Quantum-mechanical process of carbonate complex formation and large-scale anisotropy in the adsorption energy of CO2 on anatase TiO2

Abstract

Adsorption of CO 2 on a semiconductor surface is a prerequisite for its photocatalytic reduction. Owing to superior photocorrosion resistance, nontoxicity and suitable band edge positions, TiO 2 is considered to be the most efficient photocatalyst for facilitating redox reactions. However, due to the absence of adequate understanding of the mechanism of adsorption, the CO 2 conversion efficiency on TiO 2 surfaces has not been maximized. While anatase TiO 2 (101) is the most stable facet, the (001) surface is more reactive and it has been experimentally shown that the stability can be reversed and a larger percentage (up to ~ 89%) of the (001) facet can be synthesized in the presence fluorine ions. Therefore, through density functional calculations we have investigated the CO 2 adsorption on TiO 2 (001) surface. We have developed a three-state quantum-mechanical model that explains the mechanism of chemisorption, leading to the formation of a tridentate carbonate complex. The electronic structure analysis reveals that the CO 2 -TiO 2 interaction at the surface is uniaxial and long ranged, which gives rise to anisotropy in binding energy (BE). It negates the widely perceived one-to-one correspondence between coverage and BE and infers that the spatial distribution of CO 2 primarily determines the BE. A conceptual experiment is devised where the CO 2 concentration and flow direction can be controlled to tune the BE within a large window of ~1.5 eV. The experiment also reveals that a maximum of 50% coverage can be achieved for chemisorption. In the presence of water, the activated carbonate complex forms a bicarbonate complex by overcoming a potential barrier of ~0.9 eV. * Electronic address: nandab@iitm.ac.in 2 I.

Electronic Structure of Graphene/TiO$_2$ Interface: Design and Functional Perspectives

“…The dispersion corrections have been included through the semi-empirical Grimme-D2 van der Waal's correction [34]. The calculated lattice parameters of anatase in bulk phase are a = 3.794 Å, c = 9.754 Å which agree well with the previously reported experimental and theoretical values [35][36][37]. The anatase (001) surface is created using slab geometry with four TiO 2 layers (thickness ∼ 8.5 Å) which is observed to be sufficiently thick enough [21,37] and a 15 Å vacuum along the out-ofplane direction.…”

Facet Dependent Catalytic Activities of Anatase TiO$_2$ for CO$_2$ Adsorption and Conversion

“…The adsorption of CO 2 on a-TiO 2 (001)-(1×1) surface was discussed in detail in our previous studies [38] , but for comparison purposes to other surfaces, we have summarized these calculations for a 3 × 2 slab model as shown in Figure 2. As discussed in our earlier work, the CO 2 molecule is weakly adsorbed over ontop O 3f , Ti 5f and hollow positions (Figure 2(a-c)), and retains its linear configuration.…”

“…These are IV and V for unreconstructed(001); I, II and III of (1×4) reconstructed (001); III and V of (010); and I and IV of (101) surface. The strength of adsorptions for each of these cases are presented through a three state model [38] which are schematically illustrated in Figure 6. In this model, states-A and C represent the situation before and after adsorption, respectively.…”

“…Although several theoretical reports have discussed the CO 2 adsorption and coadsorption with H 2 O on (001) and (101) a-TiO 2 surface [32][33][34][35][36][37][38] , a comparative study showing the reactivity order and its correlation with the surface electronic structures have not been presented. Also, the theoretical study of CO 2 adsorption on a-TiO 2 (010) surface remains least explored [6] .…”

“…So far the research have been focused towards CO 2 adsorption and conversion on a-TiO 2 (001) surface [34,35,38] . The CO 2 adsorption on the reconstructed (001)-(1×4) surface have not been explored so far.…”