水煤气变换反应(WGS，CO(g)+H2O(g)→CO2(g)+H2(g))是CO去除和H2制备的途径[1]，也是合成气转化过程中的重要反应，如甲醇合成[2]和费托合成[3]。常见的WGS反应催化剂有Au，Cu，Pd，Ni和Pt基等催化剂[4-8]。为了提高催化剂的活性，通常通过添加助剂[9-12]或改变载体[13-15]的方法。

过渡金属碳化物是一种金属间填充型化合物，是由碳原子填隙式融进过渡性金属的晶格中形成。由于其表面性质和催化活性类似于Pt等贵金属，目前广泛应用于催化加氢、脱氢、WGS和异构化等反应[16-17]。如，Au/α-MoC用于WGS反应时，反应速率和CO转化率分别是Cu/Zn/Al2O3催化剂的354倍和17.8倍[18]，同时Au/α-MoC催化剂的活性明显优于Au/β-MoC。α-MoC对WGS反应起到了重要的作用，因此，笔者选取了α-MoC作为载体。

理论计算结果显示，α-MoC(111)载体对WGS反应没有催化效果，但是能够促进H2O的解离。当其负载Au和Cu后能够提高WGS反应在2种催化剂表面的活性[18-19]。Pd，Ni，Pt基催化剂也是WGS反应的常用催化剂，为了进一步了解α-MoC载体在WGS反应中的作用和WGS反应机理，笔者使用密度泛函理论(DFT)和动力学蒙特卡罗(KMC)方法系统的研究这3种金属负载α-MoC催化剂上的WGS反应过程，研究不同反应温度时的反应速率和催化转换频率(TOF)，并对WGS反应催化剂的优化提供指导。

1 计算方法和模型

计算使用VASP软件[20-22]，采用了GGA-PBE泛函求解电子交换相关能[23]。平面波截断能取值为 415 eV，布里渊区的k点选择 3×3×1[24]。在结构优化的过程中当力的变化小于0.1 eV/nm和能量的变化小于1.0×105 eV/atom时达到收敛标准。采用CI-NEB方法进行过渡态搜索，对达到收敛标准的过渡态进行频率分析，通过唯一虚频确定过渡态[25]。

首先对金属晶胞M(M=Ni，Pd，Pt)和α-MoC晶胞进行了优化，优化后对应的的晶格常数分别为aNi=0.351 3 nm，aPd=0.393 7 nm，aPt=0.396 7 nm，aα-MoC=0.437 0 nm，分别与实验值 aNi=0.352 4 nm，aPd=0.389 1 nm，aPt=0.392 4 nm和aMoC=0.427 8 nm接近[26-27]。因此，选择的计算方法和参数对本文的计算体系是合理的。α-MoC(111)采用7层3×3的平板模型，1.5 nm的真空层。由于H2O的快速解离，α-MoC表面的Mo位点被O占据[19]，如图1所示，图1中top,hol,bri为吸附位点。计算过程中固定底面两层，其余原子弛豫。金属M4簇负载存在2种构型:平面簇构型(2d rhombic M4)和四面体簇(3d-tetrahedral M4)，对应的吸附能分别为Ni4(2d:-8.41 eV;3d:-7.63 eV)，Pd4(2d:-8.56 eV;3d:-6.77 eV)，Pt4(2d:-10.20 eV;3d:-9.32 eV)。计算结果表明，平面簇构型更稳定，故仅考虑了2d-M4负载在α-MoC(111)表面。

2 结果与讨论

对于水煤气变换反应，反应机理主要有2种:① 氧化还原机理:CO直接与H2O分解产生的氧结合，生成CO2;② 甲酸盐途径或者羧酸盐途径:CO与H2O分解产生的羟基结合，生成甲酸盐或羧酸盐中间体，最后分解成CO2[28-30]。反应过程中所涉及到的中间体，反应物和产物的最稳定吸附构型如图2所示。

图2和表1显示，反应物、中间体和产物在3类催化剂表面的最优吸附位相近，只有*H在Pt4/α-MoC(111)和*CO2在Ni4/α-MoC(111)的吸附例外。对中间体来说(*O，*OH，*CO2，*H2O，*CHOO，*COOH)，它们在Pd4/α-MoC(111)和Pt4/α-MoC(111)的吸附能相近，吸附稳定性低于对应物种吸附在Ni4/α-MoC(111)。

反应物*CO吸附在Ni4/α-MoC(111)和Pd4/α-MoC(111)的吸附能相近(-2.23和-2.21 eV)，而在Pt4/α-MoC(111)面的吸附稳定性低于Ni4/α-MoC(111)和Pd4/α-MoC(111)面。相比CO在Ni(111)，Pd(111)和Pt(111)的吸附能，载体α-MoC提高了CO在Ni和Pd的吸附稳定性，但是降低了在Pt上的吸附稳定性[31-32]。对*H2O来说，载体α-MoC一定程度上提高了其在Ni，Pd和Pt表面的吸附稳定性[31-32]。

