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Fischer-Tropsch synthesis: Direct cobalt nitrate reduction of promoted Co/ TiO_2 catalysts

机译：费-托合成：助催化的Co / TiO_2催化剂直接还原硝酸钴

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The effect of the direct reduction of cobalt nitrate versus the more conventional calcination/reduction treatment has been investigated. Porosity properties of the catalysts are not significantly modified by avoiding the calcination step, as similar BET surface area, pore volume and pore diameter are obtained for the activated catalysts. In contrast, the cobalt reducibility decreases, but smaller cobalt particles size and higher dispersion are obtained. The reduction phenomena occurring for the uncalcined catalysts are more complex because of the additional nitrate decomposition steps. TPR-MS and TPR-XANES point out that CoOx intermediate species are formed during the reductive nitrate decomposition. However, these species are oxidized by NOX (formed by nitrate decomposition) to spinel type Co3O4, which is then converted to CoO prior to the final reduction step to Co-O. The addition of promoters (Pt, Re, Ru, Ag) improves the cobalt reducibility, especially by shifting the final reduction step (i.e., CoO to Co-O) to lower temperature. FT activity testing data show that activated uncalcined catalysts have higher CO conversion following the initial decline and leveling off period relative to the activated calcined catalyst. The best performance is achieved with uncalcined Pt-12%Co/TiO2. This catalyst has the highest CO steady state conversion, which is 1.2 times higher than the Pt-promoted calcined catalyst. Moreover, its deactivation rate is 0.13%/h compared to 0.2%/h for the corresponding calcined catalyst. The difference in the catalytic activity is even higher for the un-promoted samples, where the activated uncalcined catalyst has almost double the CO conversion as compared to its calcined counterpart. Finally, the addition of other promoters such as Ru, Re and Ag has no significant effect on catalytic activity.
机译：已经研究了直接还原硝酸钴相对于更常规的煅烧/还原处理的效果。通过避免煅烧步骤，没有显着改变催化剂的孔隙率性能，因为对于活化的催化剂获得了相似的BET表面积，孔体积和孔径。相反，钴的还原性降低，但是获得了较小的钴颗粒尺寸和较高的分散性。由于额外的硝酸盐分解步骤，未煅烧催化剂发生的还原现象更为复杂。 TPR-MS和TPR-XANES指出，在还原性硝酸盐分解过程中会形成CoOx中间物种。但是，这些物质被NOX（由硝酸盐分解形成）氧化为尖晶石型Co3O4，然后在最终还原成Co-O之前将其转化为CoO。促进剂（Pt，Re，Ru，Ag）的添加改善了钴的还原性，特别是通过将最后的还原步骤（即从CoO转变为Co-O）降低到较低的温度。 FT活性测试数据表明，相对于活化的煅烧催化剂，活化的未煅烧催化剂在初始下降和稳定期后具有更高的CO转化率。未经煅烧的Pt-12％Co / TiO2可获得最佳性能。该催化剂具有最高的CO稳态转化率，是Pt促进的煅烧催化剂的1.2倍。此外，其失活率为0.13％/ h，而相应的煅烧催化剂为0.2％/ h。对于未助催化的样品，催化活性的差异甚至更高，与未煅烧的催化剂相比，未活化的催化剂的CO转化率几乎翻了一番。最后，其他助催化剂如Ru，Re和Ag的添加对催化活性没有显着影响。

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