汽车尾气脱除催化剂的表征 Yang Bo 2012.6.26. Introduction The writer report the first...
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Transcript of 汽车尾气脱除催化剂的表征 Yang Bo 2012.6.26. Introduction The writer report the first...
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汽车尾气脱除催化剂的表征
Yang Bo
2012.6.26
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Introduction The writer report the first study of the hydrolysis of tetraethyl ortho-silicate (TEOS) in an aqueous solution of N, N, N-trimethyl-1-adamantammonium (TMAda) hydroxide, the clear sol precursor for the preparation of the high-silica zeolite SSZ-13 (CHA). The initial stages of the hydrolysis of TEOS were monitored by quantitative Si and Al nuclear magnetic resonance (NMR) and small-angle X-ray scattering (SAXS). Si NMR allowed quantitative characterization of Si in nanoparticles and dissolved oligomers and measuring the average Si − O−Si connectivity. The writer try to elucidate the effect of the organocation TMAda+ and the presence of Al on chemical composition, internal connectivity, and stability of precursor nanoparticles as well as soluble species.
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Result and Discussion
This has also been observed with the silicalite-1 and silicalite-2 systems when TPAOH and TBAOH are used as SDA, and thus, this seems to be a general pattern for alkylammonium-based silicate sols
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Figure 2. (a) Si-distribution in TEOS (■), oligomers ( □), and nanoparticles ( ●) during the hydrolysis of TEOS in TMAdaOH. (b) Comparison of the Si distribution for CHA,MEL, and MFI systems. TEOS decreases as a straight line similar tothat in panel a.
Si-distribution
At this stage , the organic SDA acts as the hydroxide ion source adjusting the pH of the medium , and affecting the stability of the silicate species present in solut ion by shifting the hydrolysis , condensation , and precipitation equilibria .
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Si-distribution
In identifying the different Si species, we have employed the following chemical shift ranges: Q0= −71 to −72 ppm, Q1andQ2Δ= − 79 to − 83 ppm, Q2and Q3Δ= −86 to −91 ppm, Q3=− 92 to − 100 ppm, and Q4= −100 to −108 ppm.
Figure 3. Si-distribution in (a) all oligomers and (c) nanoparticles during the hydrolysis ofTEOS in TMAdaOH.
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SAXS patterns
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Hydrolysis of TEOS in the Aluminosilicate System
Figure 7. Evolution of (a)27Al NMR spectra of clear sols/solutionswith the progress of TEOS hydrolysis. (b) Evolution of chemical shifts of qn Al sites with [Si]/[TMAdaOH] ratio.
77 to 73,72 to 67, 66 to 62, and 61 to 54 ppm, assigned, respectively, to q0/q1and q2,q3, and q4
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The role of the aluminum is to connect oligomers within the nanoparticles into core shell elements. As aluminum exhibits a lower activation energy than Si for metal oxygen bond breaking reformation, it catalyzes the core shell networking within the nanoparticles.
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27Al NMR
The 27Al spectra are only affected by the change in Si/TMAdaOH ratio and the connectivity of Si. This means that aluminate species behave like silicate species.
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Transformation of LEV-type zeolite into less dense CHA-type zeolite
Ikuhiro Goto, Masaya Itakura, Syohei Shibata, Koutaro Honda, Yusuke Ide, Masahiro Sadakane, Tsuneji Sano
Microporous and Mesoporous Materials 158 (2012) 117–122
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In general, medium/large pore size and high-silica zeolites are synthesized by hydrothermal treatment of amorphous aluminosilicate hydrogel as a starting material in the presence of organic structure-directing agents (OSDAs). The use of OSDAs is, however, undesirable from a practical point of view, because of their high cost as well as their large environmental impact. Hydrothermal conversion of LEV-type zeolite into CHA-type zeolite occurred in the absence of both anorganic structure-directing agent and a seed crystal.
Introduction
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Fig. 2. SEM images of (a) starting LEV, (b) CHA (Sample No. 1), (c) CHA (Sample No. 8),
SEM Images
Fig. 2 (b) and (c) show SEM images of the obtained CHA-type zeolites. The crystal morphology was cubic, and the crystals were 200–400 nm in size, which is smaller than the crystal size of the starting LEV-type zeolite, as shown in Fig. 2(a)
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.When the hydrothermal conversion of LEV-type zeolite was carriedout at 125–170℃for 1.5 h, the LEV-type zeolite was transformedinto CHA-type zeolite. At 200℃, pure ANA-type zeolite was ob-tained, suggesting that CHA-type zeolite transformed into the moststable zeolite. At 90 ℃, pure CHA zeolite was obtained when thesynthesis time was prolonged to 12 h. The morphology and crystalsize of the CHA-type zeolite crystals obtained at 90 ℃were similarto those obtained at 125 ℃.
Influence of the Synthesis Temperature
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Influence of the Si/Al Ratio
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LEV–CHA and LEV–LTA
LEV- and CHA-type zeolites have similar composite buildingunits because these two zeolites both belong to chabazite group,whereas there was less similarity between the composite units of LEV- and LTA-type zeolites.
It was suggested that locally ordered aluminosilicate species (nanoparts) produced by decomposition/dissolution of the starting LEV-type zeolite contribute to the transformation process.
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Thanks for your attention