The relationship between adsorption energies of methyl on metals and the metallic electronic properties: A first‐principles DFT study

Abstract

A theoretical study of CH3 adsorbed on the (111) surface of some transition and noble metal surfaces M (M = Cu, Ni, Rh, Pt, Pd, Ag, Au) and on the Fe(100) is presented. We find that the hollow site is preferred more than the top one for Fe, Ni, Rh, and Cu, but it is the other way for Pt, Pd, Au, and Ag. In addition, a good linear relationship was observed between the chemisorption energy and d-band center for Group VIII metals or the square of the coupling matrix element for Group IB metals at the hollow site. Interestingly, with a detailed comparison of the adsorption energies at the top and hollow sites, we find that the adsorption energies among each group are very similar on the top site, which supports the theoretical model of Hammer and Norskov that the coupling between the HOMO of adsorbate and sp states of the metal is dominant and almost equal, and that the second coupling to the d-band contributes less but reflects the change of the adsorption energy. It confirms that the coupling to the d band comprises two opposite factors, that is, the d-band center was attractive and the square of the coupling matrix element was repulsive, such that the contributions from the two factors can counteract each other at the top site.

The relationship between formate adsorption energy and electronic properties: A first principles density functional theory study

“…The trend of our adsorption energy successfully satisfies the above linear relationship (Figure 4) for the chemisorption systems of HCOO adsorbed on these 9 metal surfaces, just as previous studies on CH 3 O as well as CH 3 adsorbed on the same metal surface site [9,10] . Moreover, we also found a similar formula of adsorption energy (Q) with ε d and 2 ad V as two parameters as follows:…”

“…Both ε d and 2 ad V reflect the properties of metal d-band, but we must determine which effect is the dominant factor for a given metal, because their effect is different: ε d will be attractive and 2 ad V will be repulsive. According to previous model [7,9,10] , it showed that the closer the d-band center to the Fermi level, the higher the adsorption energy. On the contrary, the larger the 2 ad V , the stronger the overlap, and the larger the repulsion, then inducing a smaller adsorption energy.…”

“…In our previous work [9,10] we found that two parameters (the d-band center (ε d ) or the square of coupling matrix element( 2 ad V )) of the surface could be successfully correlated with its chemisorption and reaction properties for the given adsorption site, on the basis of the theoretical model of Hammer and Nørskov [7,8] . In their model, the formation of bond between adsorbate and metal surface is divided into two steps (as shown in Figure 3): cou- Figure 3 Schematic illustration of the interaction between two electronic states; E F1 and E F2 are the Fermi level of noble metals and transition metals, respectively [9] .…”

“…For the noble metals (group IB), such as copper, silver and gold, they have a filled and deep-lying d band well below the Fermi level, so the coupled anti-bonding state is completely filled, and is difficult to be pushed above the Fermi level. It has been tested that the variation in adsorption strength from copper to gold is given mainly by the Pauli repulsion [7,9,10] . This can be easily un-derstood that, as a general rule, the 5 d orbital are more extended than 4 d orbital, which are in turn more ex-tended than the 3 d orbital, and it means that the extent of the orbital overlap increases along the Cu, Ag, Au, so does the coupling element and repulsive interaction.…”

“…However, in some cases, the adsorption energy has the opposite trend, i.e., the closer the d-band center is to the Fermi level, the lower the adsorption energy is, suggesting that the metal sp band also has an important effect as the d-band does in determining the adsorption strength [3] . In our previous work, we have found that the adsorption energy of CH 3 O as well as CH 3 can be correlated with the d-band center for the transition metals or the square of couple matrix element for the IB metals [9,10] . In the present work, we will extend our conclusion to the formate adsorption, i.e., to test if there is a correlation of formate (HCOO) adsorption on some metals such as Cu, Ag, Au, Pd, Pt, Ni, Fe, and Mo with the corresponding electronic properties such as d-band center as well as the couple matrix element.…”