本論文利用傅立葉轉換紅外光譜儀（FT-IR）研究甲脒鹽酸鹽（Formamidine hydrochloride）與甲醯肼（Formic hydrazide）兩種化合物在二氧化鈦表面上的吸附與反應。
甲脒鹽酸鹽遇水會分解並以HCOO(a)與NH3(a)的形式吸附於二氧化鈦表面上，加熱至250 oC時開始生成CO(g)，若有氧的環境下則大約在300 oC時會形成CO2。未分解的甲脒鹽酸鹽分子以N―Ti的形式吸附於二氧化鈦表面上，在有氧的環境下照光，5分鐘左右即會開始生成HCOO、CO2與NO3-。
甲醯肼在二氧化鈦表面上主要以O―Ti形式吸附於二氧化鈦表面上，吸水後有部分分子反應形成HCOO及NH3吸附於表面上。此分子加熱至大約200 oC時產生CO2，250 oC時NO3-及含CN的物種。有氧環境下加熱則會生成HCOO、N2O(a)、CO及CO2。在照光反應中，大約20分鐘時產生CO(a)，而照光完進行加熱大約250 oC時會形成NCO(a)。若有氧參與照光反應，大約1分鐘時即開始出現CO(a)與HCOO(a)，並且促成CO反應成CO2，照光完加熱則同樣於250 oC時生成NCO。

Adsorption and Reactions of Formamidine Hydrochloride and Formic hydrazide on Powdered TiO2

Yu-Yin Chung
Jong-Liang Lin
Department of Chemistry, National Cheng Kung University

SUMMARY

Fourier transform infrared spectroscopy has been employed to study the thermal and photochemical reactions of formamidine hydrochloride and formic hydrazide on powdered TiO2, with the aid of density functional theory calculations. The calculation shows that formamidine hydrochloride is adsorbed on anatase TiO2(101) with the interactions of HN---Ti and N–H---O. Formamidine hydrochloride can decompose into HCOO aqnd NH3 on powdered TiO2 at 35 oC. In the photoreaction, formamidine hydrochloride decomposes to form HCOO, CO2 and NO3- on TiO2. Formic hydrazide is adsorbed on TiO2(101) via C=O---Ti and N–H---O. Adsorption of formic hydrazide on TiO2 (35 oC) can form NH3, HC(=O)NH and/or HCOO. Thermal decomposition of formic hydrazide on TiO2 at higher temperatures generates CO2, N2O(a) and NO3-. Photocatalytic reaction of formic hydrazide on TiO2 generates CO. Heating the surface subjected to the phototoirradiation produces a surface species of NCO.

Key words: TiO2, formamidine hydrochloride, formic hydrazide, FTIR

INTRODUCTION

In the past, precise determination for the surface reaction mechanisms of N-containing heterocyclic compounds, using infrared spectroscopy, often faced challenges, because it is difficult to correctly identify the surface intermediates on the basis of the detected similar peak frequencies. Therefore, it would be helpful to study simple, smaller molecules with similar structure to the N-containing heterocyclic compounds, for example, formamidine hydrochloride and formic hydrazide. They are used as precursors for organic synthesis and haven’t been studied on TiO2. In this research, we investigated the adsorption, thermal reactions, and photoreactions of formamidine hydrochloride and formic hydrazide on TiO2.

