| 摘要: | 在常壓下製備的氧化銥上，甲醇的分解實驗仍未被完整的研究。在我們之前的研究中，常壓下製備的氧化銥的晶體和表面結構隨著壓力的變化而變化。因此，不同條件下製備的氧化銥上甲醇的脫附特性和反應路徑可能會受到氧化物結構的影響。在本研究中，我們選擇了在775 K下，在1 × 10-6 Torr、1 × 10-2 Torr、1 × 10-1 Torr、1 Torr和5 Torr O2下製備的5種銥氧化物結構，以研究甲醇的分解，並採用了熱脫附質譜術（TPD）、紅外線吸附光譜（IRAS）、近大氣壓-光電子能譜（NAP-PES）和近大氣壓-質譜術（NAP-MS）等技術。
在氧氣壓為10-6 Torr下生長的銥氧化物上，CD3OD的反應路徑清晰可見，顯示出CD3OD在180 K時脫氫生成CD2O，在220 K生成CO，隨後在300 K氧化為DCOO，最終在370 K完全脫氫生成CO2。相比於2 × 1氧吸附的Ir，低曝量的CD3OD在成長條件為10-2和10-1 Torr氧氣的銥氧化物上難以與表面氧結合並形成CO2。隨著氧化物生長條件達到1 × 10-2 Torr，含碳物質更容易從氧化物中脫附。過渡態和簇狀銥氧化物上的甲醇反應機制是分解為CO和D2，這與單晶銥上的反應差不多 [2]。在近常壓的甲醇實驗中，在低於1 × 10-1 Torr下生長的銥氧化物，CH3OH的主要反應機制是在300 K先是分解為CHxO，然後形成主要產物為CO和少量產物CHx（x = 0 ~ 4），與低曝量甲醇反應機制一致。在1 Torr和5 Torr氧氣下生長的銥氧化物上，CD3OD的反應更偏向於氧化，且在低溫下容易脫附。與2 × 1氧吸附的Ir(100)相比，3維雙相和2 & 3維混相的銥氧化物上CD2O的形成和CO的脫附發生在更低的溫度。在近常壓實驗中，在300 K的溫度和氧覆蓋較高的氧化物上，CO和CHx的生成量進一步減少，但在1 Torr以上生長的銥氧化物上CHxO (x = 1 ~ 3)和C*仍略有增加。生長在10-2 Torr O2以上的銥氧化物上，氣體生成物在400 K時對CD2O的選擇性更高，而非CO。
;The methanol decomposition on the iridium oxide prepared under ambient pressure hasn’t been studied fully. In our previous studies [1], the crystal and surface structure of iridium oxide prepared under ambient pressure varied with pressures. Therefore, the desorption property and reaction pathway of methanol on the iridium oxide prepared under different conditions might be affected by the oxide structure. In this studies, 5 structures of Ir oxides prepared under 1 × 10-6 Torr, 1 × 10-2 Torr, 1 × 10-1 Torr, 1 and 5 Torr O2 at 775 K are selected for the methanol decomposition investigation by the temperature programmed desorption (TPD), the infrared reflection adsorption spectroscopy (IRAS), near ambient X ray photoelectron spectroscopy (NAP-PES) and the near ambient pressure- mass spectroscopy (NAP-MS).
The clear reaction pathway of CD3OD on Ir oxide grown under 10-6 Torr oxygen (2 × 1 oxygen adsorbed Ir(100)) demonstrates the CD3OD dehydrogenation from CD2O at 180 K and CO at 220 K, oxidation to DCOO at 300 K CO2, to then CO2 from total dehydrogenation at 370 K. The Langmuir dose CD3OD is difficult to combine with surface oxygen and form CO2 on Ir oxide grown under 10-2 and 10-1 Torr oxygen (Ir(100)-(2 × 1)-O + IrO2(100) and IrO2(100)), compared to TPD results on 2 × 1 oxygen adsorbed Ir. Since oxide growth condition up to 1 × 10-2 Torr, carbon species is more facile to desorb from the oxide. The reaction mechanisms of methanol on Ir(100)-(2 × 1)-O + IrO2(100) and IrO2(100) are simply decomposing to CO and D2, as that on single crystal Ir [2]. In ambient methanol experiments, the main mechanism of CH3OH is decomposition to CHxO at 300 K, and then main product CO and the minor product CHx (x = 1 ~ 3) on the Ir oxide grown under below 1 × 10-1 Torr. It is consistent with the Langmuir doses experiment. The reactions of CD3OD are preferential to oxidation with facile desorption at low temperature on Ir oxides grown under 1 and 5 Torr oxygen (IrO2(102) & (10-2) and IrO2(110)). The CD2O formation and CO desorption occur at lower temperature on IrO2(102) & (10-2) and IrO2(110), compared to 2 × 1 oxygen adsorbed Ir(100). In ambient pressure experiment, the CO and CHx products is further reduced on higher oxygen-covered oxide at 300 K, but CHxO (x = 1 ~ 3) and C* still slightly increases on Ir oxides grown under above 1 Torr oxygen. Gaseous productions on the Ir oxides grown above 10-2 Torr O2 have higher selectivity to CD2O at 400 K, rather than CO. |
