Relationship of Pt Particle Size to the NO_x Storage Performance of Thermally Aged Pt/BaO/Al₂O₃ Lean NO_x Trap Catalysts

Abstract

The relationship between Pt particle size and NO x storage performance has been investigated over a simple model Pt/BaO/Al 2 O 3 and an enhanced lean NO x trap catalyst (LNT). These catalysts were treated at elevated temperature to mimic the effect of thermal aging encountered during the desulfation step in NO x trap catalyst regeneration. A combination of in situ time-resolved X-ray diffraction (TR-XRD), diffuse reflectance Fourier transform infrared spectroscopy (DRIFT) measurements after CO adsorption, and high-resolution transmission electron microscopy (HRTEM) observations of oxidatively thermally aged samples clearly shows the sintering behavior of Pt crystallites as a function of aging time and temperature. At elevated temperatures, Pt crystallite growth occurs rapidly for an initially short period (i.e., 1.5 h) followed by a further gradual increase with time. A direct correlation of NO x storage performance data, obtained after successive thermal treatments of the LNT catalysts under oxidizing conditions, with the results of in situ XRD experiments allows us to clearly attribute performance degradation to observed phase changes. Comparing a simple model Pt/BaO/Al 2 O 3 sample with an enhanced formulation, which displays significantly less Pt sintering, we conclude that Pt crystallite size plays a critical role in determining NO x storage activity. In other words, the retention of small Pt particles after thermal aging is crucial to maintaining high activities. Thus, the prevention of Pt sintering, especially during high-temperature operation, must be a key factor in the design of more durable LNT catalysts.

Evaluating the NOx Storage Catalysts (NSC) Aging: A Preliminary Analytical Study with Electronic Microscopy

“…On these bases, with the aim of evaluating separately the contributions to the aging of the thermal treatment and the gaseous environment exposure, the samples listed in Table 1 were prepared, in which some slices of the catalyst were subjected to heating under argon atmosphere (namely NSC_8_Ar and NSC_24_Ar), to assess the sole thermal effect, while other ones where exposed, under the same thermic conditions, to a humid oxygen atmosphere (namely NSC_8_O2 and NSC_24_O2) to evaluate the influence of a chemical oxidizing gaseous environment [7,[9][10][11][12] at 800 °C, which is higher than the maximum temperature used for NSC catalysts. All samples were subjected to SEM-EDS and TEM analyses and compared with those of the pristine material.…”

“…Samples Preparation: The Diesel 2000 catalyst was cut in slices that were thermally aged in a furnace at 800 • C for 8 and 24 h under an argon atmosphere [5,9,12].…”

“…For a Pt-BaO/Al 2 O 3 catalyst, Kim et al observed that the sintering of platinum particles increases exponentially with both the aging time and the temperature [9], and this not only decreases the conversion of the reactions but also affects the NOx storage process [10].…”

“…A further drawback of Pt sintering is the decrease in the contact surface between Pt and BaO, resulting not only in a lower activity for the NO oxidation but also in a reduced rate of NO 2 spillover from Pt to the neighboring barium adsorption sites, which in turn decrease the NOx storage activity [9][10][11].…”

“…Most of the previous works have shown that the sintering behavior of Pt is a function of time, temperature, and composition of gaseous atmosphere to which the catalyst is exposed [9][10][11][12]. However, the sintering dependence on the gaseous environment has commonly been evaluated on model catalysts and has been based on NOx conversion efficiency, asserting that the oxidizing atmosphere tends to strongly increase the aging phenomenon [4,12].…”