(734e) Investigating the Effect of Alloying Sn and Pd on Direct Synthesis of H2O2

Direct synthesis (DS) of H₂O₂ has the potential to replace the current method of H₂O₂ production, the energy- and cost-intensive anthraquinone auto-oxidation (AO) process. DS on Pd, the most commonly used catalyst for DS, is not as selective towards H₂O₂ formation as AO (> 90% by AO vs. < 60% using DS) in the absence of caustic additives like acids and halides.¹ Alloying metals (such as Au, Zn, and Sn) with Pd increases H₂O₂ selectivity, however, the reasons for these changes remain unknown and likely depends on the identity of the second metal.^2,3 Here, we report and compare activation enthalpies for H₂O₂ and H₂O formations and the mechanisms of these reactions among Pd and PdSn_x catalysts (where 0.5 ≤ x ≤ 10) to decipher 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 treatments.

H₂O₂ formation rates on Pd and Pd₂Sn in a protic solvent (e.g., methanol) are significant, whereas those in aprotic solvents (e.g., acetonitrile) are immeasurable. These results are consistent with a proton-electron transfer (PET) mechanism on Pd in which the presence of a proton is necessary to facilitate DS.⁴ Steady state H₂O₂ and H₂O formation rates over silica supported PdSn_x nanoparticles (7-15 nm in diameter) synthesized by colloidal methods depend on the number of oxidative and reductive treatments as well as the final treatment used prior to direct synthesis, however, Pd nanoparticles (~10 nm in diameter) do not. Activation enthalpies (∆H^Ç‚) were calculated from rates measured as a function of temperature (281-305 K) and show that PdSn_2 catalysts have greater ∆H^ǂ for H₂O₂ (9 kJ mol^-1) and H₂O (15 kJ mol^-1) formation than do Pd nanoparticles (-6 kJ mol^-1, 11 kJ mol^-1). These differences in ∆H^ǂ values suggest 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 PdSn_x 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 disrupts the ensembles of surface Pd atoms that are proposed to be necessary for O-O bond rupture and subsequent H₂O formation. Together, the values and spectroscopic data suggest that the addition of Sn to Pd increases the selectivity towards H₂O₂ production predominantly through ensemble effects, which differs significantly from the electronic effects that are largely responsible for the high H₂O₂ selectivities observed on PdAu_x catalysts.³

Apart from alloying metals, 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.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 serve as “promoters.” The contributions of both homogeneous and heterogeneous Pd can be distinguished by the further addition of specific titrants that render either heterogeneous or homogeneous catalysts inactive (e.g., Hg, CS₂, C₄H₄S).^5 These results will show that comparisons between direct synthesis 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.