Surface structure and segregation of nanoalloy catalysts probed by in situ and operando IR

SSCI-VIDE+ING+FRMInternational audienceCharacterizing the surface of supported nanoalloys is not trivial, especially under conditions relevant to catalytic reactions. Examples are given here of reactions involving CO that offer unique opportunities to probe the nanoalloy surface by IR spectroscopy under reaction (operando) conditions, in addition to traditional in situ studies using CO adsorption.Alloyed nanoparticles (e.g. Au-Ag, Au-Pt, Pt-Ag, Pt-Sn, Pd-Sn, Pd-Zn, Co-Sn) were supported over standard supports (e.g. silica, alumina, ceria), as well as being embedded inside the mesopores of single-hollow [1] or multi-hollow zeolites [2].Au, Ag and Au-Ag alloy nanoparticles were embedded in silicalite-1 multi-hollow single crystals [2]. Au and Au-Ag nanoparticles exhibited similar activity for CO oxidation at room temperature. Operando diffuse reflectance FT-IR spectroscopy (DRIFTS) revealed the presence of both metals at the surface of the Au-Ag particles, Ag replacing a large fraction of the Au atoms otherwise located at low coordination sites.The structure of alumina-supported Pt, Au and Au-Pt nanoparticles were investigated by TEM, XRD and CO adsorption monitored by DRIFTS. The DRIFTS interpretation was hampered by the restructuring of Au in the presence of CO, leading to bands similar to those observed on Pt [3]. The kinetics of CO desorption enabled highlighting alloyed phases: Pt-CO were stable at 50°C in He, while CO readily desorbed from Au. Au-Pt samples exhibited complex intermediate behaviors, suggesting surface segregation during the course of desorption.The formation of intermetallic compounds by combining noble metals such as Pt and Pd with base metals such as Zn and Sn has often been used to modify catalytic properties. The benefits of these alloying in terms of geometric and electronic effects will be discussed for Pd-Sn [4], Pd-Zn [5], Co-Sn [6] and Pt-Sn [7].Interestingly, several modes of segregation have been observed over Pt-Sn. The oxidation of Sn by O2 during the preferential oxidation of CO in the presence of large excess of H2 has been observed, leading to segregated Pt and SnOx phases [8]. CO was shown to trigger a similar phase segregation, when dissociating to C and O under specific conditions [9] and also led to surface segregation in Sn-rich samples by pulling more Pt to the surface [10].REFERENCES1. S. Li, A. Tuel, J.-L. Rousset, F. Morfin, M. Aouine, L. Burel, F. Meunier and D. Farrusseng, ChemNanoMat 2, 534-539 (2016).2. R. Atwi, A. Tuel, M. Maffre, L. Burel, J.-L. Rousset and F. Meunier, Appl. Catal. A : Gen. 569, 86-92 (2019).3. R. Atwi, T. Elgayyar, F.J. Cadete Santos Aires, A. Tuel and F. Meunier, Top. Catal. 63, 1596–1605 (2020).4. I. Jbir, J. Couble, S. Khaddar-Zine, Z. Ksibi, F. Meunier and D. Bianchi, ACS Catal. 6, 2545-2558 (2016).5. F. Meunier, M. Maffre, Y. Schuurman, S. Colussi and A. Trovarelli, Catal. Commun. 105, 52-55 (2018).6. A. Paredes-Nunez, D. Lorito, L. Burel, D. Motta-Meira, G. Agostini, N. Guilhaume, Y. Schuurman, F. Meunier, Angew. Chem. Int. Ed. 57, 547-550 (2018).7. A. Moscu, Y. Schuurman, L. Veyre, C. Thieuleux and F. Meunier, Chem. Commun. 50, 8590-8592 (2014).8. A. Moscu, L. Veyre, C. Thieuleux, F. Meunier and Y. Schuurman, Catal. Today 258, 241-246 (2015).9. A. Moscu, L. Cardenas, C. Thieuleux, D. Motta-Meira, G. Agostini, Y. Schuurman and F. Meunier, J. Catal. 359, 76-81 (2018).10. Q. Wang, D. Tichit, F. Meunier and H. Guesmi, J. Phys. Chem. C 124, 9979-9989 (2020)