| 1. |
- Acke, Filip, et al.
(författare)
-
Study of the reduction and reoxidation of a CaO surface
- 1997
-
Ingår i: J Phys Chem B. - 1520-5207 .- 1520-6106. ; 101:33
-
Tidskriftsartikel (refereegranskat)abstract
- The reduction of NO with CO and H2 is shown to comprise two basic reactions: a surface oxygen abstraction by the reducing agent and a reoxidation of the surface by NO. The former reaction step has been demonstrated by transient CO2 formation during CO exposure of oxidized CaO surfaces, while the latter was demonstrated by N2 and/or N2O transient formation during NO exposure of a prereduced CaO surface. It was shown that at low temperatures (between room temperature and 500 °C) both N2 and N2O were formed, but at temperatures above 500 °C only N2 was observed. The activation energies of the respective steps have been determined using temperature-programmed reaction experiments. The activation energy of the surface oxygen abstraction was determined to be 25 kcal/mol and is similar to the apparent activation energy of the overall reaction. The activation energy of the NO bond breakage was determined to be maximum 10 kcal/mol as measured by N2O formation. The importance of an N2O2- or N2O22- intermediate in the formation of N2O will be discussed, and the importance of N2O decomposition in forming N2 at temperatures above 500 °C will be compared with a N surface diffusion mechanism.
|
|
| 2. |
- Snis, Anders, et al.
(författare)
-
Abstraction of preadsorbed oxygen from CaO(100) by carbon monoxide
- 1994
-
Ingår i: Surf Sci Lett. - 0039-6028. ; 310:1-3, s. L579-L582
-
Tidskriftsartikel (refereegranskat)abstract
- The mechanism for abstraction of preadsorbed oxygen from CaO(100) by CO(g) to form CO2(g) is investigated by electronic structure calculations. The reaction mechanism is assumed to be the reverse of the one obtained for the decomposition of N2O(g) at CaO(100) [Surf. Sci 292 (1993) 317]. A 32 kcal/mol barrier for reaction is calculated. Adsorbed oxygen, Oads, to surface vibration frequencies in the range 840–930 cm−1 are calculated. These values are in qualitative agreement with experiment.
|
|
| 3. |
- Snis, Anders, et al.
(författare)
-
N2O adsorption and decomposition at a CaO (100) surface studied by means of theory
- 1993
-
Ingår i: Surf Sci. - 0039-6028. ; 292:3, s. 317-324
-
Tidskriftsartikel (refereegranskat)abstract
- The adsorption and decomposition of an N2O molecule at different sites on a CaO(s) surface are investigated by means of ab initio quantum chemistry. The calcium, Ca2+, and oxygen, Os2−, sites at a perfect (100) surface and at a corner position, Oc2−, are considered. Adsorption energies at different sites are calculated and the largest value, 6 kcal/mol, is obtained for a corner site. The barrier for dissociation is calculated to 26 and 27 kcal/mol at the Oc2− and Os2− sites, respectively. These values are some 10 kcal/mol lower than the experimental estimate, and the discrepancy is understood from methodological difficulties to describe the free N2O molecule. A mechanism for the dissociation over an O2− site is proposed, whereby the transfer of the O atom goes via a linear N-N … O … O2− transition state.
|
|
| 4. |
- Acke, Filip, et al.
(författare)
-
Activation energy of calcination by means of a temperature programmed reaction technique
- 1998
-
Ingår i: Thermochimica Acta. - 0040-6031. ; 306:1-2, s. 73-76
-
Tidskriftsartikel (refereegranskat)abstract
- Calcination of limestone is considered to demonstrate the performance of a temperature programmed reaction technique to determine apparent activation energies for decomposition in the solid state. Influences of sample weight, particle size and sweep gas (He and Ar) are discussed, and connections are made to thermogravimetry studies in the literature.
|
|
| 5. |
- Acke, Filip, et al.
(författare)
-
Effects of O-2 on the reduction of NO over prereduced CaO surfaces: A mechanistic understanding
- 1999
-
Ingår i: Journal of Physical Chemistry B Materials. - 1089-5647. ; 110:12, s. 2195-2201
-
Tidskriftsartikel (refereegranskat)abstract
- The effect of O-2 on the reduction of NO over prereduced CaO surfaces is investigated. The experimental results suggest the existence of at least three different reaction channels, of which two are related to the high-temperature reduction of the CaO surfaces and involve the use of extra electrons in breaking the NO bond. The third reaction channel does not employ extra electrons for bond breaking, but the activity is affected by the amount of adsorbed surface oxygens. The difference between the former two reaction channels is found in the temperature needed for an observable activity. The reaction channel which is already active at low temperatures is described by a model based on F-centers, whereas the one which needs elevated temperatures involves a hole transport through the bulk. The activation energy for this transport is determined experimentally using a temperature-programmed reaction technique as well as theoretically by means of ab initio quantum chemistry calculations. Room-temperature exposure to O-2 is suggested to result in a poisoning of the F-centers, but has only a minor effect on the reaction channel proposed for high temperatures. Effects on the reduction of NO of time as well as temperature for the O-2 exposure step are also investigated and found to be consistent with an understanding based on the coexistence of different reaction channels.
|
|
| 6. |
- Acke, Filip, et al.
