Segregation Phenomena in Size-Selected Bimetallic CuNi Nanoparticle Catalysts

Abstract

Surface segregation, restructuring, and sintering phenomena in size-selected copper-nickel nanoparticles (NPs) supported on silicon dioxide substrates were systematically investigated as a function of temperature, chemical state, and reactive gas environment. Using near-ambient pressure (NAP-XPS) and ultrahigh vacuum X-ray photoelectron spectroscopy (XPS), we showed that nickel tends to segregate to the surface of the NPs at elevated temperatures in oxygen- or hydrogen-containing atmospheres. It was found that the NP pretreatment, gaseous environment, and oxide formation free energy are the main driving forces of the restructuring and segregation trends observed, overshadowing the role of the surface free energy. The depth profile of the elemental composition of the particles was determined under operando CO hydrogenation conditions by varying the energy of the X-ray beam. The temperature dependence of the chemical state of the two metals was systematically studied, revealing the high stability of nickel oxides on the NPs and the important role of high valence oxidation states in the segregation behavior. Atomic force microscopy (AFM) studies revealed a remarkable stability of the NPs against sintering at temperatures as high as 700 °C. The results provide new insights into the complex interplay of the various factors which affect alloy formation and segregation phenomena in bimetallic NP systems, often in ways different from those previously known for their bulk counterparts. This leads to new routes for tuning the surface composition of nanocatalysts, for example, through plasma and annealing pretreatments.

Structure and activity of supported bimetallic NiPd nanoparticles: influence of preparation method on CO2 reduction

“…[35] Strong chemical bonds between Ni and hydrogen-containing adsorbates (such as surface hydroxyls and amine-groups of APTES) can also drive alloy segregation. [36] On the contrary, for the SI samples, the Pd 3d region showed lower PdO contribution and the BE values agree better with that expected for Pd, except for Ni 5 Pd 5 SI sample that shows a higher contribution of PdO. The Ni 2p spectra showed mainly NiO species, with some charge transfer occurring on the Ni 1 Pd 9 that can result from interaction with Pd, leading to shifts on the spectrum to higher BE values.…”

“…Due to the absence of capping ligands, a detrimental interdiffusion process can occur and lead to element reconfiguration in the bimetallic nanoparticles . Strong chemical bonds between Ni and hydrogen‐containing adsorbates (such as surface hydroxyls and amine‐groups of APTES) can also drive alloy segregation . On the contrary, for the SI samples, the Pd 3d region showed lower PdO contribution and the BE values agree better with that expected for Pd, except for Ni 5 Pd 5 SI sample that shows a higher contribution of PdO.…”

Surface Segregation in CuNi Nanoparticle Catalysts During CO2 Hydrogenation: The Role of CO in the Reactant Mixture

“…18−20 They are created during the initial O 2 plasma treatment and have been shown to play an important role in determining the segregation trend in CuNi catalysts. 14 Another possibility that cannot be excluded is that the peak at 856.8 eV is due to Ni 2+ species (NiO or mixed Cu x Ni 1– x O), with a spectral contribution shifted to higher binding energies due to nanoconfinement effects.…”

“…It is noted that the HCOO species which are formed in the CO-free reactant mixture in principle also induces minimal surface segregation of Cu (segregation energy of 0.03 eV), but the lower stability of this species and the nearly zero segregation energy value mean that its presence is barely sufficient to invert the surface segregation of Ni observed in the CO 2 + H 2 mixture upon reduction in H 2 . 14…”

“…The Cu 2p 3/2 and Ni 2p 3/2 regions were fitted with Gaussian–Lorentzian line shapes (ratio 0.3) and a linear background. For details of the fitting procedure, the reader is referred to the SI, as well as to ref (14).…”