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1.
SiO2—Al2O3和MgO催化剂上吸附CH3NO2的TPD和IR研究   总被引:1,自引:1,他引:0  
用TPD和IR方法研究了CH_3NO_2在典型固体酸SiO_2-Al_2O_3和固体碱MgO催化剂上的吸附分解。结果表明,在SiO_2-Al_2O_3表面CH_3NO_2吸附转化为表面甲酰胺物种,后者在高温下分解为CO_2和NH_3。在MgO表面CH_3NO_2吸附形成多种表面化学物种,它们在升温过程中脱附,并通过表面亚硝基甲烷物种分解为NO、C_2H_4、C_2H_6和N_2O.讨论了CH_3NO_2分解过程中表面酸、碱中心的作用。  相似文献   

2.
张军旗  杨永进  张劲松  刘强 《化学学报》2002,60(11):1973-1980
采用脉冲微波强化丝光等离子体反应装置,研究了甲烷氧化偶联与二氧化碳重 整制合成气(CO+H_2)副产乙炔、乙烯的反应。常压下,当CH_4和CO_2流量分别为 120,80mL/min,微波峰值功率120W,脉冲通断比为100/100ms时,CH_4和CO_2转化 率分别为70.8%,68.8%;CO, C_2H_2,C_2H_4选择性分别为75%,17.8%和4.1%,产物 中没有积炭。H_2/CO摩尔比值随原料气中甲烷比例的增加而增大,当CH_4/CO_2摩 尔比为2:1时,H_2/CO摩尔比达到2,这种比例的合成气能方便地用于下一步的 Fischer-Tropsch反应和其他化学品的合成。与其他等离子体反应相比,采用脉冲 强化常规丝光等离子体进行CH_4脱氢偶联与CO_2重整反应,能量效率明显提高,这 对于促进微波等离子体技术在C1化学中的应用具有重要的意义。  相似文献   

3.
通过密度泛函理论(DFT)计算研究了愈创木酚在Fe(211)表面上的吸附活化行为和加氢脱氧(HDO)反应性能.讨论了Pd的掺杂和H_2O~*的参与对Fe催化剂活性和选择性的影响.计算结果表明,通过苯环水平吸附在催化剂表面的愈创木酚的稳定性高于仅通过羟基的垂直吸附构型,这有利于苯环, C_(Ar)-OCH_3键和O-CH_3键的活化.在Fe(211)表面上,愈创木酚通过脱甲基再加氢生成邻苯二酚在动力学上比通过脱甲氧基生成苯酚和通过脱羟基生成苯甲醚更有利. Pd掺杂对愈创木酚的吸附稳定性影响较小(0.05 eV),但增加了其加氢脱氧反应的活化能垒,抑制了C_(Ar)-OCH_3, O-CH_3和C_(Ar)-OH键的断裂以及随后加氢生成苯酚,邻苯二酚和苯甲醚的反应过程.在Fe(211)表面上, H_2O~*通过与-CH_3形成氢键作用(H-bonding机理)对反应产生影响,从而降低了愈创木酚脱甲基和脱甲氧基反应的活化能垒.在Fe(211)-1Pd表面上, H_2O~*通过H转移参与反应(H-shuttling机理),促进了愈创木酚向邻苯二酚和苯酚产物的转化,并提高了加氢脱氧反应对苯酚的选择性.  相似文献   