(författare)
-
Kinetic study of heterogeneous oxygen-exchange reactions and bulk self-diffusion of oxygen
- 1998
-
Ingår i: Journal of Physical Chemistry B Materials. - 1089-5647. ; 102:26, s. 5158-5164
-
Tidskriftsartikel (refereegranskat)abstract
- Apparent activation energies are determined for oxygen-isotope exchange reactions between O2, CO, or NO and preoxidized or prereduced CaO surfaces. Oxygen exchange between N16O or C16O and isotope labeled Ca18O surfaces produced an apparent activation energy of 2 kcal/mol. Similar values are obtained for single- isotope-exchange between 18O18O and prereduced Ca16O surfaces. Apparent activation energies of 15−18 kcal/mol are found for single and double exchange between 18O18O and preoxidized Ca16O surfaces, as well as for double exchange between 18O18O and prereduced Ca16O surfaces. The low apparent activation energies are believed to result from adsorbed intermediates, whereas the high values may involve the formation of a singlet O2 transient. It is shown that eventually the self-diffusion of oxygen ions in the bulk becomes the rate-determining step in the isotope-exchange reaction. Apparent activation energies are determined, and the values are found to depend on surface treatment, (i.e., 44 kcal/mol under reducing and 78 kcal/mol under oxidizing conditions). The involvement of oxygen vacancies under reducing conditions is discussed.
|
|
| 7. |
- Acke, Filip, et al.
(författare)
-
Promoting effects of Na and Fe impurities on the catalytic activity of CaO in the reduction of NO by CO and H-2
- 1998
-
Ingår i: Journal of Physical Chemistry B Materials. - 1089-5647. ; 102:26, s. 5127-5134
-
Tidskriftsartikel (refereegranskat)abstract
- The heterogeneous reduction of NO by H-2 and CO over different CaO materials is investigated. The dependence of the specific NO reduction rate on the impurity content is demonstrated for both reducing species. The roles of two specific impurities, i.e., Na and Fe, as well as their combined effect are investigated. The apparent activation energies for the NO + CO and NO + Hz reactions are determined for three different calcium oxides. Values between 26 and 28 kcal/mol are obtained. The influence of impurity content is found in the preexponential factor of the Arrhenius equation. A reaction mechanism based on a rate-determining surface-oxygen-abstraction step is suggested. This mechanistic understanding is explored to compare the activities of other alkaline-earth oxides. Particularly, a linear correlation between the apparent activation energy and the lattice parameter is observed.
|
|
| 8. |
- Andrews, L, et al.
(författare)
-
Infrared spectra of cis and trans-(NO)(2)(-) anions in solid argon
- 1998
-
Ingår i: Journal of Chemical Physics. - 1089-7690 .- 0021-9606. ; 109:1, s. 177-185
-
Tidskriftsartikel (refereegranskat)abstract
- Laser-ablation of over 20 different metal targets with concurrent 10 K codeposition of Ar/NO mixtures produces metal independent infrared bands at 1589.3 cm−1 due to (NO)2+, a new absorption at 1221.0 cm−1, and a band set at 1300.3, 1222.7, 884.4 cm−1. The latter bands decrease more on annealing than the 1221.0 cm−1 band. Isotopic substitution (14NO,15NO, 15N18O, and mixtures) shows that these new vibrations involve two equivalent N–O oscillators, which identifies two new (NO)2 species. The excellent agreement with frequencies, intensities, and isotopic frequency ratios from density functional theory calculations substantiates assignment of the 1221.0 cm−1 band to trans-(NO)2− and the three band set to cis-(NO)2−. The observation of a weak combination band at 2492.0 cm−1 further substantiates assignment of the two N–O stretching modes in cis-(NO)2−
|
|
| 9. |
- Antonsson, H, et al.
(författare)
-
VIBRATIONAL MOTION AND GEOMETRICAL STRUCTURE IN ADSORBED CO STUDIED BY CORE LEVEL PHOTOELECTRON-SPECTROSCOPY
- 1990
-
Ingår i: Journal of Electron Spectroscopy and Related Phenomena. - 0368-2048. ; 54, s. 601-613
-
Tidskriftsartikel (refereegranskat)abstract
- High resolution core level spectra from CO adsorbed on clean and hydrogen precovered Ni(100) and CO adsorbed on Cu(100) are presented. The core level binding energy is shown to be sensitive to the adsorption site. Cluster calculations reproduce the general trend of the binding energy shifts between the on top and hollow sites of CO/Ni(100). In the coadsorption system CO/H/Ni(100) three different adsorption sites have been observed with a maximum binding energy shift of 2.6 eV for the Ols level. The temperature dependence of the Cls and Ols line profiles in CO/Ni(100) has been carefully investigated. The temperature dependent broadening is due to thermal excitations of frustrated translations parallel to the surface. The spectra from CO on Cu(100) show no temperature dependence below 200K.
|
|
| 10. |
- Bauschlicher, C.W., et al.
(författare)
-
A further study of the products of scandium and dioxygen reactions
- 1999
-
Ingår i: Journal of Physical Chemistry A. - 1089-5639 .- 1520-5215. ; 103:28, s. 5463-5467
-
Tidskriftsartikel (refereegranskat)abstract
- The products of the reaction of Sc and dioxygen have been reinvestigated. By adding the electron-trapping molecule CCl4 additional information about the IR spectra has been obtained, as well as the observation of new bands. New ab initio calculations are also performed on possible products of the Sc plus O-2 reaction. The previously observed band at 722.5 cm(-1) is assigned as the b(2) mode of ScO2-. Bands arising from ScO+, Sc(O-2)(+), and(O-2)ScO are also assigned. We are still unable to assign any bands to OScO. The problems associated with the computational study of ScO2 are discussed.
|
|
