presence of H₃PO₄ respectively) much more than barriers for H₂O₂ formation (ΔH(H₂O₂)) (1±1 and 9±1 kJ mol^-1 in absence and presence of H₃PO₄ respectively) which partly explains the ~2 fold increase in selectivity in the presence of H₃PO₄ at 10^-4 M NaBr. This result indicates that O-O binds more weakly to the active sites in the presence of both H₃PO₄ and NaBr and hence must overcome larger barrier for dissociation.

It is believed that the halides block sites responsible for O-O dissociation [3]. The results shown here suggest that bromide reduces electron backdonation to the 2Ï€* antibonding orbitals of O₂, resulting in weaker adsorption of O₂ to surfaces but also greater barriers for O-O bond rupture leading to enhanced H₂O₂ selectivities. We will attempt to elucidate the true role of halides by making comparisons of activation enthalpies across a range of concentrations. Finally, the role of acids will be tested by performing activation enthalpy measurement in varying concentrations of H₃PO₄ in absence of NaBr to deconvolute the effects of acids and halides. Hence, this detailed study on the promotional effect of acids and halides will help us understand the role of promoters in affecting catalysis separately as well as together which in turn will help us design more productive, selective and stable process for H₂O₂ formation by direct synthesis.

References

1. Flaherty, D.W., ACS Catal., 2018, 8, 1520.
2. Chinta, S., Lunsford, J.H., J. Catal., 2004, 225, 249.
3. Bassler, P., Weidenbach, M., Goebbel, H., Chem. Eng. Trans., 2010, 21, 571.