Why Must Oxygen Atoms Be Activated from Hollow Sites to Bridge Sites in Catalytic CO Oxidation?

Abstract

There is no doubt that oxidation involving oxygen is one of the most important catalytic reactions: [1][2][3][4][5] Oxygen is a very active species and participates in many catalytic reactions, such as CO oxidation, epoxidation and fuel reforming. Taking catalytic CO oxidation as an example, which is very important not only technologically (in car exhaust emission control, CO 2 lasers, and air purification) but also scientifically, the elementary steps of CO oxidation at low and medium pressures are believed to be as follows: 1 (i) O 2 molecules dissociate on a metal surface, resulting in O atom chemisorption; (ii) CO molecules adsorb on the metal surface and then react with the chemisorbed O atoms, forming CO 2 (the Langmuir-Hinshelwood mechanism); and (iii) the product, CO 2 , desorbs from the metal surface. Recently, Zhang, Hu, and Alavi 6 have found theoretically that there are two crucial events in CO oxidation on a Ru surface. First, O atoms must be activated from hollow sites (usually the most stable site) to bridge sites, and second, CO molecules have to approach the activated O atoms from the correct direction and at an appropriate time. The reaction barrier is predominately determined by O-metal bond breaking. 6,7 Experimental evidence 8-10 also suggests that O activation is essential. Then fundamental questions are the following: Why must O atoms be activated from hollow sites to bridge sites for CO oxidation to occur? Is it likely to be true for other oxidation reactions in heterogeneous catalysis? Obviously, these issues are fundamental for understanding catalytic oxidation and may also be important to catalysis in general. In this paper, we show that the necessity of the O activation is also true for CO oxidation on Rh(111). We aim to answer the above questions.We carried out Density Functional Theory calculations for CO oxidation on Rh(111). A generalized gradient approximation 11 was utilized in the calculations. The electronic wave functions were expanded in a plane wave basis set and the ionic cores were described with ultrasoft pseudopotentials. 12 The surface was modeled by a p(2 × 2) unit cell with a slab of three layers of Rh(111). The vacuum region between slabs was 10 Å. A cutoff energy of 300 eV and 2 × 2 × 1 k-point sampling within the surface Brillouin zone were used. Recent work shows 13-16 that this set-up provides sufficient accuracy. In all the calculations, the bottom two layers of Rh atoms were held fixed in their bulk positions, while the top layer of surface atoms was allowed to relax. Transition states (TS's) were searched with a constrained minimization technique. 6,7,16 The TS is identified when (i) the forces on the atoms vanish and (ii) the energy is a maximum along the reaction coordinate, but a minimum with respect to all remaining degrees of freedom.Two TS's for CO oxidation on Rh(111) have been identified, which are shown in Figure 1a,b. The reaction barriers from these two TS's are very similar, being 0.99 and 1.13 eV for TS's (a) and (b), respectively. These two TS's...

Chemical reactions at surfaces and interfaces from first principles: Theory and application

“…At the FS, the nascent CO 2 sits on the top site of Pt{111}, weakly bonded on the surface. In addition, CO oxidation on other close-packed metal surfaces, Ru{0001}, Rh{111}, Pd{111}, Ir{111}, and an alloy surface Cu 3 Pt{111} have been performed [47][48][49][50][51][52][53][54][55][56]. It appears that the reaction pathways on the close-packed metal surfaces are very similar [49], which can be generalized as three common features.…”

“…• No bonding competition TS geometry [50]. This is because at the TS, no surface atom bonds simultaneously with both the CO and O (see Fig.…”

“…By comparing the local density state of the O on the hollow and the bridge site, Zhang and Hu [50] explained why the O needs to be activated. At the hollow site, the O bonds with three metal atoms and its p orbitals are largely saturated.…”

“…For example, CO oxidation on Pt{111} has the lowest barrier (0.7 ~ 0.8 eV), and on Ru{0001} has the highest barrier (~1.5 eV); CO oxidation on Pd{111} is sensitive to the CO coverage: at a high CO coverage, the barrier can be much lower (~0.9 eV) compared to the ~1.4 eV at a low CO coverage (this is due to the change of initial CO position on the surface at different CO coverages [50,53,54]). It should be noted that due to the usages of different calculation setups (basis sets, pseudopotential, etc.)…”

Carboxylic acid formation by hydroxyl insertion into acyl moieties on late transition metals

“…A shift in adsorption site from hollow to bridge position of a (bare) oxygen atom in preparation for a transition state of O insertion into CO on a Rh(111) surface has previously been rationalized as a way to "free" the p y orbital of the O atom to form the new C-O bond. 34,35 In its equilibrium structure, the oxygen atom of OH extends one bond to the H atom and the remaining valence p orbitals are involved in forming M-O 2 H bonds. Assuming the situation to be sufficiently analogous to O insertion into a CO adsorbate on Rh(111), 34,35 we thus expect the number of M-O 2 H interactions to be reduced during OH insertion.…”