CO formation

The hydrogenation of CO and C02 on transition metal surfaces is a promising area for using NEMCA to affect rates and selectivities. In a recent study of C02 hydrogenation on Rh,59 where the products were mainly CH and CO, under atmospheric pressure and at temperatures 300 to 500°C it was found that CH4 formation is electrophobic (Fig. 8.54a) while CO formation is electrophilic (Fig. 8.54b). Enhancement factor A values up to 220 were... 

The observed increase in CH formation and simultaneous decrease in CO formation with increasing catalyst potential and work function i.e. with increasing supply of O2 to the catalyst is remarkable and can be attributed to the preferential formation on the Rh surface of electron donor hydrogenated carbonylic species leading to formation of CH4 and to the decreasing coverage of more electron acceptor carbonylic species resulting in CO formation.59... 

The reaction was investigated under atmospheric pressure and at temperatures 500°C to 600°C, where the only product was CO, as Pd, contrary to Rh, does not adsorb C02 dissociatively.59 This difference in reaction pathway is also reflected in the NEMCA behaviour of the system, since in the present case CO formation is enhanced (by up to 600%) not only with decreasing catalyst potential and work function, but also enhanced, although to a minor extent, via catalyst potential increase (Fig. 8.56). Enhancement factor A values up to 150 were measured. The reaction exhibits typical inverted volcano behaviour, which is characteristic of the weak adsorption of the reactants at the elevated temperature of this investigation, and thus of promotional rule G4. 

C02 hydrogenation on Pd was investigated29 under atmospheric pressure and at temperatures 540°C to 605°C. The CO formation rate (reverse water-gas shift reaction) exhibits purely electrophilic behaviour with a rate increase by up to 600% with increasing sodium coverage (Fig. 9.20). This purely electrophilic behaviour is consistent with low reactant coverages and enhanced electron acceptor C02 adsorption on the Pd surface with increasing sodium coverage (Rule G2). 

Fig. 4. Effect of pinluile diameter on gaseous formation rates ai on carbon selectivity O, energy efficiency basal on LHV A, aietgy efficiency based on HHV , uqnit power 9, H2 formation rate A, CO formation rate gap distaiKe 6.0 mm QjHsOH cone., 50 mol% diaphragm fhickness, 1.0 mm.

The second channel, producing CO, was first observed by Seakins and Leone [64], who estimated 40% branching to this channel. Later measurements by Lockenberg et al. [65] and Preses et al. [66] concluded the branching to CO is 18%. Note that decomposition of formaldehyde formed in reaction (26a) is not a possible source of CO due to the large barrier for formaldehyde decomposition. Marcy et al. [67] recently combined time-resolved Lourier spectroscopy experiments with direct dynamics classical trajectory calculations to examine the mechanism of the CO product channel. They observed two pathways for CO formation, neither of which involve crossing a TS. 

The following isotopic labeling experiment was performed in order to quantify the contribution of the direct and indirect reaction routes to CO formation After steady-state reaction with CH4/02/He was achieved, an abrupt switch of the feed from CH4/02/He to an isotopic mixture of CH4/1 02/ C 02/He was made, in which the partial pressures of CH4 and 62 were kept exactly the same as in the ordinary CH4/02/He mixture, so as not to disturb the steady-state condition. However, C 02 was added to the isotopic mixture in an amount corresponding to approximately 10-15% of the CO2 produced during reaction of the mixture. The purpose was to measure the production of C 0 due to reforming of CH4 with C 02 only (indirect reaction scheme) under steady-state conditions of the working catalyst surface. Figure 3 shows the transient responses of and C O... 

The faradaic yield of CO formation on group II metals strongly depends on the value of the electrode potential. On silver and gold at definite potentials, yields up to 90 to 100% can be achieved. On zinc also, high yields (80%) were reported. 

The FTIR studies revealed that the formation of CO2 is only detected when the CO starts to be oxidized (Fig. 6.18). Therefore, it was proposed that the mechanism has only one path, with CO as the C02-forming intermediate [Chang et al., 1992 Vielstich and Xia, 1995]. This has two important and practical consequences. First, methanol oxidation will be catalyzed by the same adatoms that catalyze CO oxidation, mainly ruthenium. Second, since the steric requirements for CO formation from methanol are quite high, the catalytic activity of small (<4nm) nanoparticles diminishes [Park et al., 2002]. 

Cuesta A. 2006. At least three contiguous atoms are necessary for CO formation during methanol electrooxidation on platinum. J Am Chem Soc 128 13332-13333. 

