用硫醇在Ni(100)面上的分解和脱附

THE THERMAL DECOMPOSITION AND DESORPTION OF METHANTHIOL ADSORBED ON Ni (100)

用程序升温脱附(TPD)和俄歇电子能谱(AES)在80～773K范围内研究了甲硫醉在Ni(100)面上的脱附和分解.结果表明:当甲硫醇暴露度≤3L时(1L=1.33×10~4Pa.S),甲硫醇在表面分解的脱附产物为氢和甲烷;甲硫醇暴露度≤4L时,除有表面反应生成的氢和甲烷脱附外还伴随有甲硫醇的脱附.AES测量表明,由甲硫酸分解产生的硫原子被强吸附在Ni(100)面上,并对Ni(100)面起化学改性的作用.在甲硫醇暴露度为0.5～3L范围内,滞留在表面的强吸附硫量随甲硫醇的暴露度成正比增加.当甲硫醇暴露度等于10L时,强吸附硫量接近饱和值.表面硫的存在对甲硫醇在Ni(100)面上的反应和吸附性能有明显的影响,硫化学改性的主要作用是阻塞了Ni(100)面上的四重穴中心,降低了Ni(100)面对C-H键、C-S键、和S-H键的裂解活性.

The thermal decomposition and desorption of methanthiol adsorbed on Ni(100) has been studied using temperature programmed desorption ( TPD) and Auger electron spectroscopy (AES). TPD experiments were performed from 80 to 773 K at a heating rate of 10K /s in an UHV system. We have found that the thermal decomposition reaction pathway and desorption states after adsorption at 80 K depend strongly on the initial CH3SH exposure. AES analysis of the samples after TPD shows that the sulfur atoms formed by dissociation are resided on the surface. The sulfur coverage increases roughly proportional to the increase of CH3 SH exposure, and saturation occurs at exposure of 10 L CH3 SH. In combination with LEED patterns it is revealed that sulfur atoms are probably adsorbed in the fourfold hollows of Ni(100) in an order which transfers from P(2×2) to C(2×2) superstructure with the increase of CH3SH exposure. The sulfur bonded strongly to Ni(100) surface can change adsorption characteristics and decomposition pathway of CH3SH by a site blocking effect or / and electronic effect. At low exposure (<1 L CH3SH), a large peak of α-H2at350 K and a weak peak of CH4 in range of 220 to 380 K are observed during TPD. AES measurements indicate that in addition to sulfur there is some carbon deposited on the surface after TPD. These results reveal that CH3SH almost completely dissociated into Cads, Sads and Hads at low exposure. At median exposure (i.e. 2 to 4 L CH3SH) , a large amount of CH4desorbs in a second order peak around 250 Kand two new β1,-H2 and β2-H2 peaks appear respectively at 250 and 200 K accompanied by lowering of α-H2 peak. Except at 2L CH3 SH exposure there is no carbon deposit on the surface . This means that the dissociation of C-H bond is inhibited by sulfur poison. In this case CH3 SH entirely dissociates into CH3ads, Hads and Sads, and thenCH4 is formed via recombination of CH3ads, and Hads. The temperature of CH4 desorption is the sameas the reaction temperature of CH4 formation because CH4 not be adsorbed on Ni( 100) surface. It is thus seen from the above consideration coupled with the TPD spectrum of H2 adsorbed on the clean Ni(100) , that the α-H2 is adsorbed in the fourfold hollow sites. The lowering of α-H2 peak with increasing exposure is probably due to the blocking of fourfold hollow by more and more sulfur leading to the formation of β1-H2 and β2-H2 states, which may be weakly bonded to the bridge site and the top of Ni-atom respectively. At saturation (10 L CH3SH) , desorption of large amount of molecular CH3SH occurs in the temperature range of 200 to 650 K and CH4 peak exhibits a high temperature shoulder, which can probably be attributed to the lowering of splitting activity of C-S and S-H bond by sulfur poison on Ni (100) . All TPD spectra show that small desorption peaks of H2, CH4and CH3 SH appeared below 180 K. They probably stem from CH3SH adsorbed on the crystal rim and holder. In summary, the reaction pathway of decomposition and desorption of methanthiol adsorbed on Ni(100) may be formulated as follows: First step