| 摘要: | 我們建立了一套安裝於超高真空腔體內的近常壓反應室，透過搭配的飛秒解析超寬頻紅外光譜以研究表面催化反應的超快動力學。反應室以這樣的方式安裝因為樣品，催化模型系統是在超高真空條件下製備以防止汙染，而反應是在近常壓條件下進行的。反應室與超高真空腔體採用橡膠面密封隔離，如此一來近常壓實驗就可以在反應室內進行並保持腔體的超高真空環境，並且橡膠面密封可以維持約八個量級的壓差。此外，反應室也連接質譜儀，因此我們可以同時用紅外光譜和質譜監測反應。我們用這套近常壓反應室執行了有關質譜和紅外光譜量測的測試。質譜方面，氣態氮氣及甲醇的訊號隨著反應室內的氣壓上升可以被量測到，並且最終在約 1 mbar 飽和。然而，我們發現氣態甲醇的吸收強度不足以套用近常壓反應室而被紅外光譜量測到。因此，我們將甲醇曝進腔體內去監測紅外光吸收特徵。氣態甲醇的吸收特徵可以在氣壓高於 1 mabr 時被觀察到，並且隨著氣壓到 3 mbar時變得明顯。將腔體內的甲醇抽掉後，氣態甲醇吸收特徵便消失。
另外，我們研究了層狀 VSe2 對甲醇的反應性。表面結構藉由反射式高能電子繞射(RHEED)和光電子能譜(PES)來表徵，而反應、中間產物、氣態產物則是透過光電子能譜、近常壓光電子能譜(NAP-PES)、近常壓質譜(NAP-MS)來監測。我們透過氬離子轟擊來製造表面缺陷並且能藉由控制氬離子的劑量來調控表面缺陷的數量。藉由光電子能譜我們看到甲醇在超高真空環境下的 VSe2 表面上分解並產生 CHxO 和 CHx兩種中間產物，並且它們的產量隨著表面缺陷的數量而發生改變，其中缺陷數量越少時表現出較好的表面反應能力。因此我們可以透過改變氬離子轟擊的時間來控制缺陷數量，進而影響 VSe2 表面的反應能力。由近常壓光電子能譜可以觀察到，甲醇分解並產生的兩種中間產物之相對比例，和超高真空環境下的情形是相反的，意味著氣態產物在反應中產生並從表面脫附，進而改變了殘留在表面上的中間產物相對比例。此外通過近常壓質譜的量測，氬離子轟擊產生的表面缺陷促進了 VSe2 表面對於甲醇分解的反應能力，並最終產生幾種氣態產物包括 D2(g)、D2O(g)/CD4(g)、CO(g)、CD2O(g)。
;We designed a near-ambient-pressure (NAP) reaction cell installed inside an ultrahigh vacuum (UHV) chamber to investigate the instantaneous dynamics of surface catalytic reactions with the coupled femtosecond-resolved ultra-broadband IR spectroscopy. The reaction cell is installed in this way, because the samples, catalytic model systems, are prepared under UHV conditions to prevent contamination, and the reactions are conducted under NAP conditions. The reaction cell is isolated from the UHV chamber with rubber face sealing, so that we can perform NAP experiments in the reaction cell and maintain an UHV environment for the chamber, and the rubber face sealing can maintain about eight orders pressure difference. Besides, a mass
spectrometer is also connected to the reaction cell, so we can monitor reactions with IR and mass spectroscopies simultaneously. We conducted testing related to the measurements of mass and IR spectroscopy with the NAP reaction cell. For mass spectroscopy, the signals of gaseous nitrogen and methanol can be measured with increased pressure inside the reaction cell and finally saturated
at about 1 mbar. For IR spectra testing, however, we found the absorption intensity is not enough to be measured with the reaction cell. Therefore, we introduced methanol to the entire chamber and monitored the IR absorption feature. The absorption feature of gaseous methanol could be observed at pressure above 1 mabr and became obvious as pressure up to 3 mabr. After evacuated methanol from the chamber, the gaseous methanol absorption feature then vanished.
In addition, we studied the reactivity of layered VSe2 toward methanol decomposition. The surface structures were characterized using reflective high energy electron diffraction (RHEED) and photoelectron spectroscopy (PES) while the reactions, intermediates, and gaseous products
were monitored by PES, near-ambient-pressure photoelectron spectroscopy (NAP-PES), and near ambient-pressure mass spectroscopy (NAP-MS). We generated the surface defects through Ar+ bombardment and controlled the defect concentration by tuning the Ar+ dosage. With PES spectra,
we observed that on VSe2 surface under UHV environment, the methanol decomposed and produced CHxO and CHx, and the production of these two intermediates altered with the surface defect concentration, where the better reactivity was presented at less defect concentration. Therefore, the surface reactivity of VSe2 can be manipulated via controlling the surface defect concentration through Ar+
bombardment. With NAP-PES spectra, the relative ratio of two
produced intermediates from methanol decomposition is opposite to the UHV case, implying some gaseous products were generated during the reaction and desorbed from the surface, and altered the relative ratio of intermediates remaining on surface. Moreover, based on measurements of
NAP-MS, the surface defects generated from Ar+ bombardment promoted the reactivity of VSe2 surface toward methanol decomposition under near ambient pressure conditions with several ultimately produced gaseous products, including D2(g), D2O(g)/CD4(g), CO(g), CD2O(g). |
