本篇論文報導在超高真空(UHV)系統中，2-氯吡嗪(2-chloropyrazine, C4H3ClN2)於Cu(100)單晶表面之熱分解反應，並探討其中間態與反應氣相產物。所使用的表面分析技術包含以下幾種：程序控溫反應/脫附(TPR/D)、反射式紅外光吸收光譜分析(RAIRS)和X-光光電子能譜分析(XPS)。
在120 K乾淨Cu(100)上，吸附的2-氯吡嗪以4N原子鍵結於表面，從RAIRS與XPS圖譜分析得知2-氯吡嗪在200-300 K間發生2C-Cl鍵的斷裂，形成pyrazinyl (C4H3N2)中間物並以1N與2C原子鍵結在Cu(100)表面。當溫度達約510 K時，表面pyrazinyl會進行去氫及抓取H形成吡嗪(C4H4N2)產物脫附，隨後在約580 K出現了H2產物，接著發生破環反應並依序生成HCN (660 K)與C2N2 (830 K)。
在有氧預吸附的Cu(100)上，2-氯吡嗪於120 K一樣以4N原子鍵結於表面，升溫至約225 K產生H2O，顯示表面上預吸附之O原子(O(a))能抓取2-氯吡嗪環上的H原子，最終以H2O脫附。另外根據XPS圖譜數據顯示，2-氯吡嗪的2C-Cl在300 K已全部斷裂。故在300 K時，表面應存在C4H2N2中間物，來自於斷了2C-Cl和C-H鍵的2-氯吡嗪。在較高溫時，此中間物分解並產生了H2O (480 K)、H2 (560 K)、CO和CO2 (590 K)、N2 (815 K)以及NO (830 K)的產物脫附。

SUMMARY

The thermal chemistry of 2-chloropyrazine (C4H3ClN2) on Cu(100), and oxygen-precovered Cu(100) surfaces has been studied using temperature-programmed reaction/desorption (TPR/D) and reflection-absorption infrared spectroscopy (RAIRS) and X-ray photoelectron spectroscopy (XPS). Our experimental results indicate that 2-chloropyrazine is adsorbed predominantly in molecular form on Cu(100) at 120 K. As the surface temperature is increased to 250 K, a portion of the surface molecules undergoes 2C-Cl bond scission, forming pyrazinyl (C4H3N2) intermediate. At 350 K, all of the 2C-Cl bonds have broken. Upon heating to 510 K, dehydrogenation and hydrogenation of the pyrazinyl groups occur to generate pyrazine (C4H4N2) product. Other reaction products evolve at higher temperatures, H2 (580 K), HCN (680 K) and C2N2 (830 K), involving breakage of the aromatic ring. On the surface of O/Cu(100), H2O desorption occurs at 220 K, due to H-abstraction from 2-chloropyrazine by the preadsorbed oxygen atoms. It is also found that the C-Cl bonds of the adsorbed 2-chloropyrazine molecules no longer exist on the surface at 300 K. Therefore, the surface at 300 K is proposed to be covered with C4H2N2 groups from the C-Cl and C-H bond dissociation of the 2-chloropyrazine adsorbates. This C4H2N2 intermediate further decomposes at high temperatures to generate H2O (480 K), H2 (560 K), CO and CO2 (590 K), N2 (815 K) and NO (830 K).

Keywords: 2-chloropyrazine, temperature-programmed reaction/desorption (TPR/D), X-ray photoelectron spectroscopy (XPS), reflection-absorption infrared spectroscopy (RAIRS), Cu(100).

INTRODUCTION

Catalytic reactions of heterocyclic and aromatic molecules are important to the chemical industry, for example, fuel reforming and environmental problems. Owing to the key role of surface interactions in heterogeneous catalysis, the adsorption of six-membered-ring molecules (such as pyridine, pyrimidine and pyirdazine) has been studied on many kinds of metal surfaces. Although pyrazine molecules are reported to be the reaction intermediates in lots of catalytically important processes, it has received comparatively less attention in surface science research. We are interested in the adsorption of 2-chloropyrazine, a six-membered-ring heterocyclic compound, on Cu(100) and O/Cu(100), and the thermal reaction pathways, including the identification of the intermediates, bonding geometry and the reaction products.

