Adsorption of CO on Ni/Cu(110) bimetallic surfaces

The adsorption behavior of CO on bimetallic Ni/Cu(110) surfaces has been studied experimentally by thermal-desorption spectroscopy and theoretically by density-functional theory (DFT) calculations. The bimetallic surfaces were produced either by evaporation of nickel or by decomposition of $\text{Ni}{\left(\text{CO}\right)}_4$ on Cu(110). Adsorption of CO at 180 K on such a bimetallic surface yields three new adsorption states with adsorption energies between that of CO on clean Cu(110) and clean Ni(110). The new desorption peaks from the bimetallic surface, designated as ${\beta }_1\text{−}{\beta }_3$, can be observed at 250, 300, and 360 K, respectively. These new states are most pronounced when $\frac{1}{2}$ monolayer of nickel is present on the copper surface. DFT calculations, using the Vienna ab initio simulation package code, were performed to identify the most probable Ni/Cu atomic arrangements at the bimetallic surface to reconcile with the experimental results. It turned out that CO adsorption on nickel dimers consisting of in-surface and adjacent subsurface atoms can best explain the observed experimental data. The result shows that CO adsorption is determined by local (geometric) effects rather than by long-range (electronic) effects. These findings should contribute to a better understanding of tailoring catalytic processes with the help of bimetallic catalysts.

Figure 1

Set of CO desorption spectra after dosing the initially clean Cu(110) surface with various amounts of the $\text{CO}+\text{Ni}{\left(\text{CO}\right)}_4$ gas mixture at 180 K. Heating rate: 2 K/s.

Figure 2

Uptake curves for CO when dosing the Cu(110) surface with the $\text{CO}+\text{Ni}{\left(\text{CO}\right)}_4$ gas mixture, as obtained from a series of TDS, including those in Fig. 1.

Figure 3

CO desorption spectra after dosing 3 L CO at 130 K on a Cu(110) surface, covered with various amounts of nickel. Nickel was evaporated onto the surface at 175 K. (a): clean surface, (b): 0.05 ML Ni, (c): 0.19 ML Ni, (d): 0.27 ML Ni, (e) 1.02 ML Ni, and (f): 4.4 ML Ni.

Figure 4

Change in the amount of adsorbed CO in the individual adsorption sites after preparing the Ni/Cu(110) bimetallic surface by evaporation of 0.2 ML nickel at different substrate temperatures. Subsequent CO exposure at 130 K is 3 L.

Figure 5

Compilation of saturation CO desorption spectra after dosing (a) 3 L of pure CO on the clean Cu(110) surface $\left(T_{\text{ad}}=110\text{K}\right)$, (b) 250 L of the $\text{CO}+\text{Ni}{\left(\text{CO}\right)}_4$ gas mixture on the Cu(110) surface $\left(T_{\text{ad}}=180\text{K}\right)$, and (c) 3 L pure CO on the epitaxially grown Ni(110) surface $\left(T_{\text{ad}}=110\text{K}\right)$.

Figure 6

(Color online) Relaxed CO molecule configuration on a Cu(110) surface for a coverage of 0.75 ML. A $\left(2\times 2\right)$ surface unit cell was used for the calculations and the molecules in the basis of the unit cell are numbered according to their sequential removal.

Figure 7

(Color online) Relaxed CO molecule configuration on a Ni(110) surface for a coverage of 1.0 ML. A $\left(2\times 2\right)$ surface unit cell was used for the calculations and the molecules in the basis of the unit cell are numbered according to their sequential removal.

Figure 8

(Color online) [(a)–(d)]: Relaxed CO molecule configuration on a Ni/Cu(110) bimetallic surface with $\frac{1}{2}$ ML nickel adsorbed in a $\left(1\times 2\right)$ superstructure. (a) 1.0 ML CO, (b) 0.75 ML CO, (c) 0.5 ML CO, and (d) 0.25 ML CO.

Figure 9

(Color online) [(a)–(d)]: Relaxed CO molecule configuration on a 1:1 Ni/Cu surface alloy (a) 1.0 ML CO, (b) 0.75 ML CO, (c) 0.5 ML CO, and (d) 0.25 ML CO.

Figure 10

(Color online) [(a)–(d)]: Relaxed CO molecule configuration on a Ni/Cu(110) bimetallic surface with $\frac{1}{2}$ ML nickel in subsurface sites, forming a $\left(1\times 2\right)$ superstructure. (a) 1 ML CO, (b) 0.75 ML CO, (c) 0.5 ML CO, and (d) 0.25 ML CO.

Figure 11

(Color online) [(a)–(d)]: Relaxed CO molecule configuration on a Ni/Cu(110) bimetallic surface with $\frac{1}{4}$ ML adsorbed nickel and $\frac{1}{4}$ ML nickel in surface sites. (a) 1 ML CO, (b) 0.75 ML CO, (c) 0.5 ML CO, and (d) 0.25 ML CO.

Figure 12

(Color online) [(a)–(d)]: Relaxed CO molecule configuration on a Ni/Cu(110) bimetallic surface with $\frac{1}{4}$ ML nickel in surface sites and $\frac{1}{4}$ ML in subsurface sites. (a) 1 ML CO, (b) 0.75 ML CO, (c) 0.5 ML CO, and (d) 0.25 ML CO.

Figure 13

Total energies of five-layer copper slabs with $\left(2\times 2\right)$ unit cells, including two Ni atoms in various positions and different amounts of adsorbed CO. The arrangements of the nickel atoms correspond to the scenarios as depicted in Figs. 8, 9, 10, 11, 12. It is evident that without CO (upper curve) the Ni atoms tend to go into subsurface layers, whereas with adsorbed CO (lower curves) the Ni atoms tend to stay on the surface. For details see the text.

Figure 14

(Color online) Compilation of the experimental CO desorption peaks and the associated adsorption sites on the Ni-Cu(110) bimetallic surface, as obtained by DFT calculations. The experimental and the theoretical values for the CO desorption energies of the $\beta$ states are also inserted. Note that the theoretical values are those from Table , divided by the factor of 1.4, due to the overestimation of the CO binding energy, as described in the text.