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摘要: 負載貴金屬的γ-Al2O3催化劑具有優異的有機物催化降解能力，被廣泛用于處理固定源和移動源排放產生的廢氣，高溫燒結是導致催化劑失活的重要因素之一，如何提高催化劑抗燒結性能備受關注。本文介紹了負載貴金屬的γ-Al2O3催化劑高溫燒結產生的原因和機理，分析表明高溫致使貴金屬發生Ostwald熟化和液化團聚以及γ-Al2O3晶相轉變降低催化劑比表面積，降低了催化劑活性。在此基礎上從貴金屬、載體以及載體與貴金屬間作用三個方面回顧和整理了提高催化劑高溫熱穩定性的方法，并重點闡述了貴金屬修飾、載體改性以及改變金屬與載體相互作用來達到提高熱穩定性的方法。此外，還介紹了其他如限域法、晶面控制等實現催化劑穩定性提高的方法，為催化劑的設計提供了新思路。最后，對γ-Al2O3基氧化催化劑的未來發展方向進行了展望。Abstract: γ-Al2O3 is an enormously important industrial material, especially used as catalysts, catalyst supports, and adsorbents due to its attractive structural, surface, and dielectric properties. Particularly, catalytic reduction of pollutants such as nitric oxide, as well as oxidation of hydrocarbons, is accomplished with precious metals such as platinum or palladium dispersed on the γ-Al2O3 surface. γ-Al2O3 loaded with precious metals has an excellent catalytic degradation ability of organic matter and is widely used to treat exhaust gas from stationary and mobile sources. High-temperature sintering is a major cause of catalyst deactivation. For example, at higher treatment temperatures (>800 ℃), γ-Al2O3 transforms into δ-Al2O3 and θ-Al2O3, decreasing in surface area and a change in dielectric properties. Additionally, in the reaction environment, supported metal nanoparticles grow in size, leading to the loss of catalyst activity. How to improve the anti-sintering performance of catalysts is a particular concern of this field. This review analyzes the reason and mechanism of the high-temperature sintering of γ-Al2O3 loaded with precious metal. A high temperature leads to Ostwald ripening and particle migration, coalescence of precious metals, and phase transformation of γ-Al2O3, reducing the specific surface area and activity of the catalyst. On this basis, the approaches for improving the high-temperature thermal stability of catalysts were reviewed and sorted out from three aspects, namely, precious metals, supports, and the interaction between them. First, the focus is on precious metal modification, carrier modification, and changing the interaction between them to improve thermal stability. Additionally, other methods, such as the confinement method and crystal plane control, are thoroughly examined and explained. These strategies provide new insights into catalyst design. Finally, the developmental trends of γ-Al2O3-based oxidation catalysts are broadly forecasted.
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Key words:
- air pollution /
- catalysis /
- sintering /
- alumina /
- precious metal
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圖 2 整體式VOCs氧化催化劑失活類型.（a）理想型；（b）貴金屬燒結；（c）載體燒結；（d）中毒失活
Figure 2. Types of deactivations of monolithic VOCs oxidation catalysts: (a) ideal catalyst; (b) sintering precious metal of catalysts; (c) sintering γ-Al2O3 of catalyst; (d) poisoned catalyst
圖 3 貴金屬顆粒包覆SiO2示意圖及性能對比.(a)Pt–Pd合金被二氧化硅包覆形成核殼結構; (b) 核殼結構提高甲烷催化性能[29]
Figure 3. Schematic showing the encapsulated PdPt@SiO2 catalyst and its hydrothermal stability for lean CH4 combustion: (a) Pt–Pd alloy is coated with silica to form a core-shell structure; (b) core-shell structure improves the catalytic performance of methane[29]
圖 8 三種不同結構催化劑在10 ℃·min–1的升降溫過程中甲烷催化氧化效率與溫度關系.(a) Pd@CeO2/疏水改性的γ-Al2O3；(b)Pd/CeO2；(c)Pd/CeO2/Al2O3[52]
Figure 8. Light-off curves of CH4 conversion against the temperature for the three catalysts formulations used at heating and cooling of 10 ℃·min–1: (a) Pd@CeO2/H-Al2O3 core-shell catalyst; (b) Pd/CeO2-IWI; (c) Pd/CeO2/Al2O3-IMP[52]
表 1 貴金屬單質特征溫度
Table 1. Characteristic temperature of precious metal
Precious metal Tm/℃ 0.3Tm/℃ 0.5Tm/℃ Ru 2250 484 988 Rh 1964 398 846 Pd 2970 700 1349 Ir 2450 544 1088 Pt 1772 340 750 Au 1064 128 396 
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表 2 負載貴金屬γ-Al2O3催化劑的不同抗燒結策略
Table 2. The sintering resistance strategy of γ-Al2O3 loaded with precious metals
The sintering resistance strategy Advantages Disadvantages Modification of γ-Al2O3 Introducing anion PO43– Improvement the stability of γ-Al2O3 Lowering catalytic activity, unstable
bulk phaseIntroducing cation Mg2+, La3+
et alHigher calcination temperature (>900 ℃),
unstable bulk phaseCrystal phase transition Improvement the stability of support
bulk phaseHigh calcination temperature (1200 ℃),
much lower surface areaModification of
precious metalsShell coating Improvement the stability of
precious metalsComplex preparation process Improving Tm Lowering cost, tuning electrons
of d oribitsSmall increase in thermal stability Space confinement Micro/mesoporous confinement Retarding agglomeration rate
of precious metalSmall increase in thermal stability Introducing metal into the crystal lattice Only suitable for perovskite structure Adjustment of SMSI Increasing unsaturated coordination Al3+ Prevent precious metal
nanoparticles migrationDecrease the stability of support Reconfiguring the support interface Improvement the stability of
catalysts and its activityComplex preparation process
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