4.
大气中CO_2浓度增加导致的温室效应以及化石燃料的匮乏正日益受到世界范围的关注.由于CO_2较强的惰性以及较高C–C偶联能垒,迄今为止大部分研究都集中在CO_2催化加氢制备各种C1化学品(如CH_4,CH3OH,CO等),鲜有研究关注合成液态燃料(C_(5+)碳氢化合物).目前,CO_2加氢直接合成烃类主要通过CO_2基费托合成反应(CO_2-FTS)实现,即先通过逆水煤气变换反应(RWGS)将CO_2还原成CO,随后CO通过传统费托反应(FTS)加氢生成烃类化合物.在两种工业化FTS催化剂(Fe和Co基催化剂)中,钴基催化剂具有更高的反应活性和链增长能力,以及较高的机械强度和稳定性.然而,由于CO_2的惰性,造成催化剂表面物种的加氢程度更高,使得甲烷更容易生成.因而,高反应活性、高选择性催化剂的开发是实现该过程的关键.本文采用沉积沉淀法制备了一系列双金属CoCu/TiO_2催化剂,再通过初湿浸渍法对其进行碱金属助剂(Li,Na,K,Rb和Cs)改性,并用多种表征手段系统研究了碱金属助剂对催化剂物化性质及其催化CO_2加氢制备长链烃反应的影响.结果表明,碱金属的加入对催化剂织构性质影响不大,它们在催化剂表面发生富集,且富集程度随碱金属原子序数的增加而降低.另外,碱金属的加入增强了CO_2的吸附,其中,Na改性的CoCu/TiO_2催化剂的碱性最强;同时还降低了H_2的脱附量,尤以K,Rb和Cs改性的催化剂为甚.在250 ℃,5 MPa,空速3000 mL·g_(cat)~(–1)·h~(–1)和H_2/CO_2=3的反应条件下,对不同碱金属助剂改性的催化剂进行评价.结果表明,不加助剂的CoCu/TiO_2催化剂上CO_2转化率高达23.1%,但产物主要是CH_4,此时CO_2在Co活性中心上直接发生甲烷化反应;碱金属助剂的引入显著抑制了CH_4的生成,提高了长链烃的选择性,但同时也降低了CO_2转化率,并且随着碱金属原子序数增大呈现先下降后上升的趋势,表明合适的碱性强度可以更好地改性催化剂性能.其中,Na助剂改性的CoCu/TiO_2催化剂的碱性最强,且H_2的脱附量降低幅度较小,因此,该催化剂具有最高的C_(5+)烃类收率,达到5.4%;同时CO_2转化率为18.4%,烃类产物中C_(5+)烃类选择性为42.1%.Na助剂改性的CoCu/TiO_2催化剂还展现了良好的催化稳定性,反应200 h后,CO_2转化率和C_(5+)选择性分别保持18%和40%.基于碱金属助剂对催化剂物化性质与反应性能的调变规律,可进一步指导CO_2加氢直接合成长链碳催化剂的设计与合成.  相似文献   

5.
用原位FT-IR研究了甲烷和氧与纯CaO,La_2O_3和SrO氧化物以及LC和SLC催化剂的相互作用和反应.当不存在气相氧时,引入的甲烷与表面晶格氧反应生成碳酸盐物种。在室温或高温下,在这些氧化物和催化剂上不能检测到CH_4或O_2的吸附物类。但是,当CH_4和O_2同时存在时,在La_2O_3和LC催化剂上能检测到1118cm~-1的新谱带、这一谱带可能来自于表面碳酸盐在高温下氧气氛中的分解,并可归属为物种。甲烷与这一活性氧物种反应生成C_2H_4。但对SLC催化剂,在高温下不能检测到物种,而甲烷和氧在高温下反应也能产生表面碳酸盐并在气相中形成乙烯,这就表明,气相氧对这些催化剂也起着关键作用,但是在LC和SLC催化剂上甲烷氧化偶联反应可能有本质上的差别。  相似文献   

6.
甲烷氧化偶联La—Mn—Li系复合氧化物催化剂的研究   总被引:1,自引:0,他引:1  
用XRD、IR、XPS和SEM等方法研究了混合氧化物LiLa_(1-x)Mn_xO_2的结构和它们对甲烷氧化偶联的催化性能。结果表明,随着MnO_2的变化,可形成一系列复合氧化物,其中三元复合氧化物La_2Mn_(1-y)Li_yO_4是甲烷氧化偶联的活性相,由于Li~+部分取代Mn~(2+)形成Li~+-O~--Mn(2+)缺陷簇,增加了活性氧种的浓度和再生速度是这种氧化物具有较高甲烷偶联活性的主要原因。脉冲实验证明,CH_4脱氢生成CH_3·偶联生成C_2H_6,进一步氧化脱氢生成C_2H_4都可在催化剂表面完成,而CO和CO_2是在气相反应中生成的。在780℃C_2收率可达23.9%。  相似文献   

