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

Authors: 
Priyadarshini, P., University of Illinois, Urbana-Champaign
Flaherty, D., University of Illinois, Urbana-Champaign
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 PdClx(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 2nd 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.
Topics: