Pyridine adsorption and acid/base complex formation on ultrathin films of γ-Al2O3 on NiAl(100)
Pyridine adsorption and acid/base complex formation on ultrathin films of γ-Al2O3 on NiAl(100)
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DOI：
10.1021/jp030046p
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发表时间：
2003-07
影响因子：
3.3
通讯作者：
Kathryn A. Layman;M. Ivey;J. Hemminger
中科院分区：
文献类型：
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作者：
Kathryn A. Layman;M. Ivey;J. Hemminger
用高分辨电子能量损失谱（HREELS）研究了吡啶在NiAl（100）和γ-Al 2 O3薄膜上的吸附.吡啶吸附在NiAl（100）上，在低覆盖度下其分子轴平行于表面平面，在高覆盖度下其分子平面更倾向于表面法线。在羟基化和非羟基化的γ-Al 2 O3薄膜上，吡啶通过氮的孤对电子与配位不饱和的Al 3 +阳离子相互作用。吡啶与羟基化γ-Al 2 O3薄膜表面的羟基相互作用，形成C5 H5 N-HO复合物。复合物的形成导致OH键强度降低，OH伸缩振动从3711 cm - 1（未复合的孤立OH的特征）向较低频率移动。在低覆盖率，吡啶只与更酸性的表面OH基团，那些位于3倍铝3 +阳离子网站。这种相互作用形成C5 H5 N-HO复合物，其在2920 cm - 1处具有O-H伸缩。随着覆盖度的增加，吡啶和与2个Al 3 +阳离子结合的酸性较低的OH基团之间的相互作用形成了额外的C5 H5 N-HO络合物。该C5 H5 N-HO复合物的O-H伸缩振动频率为3150 cm - 1。随着C5 H5 N-HO复合物的O-H伸缩强度的增加，3711 cm - 1处的自由O-H伸缩强度降低。与表面羟基的相互作用是可逆的，证实了所观察到的O-H伸缩频率的变化是由弱键合的酸碱复合物的形成引起的。在290 K时，大部分吡啶从γ-Al 2 O3薄膜上脱附，而在290 K以上退火时，少量吡啶脱氢并形成表面OH基团，其O-H伸缩为3742 cm - 1。
Pyridine adsorption on NiAl(100) and ultrathin films of γ-Al 2 O 3 was studied using high-resolution electron energy loss spectroscopy (HREELS). Pyridine adsorbs on NiAl(100) with its molecular axis parallel to the surface plane at low coverages and with its molecular plane inclined more toward the surface normal at higher coverages. On the hydroxylated and nonhydroxylated thin films of γ-Al 2 O 3 , pyridine interacts with the coordinatively unsaturated Al 3 + cations via its nitrogen lone-pair electrons. Pyridine was also found to interact with the surface hydroxyl groups on the hydroxylated γ-Al 2 O 3 thin films, forming C 5 H 5 N-HO complexes. Complex formation causes the OH bond strength to decrease and the OH stretch vibration to shift from 3711 cm - 1 , which is characteristic of uncomplexed, isolated OH, to lower frequency. At low coverages, pyridine only interacts with the more acidic surface OH groups, those located on 3-fold Al 3 + cation sites. This interaction forms a C 5 H 5 N-HO complex, which has an O-H stretch at 2920 cm - 1 . As the coverage is increased, an additional C 5 H 5 N-HO complex is formed from an interaction between pyridine and the less acidic OH groups, which are bonded to 2 Al 3 + cations. The vibrational frequency for O-H stretch of this C 5 H 5 N-HO complex is 3150 cm - 1 . As the intensities for the O-H stretches of the C 5 H 5 N-HO complexes increase, the intensity for the free O-H stretch at 3711 cm - 1 decreases. The interaction with the surface hydroxyl groups is reversible, confirming that the observed shifts in the O-H stretching frequency result from the formation of weakly bonded acid-base complexes. While most of the pyridine desorbs from the γ-Al 2 O 3 thin films by 290 K, annealing the pyridine-dosed γ-Al 2 O 3 thin films to temperatures above 290 K, results in a small amount of pyridine dehydrogenation and the formation of surface OH groups with an O-H stretch at 3742 cm - 1 .