7.
甲烷催化部分氧化制合成气的反应机理   总被引:6,自引:0,他引:6  
借助脉冲反应、质谱-程序升温表面反应(MS-TPSR)等技术研究了Ni/α-Al2O3催化剂上甲烷催化部分氧化制合成气(POM)的反应机理.结果表明,NiO上CH4不能解离产生H2只有当NiO被CH4还原为Ni0后,CH4才能解高产生H2,Ni0是CH4活化和POM反应的活性相;POM反应机理遵循直接氧化机理,CH4和O2均在Ni0上活化,活化过程形成的Ni…C和Niδ…Oδ物种是反应历程中的关键物种,Niδ …Oδ物种高选择性地与CH4解离产生的碳物种Ni…C反应生成CO.  相似文献   

8.
梁湦  何秋月  孙宝珍 《分子催化》2017,31(6):553-566
采用密度泛函理论结合周期平板模型方法系统地研究了水煤气变换反应在Cu_2O(111)表面上的反应机理,包括氧化还原机理、羧基机理和甲酸根机理.结果表明,在Cu_2O(111)表面,羧基机理和甲酸根机理均可行,且甲酸根机理更为有利,其最佳反应途径为H_2O~*→H~*+OH~*;CO(g)+H~*+OH~*→trans-HCOOH~*(1)→cis-HCOOH~*→CO_2~*+H_2(g).其中trans-HCOOH~*(1)→cis-HCOOH~*为其决速步,该基元反应的能垒仅为59 kJ·mol~(-1).羧基机理的最优反应路径同样是以H_2O的解离反应开始,随后CO(g)+OH~*→cis-COOH~*→trans-COOH~*→CO_2(g)+H~*,最后产生的两个吸附的H原子先迁移再结合生成H_2,整个反应的控速步骤为H原子的迁移,迁移能垒为96 kJ·mol~(-1).氧化还原机理则由于OH解离需要越过一个很高的能垒(254 vs.187 kJ·mol~(-1))而不可行.  相似文献   

9.
负载型Ni Fe/γ-Al_2O_3双金属催化剂的物理化学性质明显受还原温度的影响,进而影响月桂酸甲酯的加氢活性和产物选择性。金属Ni活性中心主要促进脱羰/脱羧(DOC)反应,Fe的加入能促进月桂酸甲酯发生加氢脱氧反应,促进C_(12)烷烃化合物生成。H_2-TPR、XRD、H_2-TPD和BET结果表明,高的还原温度有利于金属或合金活性中心形成,NiFe双金属催化剂的加氢活性取决于金属Ni、Fe和NiFe合金的含量;NiFe双金属催化剂吸附与活化H_2分子的能力明显受还原温度的影响。在研究的温度范围内,Ni活性中心具有优异的加氢和裂解性能,Fe物种的引入能有效抑制裂解性能。双金属催化剂的加氢活性顺序:NF420NF360NF450NF300,在420℃下经H_2还原制得的NF420催化剂具有最佳的月桂酸甲酯加氢性能,在反应温度为380℃时,月桂酸甲酯加氢转化率和烷烃化合物选择性分别高达93.3%和90.0%。  相似文献   

10.
CO2在金属表面活化的UBI-QEP方法研究   总被引:1,自引:0,他引:1  
应用UBI-QEP方法估算了金属表面上形成的活化吸附态CO-2在Cu(111), Pd(111), Fe(111)和 Ni(111)表面上的吸附热, 计算了各种相关反应的活化能垒. 结果表明, CO-2在4种过渡金属表面的相对稳定性的顺序为Fe>Ni>Cu>Pd; 在Fe和Ni表面上CO-2较易生成, 且容易进一步发生解离反应, 在Fe表面会解离成C和O吸附原子, 而在Ni表面上解离的最终产物为CO和O; 在Cu表面上, CO-2虽较难形成, 但其加氢反应的活化能比解离反应低, 因此加氢反应是其进一步活化的有效模式; 在Pd表面上, CO-2吸附态在能量上很不稳定, 所以CO2在Pd表面上不容易活化.  相似文献   

