Oxygen Storage Capacity Of Catalysts Research Articles

Detailed experimental investigation of the NOx reaction pathways of three-way catalysts with focus on intermediate reactions of NH3 and N2O

In this work possible reaction pathways of NOx were investigated for a three-way catalyst on a synthetic gas test bench with a focus on intermediately produced NH3 and N2O. NH3 formation during a typical lean-to-rich transient was verified. Further experiments were carried out in Ar atmosphere allowing to directly determine the N2 selectivity. It could be shown that the catalyst's oxygen storage capacity (OSC), which is implemented by addition of CeO2 to the washcoat material, plays a significant role and that NO can fill the oxygen storage in the absence of O2. Further, no nitrogen species were found to be stored. Also, intermediately produced NH3 was oxidized by stored oxygen with a high selectivity to N2. As soon as the OSC was fully exhausted, NH3 dissociated, leading to the formation of N2 and H2. The presence of side products and secondary emissions strongly depends on the actual operating conditions.

Enhancement of the Generation and Transfer of Active Oxygen in Ni/CeO2 Catalysts for Soot Combustion by Controlling the Ni-Ceria Contact and the Three-Dimensional Structure.

The effect of the three-dimensionally ordered macroporous (3DOM) structure and the Ni doping of CeO2 on the physicochemical properties and catalytic activity for soot combustion was studied. Moreover, the way in which Ni is introduced to the ceria support was also investigated. For this, CeO2 supports were synthesized with uncontrolled (Ref) and 3DOM-structured morphology, and their respective Ni/CeO2 catalysts were prepared by impregnation of the previously synthesized supports or by successive impregnation of both precursors (Ni and Ce) on the 3DOM template. Conclusions reached in this study are: (1) the 3DOM structure increases the surface area of the catalysts and improves the catalyst-soot contact. (2) The doping of CeO2 with Ni improves the catalytic activity because the NiO participates in the catalytic oxidation of NO to NO2, and also favors the production of active oxygen and the catalyst oxygen storage capacity. (3) Ni incorporation method affects its physicochemical and catalytic properties. By introducing Ni by successive infiltration in the solid template, the CeO2 crystal size is reduced, Ni dispersion is improved, and the catalyst reducibility is increased. All of these characteristics make the catalyst synthesized by successive infiltration to have higher catalytic activity for soot combustion than the Ni-impregnated CeO2 catalyst.

Enhancement of Mn/Ce/W/Ti catalysts through control of pH and oxygen mobility during their preparation

The purpose of this study is to investigate the effects of pH control on the oxidation state and oxygen storage capacity of catalysts, and the correlation with low-temperature selective catalytic reduction (SCR) activities using Mn/Ce/W/Ti catalysts. The activity of the catalysts showed increases in low-temperature SCR reaction activity at (120–180) °C, when the pH of the slurry of active metal and support TiO2 was controlled to low levels during the preparation of Mn/Ce/W/Ti catalysts. To characterize these catalysts, NH3-TPD, FT-IR DRIFT, XPS, and O2-chemisorption were analyzed. In the DRIFT experiment using FT-IR, the NH3 adsorption on acid sites of the Mn/Ce/W/Ti catalysts, which exhibited excellent reaction activity in the SCR reaction, was increased, compared with those of the Mn/Ce/Ti catalysts. In particular, the NH4+ adsorbed on Brønsted acid site was greatly increased. When the adsorbed NH3 was reacted with injected NO + O2, the adsorbed NH3 species was rapidly decreased on Mn/Ce/W/Ti catalysts whose pH was controlled to low levels, and the reaction rate was greatly increased. According to XPS analysis, the increase of the O2 storage capacity of the catalyst is due to the decrease of the lattice oxygen present on the surface, and the increase of adsorbed oxygen species as the nonstoichiometric species Ce3+ increases. In addition, the O2-chemisorption showed that oxygen mobility was increased in the catalysts whose pH was controlled to low levels. These results indicate that oxygen mobility was increased due to the increases in the adsorbed oxygen species, which was found by XPS analysis. Therefore, the control of pH in the preparation of Mn catalyst increases the Brønsted acid site on the catalyst surface and increases oxygen mobility, due to the increase of adsorbed oxygen species, resulting in an increase in the SCR activity.

