Al-Zr Mixed Oxide Catalyst for Thermal Stability
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Solution Overview
Problem
Existing catalysts for exhaust gas treatment in internal combustion engines face challenges in maintaining thermal stability, specific porosity, and efficient DeNOx activity over time, especially under harsh conditions such as high temperatures and the presence of aggressive gases.
Innovation Solution
A mixed oxide composed of aluminium, zirconium, lanthanum, and optionally rare-earth metals other than cerium and lanthanum, with specific weight proportions, is developed. This mixed oxide is characterized by a high specific surface area and specific porosity after calcination, which helps in maintaining thermal stability and catalytic activity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a cerium zirconium based mixed oxide is used as a co-catalyst and support, then the catalyst provides good oxidation capacity and supports precious metal dispersion, but the DeNOx activity deteriorates because Rh0 is oxidized into RhIII from the desorbed oxygen from CeO2
Solution Approach 1:
The patent removes cerium from the mixed oxide composition entirely, extracting the harmful element that causes Rh oxidation. The invention uses aluminium-zirconium-based mixed oxide without cerium, thereby eliminating the source of desorbed oxygen that oxidizes Rh0 to RhIII and reduces DeNOx activity.
Solution Approach 2:
The patent changes the chemical composition parameters of the mixed oxide by replacing cerium-zirconium system with aluminium-zirconium system. This parameter change fundamentally alters the oxidation behavior, preventing Rh oxidation while maintaining thermal stability and catalytic performance.
2Reliability
If zirconia is used as a support for rhodium, then the catalyst achieves good dispersion and stabilization of Rh0, but the thermal stability deteriorates under harsh conditions with loss of porosity and surface area
Solution Approach 1:
The patent creates a composite mixed oxide material combining aluminium oxide and zirconium oxide in specific proportions (Al2O3: 20-45 wt%, ZrO2: 50-70 wt%). This composite structure leverages the thermal stability of zirconia and the porosity control capabilities of aluminium oxide to achieve both good Rh dispersion and thermal stability under harsh conditions.
Solution Approach 2:
The patent optimizes the local composition and pore structure of the mixed oxide to maintain specific porosity characteristics (V/Vtotal ≥ 0.80, Vtotal ≥ 0.35 mL/g) while achieving thermal stability. The local quality of pore distribution and surface area is preserved through controlled calcination processes.
3Stability of the object's composition
If the catalyst is subjected to severe ageing at high temperatures, then the catalyst structure stabilizes, but the specific porosity and catalytic activity are lost due to sintering encapsulation of precious metal
Solution Approach 1:
The patent performs beforehand cushioning by pre-forming a mixed oxide with optimized porosity and surface area through controlled calcination at 900-1050°C for 2-5 hours before catalyst deployment. This pre-treatment creates a thermally stable framework that cushions against subsequent sintering and encapsulation during severe ageing, maintaining porosity and catalytic activity.
Solution Approach 2:
The patent changes the thermal treatment parameters by conducting calcination at specific temperatures (900-1050°C) for specific durations (2-5 hours) to optimize the balance between crystallinity development and porosity retention. This parameter optimization ensures the mixed oxide maintains its structural integrity and catalytic properties under severe ageing conditions.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The mixed oxide catalyst retains its porosity and thermal stability at high temperatures, ensuring sustained DeNOx activity and effective mass transfer, thus addressing the limitations of traditional catalysts.
Implementation Method 1
after calcination in air at 1100° C. for 5 hours, the specific surface area (BET) of the mixed oxide is at least 25.0 m2/g; and in that after calcination in air at 950° C. for 3 hours, the porosity of the mixed oxide determined by N2 porosimetry
Implementation Method 2
The mixed oxide shall withstand temperatures as high as 1100° C. or 1200° C. In particular, to prevent that the catalytically active precious metal (notably Rh) is encapsulated due to the sintering effect, the structure of the mixed oxide shall weather the thermal stress.
Implementation Method 3
Rh is known to be an efficient precious metal to reduce the NOx content from the exhaust gas. Rh0 is preferred over Rh in high oxidation state like RhIII because it provides a better DeNOx activity.
Data Source
AI summary
The present invention relates to a mixed oxide of aluminium, of zirconium, of cerium, of lanthanum and optionally of at least one rare-earth metal other than cerium and other than lanthanum that makes it possible to repair a catalyst that retains, after severe ageing, a good thermal stability and a good catalytic activity. The invention also relates to the process for preparing this mixed oxide and also to a process for treating exhaust gases from internal combustion engines using a catalyst prepared from this mixed oxide. The mixed oxide exhibits at least one of the 3 characteristics (i), (ii), (iii) below:—(i) Δ is lower than 82.0%, Δ being calculated by the following formula: Δ=(S950° C./3 h−S1200° C./5 h)/S950° C./3 h×100; (ii) Δ* is lower than 55.0%, Δ* being calculated by the following formula: Δ*=(S950° C./3 h−S1100® C/5 h)/S950° C./3 h×100; (iii) S1200° C./5 h is strictly higher than 15.0 m2/g (>15.0 m2/g).