11.
CO(2) reforming of CH(4) on Ni(111) was investigated by using density functional theory. On the basis of thermodynamic analyses, the first step is CH(4) sequential dissociation into surface CH (CH(4) --> CH(3) --> CH(2) --> CH) and hydrogen, and CO(2) dissociation into surface CO and O (CO(2) --> CO + O). The second step is CH oxygenation into CHO (CH + O --> CHO), which is more favored than dissociation into C and hydrogen (CH --> C + H). The third step is the dissociation of CHO into surface CO and H (CHO --> CO + H). This can explain the enhanced selectivity toward the formation of CO and H(2) on Ni catalysts. It is found that surface carbon formation by the Bouduard back reaction (2CO = C((ads)) + CO(2)) is more favored than by CH(4) sequential dehydrogenation. The major problem of CO(2) reforming of CH(4) is the very strong CO adsorption on Ni(111), which results in the accumulation of CO on the surface and hinders the subsequent reactions and promotes carbon deposition. Therefore, promoting CO desorption should maintain the reactivity and stability of Ni catalysts. The computed energy barriers of the most favorable elementary reaction identify the CH(4) activation into CH(3) and H as the rate-determining step of CO(2) reforming of CH(4) on Ni(111), in agreement with the isotopic experimental results.  相似文献   

12.
IntroductionItilasbeenshot'-nthattileadditionofMnOpromotertoFocatal}stcanresultinaremarkableimprovementinthesclectivit}'to11ghtalkenesforCOh}!drogenationll'l.Ho-c'cvcr.thecadetofMnOonCH4formationandCOconversionisvery'ambigUouslltolMoreover.MnOpromotergrca…  相似文献   

13.
The activity and the selectivity to light alkenes of silicalite-2 (Si-2) zeolite supported F'e catalyst tor CO hydrogenation can he improved obviously with the addition of K2O and MnO promoters. The results of CO hydrogenation, CO-TPD, CO/H2-TPSR, C2H4/H2-TPSR and C2H4/H2 pulse reaction over K-Fe-MnO/Si-2 catalysts clearly show that the K2O additive into Fe-MnO/Si-2 catalyst leads to a remarkable increase in both the capacity and strength of the strong CO ad-species that will produce much more |Cad| via their disproportionation at higher temperatures. This results in an increase in the CO conversion and the selectivity to light olefins, and a decrease in CH4 formation. Moreover, K2O can suppress the disproportionate of C2H4 that occurs during the reaction as a side-reaction Meanwhile, the MnO promoter mainly prohibits the hydrogenation of C2H4 and C3H6, which is favorable to enhancing the selectivity to C2H4 and C3H6 and decreasing the formation of C2H6, and C3H8. It is also of interest that MnO has har  相似文献   

14.
作为便携式电子设备的动力源,直接甲酸燃料电池(DFAFC)具有燃料跨界范围小、电动势大、甲酸无毒、低温下功率密度大等优点,因而引起了人们的极大兴趣.DFAFC商业化的主要挑战之一是阳极电催化剂材料的高成本和低CO耐受性.阳极通常需要高负载的贵金属电催化剂(Pt或Pd)氧化甲酸(HCOOH)以获得所需的电能.完全电氧化甲酸在Pt和Pd表面上会产生强吸附的CO,从而降低了Pt或Pd催化剂的活性.Pt和Pd储量少且价格昂贵,减少Pt和Pd含量且保持催化性能的燃料电池催化剂一直是研究者的奋斗目标.本文用周期性密度泛函理论(DFT)系统地研究了WC负载的单分子层Pd(Pd/WC(0001))催化剂对甲酸的分解机理,这可为所需的反应路径设计、筛选催化剂提供指导.Trans-HCOOH通过C-H,O-H,C-O键的活化发生分解.关于吸附,确定了可能反应中间体的最稳定吸附构型.trans-HCOOH,HCOO,mHCOO,cis-COOH,trans-COOH,CO,H2O,OH和H的吸附过程是化学吸附,而cis-HCOOH和CO2与Pd/WC(0001)表面的相互作用较弱,是物理吸附.此外,提出了trans-HCOOH分解的不同途径来探索分解机理.trans-HCOOH中O-H,C-H和C-O键的活化能垒分别为0.61,0.77和1.05 eV,O-H键断裂的能垒最小,则trans-HCOOH优先通过O-H键断裂生成HCOO.双齿HCOO是HCOOH分解的主要中间体,它可以转变为单齿HCOO,这条路线生成CO2的能垒比双齿HCOO的低0.04 eV.CO2是HCOO主要解离产物,这一步是总反应的决速步骤.对于cis-COOH和trans-COOH,CO是其主要解离产物.此外,trans-HCOOH也能直接生成CO,但克服的能垒较大.在Pd/WC(0001)表面上分解trans-HCOOH的最有利途径是HCOOH→HCOO→CO2,其中HCOO脱氢形成CO2的步骤是速率决定步骤.本文提供了HCOOH在Pd/WC(0001)表面上分解的活性中间体、能垒和机理的推测,CO形成主要是通过cis-COOH、trans-COOH及HCO的分解,CO2的形成主要是通过HCOO的分解,CO2占主导.该结论与Pd(111)面上甲酸分解结果一致,说明WC作为Pd载体没有改变Pd对甲酸的催化性能,但降低了Pd的使用量.综上,本文阐明了WC负载单分子层Pd催化剂上甲酸催化分解机理,得出甲酸分解的最佳反应路径,为直接甲酸燃料电池设计低贵金属含量、高活性的负载型Pd催化剂提供了理论指导;可用于预测不同载体负载Pd催化剂的性能,大大减少实验成本,以验证提出的实验假设.  相似文献   

