摘要
本文用ESCA法研究了羧酸与清洁及预氧化的Zn(0001)、Bi(0001)和多晶Mn表面的反应.80 K时乙酸分子吸附于清洁的Zn(0001)表面,150K即完全脱附,不发生任何化学反应;乙酸与Zn (0001)-O表面80 K便发生反应,生成乙酸基和水,此表面水于136 K脱附.甲酸分子80 K吸附在清洁Bi(0001)表面,140 K便大部脱附;在Bi(0001)-O表面甲酸80 K便分解为表面甲酸根和水,表面水于170 K脱附,甲酸根则在428 K分解脱附.清洁的多晶Mn表面有强的亲氧性,室温下甲(乙)酸在其表面分解吸附,吸附物种为晶格氧O^(2-)、甲(乙)酸根、CH_x(C_2H_x)和已经形成了碳化锰的表面碳C_(α)^(δ-).650K(550 K)时甲(乙)酸根完全分解,Mn表面仅存O^(2-)和C^(δ-).甲(乙)酸室温下以甲(乙)酸根吸附于Mn-O表面,加热可使表面甲酸根分解.
The studies of the interactions of carboxylic acids with material surfaces can provide informations of the catalytic properties of the materials. In this paper, XPS and UPS were used to study the adsorption and decomposition of the acids on Zn(0001) , Bi(0001) and polycrystalline manganese surfaces. At 80 K, acetic acid is physisorbed on clean Zn(0001) surfaces with C( ls) peaks at 285. 5 eV and 289.8 eV, corresponding to the methyl and carboxylate groups of the molecule.Complete desorption of the adsorbate occur at 150 K.On preoxidised Zn (0001) surf aces, acetic acid decomposes to give surface acetate and water at 80 K.Water is desorbed at 136 K and the surface acetate formed is stable at 473 K.
Similarly, clean Bi(0001) surfaces did not interact chemically with formic acid. Formic acid is physisorbed at 80 K and is desorbed mostly at about 140 K. On preoxidised Bi(0001) surfaces, formate and water are observed at 80 K. O(ls) components at 530.8 eV and 533.0 eV as well as C(ls) peak at 289.0 eV appear . At 170 K, the 533.0 eV O(ls) peak disappears and the 289.0 eV C(ls) peak shifts to 288.0 eV. At 428 K, both the 530.8 eV O(ls)peak and the 288.0 eV C(ls)peak decrease in intensity while the 529.0 eV peak due to surface oxide enhances. The spectroscopic results indicated the desorption of water at 170 K and the decomposition of formate at 428 K. It was concluded that oxidation of Zn (0001) and Bi(0001) surfaces increases the basicity of the surfaces and hence induces the acid-base type interactions to occur.
On clean polycrystalline manganese surfaces, formic as well as acetic acid decompose at 295 K to give oxide O2-and the corresponding formate and acetate; surface carbon C(a)δ-and hydrocarbon fragments are also observed. Surface formate and acetate decompose at 650 K and 550 K respectively and only O2-and C(a)δ-are left on the surfaces. On preoxidised manganese surfaces, formate and acetate are the main species observed, It has been shown that when the manganese surfaces are oxidized (oxygen pressure range: 10-8torr to 1 atm., substrate temperature range: 295 K to 600 K), MnO is formed and the Mn2p(3 / 2) peak shifts from 638.8 eV to 641.2 eV in binding energy. Hence the electronegative oxygen adatoms occupy the active sites on manganese surfaces, withdraw electrons from the substrate atoms and stop electron flow towards the adsorbates. Such ligand effect can inhibit the dissociation of the carboxylate species on the surfaces.
出处
《分子催化》
EI
CAS
CSCD
1989年第1期29-34,共6页
Journal of Molecular Catalysis(China)
基金
国家教委优秀年轻教师基金(1987)资助课题.
