Tailoring Alumina Surface Chemistry for Efficient Use of Supported MoS2
Tailoring Alumina Surface Chemistry for Efficient Use of Supported MoS2
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DOI：
10.1006/jcat.1997.1874
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发表时间：
1998
影响因子：
7.3
通讯作者：
J. Reardon;A. Datye;A. Sault
中科院分区：
文献类型：
--
作者：
J. Reardon;A. Datye;A. Sault
摘要 报道的MoS 2 /γ-Al 2 O 3 催化剂加氢脱硫(HDS) 的活性趋势显示，当活性归一化为Mo 含量时，Mo 负载量的活性达到最大值。相反，在TiO 2 和ZrO 2 载体上观察到归一化活性的简单单调下降。虽然早期的工作将这些不同的活性趋势归因于 MoS 2 形态的差异，但此处提供的活性测量和透射电子显微镜图像最终证明，这两种不同的趋势可以发生在产生几乎相同的 MoS 2 形态的支撑材料上。由于形态差异无法解释这一结果，因此我们提出以下化学解释，涉及在低 Mo 覆盖率下在 γ-Al 2 O 3 上形成非活性钼酸盐物质。已知水性钼酸盐与γ-Al 2 O 3 上的I-a型OH基团以最高频率反应会在低Mo覆盖率下形成稳定的MoO 4 2− 物质，其难以转化为活性MoS 2 形式。因此，标准化 HDS 活性非常低。随着 Mo 覆盖率的增加，I-a 型 OH 基团被消耗，并且更容易硫化的钼酸盐物质的形成开始占主导地位，并且标准化活性增加。最终，随着MoS 2 片状体的平均尺寸开始增长，归一化活性随着Mo覆盖率达到最大值，导致位于活性边缘位点的Mo原子比例减少。由于γ-Al 2 O 3 上的I-a型羟基与四面体配位的Al阳离子相关，因此应该可以通过从表面去除所有四面体配位的Al阳离子来防止不活泼的钼酸盐的形成，从而消除覆盖率的最大活性。这种去除是通过使用仅含有八面体配位的Al原子的α-Al 2 O 3 以及通过在负载Mo之前用异丙醇钛滴定γ-Al 2 O 3 上的I-a型羟基来实现的。在这两种情况下，均未观察到活性最大值，并且所有 Mo 负载量下的活性均高于 γ-Al 2 O 3 上的活性。 OH基团消耗的傅里叶变换红外测量与钼酸盐还原性的X射线光电子能谱测量相结合，证明γ-Al 2 O 3 与异丙氧基钛的反应优先消耗I-a型羟基，并且钼酸盐在α-Al 2 O 3 和二氧化钛涂覆的γ-Al 2 O 3 上比在纯γ-Al 2 O 3 上更容易被还原，从而支持了化学解释。因此，通过合适的改性剂（例如二氧化钛）滴定γ-氧化铝上的I-a型OH基团提供了一种简单的方法来提高负载型MoS 2催化剂的总体活性，同时保留γ-Al 2 O 3载体的有利性能，例如高表面积和热稳定性。
Abstract Reported activity trends for hydrodesulfurization (HDS) over MoS 2 /γ-Al 2 O 3 catalysts show a maximum in activity with Mo loading when activity is normalized to Mo content. In contrast, simple monotonic decreases in normalized activity are observed over TiO 2 and ZrO 2 supports. While earlier work ascribes these different activity trends to differences in MoS 2 morphology, activity measurements and transmission electron microscope images presented here conclusively demonstrate that the two different trends can occur on support materials that give rise to virtually identical MoS 2 morphologies. Since differences in morphology cannot explain this result, we instead propose the following chemical explanation involving the formation of inactive molybdate species on γ-Al 2 O 3 at low Mo coverages. Reaction of aqueous molybdates with the highest frequency, or type I-a, OH groups on γ-Al 2 O 3 is known to form stable MoO 4 2− species at low Mo coverages, which are difficult to convert into the active MoS 2 form. As a result, normalized HDS activity is very low. As Mo coverage increases the type I-a OH groups are consumed and formation of more easily sulfided molybdate species begins to predominate, and normalized activity increases. Ultimately, normalized activity goes through a maximum with Mo coverage as the average size of the MoS 2 platelets begins to grow, resulting in a decrease in the fraction of Mo atoms located at active edge sites. Since the type-I-a hydroxyls on γ-Al 2 O 3 are associated with tetrahedrally coordinated Al cations, it should be possible to prevent the formation of inactive molybdates, and thereby eliminate the maximum in activity with coverage, by removing all tetrahedrally coordinated Al cations from the surface. This removal has been accomplished through the use of α-Al 2 O 3 , which contains only octahedrally coordinated Al atoms, and through titration of the type I-a hydroxyls on γ-Al 2 O 3 with titanium isopropoxide prior to Mo loading. In both cases, no maximum in activity is observed and activity at all Mo loadings is higher than on γ-Al 2 O 3 . Fourier transform infrared measurements of OH group consumption coupled with X-ray photoelectron spectroscopy measurements of molybdate reducibility support the chemical explanation by demonstrating that reaction of γ-Al 2 O 3 with titanium isopropoxide preferentially consumes type I-a hydroxyls and that molybdates are more easily reduced on α-Al 2 O 3 and titania coated γ-Al 2 O 3 than on pure γ-Al 2 O 3 . Thus, titration of type I-a OH groups on γ-alumina by a suitable modifier, such as titania, offers a simple method for increasing the overall activity of supported MoS 2 catalysts, while retaining the advantageous properties of γ-Al 2 O 3 supports, such as high surface area and thermal stability.