Nondissociative CO Research Articles

Kinetics of the NO and CO Reaction over Platinum Catalysts: I. Influence of the Support

The kinetics of the CO+NO reaction over Pt-based catalysts was investigated using a fixed bed flow reactor at 300°C, with CO and NO partial pressure ranges of 1.5×10−3to 9×10−3atm. Pt was deposited on various supports: γ-Al2O3, Si3N4, and Cr3C2. XRD, XPS, and hydrogen chemisorption measurements seem to indicate that Pt dispersion decreases as follows: Pt/Al2O3>Pt/Si3N4>Pt/Cr3C2. Kinetic data obtained on these Pt catalysts were interpreted on the basis of four mechanisms as proposed in the literature; only one mechanism can correctly model the reactant partial pressure dependencies of the rate. This mechanism involves nondissociative CO and NO competitive adsorptions followed by a dissociation of adsorbed NO which requires a vacant nearest-neighbor adsorption site. Clearly, the adsorption equilibrium constants, λCOand λNOof CO and NO, together with the rate constant of the NO dissociation step are strongly influenced by the support, particularly λNO, which is consistently lower than λCOon Pt/Al2O3, increases notably for Pt/Si3N4, and increases even more for Pt/Cr3C2. It has been shown that Pt/Si3N4may be the most suitable catalyst if the Pt dispersion on this support can be improved. The nature of the support also has a significant effect on the selectivity of the NO transformation into N2and N2O. Pt/Cr3C2is the most selective catalyst for N2formation (much more than Pt/Si3N4and particularly Pt/Al2O3). This seems to be related to the fact that the rate constant of the step 2Nads→N2is much higher than that of the other step Nads+NOads→N2O.

FTIR study of adsorption of CO, NO and C 2H 4 and reaction of CO + H 2 on the well-dispersed [formula omitted] and [formula omitted] catalysts

Adsorption of CO, NO and C 2H 4 and reaction of CO + H 2 on the well-dispersed FeOx TiO 2(a) and FeOx γ- Al 2O 3 catalysts were studied combined with the pulse adsorption and temperature programmed desorption of NO. The results indicated that the adsorption properties on the well-dispersed FeOx are different from those on the bulk ferric oxide. For CO adsorption, the differences in the formation of surface CO complex and surface carbonates were observed. For NO adsorption, the types of the adsorbed NO species on FeOx γ- Al 2O 3 are different from those on the FeOx TiO 2(a) catalyst. The local electronic density of the well-dispersed Fe 3+ is increased which enhanced the back donation of d electrons in the FeN bonding. The formation of NO 3 −(NO 2 −) complex on FeOx γ- Al 2O 3 is also different from that on FeOx TiO 2(a) . For C 2H 4 adsorption, no π bonded C 2H 4 species developed on the well-dispersed FeOx although it had been found on the bulk α-Fe 2O 3. The weakly adsorbed C 2H 4 species on the well-dispersed FeOx did not transform either at room temperature or at elevated temperature (up to 423 K). Surface carbonates are the major products while formate or methoxy species rarely formed on the FeOx TiO 2(a) during the CO + H 2 reaction. Small amount of the nondissociative CO complex may be developed in syngas. The alternations in the adsorption behaviors must be connected with the fact that the structural environment and the electronic property for the adsorption site are somewhat changed because of the various dispersion state and the support effect.

Reactivity of CO on a TiO 2(110) defective surface studied by electron stimulated desorption

CO adsorption on a defective TiO 2(110) surface at 150 K incorporating about 1.7 × 10 14cm −2 vacancies produced by Ar + sputtering has been studied by electron stimulated desorption, ESD, and with Auger electron spectroscopy, AES, as a complementary technique. When the surface was exposed to CO up to 5 × 10 5L the AES spectra revealed a small C signal. From the O(KLL) Ti(LMM) ratio increase a CO coverage of 0.06 ± 0.01 ML was determined. In spite of this small amount of CO the ESD technique showed dramatic changes in ion yield and energy. The characteristic dominant peak at 4.0 eV and the small structure at about 7.0 eV of the ion kinetic energy distribution curve of the clean defective surface changed to a triple peak structure with maxima at 3.4, 4.3 and 7.4 eV. The first is identified with the most probable kinetic energy of O + ions ejected from non-dissociated CO molecules. The 4.3 and 7.4 eV maxima correspond to the most probable kinetic energy of O + ions desorbed from O in-plane and from the bridging O-ligand of the TiO 2(110) surface, respectively. The growth of the 7.4 eV peak upon adsorption is a consequence of the filling of the bridging O-ligand initial vacancies by dissociated O from the CO molecule. The ion yield curves show, in addition to the onset at 35 eV electron energy of the clean surface, a threshold at about 290 eV electron energy corresponding to excitation of the C 1s core level. The total ion yield also shows a decrease upon CO adsorption. The O + ion yield curves are found to be related to the secondary electron yield. The chemisorption of CO is discussed in terms of a model where the CO is molecularly adsorbed on the Ti five-fold coordination sites with a part being dissociated and leading to O adsorbed on bridging O-ligand vacancies.

Non-dissociative high temperature induced states of CO from CO 2 adsorbed on tungsten surface near room temperature

In the present work some results regarding the thermal desorption of CO 2 adsorbed on tungsten near room temperature, studied by electron stimulated desorption (ESD), are presented. At temperatures lower than 600 K, the ESD CO + and O + ion currents behave similarly to ions coming from CO, namely the CO + and O + currents disappear at temperatures lower than 500 K. On the contrary, new states yielding CO + and O + are revealed at temperatures higher than 700 K; the former species can be identified as non-dissociative CO states and the latter one comes from both CO and O. A model reaction of the implied process at the different temperatures is also presented.

Absolute coverage determination of CO on Ni(111)

The adsorption of CO on Ni(111) has been studied using LEED and calibrated Auger measurements in order to determine the absolute amount of adsorbed CO molecules. At room temperature and CO pressures of up to 1.3×10−3 Pa a saturation coverage of ϑCO?0.28, corresponding to 5.2×1014 molecules per cm2, has been obtained. This is contrary to the commonly accepted opinion that the saturation coverage of nondissociated CO on various transition metals should always amount to about 1×1015 molecules per cm2 irrespective of the underlying substrate structure.