本篇論文是利用程式控溫反應/脫附(TPR/D)、反射式紅外吸收光譜(RAIRS)表面分析技術研究在超高真空系統中(UHV)，Pyrazine在銅(100)、氧/銅(100)(O/Cu(100))及羥基/銅(100)(OH/Cu(100))表面的熱反應。於乾淨Cu(100)實驗中只觀察到少量缺陷位置的產物：C2N2 (680 K)及HCN (850 K)。有氧條件下，表面氧原子抓取pyrazine上的氫，生成產物H2O (220 K)。約500 K時，去H的pyrazine六元環開始破裂，再次與氧反應產生CO (550 K)、CO2 (550 K)及NO (830 K)。RAIRS實驗指出可能有pyrazinyl吸附物種的存在。Pyrazine/OH/Cu(100)實驗中觀察到的產物與有氧表面相同，表面OH抓氫形成水後，表面溫度大於500 K時出現CO、CO2及NO的產物。我們認為銅表面有O(a)及OH(a)時，pyrazine反應中間物可能為CN(a)或是NCOx(a) (x=1,2)。本研究的另一主題是利用近緣X光吸收細微結構光譜(NEXAFS)分析技術，探討1H-pyrazole的吸附位向，結果顯示吸附的1H-pyrazole及去氫(來自斷N-H鍵)pyrazole與Cu(100)表面夾角約60~90°，而在有氧吸附條件時與表面夾角呈60~70°。

SUMMARY

Thermal chemistry of pyrazine (C4H4N2) on Cu(100), O/Cu(100) and OH/Cu(100), was studied with temperature-programmed reaction/desorption (TPR/D) and reflection-absorption infrared spectroscopy (RAIRS). We observed the formation of C2N2 and HCN from defect sites on Cu(100) surface. For pyrazine on the oxygen-precovered surface (O/Cu(100)), its decomposition involved dehydrogenation and ring opening reaction. The surface-bound O atoms abstracted hydrogen from pyrazine and formed H2O (220 K). Upon heating to a temperature higher than 500 K, the ring of the dehydrogenated pyrazine began to decompose and produced CO and CO2 at ~550 K. At ~830 K, NO desorption was also observed. The RAIRS study suggested that pyrazinyl intermediate may be generated from dehydrogenation of the pyrazine. In the case of pyrazine/OH/Cu(100), we observed the same products as those from O/Cu(100). CN(a) and NCOx(a) (x=1,2) were suggested to be the intermediates in the reaction of pyrazine on the O/Cu(100) and OH/Cu(100) surfaces.
Using near edge X-ray absorption fine structure (NEXAFS), we studied the adsorption orientation of 1H-pyrazole and the dehydrogenated pyrazole, the measured tilting angle for the rings was 60~90° with respect to the Cu(100) surface and 60~70° for the O/Cu(100).

Keywords: pyrazine, 1H-pyrazole, temperature-programmed reaction/desorption (TPR/D), reflection-absorption infrared spectroscopy (RAIRS), near edge X-ray absorption fine structure (NEXAFS), Cu(100)

INTRODUCTION

Pyrazine is a heterocyclic compound which has a six-membered ring of two nitrogen atoms and four carbon atoms. The inductive effect of the nitrogen atoms induces a partially positive charge on the carbon atoms, with a pKa = 0.65. Pyrazine derivatives are found in nature and can be used as flavoring ingredients and medical drugs (e.g., acetylpyrazine, pyrazinamide). Adsorption and thermochemistry of pyrazine on smooth and roughened Ag(111) surfaces have been investigated, but only showing reversible adsorption on both the surface. Pyrazine is suggested to stand at an upright orientation on Cu(l10), Cu(111) and Ag(111). Pyridine, an analog of pyrazine, on Pt(111) surface decomposes to form the products of hydrogen (H2), hydrogen cyanide (HCN), cyanogen (C2N2) and ammonia (NH3).
To the best of our knowledge, no thermal reactions of pyrazine on Cu(100) and O/Cu(100) have been reported previously, therefore intermediates, products formed in the reaction processes and bonding of the surface species are worthy of exploring. We mainly present the results of thermal chemistry of pyrazine on Cu(100) and O/Cu(100) surfaces.