Structure and oxygen storage capacity of Pd/Pr/CeO2-ZrO2 catalyst: effects of impregnated praseodymia

Praseodymium (Pr) was impregnated to CeO2-ZrO2 solid solution by an impregnation method. The as-obtained Pr modified CeO2-ZrO2 was impregnated with 1 wt.% Pd to prepare the catalysts. The structure and reducibility of the fresh and hydrothermally aged catalysts were characterized by X-ray diffraction (XRD), Raman, X-ray photoelectron spectroscopy (XPS), CO chemisorption and H2 temperature-programmed reduction (H2-TPR). The oxygen storage capacity (OSC) was evaluated with CO serving as probe gas. Effects of impregnated Pr on the structure and oxygen storage capacity of catalysts were investigated. The results showed that the aged Pr-impregnated samples had much higher OSC and better reducibility than the unmodified ones. The scheme of structural evolutions of the catalysts with and without Pr was also established. Partial of the impregnated Pr diffused into the bulk of CeO2-ZrO2 during ageing, which inhibited the sintering, and increased the amount of oxygen vacancies in CeO2-ZrO2 support. Furthermore, those impregnated Pr species which covered on the surface of the support obstructed the strong metal-support interaction between Pd and Ce so as to reduce the encapsulation of Pd as well as the back spill-over of the oxygen during the catalytic process.

The Effect of Potassium Carbonate on the Catalytic Properties for Methane Conversion with Ceria

Pure CeO2and a series of (x %) K-CeO2(x=1, 2, 3, 4) catalysts were respectively prepared by the precipitation and incipient wetness impregnation methods, and characterized by means of XRD, BET and H2-TPR techniques. The catalytic activity was investigated by the gas-solid reaction with methane in the absence of gaseous oxidant in a fixed bed reactor at 800 °C. The XRD measurement showed that doping of K2CO3did not change the structure of CeO2with the addition of K2CO3 without formation of Ce-K-O solid solution in these materials. Surface area of catalysts wasSubscript textdecreased with the impregnation amount of K2CO3. Reducibility of catalysts was obviously enhanced by the addition of K2CO3as shown in H2-TPR tests. The catalysts activity tests indicated that adding K2CO3to CeO2could promote the oxygen storage capacity of catalysts. K species in CeO2could affect the CO formation in methane oxidation.

Soot oxidation via CuO doped CeO 2 catalysts prepared using coprecipitation and citrate acid complex-combustion synthesis

This work deals with soot oxidation under SO 2 presence over CuO doped CeO 2 prepared using two routes, coprecipitation (CP) and citrate acid complex-combustion synthesis (CA). TPO, XRD, BET, H 2-TPR, O 2-TPD, FTIR and XPS are utilized to correlate the performance and physicochemical features of catalysts. The results show that solid solutions are formed due to Cu 2+ cations entering into the ceria lattice. This reorganization increases the amount of oxygen vacancies and then creates more surface active species (Ce 4+/Ce 3+ and O*), and increases the reducibility and oxygen storage capacity of catalysts, thus promoting catalytic activity for soot oxidation. Both for Cu 0.05Ce 0.95-CA and Cu 0.05Ce 0.95-CP after TPO in the presence of various concentrations of SO 2, more Ce 4+/Ce 3+ redox couples, and more active oxygen species, detected by XPS technique and FTIR, respond to the better activity. When compared to Cu 0.05Ce 0.95-CP, the better reducibility and greater OSC, and active oxygen species of Cu 0.05Ce 0.95-CA coincide with its better activity. A possible pathway of soot oxidation over Cu 0.05Ce 0.95-CA is proposed, based on the relationship of the performance and the physicochemical features of catalysts, which pathway follows the mechanism associated with redox couple and oxygen spill over effect.

Probability density diagnostic metric for an integrated three-way catalyst controller and monitor

An integrated model-based methodology for three-way automotive catalyst control and diagnostic monitoring is presented in this work. The catalyst controller and monitor both utilize a limited integrator catalyst oxygen storage model with an adaptive integral gain. This adaptive catalyst gain, which is a measure of the catalyst oxygen storage capacity, is used by the controller to provide information on the dynamic catalyst behaviour and by the diagnostic monitor to provide information on long-term catalyst deactivation and short-term emission control device failure. A statistical classification technique based on the fraction of time that the catalyst gain values in a moving window are within a threshold of zero is employed as the test metric for on-board diagnostic monitoring. The performance of the catalyst monitor is demonstrated with experimental vehicle test data from a 4.6 l ULEV II gasoline engine operated over a series of Environmental Protection Agency Federal Test Procedure drive cycles with differently aged catalysts. Preliminary results indicate that it is possible to perform very accurate discrimination between catalyst operation, even near the on-board diagnostic detection threshold, using this technique.

Correlation between activity and oxygen storage capacity of ceria on Pd-only three-way catalysts

One way of enhancing the thermal stability and NOx, THC and CO conversion of a catalyst is to improve the thermal stability and oxygen storage capacity (OSC) of the ceria. The appropriate mixing ratios of bulk and stabilized ceria are especially very important for designing Pd-only three-way catalysts. In this paper, we discuss the surface phenomena of stabilized and unstabilized (bulk) ceria, the OSC of catalysts, and the correlation between activity and OSC of Pd-only catalysts with mixing ratios of bulk ceria and stabilized ceria.