CHx hydrogenation on Co(0001): A density functional theory study

Hydrogenation is an important process in the Fischer-Tropsch synthesis. In this work, all the elementary steps of the hydrogenation from C to CH4 are studied on both flat and stepped Co(0001) using density functional theory (DFT). We found that (i) CH3 hydrogenation (CH3+H-->CH4) is the most difficult one among all the elementary reactions on both surfaces, possessing barriers of around 1.0 eV; (ii) the other elementary reactions have the barriers below 0.9 eV on the flat and stepped surfaces; (iii) CH2 is the least stable species among all the CHx(x=1-3) species on both surfaces; and (iv) surface restructuring may have little effect on the CHx(x=0-3) hydrogenation. The barriers of each elementary step on both flat and stepped surfaces are similar and energy profiles are also similar. The reason as to why CHx hydrogenation is not structure-sensitive is also discussed. (C) 2005 American Institute of Physics.

CO dissociation and O removal on Co(0001): a density functional theory study

CO dissociation and O removal (water formation) are two important processes in the Fischer-Tropsch synthesis. In this study, both processes are studied on the flat and stepped Co(0 0 0 1) using density functional theory. It is found that (i) it is difficult for CO to dissociate on the flat Co(0 0 0 1) due to the high barrier of 1.04 eV relative to the CO molecule in the gas phase; (ii) the stepped Co(0 0 0 1) is much more favoured for CO dissociation; (iii) the first step in water formation, O + H --> OH, is unlikely to occur on the flat Co(0 0 0 1) due to the high barrier of 1.72 eV, however, this reaction can become feasible on steps where the barrier is reduced to 0.73 eV; and (iv) the barrier in the second step, OH + H --> H2O, on steps is higher than that on the flat surface. (C) 2004 Elsevier B.V. All rights reserved.

Insight into electron-mediated reaction mechanisms: Catalytic CO oxidation on a ruthenium surface

Ruthenium is one of the poorest catalysts for CO oxidation under normal conditions (low or medium O coverage and normal temperature). However, a recent study [Science 285, 1042 (1999)] reveals that, under femtosecond laser irradiation, CO2 can be formed on the Ru surface, and the reaction follows an electron-mediated mechanism. We carried out density functional theory calculations to investigate CO oxidation via an electron-mediated mechanism on Ru(0001). By comparison to the reaction under normal conditions, following features emerge in the electron-mediated mechanism: (i) more reaction channels are open; (ii) the reaction barrier is significantly lowered. The physical origins for these novel features have been analyzed. (C) 2001 American Institute of Physics.

Combined studies of DFT atomistic modelling and in situ FTIR spectroscopy on surface oxidants and CO oxidation at Ru electrodes

We report the combined studies of density functional theory (DFT) calculations and electrochemical in situ FTIR spectroscopy on surface oxidants and mechanisms of CO oxidation at the Ru(0001) electrodes. It is shown that CO can co-adsorb with both O and OH species at lower potential region where a low coverage of the (2 x 2)-O/OH adlayer formed; the oxidation of CO adsorbates takes place at higher potentials where a high coverage of the (1 x 1)-O/OH adlayer formed. Surface O species are not the active oxidants under all coverages studied, due to the high reaction barriers between CO and O (>1 eV). However, surface OH species with higher coverage are identified as the active oxidants, and CO oxidation takes place via a two-steps' mechanism of CO + 3OH -> COOH + 2OH -> CO2 + H2O + OH, in which three nearby OH species are involved in the CO2 formation: CO reacts with OH, forming COOH; COOH then transfers the H to a nearby OH to form H2O and CO2, at the same time, another H in the H2O transfers to a nearby OH to form a weak adsorbed H2O and a new OH. The reaction barrier of these processes is reduced significantly to around 0.50 eV. These new results not only provide an insight into surface active oxidants on Ru, which is directly relevant to fuel cell catalysis, but also reveals the extra complexity of catalytic reactions taking place at solid/liquid electrochemical interface in comparison to the relatively simpler ones at solid/gas phase. 

A density functional theory study of CO oxidation on Ru(0001) at low coverage

We have performed ab initio density functional theory calculations with the generalized gradient approximation to investigate CO oxidation on Ru(0001). Several reaction pathways and transition states are identified. A much higher reaction barrier compared to that on Pt(111) is determined, confirming that the Ru is very inactive for CO oxidation under UHV conditions. The origin of the reaction barrier was analyzed. It is found that in the transition state the chemisorbed O atom sits in an unfavorable bonding site and a significant competition for bonding with the same substrate atoms occurs between the CO and the chemisorbed O, resulting in the high barrier. Ab initio molecular dynamics calculations show that the activation of the chemisorbed O atom from the initial hcp hollow site (the most stable site) to the bridge site is the crucial step for the reaction. The CO oxidation on Ru(0001) via the Eley-Rideal mechanism has also been investigated. A comparison with previous theoretical work has been made. (C) 2000 American Institute of Physics. [S0021-9606(00)31223-5].

Integration of design of experiment, surface response methodology and multi-layer validation to predict the effect of blanching on colour of tomato juice

Response surface methodology was used to develop models to predict the effect of tomato cultivar, juice pH, blanching temperature and time on colour change of tomato juice after blanching. The juice from three tomato cultivars with adjusted pH levels ranging from 3.9 to 4.6 were blanched at temperatures from 60-100 °C for 1-5 min using the central composite design (CCD). The colour change was assessed by calculating the redness (a/b) and total colour change (∆E) after measuring the Hunter L, a and b values. Developed models for both redness and ∆E were significant (p<0.0001) with satisfactory coefficient of determination (R2 = 0.99 and 0.97) and low coefficient of variation (CV% = 1.89 and 7.23), respectively. Multilevel validation that was implemented revealed that the variation between the predicted and experimental values obtained for redness and ∆E were within the acceptable error range of 7.3 and 22.4%, respectively