Surface reactions that give rise to the electric crurent measmed depend on the electrode potential range and the type of measurements reported. The dominating reactions contributing to the voltammetric data in Fig. 13a-d are chemisorbed CO oxidation, oxidation of methanol to CO2 (on both scans), and chemisorbed CO formation on the reverse mn ... 

Overall, we demonstrated electrode potential- and time-dependent properties of the atop CO adsorbate generated from the formic acid decomposition process at three potentials, and addressed the issues of formic acid reactivity and poisoning [Samjeske and Osawa, 2005 Chen et al., 2003,2006]. There is also a consistency with the previous kinetic data obtained by electrochemical methods the maximum in formic acid decomposition rates was obtained at —0.025 V vs. Ag/AgCl or 0.25 V vs. RHE (cf. Fig. 12.7 in [Lu et al., 1999]). However, the exact path towards the CO formation is not clear, as the main reaction is the oxidation of the HCOOH molecule ... 

Kinetic analysis shows that the formation of tropone through a hydroxyphenyl-carbene intermediate (which exhibits the lowest activation energy 69.3 kcal/mol) dominates o-QM decomposition process up to 1500 K, with fulvene + CO formation becoming competitive at higher temperatures. In fact, the latter decomposition mode although disfavored by its higher activation enthalpy (75.4 versus 69.3 kcal/mol) becomes competitive due to its more positive activation entropy. 

Hawecker et a/.141 used Re(bpy)(CO)3X (X = Cl, Br) complexes as photosensitizers and succeeded in improving markedly the efficiency of CO formation using a system similar to that described above, where DMF was used as a solvent and 2,9-dimethyl- 1,10-phenanthroline was added, as a ligand for the cobalt ion, to a solution containing Ru(bpy)3+, Co2+, and triethanolamine. 

Oxalate is found predominantly via the dimerisation of the radical (cf. the results of Aylmer-Kelly et ai (1973) with CO formation being via route 3 followed by step 5. ... 

The authors concluded that steps (ii) and (iii) are certainly fast but that step (iv) must be sufficiently slow for the carboxylato complex to diffuse away from the electrode before CO formation takes place. 

Hu and Ruckenstein s results (130) showed that on the reduced nickel-containing catalyst, the reaction took place by a Langmuir-Hinshelwood mechanism involving adsorbed CH4 and oxygen species. Furthermore, they indicated that a slow dynamic redox process consisting of lattice oxygen formation and its reduction by carbon species was at least partly responsible for the CO formation. 

Fig. 11. CO formation rates determined from reactant conversions and product selectivities in a fixed-bed flow reactor for C02 reforming of CH4. The catalysts were nickel supported on La203, y-Al203, or CaO. Each catalyst contained 17 wt% Ni. Before reaction, the catalyst was reduced in flowing H2 at 773 K for at least 5 h and then at 1023 K for 2 h. Reaction conditions pressure, 1.0 atm temperature, 1023 K feed gas molar ratio, CH4/C02/He = 2/2/6 GHSV, 1,800,000 mL (g catalyst)-1 h-1 (227).

To allow for CO formation in the H adsorption region one can modify the reaction proposed earlier for formation of COH (31) ... 

From the comparison by Nyman et al. between plasma Hp level and different red cell destruction indices (half-life of CrB1-tagged erythrocytes, endogenous CO formation, stercobilin (urobilin) excretion in feces) it is evident that anhaptoglobinemia is a very common, but not an invariable accompaniment of increased red cell turnover. The exceptions are possibly persons with increased Hp synthesis. Individual differences in efficiency to destroy red cells outside the circulation without Hb return may be reflected by the wide normal variation range of plasma Hp as well by the promptness with which anhaptoglobinemia develops on even slight increase in Hb turnover. 

Ethanol and methane steam reforming reactions were studied assuming that the exit composition of the ethanol reformer depends on the steam reforming of methane. The competition for the same active site for ethanol and methane reforming maximizes the H2 and C02 production and minimizes the CO formation Catalysts were prepared by incipient wet impregnation. 20 wt% Ni supported on ZnO exhibited better performance compared to that supported on La203, MgO and A1203... 

H2 production technologies based on natural gas. Operating the reaction at relatively lower temperature, between 300 and 450 °C could minimize the CO formation because the equilibria for WGS and CO oxidation reactions are thermodynamically more favorable at lower temperatures. In order to achieve this goal, highly selective catalysts that are specific for reforming via acetaldehyde formation rather than ethanol decomposition to CH4 and/or ethylene are required. The success in the development of ethanol-based H2 production technology therefore relies on the development of a highly active, selective and stable catalyst. 

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