Interaction of carbon dioxide with Ni(110): A combined experimental and theoretical study

We present a combined experimental and theoretical study of the $\mathrm{C}{\mathrm{O}}_2$ interaction with the Ni(110) surface. Photoelectron spectroscopy, temperature-programmed desorption, and high-resolution electron energy loss spectroscopy measurements are performed at different coverages and for increasing surface temperature after adsorption at $90\mathrm{K}$ with the aim to study the competing processes of $\mathrm{C}{\mathrm{O}}_2$ dissociation and desorption. Simulations are performed within the framework of density functional theory using ab initio pseudopotentials, focusing on selected chemisorption geometries, determining the energetics and the structural and vibrational properties. Both experimental and theoretical vibrational frequencies yield consistent indications about two inequivalent adsorption sites that can be simultaneously populated at low temperature: short-bridge site with the molecular plane perpendicular to the surface and hollow site with the molecular plane inclined with respect to the surface. In both sites, the molecule has pure carbon or mixed oxygen-carbon coordination with the metal and is negatively charged and bent. Predicted energy barriers for adsorption and diffusion on the surface suggest a preferential adsorption path through the short-bridge site to the hollow site, which is compatible with the experimental findings. Theoretical results qualitatively support literature data concerning the increase of the work function upon chemisorption.

Figure 1

TPD spectra obtained on Ni(110) as a function of the initial $\mathrm{C}{\mathrm{O}}_2$ exposure at $90\mathrm{K}$. In the inset, a plot of the total and relative $\mathrm{C}{\mathrm{O}}_2$ desorbing coverage is shown, together with a quantification of the formed CO. The latter concentration is calculated via a cross-link between the TPD and the XPS data.

Figure 2

Oxygen $1s$ (left) and carbon $1s$ (right) XPS core level spectra $(h\nu =1253.6\mathrm{eV})$ upon exposure of the Ni(110) surface to $4\mathrm{L}$ of $\mathrm{C}{\mathrm{O}}_2$ at $90\mathrm{K}$ and subsequent stepwise annealing.

Figure 3

(Color online) HREELS spectra on Ni(110) (a) as a function of the initial $\mathrm{C}{\mathrm{O}}_2$ exposure at $87\mathrm{K}$ ($\mathrm{C}{\mathrm{O}}_2$ uptake, upper panel) and (b) of the surface initially exposed to $0.22\mathrm{L}$ of $\mathrm{C}{\mathrm{O}}_2$ at $87\mathrm{K}$ and subsequently heated to different temperatures ($\mathrm{C}{\mathrm{O}}_2$ annealing, lower panel).

Figure 4

Relative surface coverage (right scale) of the physisorbed and chemisorbed $\mathrm{C}{\mathrm{O}}_2$ species as a function of the total surface coverage, obtained by cross-linking CO and $\mathrm{C}{\mathrm{O}}_2$ XPS data with the TPD series obtained as a function of the $\mathrm{C}{\mathrm{O}}_2$ exposure at low temperature. Data for work function variation in the same low-temperature conditions from Bartos (Ref. 5) are also reported (left scale).

Figure 5

(Color online) Geometries of $\mathrm{C}{\mathrm{O}}_2$ adsorbed on different sites on Ni(110) surface [short bridge ($\mathrm{SB}\text{−}{C}_{2v}$ and $\mathrm{SB}\text{−}{C}_s$) and hollow up (HU)], transition states along the gas $\text{phase}\to \mathrm{SB}$ path and along the $\mathrm{SB}\leftrightarrow \mathrm{HU}$ diffusion path (${\mathrm{TS}}^{\mathit{ads}}$ and ${\mathrm{TS}}^{\mathit{diff}}$), and linear weakly bound (“phys.”) configuration (see text). Left panel, schematic top view of the $(3\times 2)$ cell; right panels, ball and stick models generated using XCRYSDEN (Ref. 40). Distances are in Å.

Figure 6

(Color online) Energy diagram for different $\mathrm{C}{\mathrm{O}}_2$ adsorption sites on Ni(110) and relative barriers. The data reported are obtained from GGA double-side five-layer supercell calculations. The zero of the energy is set to the nondissociated molecule at infinite distance from the surface.