RESUMO
Experimentally, steric and inductive effects have been suggested as key parameters in the adsorption and reactivity of alcohols on transition-metal (TM) surfaces, however, our atomistic understanding of the behavior of alcohols in catalysis is far from satisfactory, in particular, due to the role of hydroxy groups in the adsorption properties of C3 alcohols on TM surfaces. In this study, we investigated those effects through ab initio calculations based on density functional theory employing a semilocal exchange-correlation functional within van der Waals corrections (the D3 framework) for the adsorption of C3 alcohols with different numbers and positions of OH groups, namely, propane, 1-propanol, 2-propanol, 1,2-propanediol, 1,3-propanediol and glycerol, on the compact Ni(111), Pd(111) and Pt(111) surfaces. As expected, we found that the adsorption energy is affected by the number of hydroxy groups with similar values for each pair of regioisomers, which clearly indicates the effect of the number of OH groups. Based on Bader charge analysis, we found an effective charge transfer from the C3 molecules to the substrates, which can explain the reduction in the work function due to adsorption. Upon adsorption, the alpha carbon to the OH group closest to the surface and the central carbon are the most positively charged atoms, which increases the lability of their bonded H atoms. In addition, the depletion of electron density between the C-H and O-H bonds closer to the surfaces corroborated their stretching, suggesting that the proximity of the adsorbates to the surfaces affects the acidity of these H atoms, as well as inductive effects within the molecules.
RESUMO
Experimental and theoretical studies have suggested the use of strain effects to design efficient catalysts for direct alcohol fuel cells. However, our atomistic understanding of the adsorption of alcohols on strained transition-metal (TM) catalysts is still far from satisfactory. Here, we report an ab initio investigation based on density functional theory within the van der Waals D3 correction to explore the adsorption properties of methanol, ethanol and glycerol on Pt3Ni(111) alloys under different conditions. For that, we selected five TM substrates, namely, (i) Ni(111), (ii) Pt12Ni4/Pt12Ni4/Ni(111) (compressive strain), (iii) Pt16/Pt8Ni8/Pt3Ni(111) (without strain), (iv) Pt16/Pt8Ni8/Pt(111) (tensile strain) and (v) Pt(111). As expected, the physical and chemical properties of the Pt3Ni thin-layers are affected by the strain induced by the underlayer TM substrate, and hence, we can tune the adsorption properties of alcohols. In general, the magnitude of the alcohol adsorption energy increases in the following order Pt16/Pt8Ni8/Pt3Ni(111) < Pt16/Pt8Ni8/Pt(111) < Pt12Ni4/Pt12Ni4/Ni(111), which correlates with the d-band center and the effective charge on the adsorption sites, i.e., the coulombic contribution plays an important role in the adsorption. Structural and electronic density analyses indicate that, upon adsorption, the O-H and C-H bonds weaken and their breaking should be the first steps in the decomposition of alcohols. From the Bader charge analysis, we found that the TM atom directly below the bonding O loses charge to neighboring atoms, which polarizes the surface and changes the substrate work function. Although a significant enhancement of energetic and structural properties was found, the addition of the D3 correction does not change our qualitative results except for improving the dependence of the adsorption energy with the alcohol size.
RESUMO
The adsorption of Zr on the CeO2 surfaces can lead to the formation of ZrO2-like structures, which can play a crucial role in the catalytic properties of Ce x Zr1-x O2 as support for transition-metal catalysts; however, our atomistic understanding is far from satisfactory, and hence, it affects our capacity to engineer the combination of ZrO2-CeO2 for catalysis applications. Here, we investigate the adsorption of Zr n (n = 1 - 4) atoms on CeO2(111) surfaces through density functional theory with the Hubbard model and bring new insights into the Zr-CeO2 interaction and the formation of ZrO2-like structures on ceria. We found that the Zr atoms oxidize to Zr4+ and strongly interact with the O2- anions, reducing the surface Ce4+ cations to Ce3+ (4 Ce atoms per Zr adatom), which stabilizes the system by more than 10 eV per Zr. As more Zr is adsorbed, the O2- species migrate from the sub-surface to interact with the on-surface Zr adatoms in hcp sites, producing a full ZrO2-like monolayer, which contributes to reduce the strain induced by the increased size of the Ce3+ cations compared with Ce4+. The simulated partial and full ZrO2-like structure thicknesses agree with the experimental measurements. In addition, we found an unprecedented trend for the on-surface Zr atoms: our calculations show that they are less stable than Zr replacing Ce3+ atoms from the first cation layer. Therefore, under sufficiently high temperatures, one expects the formation of a Ce2O3-like/c-ZrO2/CeO2 structure, which may completely change the reactivity of the surface.