MATERIALS AND METHODS

Formamidine hydrochloride (98%, Alfa), formic hydrazide (98%, Aldrich), acetone (99.98%, Honeywell), oxygen (99.998%, Matheson) and TiO2 powder (Degussa P25, ~50 m2/g) were used in this study. The TiO2 powder was dispersed in water/acetone solution and then sprayed onto a tungsten mesh. The obtained TiO2/W was placed inside the IR cell and heated to 450 oC under vacuum for 24 h. The temperature of the TiO2/W was measured by a K-type thermocouple spotwelded on the tungsten mesh. Before each experiments, the TiO2/W was heated to 350 oC for 30 mins in the presence of O2 and then maintained at 450 oC under vacuum for 2 h to remove surface impurity. After the heating, 5.0 Torr O2 was introduced into the cell as the TiO2/W was cooled to 70 oC and the cell was evacuated for gas dosing when the temperature reached 35 oC. The IR cell with two KBr windows for IR transmission down to ~400 cm-1 was connected to a gas manifold maintained by a turbomolecular pump at a base pressure of ~1×10-7 Torr. A capacitance manometer and thermocouple gauge were used to monitored the pressure of IR cell. In the photochemistry study, both the UV and IR beams were 45o to the normal of the TiO2/W. The light source was a combination of a 350 W Hg arc lamp, a water filter and a ~ 325 nm band-pass filter. IR spectra were obtained with a 4 cm-1 resolution by Bruker FT-IR spectrometer with an MCT detector.

RESULTS AND DISCUSSION

After depositing formamidine hydrochloride and H2O on TiO2/W, the infrared spectrum taken under vacuum shows the infrared absorptions at 1180, 1360, 1380, 1412, 1554, 1621, 1668, 1718, 2954 and 2873 cm-1. According to previously reported literatures these peaks can be attributed to surface species as follows: 1180 cm-1 to NH3; 1360, 1380, 1412, 1554, 2954, 2873 to HCOO(a); 1621 cm-1 to H2O. In addition, for the TiO2 surface directly incontact with formamidine hydrochloride vapor only, we have measured an infrared spectrum with peaks at 1172, 1241, 1288, 1363, 1375, 1444, 1558, 1621, 1680 and 2975 cm-1. The 1680 cm-1 peak may be assigned to N=C–N asymmetry stretching. In the photocatalytic reaction, there are no obvious spectra after the photoirridiation at 325 nm changes in the absence of oxygen. But, in the presence of oxygen, the N=C–N bond is broken, generating CO2(g) (2349 cm-1), HCOO(a) (1353 and 1558 cm-1) and NO3- (1251 and 1436 cm-1). The proposed reactions of formamidine hydrochloride on TiO2 are shown in Scheme 1.

Scheme 1

Adsorption and reactions of formamidine hydrochloride on TiO2.

The infrared spectrum of formic hydrazide/TiO2 shows the absorptions at 1160, 1245, 1360, 1380, 1438, 1473, 1565, 1595, 1641, 1672, 2732, 2873, 2973, 3263, 3384 and 3573 cm-1. Among them, the set of 1380, 1565, 2732, 2829 and 2873 cm-1 may be attributed to HCOO(a); 1160 and 1595 cm-1 to NH3(a). Scheme 2 shows the proposed catalytic reactions of formic hydrazide on TiO2. In the thermal reactions, enhanced absorptions at 1438, 2281, 2310, 2348 cm-1 are measured, suggesting the formation of NO3-, CN-containing species and CO2(g). In the presence or O2, the absorptions at 1355, 1380 and 2231 cm-1 at 200 oC are due to HCOO(a) and N2O and the 2117 cm-1 at 250 oC may be due to CO(a). In the photocatalytic reaction, a new peak at 2189 cm-1 is found to grow with time, but decreases when the TiO2/W is heated to 100 oC after the photoirradition. We surmise that it is from photogenerated CO. The 2214 cm-1 peak measured at 250 oC can be ascribed to NCO(a). In the presence of oxygen, the CO(a) is further photooxidized to CO2(g).

 
Scheme 2

Adsorption and reactions of formic hydrazide on TiO2.

CONCLUSION

The photoreaction of formamidine hydrochloride on TiO2 forms CO2(g), HCOO(ad) and NO3- in the presence of oxygen. In the thermal reactions of formic hydrazide on TiO2 without O2, the products are found to be CO2, NO3- and CN-containing species, but HCOO(ad), N2O(ad), CO(ad) and CO2(g) are generated in the presence of oxygen. In the photocatalytic reaction of formic hydrazide, CO(ad) is found but desorption when the TiO2/W is heated to 100 oC after the photoirradition. At 250 oC, a new product ascribed to NCO(a) is found on TiO2. In the presence of oxygen, the CO(a) is further photooxidized to CO2(g).

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