Evolution of the Structure and Chemical State of Pd Nanoparticles during the in Situ Catalytic Reduction of NO with H₂

Abstract

An in-depth understanding of the fundamental structure of catalysts during operation is indispensable for tailoring future efficient and selective catalysts. We report the evolution of the structure and oxidation state of ZrO(2)-supported Pd nanocatalysts (∼5 nm) during the in situ reduction of NO with H(2) using X-ray absorption fine-structure spectroscopy and X-ray photoelectron spectroscopy. Prior to the onset of the reaction (≤120 °C), a NO-induced redispersion of our initial metallic Pd nanoparticles over the ZrO(2) support was observed, and Pd(δ+) species were detected. This process parallels the high production of N(2)O observed at the onset of the reaction (>120 °C), while at higher temperatures (≥150 °C) the selectivity shifts mainly toward N(2) (∼80%). Concomitant with the onset of N(2) production, the Pd atoms aggregate again into large (6.5 nm) metallic Pd nanoparticles, which were found to constitute the active phase for the H(2)-reduction of NO. Throughout the entire reaction cycle, the formation and stabilization of PdO(x) was not detected. Our results highlight the importance of in situ reactivity studies to unravel the microscopic processes governing catalytic reactivity.

NO dissociation and reduction by H 2 on Pd(1 1 1): A first-principles study

“…In addition, interestingly, (NO) 2 dimer was observed at room temperature by using infrared spectroscopy on Pd(1 1 1) surface recently, as demonstrated in the literature on some other surfaces including Au [4], Ag [26], Cu [27][28][29], Mo [30], Rh [31], Al 2 O 3 [32], MgO [33], and Si-doped graphene [34]. In experiment, N 2 O was observed at low temperature on ZrO 2 -supported Pd nanocatalysts [3]. So what is the mechanism for the N 2 O formation on Pd(1 1 1), via the direct mechanism (NO ?…”

“…The emission of NO not only causes a series of environmental problems such as photochemical smog, acid rain and ozone depletion [1][2][3], but also has adverse effect on humans. Therefore, the catalytic removal of NO has become one of the most important problems for air pollution control and attracted remarkable attention in both academic and applied research fields [4].…”

“…Also, the combustion of H 2 producing water makes H 2 -SCR to be a green environmental technology. Therefore, the SCR of NO by H 2 over the noble metal-based catalysts has attracted much attention experimentally and theoretically [3,[12][13][14][15][16][17][18][19][20][21][22].…”

“…Ma et al [16] studied the surface nitrogen removal pathways of NO + H 2 reaction on Pd(1 1 0) and concluded that the reduction of NO by H 2 shows not only a higher reactivity but also a higher selectivity toward N 2 compared with the reduction by CO. Among the studies of NO + H 2 reaction on different Pd-based catalysts, the Pd/TiO 2 catalyst was proved to possess higher NO x conversion than Pd/Al 2 O 3 [21], and the ZrO 2 -supported Pd nanocatalysts were reported to present both high reactivity for NO and high selectivity toward N 2 [3]. In addition, for the different metal-based catalysts for NO reduction by H 2 , Roy et al [19] studied four various TiO 2 -supported metallic catalysts such as Ru, Rh, Pt, and Pd, and found that Pd/TiO 2 catalyst possess both the low-temperature catalytic activity and selectivity.…”

Understanding the Role of Hyponitrite in Nitric Oxide Reduction

“…18 Even before these challenges were recognized, though, there had been considerable interest in understanding the mechanism of heterogeneous NO reduction in catalytic converters. [20][21][22][23][24][25] Many research groups have examined this reaction, and the generally accepted mechanism is thought to involve initial NO dissociation to form surface bound nitride and oxide (Scheme 1a). 18,[24][25][26][27][28][29][30][31] However, more recent experimental work suggests an 4 alternate mechanism could also be operative.…”