Reactive and kinetic properties of carbon monoxide and carbon dioxide on a graphite surface

Abstract

Temperature programmed desorption (TPD) results after chemisorption of carbon monoxide (CO) and carbon dioxide (C0 2 ) on polycrystalline graphite are presented. CO adsorbs onto graphite 'with a very low sticking coefficient « 10-12 ).After CO chemisorption, CO (mass 28 amu) desorbs in two temperature regions, between 400 and 700 K, and between 1000 and 1300 K, and CO 2 (mass 44 amu) des orbs below 950 K. The intensity of the CO 2 signal is less than one order of magnitude lower than the CO intensity. After CO 2 adsorption the major desorption product is CO at high temperatures (lOOO<T( K)<1300), whereas a small amount of CO 2 desorbs around 450 K. The adsorption of a C 16 0 2 and C1 8 0 2 mixture leads to a nearly total oxygen scrambling of the CO 2 desorbed.A mechanism for CO and CO 2 interconyersion oil the graphite surface is presented in term of surface oxide species, mainly lactones and semi-quinones, and their relative stability. Assignments of the TPD features is proposed accordingly.Reaction studies on the CO 2 gasification of clean graphite and the CO disproportionation (Boudouard reaction) haye been performed. A comparison between the activation energies obtained and the desorption energies calculated from the analysis of the TPD results indicate that both reactions are controlled by the desorption of the products . t:Permanent address

The Boudouard–Bell reaction analysis under high pressure conditions

“…The results are given in Table 3. Despite the fact that some simplification was performed, the results obtained are in good agreement with literature data [16,22,27,35].…”

“…23, one can determine the activation energy, and the values obtained are in compliance with the literature data [16,24,27,35].…”

Importance of the oxygen bond strength for catalytic activity in soot oxidation

“…This value is relatively high compared to reported activation energies for O 2 dissociation on defected graphite [72], but in the low end of the reported activation energies for CO and CO 2 desorption from semiquinone and lactone species in graphite surfaces [72][73][74]. It is therefore difficult on this basis to unambiguously assign a rate limiting step for the non-catalytic oxidation.…”

“…The reaction leaves a reduced surface site in the metal oxide, which is reoccupied according to R1. It is known from TPD studies [73,74] that the semi-quinone species can desorb as CO (R3), but the activation energy for desorption of CO 2 from carbon bound to two oxygen atoms is significantly lower [73,74], and, when possible, a second transfer of oxygen (R4) should be a more favourable pathway. In the catalytic oxidation the fraction of carbon released as CO is low compared to the non-catalytic oxidation (supplementary information, figure S13), but as secondary catalytic oxidation of released CO is also a possibility, it is difficult to identify the immediate product of the carbon oxidation.…”

“…The activated oxygen then migrates (by surface diffusion or through the gas phase) to the surface of the carbonaceous material, where it creates an oxidized surface species, here again exemplified by a semi-quinone species (R5). As discussed in connection with the tight contact reaction CO can then desorb from the semiquinone species (R3) [73,74]. Alternatively, an attack on the oxidized carbon atom by a second oxygen atom can result in the formation of CO 2 (R6).…”