(461g) Understanding the Promotional Role of Acids and Halides on Direct Synthesis of H2O2

Direct synthesis (H₂ + O₂ → H₂O₂) is a promising reaction to produce H₂O₂ (replacement for chlorinated oxidants in industrial processes) instead of the cost- and energy- intensive incumbent technology, the anthraquinone autoxiation (AO) process. Yet, direct synthesis suffers from low H₂O₂ selectivities (< 60%) on standard Pd catalysts [1]. Introduction of a cocktail of acids (e.g. HCl, H₂SO₄, H₃PO₄) and halide salts (KCl, KBr, NaBr) in the aqueous or alcoholic reaction mixture have improved the H₂O₂ selectivities to >80% [2]. Recently, H₂O₂ selectivities approaching 100% were achieved by introducing complex combinations of HCl, H₂SO₄ and Br^-, yet this comes at the expense of significant rates of metal dissolution [1, 3]. The intrinsic manner by which these promoters influence catalysis is still unclear despite the large number of publications reporting their effects on catalyst performance. Here, we aim to understand the fundamental role of these promoters in enhancing the rates and selectivities in direct synthesis of H₂O₂ by conducting steady state rate measurements as a function of promoter concentrations (NaBr and H₃PO₄), O₂ pressures and temperature.

Steady-state H₂O₂ and H₂O formation rates were measured within a fixed bed, plug flow reactor as functions of NaBr concentration (0 – 10^-3 M), H₂ and O₂ pressure (10-400 kPa), temperature (275-295 K), and in presence and absence of H₃PO₄ (10^-3 M) on silica-supported Pd nanoparticles (~ 7 nm in diameter). H₂O₂ selectivities increase from 15% in absence of Br^- to 40% at 10^-4 M Br^-, yet, increasing [Br^-] further to 10^-3 M decreased the selectivity to 35% indicating that there is an optimum [Br^-] to achieve maximum H₂O₂ selectivity. These changes are accompanied by rates of H₂O₂ formation that increase with increasing [Br^-] up to 10^-4 M Br^-, but which decrease 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.

Turnover rates for H₂O₂ formation increase in proportion to O₂ pressures at lower values (10-80 kPa) and change with a -0.5 order dependence on high O₂ pressures (100-400 kPa) in the presence of 10^-4 M Br^-, which differs from H₂O₂ turnover rates that do not depend on O₂ pressure (25-400 kPa) in the absence of Br^-. These differences reflect competitive adsorption processes between Br^- and O₂ on the catalyst surface. The change in apparent dependence on O₂ pressures is accompanied by differences in activation enthalpies. Barriers for H₂O₂ formation (ΔH(H₂O₂)) and H₂O formation (ΔH(H₂O)) increase when H₃PO₄ was introduced along with 10^-4 M NaBr (1±1 and 9±1 kJ mol^-1 in absence of H₃PO₄, 9±1 and 50±6 kJ mol^-1 in the presence of H₃PO₄,respectively). Yet, the presence of H₃PO₄ raised the barrier of H₂O formation by 5 times in contrast to the small increase in the barriers of H₂O₂ formation which partly explains the ~2 fold increase in selectivity in the presence of H₃PO₄ at 10^-4 M NaBr. The changes in rates of H₂O₂ formations in proportion to lower O₂ pressure (10-80 kPa) and ΔH(H₂O₂) indicate that O₂ binds more weakly to the active sites in the presence of both H₃PO₄ and NaBr and hence must overcome larger barrier for dissociation.

It has often been hypothesized in literature that the halide adatoms coadsorb on the surface of the catalyst and break the deleterious Pd-Pd ensembles that are responsible for O-O bond cleavage [4]. The results shown here demonstrate that bromide reduces electron backdonation to the 2Ï€* antibonding orbitals of O₂, which results in weaker adsorption of O₂ to surfaces but also greater barriers for O-O bond rupture. Together these differences lead to greater H₂O₂ selectivities. We will attempt to elucidate the true role of halides by making comparisons of activation enthalpies across a range of concentrations and by characterizing the catalyst before and after reaction by CO FTIR to determine what changes occur in the bond vibrations of adsorbed CO by prolonged bromide exposure in the plug flow system. Finally, acids such as H₃PO₄ change the ionic strength of the solution which can affect the stability of the transition states within the catalytic cycle [1]. This hypothesis 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. These results may evince if the acids stabilize H₂O₂ formation pathways over H₂O formation. In summary, this detailed study on the promotional effect of acids and halides will help us understand the fundamental manner in which these promoters affect catalysis separately as well as together which in turn will help us design a more robust process for H₂O₂ formation by direct synthesis.

References

1. Flaherty, D.W., ACS Catal., 2018, 8, 1520.
2. Liu Q., Bauer, J.C., Schaak, R.E., Lunsford, J.H., Angew. Chem. Int. Ed., 2008, 47, 6221.
3. Chinta, S., Lunsford, J.H., J. Catal., 2004, 225, 249.
4. Ntainjua N. E., Piccinini, M., Pritchard, J. C., Edwards, J. K., Carley, A. F., Moulijn, J. A., Hutchings, G. J., ChemSusChem, 2009, 2, 575