原位紅外線偵測二氧化碳在光觸媒上之光催化反應

Abstract

本研究運用傅立葉轉換紅外線光譜儀，原位即時研究二氧化碳吸附於純二氧化鈦與負載過渡金屬銅的二氧化鈦上，經紫外光照射後所產生的光觸媒催化反應。利用改良式溶膠凝膠法水解鈦的醇氧化物製備二氧化鈦，以及負載銅金屬之二氧化鈦。由X光繞射分析、紫外光－可見光光譜分析可證明觸媒結構以及對紫外光吸收的能力。 在500℃通入空氣進行前處理之後，觸媒表面偵測到大量過氧化物以及氫氧基。通入二氧化碳氣體後，發現二氧化碳會移除表面的水份與部分氫氧基，並與觸媒表面原有的氫氧基、氧空缺分別產生碳酸氫根與碳酸根。 觸媒照射紫外光後受激發所產生的電洞，與水解離出的氫氧離子結合形成氫氧基，或與二氧化鈦的氧離子結合形成氧分子，氧分子自觸媒表面脫附形成氧空缺；二氧化碳再與光反應產生的氫氧基、氧空缺結合形成碳酸氫根與碳酸根，吸附型式的二氧化碳有明顯的減少。觸媒在紫外光照射下受激發所產生的電子，將與水解離的氫離子結合形成氫原子，進一步將二氧化碳依序還原成甲酸鹽、甲酸、甲醛、甲醇等有機產物，但是其傅立葉轉換紅外線光譜訊號並不明顯，由此推論光催化還原的效率或反應步驟不易由紅外線光譜儀觀察得到。 由傅立葉轉換紅外線光譜分析的結果，可知二氧化碳光催化還原反應並不明顯，不過仍可見其還原後中間產物與生成物的微弱訊號，進而推測出二氧化碳在純二氧化鈦與負載過渡金屬銅的二氧化鈦上，進行光觸媒催化反應的可能機制。

Photocatalytic CO2 reaction on TiO2 and transition metal-loaded TiO2 (Cu/TiO2) catalysts under UV irradiation was studied using in situ FT-IR spectroscopy. TiO2 and Cu/TiO2 catalysts were prepared by sol-gel method via controlled hydrolysis of titanium (Ⅳ) butoxide. Copper was loaded onto TiO2 during sol-gel procedure. XRD and UV-Visible tests showed that the structures and the abilities of absorbing UV light. After treated at 500℃under air flow, a large amount of surface peroxo species and OH groups were detected on the TiO2 and Cu/TiO2 catalysts. Under CO2 flow, CO2 removed original H2O and OH groups on the surface of catalysts and formed carbonate as well as bicarbonate expeditiously via CO2 combining with oxygen-vacancy and OH groups respectively. Under UV irradiation, photogenegrated holes merged with OH- from H2O dissociation to form OH groups and combined with O2- of TiO2 to form disorbing O2 and oxygen-vacancy. CO2 combined with oxygen-vacancy and OH groups generated from UV irradiation to form carbonate or bicarbonate newly as CO2 in the adsorption sort decreased. Photogenerated electron merged with H+ from H2O dissociation to from H atom which reduced CO2 to formate, formic acid, dioxymethlene species, formaldehyde and methoxy slightly using in situ FT-IR spectroscopy. The IR signals of reduced products are weak. It is inferable that the efficiency of photocatalytic reduction and reaction mechanism is not easy to observe. Based on the FT-IR result, Photocatalytic CO2 reduction were not apparent. A possible mechanism is proposed for the photocatalytic CO2 reduction on photocatalysts from the weak signals of in situ FT-IR detecting the reduced intermediates and products on the photocatalysts.