MATERIAL AND METHODS

In this research, all TPR/D and RAIRS experiments were performed in an ultra-high vacuum (UHV) chamber with a base pressure of 2x10-10 Torr. This UHV system was equipped with a differentially pumped mass spectrometer (QMS) for TPR/D, a cylindrical mirror analyzer for Auger electron spectroscopy (AES), a Fourier-transform infrared spectrometer for RAIRS. 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. Prior to each experiment, cycles of Ar+ sputtering and annealing were performed to clean the surface until no impurities were detected by AES. C4H3ClN2 (98%, Alfa Aesar) were subjected to several freeze-pump-thaw cycles before being introduced into the chamber. The oxidized Cu(100) surface was prepared by exposing a clean Cu(100) to 30 L O2 (99.9998%, Matheson) at 500 K. TPR/D data were obtained by positioning the Cu(100) surface 1 mm from a 3 mm aperture leading to the QMS and a heating rate of 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 a temperature about 120 K, with 1000 scans and 4 cm-1 resolution. XPS experiments were carried out at the National Synchrotron Radiation Research Center of ROC. All the XPS spectra were taken at about 120 K.

RESULT AND DISCUSSION

The temperature-programmed reaction/desorption experiments show that the multilayers of 2-chloropyrazine on Cu(100) are desorbed at the temperature of 176 K. At a submonolayer coverage, the adsorbed 2-chloropyrazine molecules completely dissociate by C-Cl bond cleavage at 300 K, forming pyrazinyl groups on the surface, evidenced by the RAIRS and XPS studies. Pyrazine evolves at 510 K from dehydrogenation and hydrogenation of the pyrazinyl. At higher temperatures, the ring of the pyrazinyl intermediate decomposes and produces the product of H2 (580 K), HCN (667 K) and C2N2 (827 K). In the reflection-absorption infrared spectroscopy experiment, it is found that the 2-chloropyrazine is molecularly adsorbed on Cu(100) at 120 K. When the surface temperature reaches to 250 K, the multilayers of 2-chloropyrazine are desorbed. At the same temperature, the peak intensity of 1143cm-1 (C-Cl stretching) diminishes, indicating the breakage of the 2C-Cl bond of 2-chloropyrazine. When the temperature is increased to 350 K, all of the 2C-Cl bonds dissociate. The generated pyrazine groupsare bounded to the Cu surface via the 1N and 2C atoms in the ring. In the X-ray photoelectron spectroscopy experiment, the Cl2p spectra also shows 2C-Cl bond cleavage at 350 K. The N1s and C1s binding energies of the pyrazinyl appear at 399.3 eV and 285.1 eV, respectively.
On O/Cu(100), the thermal reaction products of 2-chloropyrazine are different from those from the clean Cu(100) surface. The reaction of 2-chloropyrazine on O/Cu(100) first produces H2O at 225 K, due to H-abstraction from the aromatic ring by the adsorbed oxygen atoms. The resulting surface OH groups can recombine to evolve H2O (2OH(a) →H2O(g) + O(a)). The C-Cl bond bonds of the adsorbed 2-chloropyrazine molecules also completely dissociate at 300 K, as revealed by the XPS study. The RAIRS study shows two set of intermediate spectra, which are different from that of the pyrazinyl. These two intermediate spectra are attributed to C4H2ClN2 and and C4H2N2 from C-H and/or C-Cl bond dissociation of the 2-chloropyrazine on O/Cu(100). At higher temperatures, the C4H2N2 further teacts on O/Cu(100) and generates the products of H2O (486 K), CO and CO2 (590 K), H2 (600 K), N2 and NO (830 K).

CONCLUSION

In the study of C4H3ClN2/Cu(100), it is found that the 2-chloropyrazine molecules predominantly remain intact on the surface at 120 K. The 2C-Cl bonds of the adsorbed 2-chloropyrazine molecules broke completely at 300 K and the generated pyrazinyl intermediate is bonded to the surface via the 1N and 2C atoms. When the temperature is increased continuously, the reaction products of C4H4N2 (511 K), H2 (580 K), HCN (667 K)and C2N2 (827 K) are generated at different temperatures. On the oxidized surface, the 2C-Cl bond scission also occurs at 300 K. Besides, O(a) can abstract H from the 2-chloropyrazine and produce H2O at 225 K. C4H2N2 is proposed as the intermediate from the 2-chloropyrazine reaction on O/Cu(100). H2O (480 K), CO (590 K), CO2 (590 K), H2 (600 K), N2 (815 K) and NO (830 K) are detected to the reaction products at higher temperatures from decomposition of the C4H2N2 on O/Cu(100).

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