Improvement of sulfur resistance of Pd/Ce–Zr–Al–O catalysts for CO oxidation

A Review on the Impact of SO2 on the Oxidation of NO, Hydrocarbons, and CO in Diesel Emission Control Catalysis

“…For example, the dissociative adsorption of O 2 on Pt site is forbidden because of weakened electron transfer from Pt to O 2 as a result of sulfur coordinated onto Pt with high coverage, and beyond which, the inertization of O in sulfates (both for sulfated metals and supports) limits oxygen transfer. 60,127 For some supports that govern oxygen transfer between active sites and supports (e.g., CeO 2 , Ce−Zr, and perovskite), they are flexible in donating/withdrawing oxygen to/from Pt/Pd to supplement consumed O or prevent overoxidation of Pt. Thus, they work better initially but would fail eventually like Al 2 O 3 when the supports are saturated with a stable sulfate, among others.…”

“…For instance, the T 50 was found to increase with decreasing Pd dispersion over sulfur-aged Pd/Ce−Zr−Al catalyst because of limited oxygen adsorption and activation. 127 In another instance, the surface hydroxylated Pd/CeO 2 via hydrothermal treatment (with 10% H 2 O at 750 °C) allows redispersion of Pd nanoparticles into reduced domain size in an increased numbers of oxidized state through migration of Pd 2+ −OH along ceria surface with a strong Pd−O−Ce interaction (Figure 4c). 140 During the CO oxidation with 100 ppm of SO 2 for the first 14 h, the catalyst obtained 100% CO conversion by hindering sulfate and carbonate accumulation (Figure 4d).…”

“…bond formation with CeO 2 , Al 2 O 3 , and SiO 2 . 88,89,127,261,265,267 For the popular Ce−Zr solid support or the ones they modified (e.g., Al 2 O 3 /Ce−Zr), it has been established that the appropriate content of Zr modification into CeO x creates defects that promote the reduction of surface sulfate at lowered temperatures, allowing the Ce−Zrsupported Pt catalyst with better regeneration ability than a pure oxide like CeO 2 . Similarly, the introduction of CeO 2 into Pd/Al 2 O 3 /Ce 0.6 Zr 0.4 O 2 decreases the decomposition temperature of sulfate by 50−100 °C.…”

“…More details regarding the support effect on sulfate formation a n d d e c o m p o s i t i o n c a n b e f o u n d i n S e c t i o n 2. 88,89,127,261,265,267,274 To add additional adsorption and activation channels for target reactants and oxygen, doping transition metals such as Fe, Mn, Cu, V, W, Co, among others, to modify PGM active components or supports are popularly applied in academic research. 98,99,275 The charge imbalance resulting from the multivalence metal dopants yields oxygen vacancies in the oxide support, which is in favor for the adsorption and dissociation of O 2 and hence the oxidation activity, such as for compensating the loss caused by SO 2 poisoning.…”

“…Except for blockage of reactant adsorption over the noble metals, another reason behind the catalytic activity loss is the restricted availability of oxygen species to the oxidation of the target adsorbate. For example, the dissociative adsorption of O 2 on Pt site is forbidden because of weakened electron transfer from Pt to O 2 as a result of sulfur coordinated onto Pt with high coverage, and beyond which, the inertization of O in sulfates (both for sulfated metals and supports) limits oxygen transfer. , For some supports that govern oxygen transfer between active sites and supports (e.g., CeO 2 , Ce–Zr, and perovskite), they are flexible in donating/withdrawing oxygen to/from Pt/Pd to supplement consumed O or prevent overoxidation of Pt. Thus, they work better initially but would fail eventually like Al 2 O 3 when the supports are saturated with a stable sulfate, among others. − Different from the cases of NO and HCs (especially unsaturated alkenes), H 2 O could also act as an O supplier in the case of CO oxidation by dissociating into active OH, such as at Pt–Ti interface, to initiate CO oxidation by forming COOH intermediate .…”