Temperature Hysteresis in Oxidation of CO on Complex Oxide Catalysts

Abstract

We have used thermal desorption (TD) with mass spectrometric detection of the desorbed species to study the state of the surface of oxide catalysts for oxidation of CO. Detection of weakly bound forms of water and the species H 3 O + and HO 2 ⋅ in the thermal desorption spectra correlates with the existence of temperature hysteresis in these samples. The data obtained are explained by the reaction occurring in a highly active state via a heterogeneous-homogeneous mechanism, and also by the effect of local superheating.In oxidation of CO on different catalysts, we observe concentration and temperature hysteresis as a consequence of the multiplicity of steady states [1]. In turn, steady state multiplicity may be due to different reasons, among which we may mention heat exchange between the gas phase and the surface of the catalyst [2, 3], nonlinearity of the kinetics with realization of a number of steady states [1,4], the possibility of the reaction occurring according to a heterogeneous/homogeneous mechanism, and a number of other factors indicated in [1].The properties of complex oxide catalysts based on 3d metals were studied in [5,6]. In tests under flow-through conditions (atmospheric pressure, gas mixture: 2% CO, 20% O 2 , 78% He) for samples of Fe-Co, Cu-Co, Fe-Mn (with Mn concentration higher than 10 mass %) and Fe-Cu-Co (for ratios Cu : Co = 50 : 50 and Cu : Co = 5 : 95, Fe up to 15 mass %) oxide systems with different compositions, we observed temperature hysteresis (Fig. 1, curves 1 and 2), while in Fe-Mn, Fe-Cu, Fe-Cu-Co (for ratios Cu : Co = 95 : 5 and Fe up to 15 mass %) oxide systems there were samples with no hysteresis (Fig. 1, curve 3). In tests under continuous circulating conditions, there was no hysteresis for all the samples. In this work, we studied the oxide catalysts by programmed thermal desorption (TD) with mass spectrometric detection of the desorbed species. The results obtained are compared with catalytic activity data to determine the nature of the temperature hysteresis in oxidation of CO.The catalysts were prepared by dissolving appropriate amounts of the metals in nitric acid, followed by drying according to the procedures described in [5,6]. After the catalytic studies, the samples were transferred to the quartz cuvet of the mass spectrometer. They were held for one hour in the reaction medium. The TD spectra were taken on an MKh7304A mass spectrometer with linear heating of the sample at a rate of 10 deg/s. The desorption activation energy (E d ) was calculated by the Cvetanovic method [7].In the TD spectra, we detected water peaks that were symmetric (b form). This suggests that the desorption process corresponds to a second-order kinetic equation (recombination). We may pick out two types of TD peaks for water: the b 1 form (desorption maximum temperature T m < 250°C, E d < 150 kJ/mol), and the b 2 form (T m > 250°C, E d > 150 kJ/mol) ( Fig. 2a and b, curves 1). 3400040-5760/05/4105-0340

Thermo-Exfoliated Graphite Containing CuO/Cu2(OH)3NO3:(Co2+/Fe3+) Composites: Preparation, Characterization and Catalytic Performance in CO Conversion

“…(H 2 O + ) shows an intensive symmetric peak corresponding to a dissociative desorption of water from the surface of the composite with T d < 723 K. TPD MS profiles of CO 2 + and O 2 + are both of asymmetric character, meaning that the molecules desorb without dissociation. It was shown in previous studies [16] that the catalytic activity in the CO conversion for the bulk CuO/Cu 2 (OH) 3 NO 3 :(Co 2+ /Fe 3+ ) composites correlates with intensity and area of peaks referring to atomic oxygen and CO 2 in the TPD MS spectra.…”

“…Copper oxides are extensively utilized due to their good catalytic performance [10,11,12,13,14,15] and low fabrication costs. Among copper oxide catalysts, a composite consisting of CuO/Cu 2 (OH) 3 NO 3 phases doped with Co 2+ and Fe 3+ , further denoted as CuO/Cu 2 (OH) 3 NO 3 :(Co 2+ /Fe 3+ ), recently obtained by us [16], exhibits sufficient activity in the CO conversion at relatively low temperatures giving a 100% conversion of CO to CO 2 ( t 100% ) at 373 K.…”

Activity of copper–cerium–zirconium catalysts in oxidation of hydrogen

“…A common feature of all the catalysts, regardless of their texture characteristics, is a detectable hysteresis, the width of which is as great as 60°-65°. The appearance of hysteresis in the case of oxidation of hydrogen on solid catalysts may be due to a number of reasons, the most likely of which in this case is local heating of active centers resulting from the highly exothermic reaction (DH 0 = -286 kJ/mol) [7,8].…”

Effect of ZrO2 morphology in copper–cerium–zirconium oxide systems on their catalytic properties in the reaction of co oxidation in hydrogen-rich mixtures

“…The presence of hysteresis is associated with the reaction going into the interior volume and realization of a heterogeneous/homogeneous mechanism for oxidation of CO, local superheating of the active sites of the catalyst during the exothermic oxidation reactions, and also formation of metastable phases containing copper oxide Cu 1+ while the reaction temperature is being raised [10,11]. As a result of formation of such phases, electron transfers Ce 4+ + Cu 1+ « Ce 3+ + Cu 2+ are facilitated and accordingly the CO oxidation rate increases [12,13].…”