Junjiang Zhu

CO Oxidation over the Perovskite-Like Oxides La2−xSrxMO4(x= 0.0, 0.5, 1.0; M = Cu, Ni): A Study from Cyclic Voltammetry

Abstract Correlations between the catalytic performance and the electrochemical properties in heterogeneous catalysis were observed, using perovskite-like oxides La2−xSrxMO4 (x = 0.0, 0.5, 1.0, M = Cu, Ni) as the model catalysts and CO oxidation as the probe reaction. For a reaction that contains reducing agent in the feed gas, the activity depends mainly on the area of redox peak in the CV curves, while has no much relation to the symmetry of the redox potentials. The irreversible cycle in the first CV curves of LaSrCuO4 suggested that the first step of the reaction is adsorption, corresponding to Mn+ (Cu2+) to M(n+1)+ (Cu3+).

Excellent performance of La1-xCexSrNiO4 (x ≤ 0.3) for no decomposition in the presence of excess oxygen

Abstract Ce involved perovskite-like oxides (La1−xCexSrNiO4, x ≤ 0.3) show high activity for NO decomposition even in the presence of excess oxygen, which might be mainly ascribed to the new active site formed in the catalyst due to the entry of Ce to the La site of LaSrNiO4.

Mechanism of NO decomposition on perovskite (-like) catalysts

NOx emitting from industrial and mobile exhaust are serious pollutant in air atmosphere, and the removal of them is an urgent task of today in environment-protection field. Although the present three-way-catalyst (TWC) can remove NOx from the mobile exhaust effectively, it will be out of work as lean-burn strategies are used to increase energy efficiency (for example, the diesel engine operated in the lean-burn condition), hence, the technology that can remove NOx in the presence of excess oxygen is desired. In addition, because the capability of noblemetal catalyst for NOx removal is weak at high temperatures (>873 K), it is thus necessary to comprehend the process of NOx decomposition, which would help to solve the problem of NOx removal. For NO decomposition reaction (2NO = N2 + O2), it is generally accepted that N2 was formed through the decomposition of N2O species, i.e. 2NO N2O + Oads and N2Oads N2 + Oads . While the way of oxygen formation is still unclear at present. According to the literatures, there are two proposed ways for oxygen desorption, that is, i) the direct desorption of two vicinal oxygen, Oads + Oads O2; and ii) the indirect desorption with NO as an intermediate, NOads + Oads NO2, NO2 + Oads NO3 ads, NO3 ads NO + O2. Accordingly, there are two ways for NO2 formation, that is, iii) reaction of gaseous NO and O2 occurring in the downstream, O2 + 2NO 2NO2; and iv) reaction of adsorbed O atom and NO occurring on the catalyst surface, Oads + NOads NO2. Recently, Iglesia et al. suggested that the O2 formation over Cu-ZSM-5 mainly occurred through the decomposition of NO3(a) species, but there is still no literature discussing the way of O2 and NO2 formation over perovskite (-like) oxides. In this work, by discussing the possible reaction steps occurring in the process of NO decomposition, we proposed that the way of NO decomposition over perovskite (-like) mixed oxides occurred with NO2 as an intermediate, and NO2 was mainly formed on the catalyst surface in the way of Oads + NOads NO2, while O2 was formed through the dissociation of NO2 species in the way of 2NO2(g) = 2NO(g) + O2(g). The samples, LaSrNiO4 and La0.4Sr0.6Mn0.8Ni0.2O3, were prepared by the conventional citric combustion method. O2-TPD experiment was carried out on a homemade apparatus equipped with a thermal conductivity detector (TCD). The samples (0.2 g) were first treated in O2 at 1073 K for 1 h and cooled to room temperature in the same atmosphere, then swept with pure He (or 0.5% O2/He) at a rate of 11.8 mL/min until the base line on the recorder remained unchanged. Finally, the sample was heated at a rate of 20 K/min in He (or 0.5% O2/He) to record the TPD profile. Steady-state activities of catalysts were evaluated using a single-pass flow micro-reactor made of quartz, with an internal diameter of 6 mm. The reactant gas (1% NO/He + 0% 10% O2/He) was passed through 0.5 g (in the absence of oxygen) or 1.0 g (in the presence of oxygen) catalysts at a rate of 25 or 40 mL/min (in all, to keep W/F = 1.2 g·s·cm ). The gas composition was analyzed before and after the reaction by an on-line gas chromatography, using molecular sieve 5A column for separating NO, N2 and O2. N2O was not analyzed here because it was difficult to form between 773 and 1123 K as reported in ref. [9]. Before the data were obtained, reactions were maintained for a period of ~2 h at each temperature to ensure the steady-state conditions. According to the literatures reported previously, the possible reaction steps occurring in the process of NO decomposition are summarized in Table 1.