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

Authors: 
Priyadarshini, P., University of Illinois, Urbana-Champaign
Flaherty, D., University of Illinois, Urbana-Champaign
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
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