15.
Ni催化剂上一氧化碳加氢反应机理研究   总被引:3,自引:0,他引:3  
Ni催化剂上一氧化碳加氢反应机理研究胡云行,万惠霖,关玉德,林恒生(厦门大学化学系,固体表面物理化学国家重点实验室,厦门,361005)(中国科学院山西煤炭化学研究所,太原)关键词脉冲法,一氧化碳,加氢,反应机理自本世纪初报道了一氧化碳加氢生成甲烷以...  相似文献   

16.
Case studies of ten reactions using a variety of standard electronic structure methods are presented. These case studies are used to illustrate the usefulness and shortcomings of these standard methods for various classes of reactions. Limited comparisons with experiment are made. The reactions studied include four radical-radical combinations, H + CH(3)--> CH(4), CH(3) + CH(3)--> C(2)H(6), H + HCO --> H(2)CO and CH(3) + HCO --> CH(3)CHO, three abstraction reactions, H + HO(2)--> H(2) + O(2), H + HCO --> H(2) + CO and CH(3) + HCO --> CH(4) + CO, a radical-molecule addition, H + HCCH --> C(2)H(3), and two molecular decompositions, H(2)CO --> H(2) + CO and CH(3)CHO --> CH(4) + CO. The electronic structure methods used are DFT, MP2, CCSD(T), QCISD(T), CASSCF, CASPT2, and CAS+1+2+QC.  相似文献   

