(582ab) Understanding the Effect of Alloying Pd and Sn on Direct Synthesis of H2O2

Authors: 
Priyadarshini, P., University of Illinois, Urbana-Champaign
Wilson, N. M., University of Illinois Urbana-Champaign
Adams, J. S., University of Illinois, Urbana-Champaign
Flaherty, D., University of Illinois, Urbana-Champaign
Direct synthesis (DS) of H2O2 has the potential to replace the current method of H2O2 production, anthraquinone auto-oxidation (AO) process, which is energy- and cost-intensive. But Pd, the most commonly used catalyst for DS, is not very selective towards H2O2 formation (> 90% by AO vs. < 60% using DS) in the absence of additives like acids and halides.1 Alloying metals (such as Au, Zn, and Sn) with Pd has been effective in increasing H2O2 selectivity, however, the reasons for these changes remain unknown and likely depends on the identity of the second metal.2,3 Here, we compare activation enthalpies for H2O2 and H2O formations and the mechanisms of these reactions among Pd and PdSnx catalysts (where 0.5 ≤ x ≤ 10) to understand the underlying cause of improved selectivity towards H2O2 production that result from the addition of Sn to Pd and different post synthesis oxidative and reductive heat treatments.

H2O2 formation rates on Pd and Pd2Sn are significant in a protic solvent (e.g., methanol), whereas those in aprotic solvents (e.g., acetonitrile) are immeasurable. These results are in agreement with a proton-electron transfer (PET) mechanism on Pd in which the presence of a proton is necessary for DS to occur.4 Steady state H2O2 and H2O formation rates over silica supported PdSnx nanoparticles (7-15 nm in diameter) synthesized by colloidal techniques depend on the number of oxidative and reductive treatments as well as the final treatment used prior to DS, however, Pd nanoparticles (~10 nm in diameter) do not. Catalysts which were subjected to a second oxidation treatment after an oxidation-reduction cycle were less selective than catalysts which were just oxidized and reduced. Activation enthalpies (∆Hǂ) were calculated from rates measured as a function of temperature (281-305 K) and show that PdSn2 catalysts have greater ∆Hǂ for H2O2 (9 kJ mol-1) and H2O (15 kJ mol-1) formation than Pd nanoparticles (-6 kJ mol-1, 11 kJ mol-1). These differences in ∆Hǂ values indicate that the addition of Sn raises the barriers of H2O2 formation much more than H2O formation, however, H2O2 selectivities on PdSn2 (45%, oxidized and reduced) exceed those on Pd (23%, reduced). Spectra of adsorbed CO on PdSnx obtained using Fourier transform infrared spectroscopy (FTIR) show that the ratio of the peak area for the bridge bound CO to that of the atop bound CO decreases from 17 on Pd to 2 on PdSn2, indicating that Sn breaks the ensembles of surface Pd atoms that are proposed to be necessary for O-O bond rupture leading to H2O formation. Together, the ∆Hǂ values and spectroscopic data suggest that the addition of Sn to Pd increases the selectivity towards H2O2 production predominantly through ensemble effects, which is significantly different from the electronic modification of Pd that are mostly responsible for the high H2O2 selectivities observed on PdAux catalysts.3

Factors such as the presence of additives in the reaction medium (i.e., acids, halides) and the type of reactor used (e.g., PFR vs. BSTR) also affect H2O2 selectivities and rates of DS apart from alloying metals.1 Such results indicate that the active form(s) of the catalyst may include soluble forms of Pd in addition to supported nanoparticles. Homogeneous Pd complexes may form by dissolution of Pd from surfaces and complexation with anionic species added to act as “promoters.” The contributions of both homogeneous and heterogeneous Pd can be distinguished by adding specific titrants into the reaction medium that deactivate either heterogeneous or homogeneous catalysts (e.g., Hg, CS2, C4H4S).These results will show that comparisons between DS catalysts operating in continuous flow or batch modes require corrections to account for more than just the simple difference between the design equations of the systems.

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

(2) Freakley, S. J.; He, Q.; Harrhy, J. H.; Lu, L.; Crole, D. A.; Morgan, D. J.; Ntainjua, E. N.; EDwards, J. K.; Carley, A. F.; Borisevich, A. Y.; Kiely, C. J.; Hutchings, G. J., Science 2016, 351, 965.

(3) Edwards, J. K.; Thomas, A.; Carley, A. F.; Herzing, A. A.; Kiely, C. J.; Hutchings, G. J., Green Chem. 2008, 10, 388.

(4) Wilson, N. M.; Flaherty, D. W., J Am Chem Soc 2016, 138, 574.

(5) Widegren, J. A.; Finke, R. G., J Mol Catal A: Chemical 2003, 198, 317.

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