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3. Results and discussion The samples were prepared following the previously optimised procedure [2,17,25] (Table 1). In order to check the effect of different activation conditions and to identify the state of promoters, a single batch of Ru/C was used for promoters deposition. No sample treatment was done after promoters impregnation, hence precursors’ decomposition was expected during catalyst activation. The latter was made by temperature programmed reduction (TPR), according to the heating programmes detailed in Table 2. The reduced samples were then analysed by oxygen pulse chemisorption. TPR with H2 surely leads to the reduction of the RuOx surface passivated layer and to the decomposition of the nitrate promoters precursor. In addition, a higher hydrogen and oxygen uptake can be expected if the alkaline promoters reduction occurs. On the other hand, if the structural action of Ba is true, a different oxygen uptake should be observed if the hypothesised Ru redistribution affects metal dispersion. A further checking for support effect during activation was made by means of the comparative sample prepared with the as supplied active carbon (AC). It should be remembered that this sample does not have any practical use, due to support instability under the ammonia synthesis reaction conditions. 3.1. TPR analysis An example of TPR pattern is reported in Fig. 1, relative to the R3 programme (Table 2). The blank Cs/GC sample showed a broad and very small H2 uptaking peak between 350 and 450℃, corresponding to nitrate decomposition. Similar patterns were observed for the other blank samples. The Ru/GC sample showed a reduction peak centred at ca. 100℃, due to the reduction of RuOx surface species. The same peak was observed when using AC as support (sample Ru/AC, not reported), but it showed much broader. The peak at higher temperature is due to incipient CH4 formation, due to poor support stability in the absence of promoters [25]. After adding the promoters the pattern radically changed, as found also by others [29,32]. The first peak shifted towards higher temperature and became much more intense. Indeed, the maximum of the first peak was found at ca. 200–250℃ for all the singly promoted samples supported on GC. By contrast, thenitrate precursor decomposition occurred at markedly lower temperature with respect to the blank samples. It should be noticed that the TCD detector does not permit a reliable quali-and quanti-fication of the various species in the case of strongly overlapping contributions. The Cs-Ru/AC sample showed the same shift of the low temperature reduction peak, whereas the higher temperature peak was practically absent. Finally, the tri-promoted sample prepared from RuCl3 gave a broader reduction pattern, with higher H2 uptake, continuing even at the highest temperature (Fig. 2). From the TPR data the following preliminary conclusions can be drawn: (i) H2 uptake strongly increases for the promoted samples; (ii) promoters precursors decomposition shifts to markedly lower temperature in the presence of Ru; (iii) the reduction profile is more complex and H2 uptake is higher for the Cs-promoted sample supported on GC, with respect to AC; (iv) the chloride containing sample shows higher H2 uptake and requires a much prolonged reduction with respect to the samples prepared from a different Ru precursor. |
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jacy_cheyn
铁杆木虫 (知名作家)
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【答案】应助回帖
Mally89: 建议虫友尽量将求助内容翻译完全！~ 2011-06-02 00:21:32
jiachuo(金币+140, 翻译EPI+1): 2011-06-03 23:24:29
jiachuo(金币+140, 翻译EPI+1): 2011-06-03 23:24:29
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3．结果与讨论 如表一所示，为由前所述最优方法来制备的样品。为了检测不同的活化条件对助剂状态的影响，我们只采用一组Ru/C 来表征助剂的沉积。助剂浸渍后没有对样品进行处理，所以可以预想在催化过程中，助剂分解了。后者采取程序升温还原（TPR）的方式，有表二可以看到升温程序的细节。用氧脉冲吸附分析还原产物。程序升温还原过程中，氢气使得RuOx 表面钝化层减少，并且使硝酸盐助剂前体分解。另外，可以预想到如果碱性助剂被还原，氧化还原的活性会提高。另一方面，如果假设Ru的再分配影响到了金属的分散，当Ba的结构是真实的，可以的到不同的氧吸收量。 通过将我们制备的样品与购买的活性炭作对比，我们可以得到在催化过程中的载体活性的进一步研究。但是我们也要明确在氮气反应条件下载体的不稳定，因而这些样品没有任何实际用处。 |
2楼2011-06-01 21:33:32
jacy_cheyn
铁杆木虫 (知名作家)
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【答案】应助回帖
★
sltmac(金币+1): 谢谢交流~~ 2011-06-03 08:47:06
sltmac(金币+1): 谢谢交流~~ 2011-06-03 08:47:06
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3.1 TPR 分析 如图一所示，相对于表二中的R3程序，为样品的程序升温还原图。空白Cs/GC 样品在350-450℃间的很宽的很小的H2吸收峰，与硝酸盐分解相一致。在其他的空白试验中也得到了相似的图形。在Ru/GC组样品中，由于RuOx表面还原作用，出现一个相似的约以100℃为中心的吸收峰。当以活性炭作为载体的时候，也观察到相似的但是更宽的吸收峰。由于初期的甲烷的形成，没有助剂时载体的不稳定性，在较高温度下也形成了一个峰。当加入助剂之后，和其他的研究者得到的结果一样，峰型发生了根本的变化。第一个峰向较高温度发生移动，并且变得更为尖锐，而且，在所有负载于GC上的单助剂的样品第一个峰的最大值大概出现在200-250℃之间。相对的，相对于空白试验的样品来说，thenitrate前提的分解温度明显低了很多。我们注意到TCD检测器不能在多重大量重复条件下提供可靠的定性定量研究. Cs-Ru/AC 样品具有相同的向低温移动的还原峰，并且没有高温的吸收峰。最后，由RuCl3制备的三重助剂的样品具有更宽的还原峰，甚至在最高的温度时候，仍然又连续的更高的H2吸收。 |
3楼2011-06-02 08:45:21
jacy_cheyn
铁杆木虫 (知名作家)
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4楼2011-06-02 08:52:02
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