(173g) Investigating the Effect of Acids and Halides on Direct Synthesis of Hydrogen Peroxide

Authors: 
Priyadarshini, P., University of Illinois, Urbana-Champaign
Flaherty, D., University of Illinois, Urbana-Champaign
H2O2 is an environmentally benign and selective oxidizer which can potentially replace chlorine and chlorinated oxidizers in many industrial applications. It is currently produced on large scale by the energy- and cost-intensive anthraquinone auto-oxidation (AO) process which makes it more expensive than chlorine. Direct synthesis (DS) of H2O2 is a greener alternative to AO with less energy and cost requirements, but DS suffers from low H2O2 selectivities on Pd (the most commonly used catalyst for DS) in the absence of caustic additives like acids and halides (> 90% by AO vs. < 60% using DS).1 Although acids and halides increase selectivity, they also lead to dissolution of Pd from the surface which complexes with the anionic species in solution to form homogeneous Pd2+ complexes (e.g., PdCl42-) that may catalyze H2O2 formation or decomposition, and contribute to the reported changes in selectivity.2,3 Consequently, it is unclear if the active sites for direct synthesis are homogeneous Pd complexes, halide-modified Pd nanoparticles, or if these species are somehow linked within a single catalytic cycle. Kinetic measurements need to be combined with spectroscopic tools to probe the relationships between rates, selectivities, and the populations of distinct homogeneous and heterogeneous Pd species to elucidate the true nature of active catalytic species in direct synthesis.

Here, we combine transient and steady-state measurements of H2O2 and H2O formation rates with in situ UV-Vis spectroscopy to probe the identity of active catalytic species that form H2O2. Pd atoms exist in equilibrium between a heterogeneous colloidal state (Pd0) and a collection of homogeneous Pd2+ complexes due to the presence of both reducing (H2) and oxidizing (e.g., O2, Cl-) environment in the solvent. Reactions are conducted in a semi-batch reactor by introducing Pd salts (e.g., 0.01- 0.05 mmol of PdCl2, PdSO4, Pd(NO3)2) into methanol (140 mL) saturated with H2 and O2 reactants (4.8 kPa H2, 4.8 kPa O2, 278 K). In situ UV-Vis spectra show that at steady state approximately 10 - 20% of the total Pd exist in the Pd2+ form while the remaining Pd atoms exist in the Pd0 state at these conditions. After the reaction attains steady state, an excess of HCl (10 mmoles) is injected into the solvent which increases the concentration of Pd2+ species to 25 – 50 %. We also observe an increase in the selectivity to H2O2 accompanied by a decrease in the rate of formation and rate of hydrogenation of H2O2. A second HCl injection increases the population of the homogeneous Pd2+ complexes to ~ 40-80% but the rates of formation and hydrogenation as well as selectivity are unaffected on subsequent HCl injection. The rate constants for H2O2 formation (kformation) and H2O2 hydrogenation (khydrogenation) are calculated by fitting the rate equation for net H2O2 formation to the H2O2 concentration as a function of time.4 The ratios of kformation and khydrogenation increase two- to six- fold after the first HCl injection for each salt tested, but remain constant after the second injection. H2 conversions decrease after each HCl addition which alludes to a loss of active sites on introduction of HCl. These observations are incompatible with soluble Pd2+ species acting as the active catalytic species for the formation of H2O2. Rather, the colloidal Pd nanoparticles produce H2O2. The Cl- ions modify the surface of the colloidal Pd0 species and these chloride-modified nanoparticles inhibit the H2O2 hydrogenation after the first HCl injection which increases the kformation/khydrogenation ratio and selectivity towards H2O2 after the first HCl spike. The decrease in the H2O2 formation rates occurs due to the reduction in number of active colloidal Pd sites due to partial conversion to Pd2+. Adding additional Cl- ions after the first injection does not have any effect other than changing the distribution of Pd2+ and Pd0 species because the surface of the colloids are already saturated with Cl- following the first HCl injection. The kformation/khydrogenation ratios are also different for each Pd salt prior to adding HCl, which shows that the corresponding anions (Cl-, SO42-, NO3-) also modify the surface of active Pd colloids altering the activity and selectivity of the process.

Having established the true nature of active catalytic species in direct synthesis, we will attempt to understand how these acid and halide promoters are affecting the selectivity of direct synthesis by conducting activation enthalpy measurements using commonly used additives like HCl, HBr and H2SO4 on supported Pd nanoparticles. These promoters possibly change the enthalpies of formation of H2O2 and H2O and the binding energy of oxygen to the nanoparticles due to which the H2O2 selectivity changes. Comparison of activation enthalpies and selectivities will help us understand the role played by these commonly used additives in improving the H2O2 selectivity.

(1) Wilson, N. M.; Bregante, D. T.; Priyadarshini, P.; Flaherty, D. W. In Catalysis; Spivey, J., Han, Y.-F., Eds. 2017; Vol. 29, p 122.

(2) Dissanayake, D. P.; Lunsford, J. H Journal of Catalysis 2003, 214, 113.

(3) Dissanayake, D. P.; Lunsford, J. H. Journal of Catalysis 2002, 206, 173.

(4) Wilson, N. M.; Priyadarshini, P.; Kunz, S.; Flaherty, D. W. Journal of Catalysis 2018, 357, 163.

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