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

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 PdSn_x 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 PdSn_x 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 (∆H^Ç‚) were calculated from rates measured as a function of temperature (281-305 K) and show that PdSn₂ catalysts have greater ∆H^ǂ 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 ∆H^ǂ 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 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 breaks the ensembles of surface Pd atoms that are proposed to be necessary for O-O bond rupture leading to H₂O formation. Together, the ∆H^ǂ 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 PdAu_x 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).⁵ 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.