共查询到18条相似文献,搜索用时 234 毫秒
1.
利用FTIR,TPSR和微量反应技术考察了CO和CO/H2在无定形、四方相和单斜相3种形态氧化锆为载体的钴基催化剂上的吸附和反应行为.结果表明,CO在以不同形态氧化锆为载体的催化剂上的吸附形式和转化行为具有较大差异.在单斜氧化锆为载体的催化剂上生成桥式和多桥式吸附的CO,并容易进一步加氢生成烃类物种,具有较高的反应活性和C5+烃的选择性.而以四方氧化锆为载体的催化剂上桥式吸附的CO较稳定,加氢反应活性较低. 相似文献
2.
3.
4.
常用的氧化物负载金属催化剂通常在水相中制备,且在使用前常常需要经过煅烧.因此,氧化物载体表面的水合和脱水过程对于负载型金属催化剂的真实建模至关重要.通过第一性原理分子动力学模拟,本文考察了温和温度下无水单斜氧化锆(■)表面在显式溶剂水中的演化.在模拟过程中,所有的双重配位桥位氧位点很快被溶剂水质子化,形成酸性羟基(HOL),并在锆原子上留下碱性羟基(HO~*).这些碱性羟基(HO~*)可以与表面未解离的吸附水分子(H2O~*)进行活跃的质子交换,进而在表面自由扩散.在273 K到373 K的温度范围下,第一性原理分子动力学水相模拟可以得到一种较为确定的、有代表性的平衡水合单斜氧化锆(■)表面,其表面锆原子上覆盖度(θ)为0.75.随后,为了模拟低于800 K的温和煅烧温度下的表面脱水过程,本文使用密度泛函理论计算了表面水分子的逐步脱附自由能.通过获得表面的脱水相图,总结了不同煅烧温度下有代表性的、部分水合的单斜氧化锆(■)表面(0.25≤θ<0.75).这些水合单斜氧化锆(■)表面具有重要的理论意义,可以方便快捷地被应用于氧化锆催化剂及... 相似文献
5.
用XRD、LRS、NH3-TPD、CO2-TPD和CO-FTIR等表征手段考察了不同温度焙烧的氧化锆表面性质的差别,特别是表面酸碱性的差异对Cu/ZrO2催化剂CO加氢反应行为的影响。结果表明,不同温度焙烧的氧化锆表面酸碱性具有较大的差异,其中以450℃焙烧的氧化锆具有较高的表面碱性和最低酸性。这些表面性质的差异对于Cu/ZrO2催化剂的CO吸附行为产生较大的影响,进而影响CO的加氢反应活性。以450℃焙烧的氧化锆为载体时,Cu/ZrO2催化剂具有较好的反应活性。 相似文献
6.
7.
Y沸石中不同状态铁离子的还原和催化性质 总被引:3,自引:0,他引:3
制备了Fe(Ⅲ)处于骨架四面体内、阳离子位及以氧化物形式分散在沸石表面的含铁Y沸石.用TPR方法测定样品的可还原性,发现不同形态的Fe(Ⅲ)其还原情况不同,在800℃以下均不能被还原成金属Fe.以异丙醇脱水和脱氢为探针,考察了样品的酸碱性,证实在沸石结构内引入不同形态的Fe(Ⅲ)可调变沸石的酸碱性.用乙苯脱氢和裂解反应考察了样品的脱氢活性.当样品表面金属离子和强酸位同时存在时,由于二者的协同作用,裂解反应有可能占主导地位.消除强酸位,增加碱性位,可显著提高脱氢的选择性. 相似文献
8.
9.
吸附量热技术和金属氧化物催化剂的表面酸碱性 总被引:4,自引:2,他引:2
探讨了吸附量热技术及其在测定固体表面酸碱性中的应用.使用NH3和CO2为探针分子,吸附量热技术能够定量地描述固体表面酸碱中心的数目和强度分布.结合原位红外光谱,还能够详细地了解表面酸碱中心的性质:金属氧化物表面的Lewis酸中心和碱中心分别是表面配位不饱和的金属离子和氧负离子,Brnsted酸中心和碱中心则是金属氧化物表面的羟基.通过测量吸附热,金属氧化物的表面酸碱性强度可与Sanderson电负性关联起来,也可以考虑使用Drago参数来描述固体表面的酸碱性.根据酸碱性与电负性的关系以及对表面配位不饱和离子的要求,有可能通过选择合适的复合金属氧化物组成,获得具有一定酸(碱)量和酸(碱)强度的固体酸(碱). 相似文献
10.
用辅以回流处理的两步法制备硫酸化氧化锆(SO2-4-ZrO2), 再用浸渍法制备Li质量含量为0.5%~15%的LiCl/SO2-4-ZrO2催化剂. 650 ℃时,在Li含量为15%的催化剂上,获得了90.6%的乙烷转化率、85.9%的乙烯选择性和77.8%的乙烯收率,在24 h的实验考察中,乙烯的收率一直保持在71%以上. 采用X射线粉末衍射、低温N2吸附、程序升温脱附和X射线光电子能谱等对催化剂的体相结构、比表面积、表面酸碱性和表面元素组成等进行了表征. 结果表明, LiCl的添加使催化剂中四方相ZrO2的含量降低,比表面积减小,表面酸性减弱,乙烷氧化脱氢催化性能明显提高,但ZrO2的体相结构对其催化性能没有明显影响. 随着反应时间的延长, LiCl慢慢流失,催化剂的乙烯选择性逐渐下降. 与未经回流处理制得的硫酸化二氧化锆相比,采用回流处理后的ZrO2制得的SO2-4-ZrO2具有较大的比表面积,单位质量的样品可负载更多的LiCl, 有利于延缓催化剂活性因LiCl流失而下降. 相似文献
11.
