Crystallographic Orientation Dependence of Surface Segregation and Alloying on PdCu Catalysts for CO₂ Hydrogenation

Abstract

The influence of the crystallographic orientation on surface segregation and alloy formation in model PdCu methanol synthesis catalysts was investigated in situ using near-ambient pressure X-ray photoelectron spectroscopy under CO 2 hydrogenation conditions. Combined with scanning tunneling microscopy and density functional theory calculations, the study showed that submonolayers of Pd undergo spontaneous alloy formation on Cu(110) and Cu(100) surfaces in vacuum, whereas they do not form an alloy on Cu(111). Upon heating in H 2 , inward diffusion of Pd into the Cu lattice is favored, facilitating alloying on all Cu surfaces. Under CO 2 hydrogenation reaction conditions, the alloying trend becomes stronger, promoted by the reaction intermediate HCOO*, especially on Pd/Cu(110). This work demonstrates that surface alloying may be a key factor in the enhancement of the catalytic activity of PdCu catalysts as compared to their monometallic counterparts. Furthermore, it sheds light on the hydrogen activation mechanism during catalytic hydrogenation on copper-based catalysts.

Synthesis of Mesoporous PdxCu1–x/Al2O3-y Bimetallic Catalysts Via Mechanochemistry for Selective N-Formylation of Amines with CO2 and H2

“…Notably, about 25% of carbon remains in the catalyst (Figure S1a), which is due to the incomplete carbonization of the precursor at 400 °C. As shown in Figure a, the Pd 3d spectra of Pd 0.25 Cu 0.75 /Al 2 O 3 -0.4 can be fitted to four peaks, corresponding to Pd(0) 3d 5/2 (335.14 eV), Pd­(II) 3d 5/2 (336.60 eV), Pd(0) 3d 3/2 (340.40 eV), and Pd­(II) 3d 3/2 (341.86 eV). , Similarly, Cu 2p spectra were fitted to three peaks corresponding to Cu 2p 3/2 (932.85 eV), Cu 2+ satellite (942.33 eV), and Cu 2p 1/2 (952.60 eV), which are close to the standard peaks of CuO. − Significantly, the binding energy of Pd 2+ 3d in Pd 0.25 Cu 0.75 /Al 2 O 3 -0.4 is positively shifted compared to the standard PdO. In contrast, the binding energy of Cu 2+ 2p in Pd 0.25 Cu 0.75 /Al 2 O 3 -0.4 is negatively shifted compared to that of standard CuO.…”

“…For example, Cu is doped into Pd-based catalysts to replace part of the Pd and to prepare efficient catalysts with low Pd loading. As a matter of fact, PdCu alloy catalysts have been investigated for the reactions of formic acid oxidation and oxygen reduction, 50 methanol oxidation, 51 toluene oxidation, 52 allylic alkylation, 53 and CO 2 hydrogenation to methanol 54,55 and formic acid. 56 It is well known that the carrier is usually essential for heterogeneous metal catalysts to exhibit excellent catalytic activity and good stability.…”

Simulating Segregation in a Ternary Cu–Pd–Au Alloy with Density Functional Theory, Machine Learning, and Monte Carlo Simulations

“…It was reported that CuPd shows different segregation behavior on different fcc surfaces such as (111), (110), and (100) . We compare the DFT segregation energies for CuPd fcc(110) and fcc(111) on a 3 × 3 × 6 slab in Table .…”

“…When we take the surface relaxation into account, the surface segregation trend is reversed. This calculation means that the surface segregation of CuPd depends on the orientation of the slab, which is consistent with the recent experimental work where the surface segregation behavior of Cu and Pd is related to the surface environment …”

“…It was reported that CuPd shows different segregation behavior on different fcc surfaces such as (111), (110), and (100). 52 We compare the DFT segregation energies for CuPd fcc(110) and fcc(111) on a 3 × 3 × 6 slab in Table 4. The inclusion of surface relaxation did not change the sign of the surface segregation energy for fcc(111).…”