优化后，产物*CO2远离Pd4/α-MoC(111)和Pt4/α-MoC(111)面，但是其吸附在Ni4/α-MoC(111)的bri位，这是由于*CO2在Ni4/α-MoC(111)面的吸附稳定性强于Pd4/α-MoC(111)和Pt4/α-MoC(111)面。总体来说，相较Ni(111)，Pd(111)和Pt(111)，α-MoC(111)提高了CO2在3种催化剂表面的吸附稳定性[31-32]。对于H2来说，其在3个表面的吸附能力很弱，载体对其吸附基本没有影响。

*H2O的解离(*H2O+*→*OH+*H)是WGS反应的第1步，在Ni4/α-MoC(111)上*H2O解离的能垒为1.16 eV。对于氧化还原机理，*O生成存在2种方式:① *OH直接裂解(*OH+*→*O+*H)能垒Ea为0.95 eV;② *OH歧化反应(*OH+*OH→*H2O+*O)能垒为0.37 eV。最后，*CO氧化生成*CO2需克服1.62 eV的能垒。对于甲酸盐和羧酸盐路径，*CO与*OH反应，生成*CHOO(Ea=2.84 eV)或*COOH(Ea=1.61 eV)。结果表明，在Ni4/α-MoC(111)上氧化还原机理容易发生，速控步骤为*CO氧化。

在Pd4/α-MoC(111)上，*H2O克服1.23 eV能垒解离生成 *OH 和 *H。当发生氧化还原反应时，*OH歧化反应的能垒为0.35 eV，而*OH直接解离的能垒为1.02 eV。最后，*CO氧化生成*CO2的能垒为0.64 eV。在发生甲酸盐或羧酸盐路径中，*CO与*OH反应生成*CHOO和*COOH的能垒分别为1.64和1.28 eV。因此，在Pd4/α-MoC(111)上，WGS反应的氧化路径为主要路径，H2O的解离为速控步骤。

在Pt4/α-MoC(111)上，H2O的解离能垒为0.86 eV。当发生氧化还原反应时，*OH+*→*O+*H和*OH+*OH→*H2O+*O的反应能垒分别为1.28和0.35 eV，*CO氧化生成*CO2的能垒为0.81 eV。在发生甲酸盐或羧酸盐路径中，*CO与*OH反应生成*CHOO和*COOH的能垒分别为1.72和0.66 eV。由于生成*COOH的能垒远远低于生成*HCOO的能垒，因此*CO和*OH反应优先生成*COOH。最后，经*COOH与*OH反应生成*CO2和*H2O(Ea=0.42 eV)。

基于此，根据WGS反应的正逆反应能垒和不同温度时的指前因子，利用KMC研究了WGS反应在Ni4/α-MoC(111)，Pd4/α-MoC(111)和Pt4/α-MoC(111)上的反应路径和催化转换频率。对于反应物和产物的吸附和脱附，考虑了熵的贡献[33]。KMC模型和计算参数等详细信息见文献[34]。

KMC结果显示，标准大气压(n(CO)∶n(H2O)=1∶1，物质的量之比)和反应温度为500 K时，在Ni4/α-MoC(111)和Pd4/α-MoC(111)催化剂上，水煤气变换的反应机理为氧化还原机理如图3所示，图3中,TS为过渡态。即CO(g)+H2O(g)→*CO+*H2O→*CO+*OH+*H→*CO+*O+*2H→*CO2+*H2→CO2(g)+H2(g)。需要指出的是，尽管*OH+*OH→*H2O+*O的反应能垒远远小于*OH+*→*O+*H的反应能垒，然而由于高的*H2O→*OH+*H反应能垒导致表面*OH覆盖度过低。因此，KMC结果显示*OH的直接解离是*O产生的反应途径。在Pt4/α-MoC(111)催化剂上，水煤气变换的反应路径为羧酸盐路径如图4所示，即CO(g)+2H2O(g)→*CO+2*H2O→*CO+2*OH+2*H→*COOH+*OH+2*H→*CO2 +*H2O+*H2 →CO2(g)+H2O(g)+H2(g)。由于H2和CO2的TOF相等，因此图5仅列出了H2的TOF(反应条件为一个大气压下，CO与H2O物质的量之比为1∶1)。总体来说，H2的TOF随反应温度的升高而升高。H2在Ni4/α-MoC(111)和Pd4/α-MoC(111)的TOF偏低。在500 K时催化剂单位活性位点上转换次数约为0和0.056 s-1，这是由于WGS反应过程中反应能垒较高造成的。对于Pt4/α-MoC(111)，值得注意的是，400 K时，H2在催化剂单位活性位点上转换次数约为0，这是因为Pt4/α-MoC(111)上CO的脱附能较高(1.07 eV)，活性位点被CO所覆盖。随着反应温度的升高，CO的脱附能(500 K下脱附能为0.83 eV)降低，H2在催化剂单位活性位点上的TOF约为6.3 s-1，WGS反应发生。结果表明，*CO的强稳定性不利于WGS反应的进行。当反应温度为500 K时，H2的TOF的排序是:Pt4/α-MoC(111)≫Pd4/α-MoC(111)>Ni4/α-MoC(111)。因此，当3种金属负载在α-MoC载体上时，Pt4/α-MoC(111)催化剂活性最好。同时，其反应活性高于Pt/Al2O3(TOF≈0.7 s-1)和Pt/TiO2(TOF≈0.2 s-1)[35]，这表明Pt/α-MoC是WGS反应的高活性催化剂。