MATERIALS AND METHODS

The ultrahigh-vacumm (UHV) apparatus used in this research is equipped with an ion gun for sputtering, a differentially pumped mass spectrometer for TPR/D, an analyzer for Auger electron spectroscopy (AES), and a Fourier-transform infrared spectrometer for RAIRS. The UHV chamber was evacuated to a base pressure of ~2×10-10 Torr by a turbomolecular pump and ion pump. The Cu(100) single crystal (1 cm in diameter) was mounted on a resistive heating element and could be cooled with liquid nitrogen to 120 K and heated to 1000 K. Before each experiment, the Cu(100) surface was cleaned by cycles of Ar+ sputtering, followed by annealing to 950 K. Pyrazine with a purity of 99% was purchased from Sigma-Aldrich. Before introducing the pyrazine vapor into the UHV chamber, the glass tube containing pyrazine was evacuated for 100 s serveral times. The oxidized Cu(100) surface was prepared by exposing the clean Cu(100) to 30 L O2 (99.9998%, Matheson) at 500 K. In the TPR/D experiments the Cu(100) surface was set ~1 mm away from the aperture leading to the quadrupole mass spectrometer and a heating rate 2 K/s was used. In the RAIRS study, the IR beam was taken from a Bruker FTIR spectrometer and a mercury-cadmium-telluride (MCT) detector was used. All the IR spectra were taken at ~120 K, with 1000 scans and a 4 cm-1 resolution. The NEXAFS experiments were conducted at the wide range spherical grating monochromator beam line (WR-SGM) at the National Synchrotron Research Center of ROC.

RESULTS AND DISCUSSION

Figure 1(a) shows the TPR/D spectra of 0.2 L pyrazine adsorbed on O/Cu(100). The reaction products are identified as H2O (m/z 18), CO (m/z 28), CO2 (m/z 44) and NO (m/z 30) according to their cracking patterns, which are consistent with their reported mass spectra reported in the NIST database. The H2O peak observed at ~160 K is caused by direct water desorption from the surface. The water may come from the background and may be originally present in the pyrazine used. However, the H2O desorption at 220 K is attributed to the OH(a) coupling, which from dehydrogenation of the adsorbed pyrazine molecules. When the temperature is higher than 500 K, CO, CO2 and NO are also detected. NC, NCO and NCO2 are suggested to be the reaction intermediates. After desorption of the CO and CO2 (550 K) N atoms are left on the surface. Above 800 K, N(a) and O(a) recombine to NO. On OH/Cu(100) surface, we observe the same reaction behavior.
Figure 1(b) shows the RAIR specta of 0.3 L pyrazine adsorbed on O/Cu(100). The spectra obtained at 120, 150 and 180 K are attributed to pyrazine molecules in multilayers and monolayer. However, the 1036 and 1126 cm-1 peaks may come from pyrazinyl froups, which need farther investigation.
In the NEXAFS studies, the area of the peak from C1s→π* as a function of incidence angle in the cases of 1H-pyrazole on Cu(100) and O/Cu(100) are displayed in Figure 2. Further analysis show that the dehydrogenated 1H- pyrazole on Cu(100) has tilting angle of 60~90° in the ring orientation relative the surface. On O/Cu(100), the tilting angle is measured in the range of 60~70°.

CONCLUSION

Pyrazine on O/Cu(100) surface decomposes to form the products of water (H2O), carbon monoxide (CO), carbon dioxide (CO2) and nitric oxide (NO). In the RAIRS experiments, pyrazine is suggested to dehydrogenate and form -OH and pyrazinyl on the oxygen-precovered surface. Dehydrogenated pyrazine began to decompose and generate CO and CO2 at ~550 K. N(a) and O(a) recombine to NO at ~830 K. NC(a) and NCOx(a) (x=1,2) are suggested to be the reaction intermediates. Using NEXAFS, we studied the adsorption orientation of the 1H-pyrazole and the dehydrogented pyrazole on the copper surface, the angle between the molecule and the surface is about 60~90° on clean Cu(100), tilting angle is 60~70° on O/Cu(100).

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