Decomposition of Al2O3-Supported PdSO4 and Al2(SO4)3 in the Regeneration of Methane Combustion Catalyst: A Model Catalyst Study

Abstract

Exhaust gas aftertreatment systems play a key role in controlling transportation greenhouse gas emissions. Modern aftertreatment systems, often based on Pd metal supported on aluminum oxide, provide high catalytic activity but are vulnerable to sulfur poisoning due to formation of inactive sulfate species. This paper focuses on regeneration of Pd-based catalyst via the decomposition of alumina-supported aluminum and palladium sulfates existing both individually and in combination. Decomposition experiments were carried out under hydrogen (10% H2/Ar), helium (He), low oxygen (0.1% O2/He), and excess oxygen (10% O2/He). The structure and composition of the model catalysts were examined before and after the decomposition reactions via powder X-ray diffraction and elemental sulfur analysis. The study revealed that individual alumina-supported aluminum sulfate decomposed at a higher temperature compared to individual alumina-supported palladium sulfate. The simultaneous presence of aluminum and palladium sulfates on the alumina support decreased their decomposition temperatures and led to a higher amount of metallic palladium than in the corresponding case of individual supported palladium sulfate. From a fundamental point of view, the lowest decomposition temperature was achieved in the presence of hydrogen gas, which is the optimal decomposition atmosphere among the studied conditions. In summary, aluminum sulfate has a two-fold role in the regeneration of a catalyst—it decreases the Pd sulfate decomposition temperature and hinders re-oxidation of less-active metallic palladium to active palladium oxide.

Fundamentals of Sulfate Species in Methane Combustion Catalyst Operation and Regeneration—A Simulated Exhaust Gas Study

“…Re-oxidation of metallic Pd to active PdO can be observed as a downward peak in Figure 4 in a temperature range of between 470 °C and 700 °C. The presence of Pd4S should bear in mind as detected in the latest experiments if the catalyst is heated under hydrogen gas [29,31]. Because steam reforming and water gas shift reactions form hydrogen during regeneration in simulated exhaust gas, Pd4S structure is possible to form during the regeneration period.…”

“…After regeneration the catalysts PS + 1.0 AS/Al 2 O 3 and 1.0 AS + PS/Al 2 O 3 show higher methane conversion activities than the catalysts PS + 0.25 AS/Al 2 O 3 and 0.25 AS + PS/Al 2 O 3 . A reason relies on the higher sulfur content of the samples, resulting in the catalysts bulk-kind-of sulfates, which decomposes in regeneration at lower temperature [31]. In fact, the PS + 0.25 AS/Al 2 O 3 model catalyst even showed a slight decrease in activity even though sulfur species were decomposed at least partially during the regeneration.…”

“…Impregnation of PdSO 4 and Al 2 (SO 4 ) 3 was carried out in cold water by mixing at least for 2 h. After impregnation, the solid was dried at room temperature, and to finalize the catalyst it was heated at 90 • C under air. Detailed preparation procedures are described in our previous work [31]. Amounts of added PdSO 4 correspond to 1 wt.% of sulfur and 4 wt.% of palladium loading, whereas X indicates the amount of sulfur in Al 2 (SO 4 ) 3 -containing catalysts, and it is 0.25 or 1.0 wt.% of sulfur.…”

Catalyst Deactivation, Poisoning and Regeneration

“…The complexity of catalyst poisoning obviously increases along with the increasing use of biomass/waste-derived/residual feedstocks [2,3] and with requirements for cleaner and novel sustainable processes, such as those implementing a catalytic assisted chemical looping approach [4,5].This Special Issue provides insight for several specific scientific and technical aspects of catalyst poisoning and deactivation, proposing more tolerant catalyst formulations and exploring possible regeneration strategies. In particular, 14 research articles focus on heterogeneous catalysts by investigating thermal [6-8], physical [9,10] and chemical [11][12][13][14][15][16][17][18][19] deactivation phenomena, and also exploring less conventional poisons related to the increasing use of bio-fuels [17]. Some regeneration strategies [11,16], together with solutions to prevent or limit deactivation phenomena [7,9,11,16], are also discussed.…”

“…Sulphur, mainly as SO 2 , remains one of the main poisons for catalytic systems treating exhaust gases from combustion processes in stationary and mobile applications, since it forms highly stable sulphates. The individual and combined decomposition of aluminum and palladium sulphates, components of DOC, TWC and MOC, is presented in [12]. The same authors also address the role of sulphates formed on the support in restoring the Pd active species under different atmospheres and under simulated exhaust gas [13].…”