3 结 论

(1)通过密度泛函理论和动力学蒙特卡洛方法探究了过渡金属Ni,Pd,Pt负载α-MoC催化剂的WGS反应的反应机理和活性。

(2)DFT计算结果发现，在Ni4/α-MoC(111)和Pd4/α-MoC(111)上，WGS反应的反应路径为氧化还原路径为CO(g)+H2O(g)→*CO+*H2O→*CO+*OH+*H→*CO+*O+*2H→*CO2+*H2→CO2(g)+H2(g);在Pt4/α-MoC(111)催化剂上，WGS反应通过羧酸盐路径发生，即CO(g)+2H2O(g)→*CO+2*H2O→*CO+2*OH+2*H→*COOH+*OH+2*H→*CO2+*H2O+*H2→CO2(g)+H2O(g)+H2(g)。

(3)动力学模拟结果表明，Pt4/α-MoC(111)具有较高的催化活性。标准大气压下，反应温度在400～500 K，H2O与CO的物质的量之比为1时，H2的TOF排序:Pt4/α-MoC(111)≫Pd4/α-MoC(111)>Ni4/α-MoC(111)，Pt/α-MoC是WGS反应的高活性催化剂。

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Theoretical study for water-gas shift reaction over Ni,Pd and Pt supported on α-MoC surface

SHI Xiao1,2,ZOU Xueyan1,2,HUANG Wei1,2,ZUO Zhijun1,2

(1.State Key Laboratory of Clean and Efficient Coal Utilization,Taiyuan University of Technology,Taiyuan 030024,China; 2.Key Laboratory of Coal Science and Technology of Ministry of Education,Taiyuan University of Technology,Taiyuan 030024,China)

Abstract:Hydrogen energy is regarded as the most promising clean energy in the 21st century due to its advantages of clean,efficient and renewable.Water-gas shift (WGS) reaction is a commonly employed reaction in the industry.It is also an important means of hydrogen production while recycling CO waste gas.Besides,it has the dual advantages of environmental governance and energy saving and emission reduction.As a catalyst support,α-MOC shows excellent properties.Pd,Ni and Pt based catalysts are commonly used in the WGS reaction.In order to further understand the role of α-MoC support in the WGS reaction and its catalytic performance under different metal loadings,the reaction mechanism and activities of Ni4/α-MoC (111),Pd4/α-MoC (111) and Pt4/α-MoC (111) were investigated by using density function theory (DFT) and kinetic Monte Carlo (KMC) simulation.The results show that the WGS reaction on Ni4/α-MoC (111) and Pd4/α-MoC (111) is redox mechanism,of which CO combines with atomic oxygen from water decomposition to generate CO2;the WGS reaction on Pt4/α-MoC (111) occurs via the carboxyl pathway,of which carboxylate intermediate is formation from CO reacts with OH produced from water decomposition.Ni4/α-MoC (111) and Pd4/α-MoC (111) show low catalytic activity because of the high energy barrier.At a low temperature,Pt4/α-MoC (111) also shows low a catalytic activity due to strong stability of CO,of which the active sites are covered by CO.With the reaction temperature increasing,the desorption energy of CO decreases,and the catalytic activity increases.At the reaction temperature varied from 400 to 500 K,Pt4/α-MoC (111) shows the highest H2 turnover frequency at 1 atm with a CO:H2O ratio of 1.Compared with the Pt/Al2O3 and Pt/TiO2,Pt/α-MoC shows the highest H2 turnover frequency.

Key words:Water-Gas Shift(WGS);Ni/α-MoC;Pt/α-MoC;Pd/α-MoC;Density Function Theory(DFT);Kinetic Monte Carlo(KMC)

中图分类号:TQ116.2;TQ426

文献标志码:A

文章编号:0253-9993(2021)04-1107-06

收稿日期:20201213

修回日期:20210208

责任编辑:黄小雨

DOI:10.13225/j.cnki.jccs.QJ20.1936

基金项目:国家自然科学基金面上资助项目(21776197);山西重点研发计划(国际科技合作)资助项目(201903D421074);山西省高等学校科技成果转化培育资助项目(2020CG012)

作者简介:史 肖(1993—)，男，河北定州人，硕士研究生。E-mail:2713457580@qq.com

通讯作者:左志军(1981—)，男，山西繁峙人，教授，博士生导师。E-mail:zuozhijun@tyut.edu.cn

引用格式:史肖，邹雪燕，黄伟，等.Ni，Pd，Pt负载α-MoC催化水煤气变换反应理论[J].煤炭学报,2021,46(4):1107-1112.

SHI Xiao,ZOU Xueyan,HUANG Wei,et al.Theoretical study for water-gas shift reaction over Ni,Pd and Pt supported on α-MoC surface[J].Journal of China Coal Society,2021,46(4):1107-1112.