17.
A diverse array of unsaturated C1 (methylene and methylidyne) and C2 (vinyl, vinylidene, ethylidene, and ethylidyne) bound to metal center(s) and surfaces has received much attention. In sharp contrast to the effort devoted to C1 and C2 ligands, complexes or surfaces bearing C3 fragments have been less explored, especially the M-C3H3 systems, which include propargyl (M-CH2C[triple bond]CH), allenyl (M-CH=C=CH2), and acetylide (M-C[triple bond]CCH3) forms. To understand the bonding and reactivity of these C3 species appended to an extended metal structure, proprargyl bromide (Br-CH2C[triple bond]CH) was utilized as a precursor to generate C3H3 fragments on a Ag(111) surface under ultrahigh vacuum conditions. The molecular transformation process was explored by a combination of temperature-programmed desorption (TPD), reflection absorption infrared spectroscopy (RAIRS), and X-ray photoemission spectroscopy (XPS) techniques. In addition, density functional theory (DFT) calculations were conducted to obtain the optimized geometries and energies for the various surface intermediates. The computed IR spectra facilitated the vibrational mode assignments. TPD spectra show that C3H3(ad) self-hydrogenates to C3H4 around 300 and 475 K, respectively. In addition to hydrogenation, a C-C coupling product C6H6 (2,4-hexadiyne) is also unveiled as part of the desorption feature at 475 K. Identification of the possible C3H4 isomers (propyne and/or allene) was equivocal, but it was circumvented by using an alpha,alpha-dimethyl-substituted propargylic species--(CH3)2(alpha)C-C[triple bond]CH, which results in hydrogenation products, alkynic (CH3)2CH-C[triple bond]CH and allenic (CH3)2C=C=CH2, distinguishable by the mass spectrometry. The substitution experiments clarify that in the normal case the convoluted TPD feature around 300 K, in fact, consists of both allene at 260 K and propyne at 310 K, while the last hydrogenation product at 475 K is solely propyne. The RAIR spectroscopy demonstrates that at 200 K C3H3(ad) on Ag(111) readily adopts the allenyl formalism involving concerted CBr bond scission and [1,3]-sigmatropic migration (i.e., Br-*CH2C[triple bond]CH --> *CH2=C=CH-Ag), in which the sigma bond moves to a new metal location across the pi-periphery. Single hydrogen incorporation to the alpha-carbon of the surface allenyl rationalizes the allene formation at 260 K. When the surface is heated to the range of 250-300 K, both RAIR and XP spectra reveal drastic changes, indicative of a new species whose spectral characteristics could be duplicated by separate measurements from 1-propyn-1-yl iodide (CH3-C[triple bond]C-I) being a direct source for the surface methylacetylide (CH3-C[triple bond]C-Ag). It is thus suggested that allenyl is further reorganized to render acetylide presumably via [1,3]-hydrogen shift (i.e., *CH2=C=CH-Ag --> *CH3=C[triple bond]C-Ag). The presence of this third Ag-C3H3 isomeric form demonstrates an unprecedented propargyl-allenyl-acetylide multiple rearrangements on a metal surface. Migration of the triple bond from the remote terminal position into the chain, through the stage of allenic structure, is driven by thermodynamic stabilities, supported by the DFT total energy calculations. Consequently, the evolutions of propyne at 310 and 475 K, as well as 2,4-hexadiyne (bismethylacetylide), can all be reasoned out.  相似文献   

18.
采用La2(CO3)3空气焙烧法制备了La2O2CO3载体、采用浸渍法制备了Ni,Fe不同比例的Ni-Fe双金属催化剂及Ni/La2O2CO3,Fe/La2 O2 CO3催化剂,考察了各催化剂从300~700℃催化乙醇水蒸气重整反应的性能,并用BET,XRD,TPR等技术对催化剂进行表征。结果表明,相对单一金属催化剂,Ni-Fe双金属催化剂均表现出更高的活性,这可能是因为高分散的Ni,Fe和LaFeyNi1-yO3的共存作用。其中Ni含量为10%,Fe含量为5%时的Ni-Fe/La2O2CO3表现出最高的活性,400℃时乙醇的转化率为100%,H2的选择性最高达到94.1%,而CO的选择性则低至1.2%。  相似文献   

19.
Mo助剂对Rh/SiO_2催化剂上甲醇吸附性能的影响──Ⅱ.程序升温脱附和分解表征李光进,徐奕德,黄家生(中国科学院大连化学物理研究所催化基础国家重点实验室,大连116023)关键词铑,二氧化硅,钼助剂,程序升温脱附,程序升温分解,甲醇吸附根据微观可?..  相似文献   

20.
The activation of adsorbed CO is an important step in CO hydrogenation. The results from TPSR of pre-adsorbed CO with H2 and syngas suggested that the presence of H2 increased the amount of CO adsorption and accelerated CO dissocia-tion. The H2 was adsorbed first, and activated to form H* over metal sites, then reacted with carbonaceous species. The oxygen species for CO2 formation in the presence of hydrogen was mostly OH*, which reacted with adsorbed CO subsequently via CO*+OH* → CO2*+H*; however, the direct CO dissociation was not excluded in CO hydrogenation. The dissociation of C-O bond in the presence of H2 proceeded by a concerted mechanism, which assisted the Boudourd reaction of adsorbed CO onthe surface via CO*+2H* → CH*+OH*. The formation of the surface species (CH) from adsorbed CO proceeded as indicated with the participation of surface hydrogen, was favored in the initial step of the Fischer-Tropsch synthesis.  相似文献   

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