Activation of Pd/SSZ-13 catalyst by hydrothermal aging treatment in passive NO adsorption performance at low temperature for cold start application

From metal-supported oxides to well-defined metal site zeolites: the next generation of passive NOxadsorbers for low-temperature control of emissions from diesel engines

“…Having that in mind, recent studies have focused on the evaluation of the metal loading, the Si/Al molar ratio and the metal dispersion treatments in different zeolites, particularly MFI and CHA, and their influence in the low-temperature NOx adsorption/desorption processes. [20][21] In a very complete experimental design, Lee et al have first evaluated the influence of the activation treatment on the metal dispersion, detecting that an oxidative treatment at 750°C allows the atomically dispersion of the Pd species within the MFI crystals (see EXAFS spectra in Figure 15), which maximizes the total amount of adsorbed NO species (∼24.6 µmol/gcatal). 20 Afterwards, a large number of Pd-containing MFI zeolites, with different Si/Al molar ratios (from 11 to 150, see Table 2) and Pd contents (from 0.5 to 5.5%wt…”

“…20 The achieved results are summarized in Table 2, where it can be concluded that the Pd/MFI zeolite prepared with a Si/Al Recently, Ryou et al have performed a similar systematic study on the Pd/CHA zeolite. 21 As it has been exposed above, the introduction of metal species with large ionic radii within small pore zeolites can suffer severe diffusion limitations, frequently precluding an optimized metal dispersion along the small pore zeolite crystals. [46][47][48] Ryou et al have evaluated different metal incorporation procedures (incipient wetness impregnation, wet impregnation, ion exchange or solid-state ion exchange) and metal loadings (from 0.2 to 4.7%wt Pd), with the aim of maximizing the metal dispersion along the CHA crystals.…”

“…[46][47][48] Ryou et al have evaluated different metal incorporation procedures (incipient wetness impregnation, wet impregnation, ion exchange or solid-state ion exchange) and metal loadings (from 0.2 to 4.7%wt Pd), with the aim of maximizing the metal dispersion along the CHA crystals. 21 The authors found that regardless the metal incorporation treatment employed, the NO adsorption ability of the Pd/CHA materials can considerably be increased if the Pd-containing samples are previously treated at 750°C with an oxidative atmosphere (15%O2, 10%H2O in N2). This hydrothermal treatment induces the transformation of bulk PdO into Pd 2+ cations along the zeolite, thus favoring the lowtemperature NOx adsorption process.…”

“…From these results, the authors have found that the optimum Pd loading in the Pd-CHA based PNA material is ∼ 2%wt, leading to an overall NOx adsorption value of ∼30 µmol/gcatal. 21…”

“…BEA, MFI or CHA). [19][20][21] However, the design of a very efficient and stable low-temperature PNA requires a rapid adsorption of NOx at temperatures below ∼100°C, and desorption of the NOx components at intermediate temperatures (i.e. 200-350°C).…”