Recent advances in gasoline three-way catalyst formulation: A review

Abstract

Development of three-way catalyst technology has been critical in maintaining air quality regulations for gasoline engines via the conversion of pollutants from the internal combustion engine exhaust. The development of improved three-way catalyst formulations is an important challenge for automotive industry. Indeed, in order to meet increasingly stringent environmental regulations around the world, the development of more efficient catalysts depends on a complete understanding of the many parameters related to three-way catalyst design. In this review paper, some of these parameters are examined in relation to three-way catalyst performance, and especially low-temperature activation performance, with a focus on more recently published work. In particular, washcoat composition, platinum group metal ratios and loading, and substrate design are considered. The effect of these parameters with regard to the conversion efficiency of carbon monoxide, unburned hydrocarbons, and nitrogen oxides pollutants is summarized.

Linking the Electrical Conductivity and Non-Stoichiometry of Thin Film Ce1−xZrxO2−δ by a Resonant Nanobalance Approach

“…In principle, pure ceria already features a high oxygen diffusion coefficient (which implies high conductivity), a high mechanical strength, and an outstanding resistance to corrosive gases [14,17], but, in Zr-substituted ceria, these properties are further improved. By the substitution of Zr for Ce, the reduction temperature can be downshifted by 200-300 • C below that of pure CeO 2 , whereas the thermal stability against aging is extended to temperatures as high as 1000 • C that are typical for operating combustion engines [24,30]. Because of the strain induced in the crystal lattice by the substitution of smaller Zr 4+ for larger Ce 4+ , the reduction enthalpies in Ce 1−x Zr x O 2−δ (CZO) are substantially decreased, so that higher degrees of non-stoichiometry and superior oxygen storage capacity can be attained at higher pO 2 conditions as compared to pure CeO 2 [9,[14][15][16][17][18]23,24,[27][28][29][30][31][32][33][34][35].…”

“…Hence, the addition of ZrO 2 essentially transforms the CZO into strongly electronic conductors [9,36,37]. By the virtue of downscaling the particle sizes to submicron dimensions or confining the catalyst material in thin films, leading to extension of active surface, the catalytic properties of ceria derivatives can be further enhanced [17,24,30,[38][39][40]. Because the redox reactions in ceria proceed in two serial processes, namely bulk diffusion of oxygen species and surface reactions [41], and, due to high oxygen diffusion coefficient of ceria, the catalytic processes in low-dimensional thin films with high surface-to-volume ratios are very fast, providing that surface reactions are not rate limiting.…”

“…Because the redox reactions in ceria proceed in two serial processes, namely bulk diffusion of oxygen species and surface reactions [41], and, due to high oxygen diffusion coefficient of ceria, the catalytic processes in low-dimensional thin films with high surface-to-volume ratios are very fast, providing that surface reactions are not rate limiting. Hence, significantly faster reduction and equilibration of oxygen non-stoichiometry with the environment can be expected in comparison to conventional bulk material, especially when operated above several hundred degrees • C. By the addition of ZrO 2 , the thermal aging effect can be minimized and the extended surface morphology of thin film CZO can be preserved with no decrement in reducibility at higher temperatures [3,17,24]. These features were successfully used for the creation of fast conductometric oxygen sensors based on nanocrystalline and thin-film ceria or ceria-zirconia to be applied in exhausts or flue gases [14][15][16].…”

“…Furthermore, the long-term stability of catalytic properties of CeO 2 at high temperatures is limited by the effect of thermal sintering and the deactivation of redox couple (aging) which reduce the oxygen storage capacity [ 3 , 14 , 15 , 16 , 17 ]. Upon the developing of ceria-based materials with tailored properties for use as the oxygen storage component in state-of-the-art TWCs, these drawbacks can be mitigated by extrinsic doping with other aliovalent (alkali, rare-earths, transition metals) or isovalent (Zr, Hf, Ti) metal cations to form solid solutions with functional point defects [ 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 ]. In this regard, the Zr-substituted ceria, i.e., Ce 1−x Zr x O 2−δ (henceforth, CZO), has attracted the increasing interest of automobile industry and researchers as potentially the most prospective catalyst material for TWCs [ 17 , 23 , 24 , 27 , 28 , 29 ].…”

“…Upon the developing of ceria-based materials with tailored properties for use as the oxygen storage component in state-of-the-art TWCs, these drawbacks can be mitigated by extrinsic doping with other aliovalent (alkali, rare-earths, transition metals) or isovalent (Zr, Hf, Ti) metal cations to form solid solutions with functional point defects [ 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 ]. In this regard, the Zr-substituted ceria, i.e., Ce 1−x Zr x O 2−δ (henceforth, CZO), has attracted the increasing interest of automobile industry and researchers as potentially the most prospective catalyst material for TWCs [ 17 , 23 , 24 , 27 , 28 , 29 ]. In principle, pure ceria already features a high oxygen diffusion coefficient (which implies high conductivity), a high mechanical strength, and an outstanding resistance to corrosive gases [ 14 , 17 ], but, in Zr-substituted ceria, these properties are further improved.…”