单斜及四方晶相ZrO2催化CO加氢反应性能的比较 总被引:7,自引:0,他引:7
研究了以纯单斜(m)和四方(t)晶相ZrO2为催化剂的CO加氢反应.尽管两种晶相催化剂均有较高的低碳烯烃的选择性,但是也发现了两者催化性能的显著差别.m-ZrO2催化剂对异丁烯有突出的选择性,而t-ZrO2催化剂,则只有乙烯和丙烯,几乎没有C4烯烃的选择性.室温下CO吸附的原位IR谱测试可见,只有在t-ZrO2催化剂上观测到不可逆含氧吸附物种.吡啶吸附的Raman谱显示出它们之间表面性质的差别,m-ZrO2催化剂表面存在等强度的Lewis和Bronsted酸中心,而在t-ZrO2催化剂表面几乎只有Bronsted酸中心.催化过程的一些模型分子电子结构计算也表明了ZrO2催化剂对低碳烯烃选择性内在的电子结构条件.我们推测CO在m-ZrO2催化剂表面的孪式吸附物种可能是导致异丁烯产物的根源 相似文献
12.
13.
14.
Zirconia powders were prepared by forming gels by desiccation of aqueous precursor solutions of zirconium acetate containing nitric or sulfuric acid at pH 2.4 and 1.4 and pyrolyzing the gels to temperatures up to 825°C. The structure development in the gels and solid pyrolysis products was investigated. The crystalline zirconia structures produced monoclinic (m), metastable cubic (c) and tetragonal (t) polymorphs. The structure transition temperatures were strongly dependent on the pH, the anions and the stoichiometry of the zirconium complex in the precursor solution. The monoclinic polymorph fraction in the zirconia formed by pyrolyzing the gel formed from the precursor solution containing sulfuric acid at pH 2.4 to 750°C approaches zero while this ratio in the zirconia formed by pyrolyzing the gel formed from the precursor solution containing nitric acid at pH 1.4 to 825°C is 0.7. 相似文献
15.
Sugeng Triwahyono Zalizawati Abdullah Aishah Abdul Jalil 《天然气化学杂志》2006,15(4):247-252
The effect of sulfate ion (SO42-) loading on the properties of Pt/SO42- -ZrO2 and on the catalytic isomerization of n-butane to iso-butane was studied. The catalyst was prepared by impregnation of Zr(OH)4 with H2SO4 and platinum solution followed by calcination at 600℃. Ammonia TPD and FT-IR were used to confirm the distribution of acid sites and the structure of the sulfate species. Nitrogen physisorption and X-ray diffraction were used to confirm the physical structures of Pt/SO42--ZrO2. XRD pattern showed that the presence of sulfate ion stabilized the metastable tetragonal phase of zirconia and hindered the transition of amorphous phase to monoclinic phase of zirconia. Ammonia TPD profiles indicated the distributions of weak and medium acid sites observed on 0.1 N and 1.0 N sulfate in the loaded catalysts. The addition of 2.0 N and 4.0 N sulfate ion generated strong acid site and decreased the weak and medium acid sites. However, the XRD results and the specific surface area of the catalysts indicated that the excessive amount of sulfate ion collapsed the structure of the catalyst. The catalysts showed high activity and stability for isomerization of n-butane to iso-butane at 200℃under hydrogen atmosphere. The conversion of n-butane to iso-butane per specific surface area of the catalyst increased with the increasing amount of sulfate ion owing to the existence of the bidentate sulfate and/or polynucleic sulfate species ((ZrO)2SO2), which acts as an active site for the isomerization. 相似文献
16.
Tingjiang Yan 《Journal of solid state chemistry》2011,184(2):357-6257
This work explored the selective synthesis of polymorphs of CdWO4 in either tetragonal or monoclinic phase by optimizing the experimental parameters. Systematic characterization indicated that both polymorphs possessed similar spherical morphologies but different structural building blocks. Electronic structures calculations for both polymorphs demonstrated the same constructions of conduction band or valence band, while the conduction band widths of both polymorphs were quite different. Both CdWO4 polymorphs exhibited good photocatalytic activity for degradation of methyl orange under UV light irradiation. When comparing to some other well-known tungstate oxide materials, the photocatalytic activity was found to follow such a consequence, monoclinic CdWO4≈monoclinic ZnWO4>tetragonal CdWO4>tetragonal CaWO4. The specific photocatalytic activity of monoclinic CdWO4 was even higher than that of commercial TiO2 photocatalyst (Degussa P25). The increased activity from the tetragonal CdWO4 to the monoclinic was consistent with the trend of the decreased symmetry, and this could be explained in terms of the geometric structures and electronic structures for both polymorphs. 相似文献
17.
18.
V. A. Matyshak O. N. Sil’chenkova V. A. Sadykov V. N. Korchak 《Kinetics and Catalysis》2016,57(2):234-242
The IR spectra of surface compounds observed in the course of the temperature-programmed desorption (TPD) of NOx and the TPD spectra are compared. The high-temperature peaks of desorption are related to the decomposition of surface nitrites and nitrates. The low-temperature peaks of NOx desorption with maximums below 140°C are caused by the decomposition of surface nitrosyls. On the heating of surface nitrosyls, the following two reaction paths are possible: desorption at low temperatures and conversion into nitrates. The shape of the TPD spectra of NO depends on the phase composition of test samples. The transition of a tetragonal phase into a monoclinic one occurred upon the surface dehydroxylation of polycrystalline particles with the formation of particles with a tetragonal nucleus and a monoclinic crust. This transition is reversible. The cooling of a sample in a moist atmosphere leads to the transition of the monoclinic crust to the tetragonal phase. 相似文献