Dissociative adsorption

Computational chemistry has reached a level in which adsorption, dissociation and formation of new bonds can be described with reasonable accuracy. Consequently trends in reactivity patterns can be very well predicted nowadays. Such theoretical studies have had a strong impact in the field of heterogeneous catalysis, particularly because many experimental data are available for comparison from surface science studies (e.g. heats of adsorption, adsorption geometries, vibrational frequencies, activation energies of elementary reaction steps) to validate theoretical predictions. 

Ligand effect CO adsorption is lowered by alloying, thus decreasing CO coverage and increasing sites available for H2 adsorption/ dissociation and oxidation. 

This section introduces the principal experimental methods used to study the dynamics of bond making/breaking at surfaces. The aim is to measure atomic/molecular adsorption, dissociation, scattering or desorption probabilities with as much experimental resolution as possible. For example, the most detailed description of dissociation of a diatomic molecule at a surface would involve measurements of the dependence of the dissociation probability (sticking coefficient) S on various experimentally controllable variables, e.g., S 0 , v, J, M, Ts). In a similar manner, detailed measurements of the associative desorption flux Df may yield Df (Ef, 6f, v, 7, M, Ts) where Ef is the produced molecular translational energy, 6f is the angle of desorption from the surface and v, J and M are the quantum numbers for the associatively desorbed molecule. Since dissociative adsorption and... 

For heterogeneous systems, the set of reactions includes adsorption, dissociation, surface diffusion, desorption, and other processes. In this case, the rates of processes can differ by many orders of magnitude. In accordance with Eq. (30), the time step is determined by the fastest process in the system. This condition strongly restricts the maximum real time in the simulation and prevents modeling of rare processes. One of the ways of overcoming this problem can be to exclude all fast processes from the table of reactions and use equilibrium distributions for these processes. For example,... 

The requirements that electrodes have to meet are different from the previous ones. Like the electrolyte, they have to be chemically and thermodynamically compatible with the neighboring phase, are however—unlike the electrolyte—not subject to substantial chemical potential gradients. Besides exhibiting high electronic conductivities, they must catalyze the electrode reaction, i.e., enable adsorption, dissociation, ionization, and charge transfer into the electrolyte to occur with sufficient reaction rates. 

Baraldi and co-workers [52] have described a wealth of dynamic XPS studies on surface reactions, including adsorption, dissociation, desorption, and even catalytic reactions, such as the epoxidation of alkenes [60], and the reduction of NO by H2 and CO [61]. 

D. CO Adsorption, Dissociation, and Oxidation on Pt(l 1 1) and Platinum Nanoparticles Supported on Si02... 

In light of the open questions related to CO adsorption/dissociation on Rh(l 1 1), Pery et al 314) carried out a systematic SFG/AES study of CO on Rh(l 1 1), at pressures from 10 to 1000 mbar and temperatures from 300 to 800 K. Figures 38a and b show a series of SFG spectra recorded at 300 K and a comparison of spectra at 10 mbar before and after the atmospheric pressure gas exposure. All spectra are dominated by a single vibrational peak at 2053-2075 cm typical of CO terminally bonded to a single Rh atom, with a small peak at about 1900 cm characterizing CO on threefold hollow sites (see, e.g., the 500-mbar spectrum). The intensity difference between the two peaks again points to the lower sensitivity of... 

SFG spectra of CO adsorbed on nickel have been reported 116,118,416,417), as have spectra characterizing NH3 adsorption/dissociation on Fe(l 1 1) 418). UHV SFG investigations of formic acid decomposition on NiO(l 1 1) were also reported 419,420). Investigations of ruthenium surfaces 147,148,157,421-425) and of CO adsorbed on Ir(l 1 1) are also available 426). 

The interaction of CO2 with group VIII metals was reviewed by Solymosi (29). At 80 K, the adsorption on a Rh field emitter exhibits an interesting crystal face dependency 30). In addition to molecular adsorption, dissociation occurs at an appreciable rate on the stepped surfaces around (111) and (100) at temperatures higher than 220 K. The field electron microscopy (FEM) patterns suggest that the surface structure of Rh has a striking influence on the ability of the metal to dissociate the CO2 molecule. From... 

Associative adsorption Dissociative adsorption Associative adsorption Dissociative adsorption... 

Fernandez et al. used a combined high throughput electrocatalyst study and DFT study to examine Pd-Co electrocatalysts for oxygen reduction. They screened an array of electrocatalysts using a scanning electrochemical microscope to assess the activity of each element. The DFT study was performed to help identify the role of Co in the system. They examined oxygen molecule adsorption, dissociation and... 

Reuter, Frenkel, and Scheffler have recently used DFT-based calculations to predict the CO turnover frequency on RuO2(110) as a function of 02 pressure, CO pressure, and temperature.31 This was an ambitious undertaking, and as we will see below, remarkably successful. Much of this work was motivated by the earlier success of ab initio thermodynamics, a topic that is reviewed more fully below in section 3.1. The goal of Reuter et al. s work was to derive a lattice model for adsorption, dissociation, surface diffusion, surface reaction, and desorption on defect-free Ru02(l 10) in which the rates of each elementary step were calculated from DFT via transition state theory (TST). As mentioned above, experimental evidence strongly indicates that surface defects do not play a dominant role in this system, so neglecting them entirely is a reasonable approach. The DFT calculations were performed using a GGA full-potential... 

The values of PZC referred to as cip and pH" in Table 3.1 are obtained by acid-base titration. The experimental procedure is basically the same (although many variants have been described), and the substantial difference is in the method of selection of the zero point. Titration gives very accurate changes in uo from one pH value to another (or at least the part of due to proton adsorption/dissociation) without assumptions, but some assumptions are necessary to obtain the zero point. Existence of CIP does not prove the absence of specifically adsorbed ions. Lyklema [44] showed that in case of specific adsorption of metal cations the shift in the CIP to pH below the pristine PZC is more pronounced for metals having stronger affinity toward the surface (e.g. Pb > Ca) and the do at CIP is also more positive for more strongly bound cations. 

It was verified in this study that NO adsorption, dissociation and adsorbed N-atom recombination is the reaction path for N2 formation on reduced supported platinum catalyst. 

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