人類活動對於化石燃料的依賴日益增加，造成CO2的過度排放，並導致全球氣候變遷。因此，CO2減量是各國努力的目標。光催化還原CO2具有能夠直接利用太陽能的優勢，是目前最有潛力的方法之一。其中，CO2吸附是進行光催化反應的初始步驟，也被認為是影響反應活性的關鍵因素。 因此，利用傅立葉轉換紅外光譜儀，即時監測CO2吸附於TO2奈米管(TiNT)及負載銅與鉑之TiNT光觸媒。
以水熱法在鹼性及高壓環境中合成TiNT，並利用光化學沉積法負載銅及白金。以掃描式電子顯微鏡(SEM)、穿透式電子顯微鏡(TEM)觀察TiNT於反應 48小時後之長度約100 nm，內、外徑分別介於9~15及3~5 nm，並具有高比表面積(120 m2/g)。均勻負載於TiNT之金屬奈米顆粒約為1-5 nm。X光電子能譜(XPS)及拉曼光譜(RAMAN)顯示光觸媒表面具氧缺陷及Ti3+活性位。程式升溫CO2脫附曲線及紅外光譜顯示，比較於TiO2 (銳鈦礦)及TiNT與CO2分子具有較強的親和性。
在即時紅外光譜之1000-1800 cm-1的範圍中，觀察到光觸媒表面具有碳酸物種的吸收峰，包含：bidentate carbonate (b-CO32-)、monodentate carbonate(m-CO32-)、及bicarbonate(HCO3-)。此外，負載銅之Cu/TiNT具有較強的CO2吸收，並偵測到光催化還原反應關鍵物種(carboxyl species, CO2-)的形成。由XPS分析結果顯示，負載銅的TiNT之表面有豐富的氧缺陷，此種表面缺陷有利於CO2吸附。
因此，由即時紅外光譜觀察結果，可以得知光觸媒表面缺陷有助於光電子的傳遞，並使CO2吸附形成CO2-進而促使光催化反應的進行，因此未來研究可針對於材料的表面進行改質，設計更具有反應活性的光觸媒材料。

Rising atmospheric levels of CO2 has given rise to great concerns in climate change. Photocatalytic reduction of CO2 is a promising pathway utilizing the abundant solar energy to convert CO2 to low-carbon fuels or chemicals. CO2 adsorption is an initial and key step for the multi-electron electron transfer photocatalytic process. It is essential to have a better understanding of the photocatalyst surface chemistry.
In this study, the one-dimensional titania nanotube (TiNT) and metal-loaded TiNT photocatalysts were studied by in situ infrared spectroscopy. TiNT was prepared by the hydrothermal method in alkaline solution. In addition, TiNT was further modified with metal co-catalysts. Copper and platinum nanoparticles were loaded on the TiNT with the chemical photodeposition method.
The SEM and TEM images show the tubular structure of TiNT with the length of 100 nm and outer/inner opening diameters of 9-15/3-5 nm, and the metal nanoparticles were uniformly dispersed on TiNT. The high surface area (120 m2/g) of the TiNT was measured by N2 adsorption-desorption isotherm. Surface defects including Ti3+ and oxygen vacancies of the photocatalyst were measured by XPS and Raman spectroscopy. The FTIR spectra and CO2-TPD data suggest that the TiNT have a strong affinity to the CO2 molecules compared to the pristine TiO2.
By in situ FTIR studies for CO2 adsorption, a number of peaks related to the adsorbed carbonate species (1200-1800 cm-1) are found, including bidentate carbonate (b-CO32-), monodentate carbonate (m-CO32-), bicarbonate (HCO3-) and carboxyl species (CO2-). For the Cu/TiNT, the defect-rich (Ti3+/oxygen vacancies) surface may provide active sites to promote the CO2 adsorption ability. Note that the adsorbed carboxylate species (CO2-) formation are observed on the TiNT.
The observations show that the adsorption capacity and formation of carboxylate species may be adsorbed on the surface-active sites. To enhance the photocatalytic activity, more research should focus on the modification of detect-rich materials to promote CO2 adsorption and electron transfer.

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