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Kinetics, Catalysis, and Reaction Engineering
Kinetics Insights and Active Sites Discrimination of Pd-Catalyzed Selective Hydrogenation of Acetylene Yueqiang Cao, Wenzhao Fu, Zhi Jun Sui, Xuezhi Duan, De Chen, and Xinggui Zhou Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b05687 • Publication Date (Web): 11 Jan 2019 Downloaded from http://pubs.acs.org on January 17, 2019
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Industrial & Engineering Chemistry Research
Kinetics Insights and Active Sites Discrimination of Pd-Catalyzed Selective Hydrogenation of Acetylene Yueqiang Cao1, Wenzhao Fu1, Zhijun Sui1, Xuezhi Duan1,*, De Chen2, Xinggui Zhou1,* 1State
Key Laboratory of Chemical Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, China
2Department
of Chemical Engineering, Norwegian University of Science and Technology, Trondheim 7491, Norway *Corresponding authors:
[email protected];
[email protected].
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Abstract: Catalysis is a kinetics behavior, and developing the kinetics-assisted discrimination of the active sites is an important yet challenging issue in the heterogeneous catalysis. Herein, we combine the multi-faceted kinetics analysis with the model calculations to discriminate the dominant active sites in Pd-catalyzed semi-hydrogenation of acetylene. The size-insensitive activation energy of ≥ 3.1 nm sized Pd catalysts with similar electronic properties suggests that only one typed active site mainly dominates the acetylene hydrogenation. The results of model calculations, based on the specific cuboctahedron shape of Pd nanoparticles on CNT, indicate that the Pd(111) facet is dominant active sites for the reaction and the formation of C4 byproduct, while the Pd corner site for the formation of ethane. Moreover, for the Pd particle size being smaller than 3.1 nm, the catalysts exhibit higher activation energy but higher TOF, due to their lower Pd0 3d binding energy and higher pre-exponential factor, respectively.
Keywords: Acetylene hydrogenation; Pd/CNT catalyst; Active site; Multi-faceted kinetics analysis; Model calculations
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Industrial & Engineering Chemistry Research
1. Introduction Semi-hydrogenation of trace acetylene in ethylene-rich feedstock using Pd-based catalysts is an important industrial process in polyethylene industry to avoid poisoning the Ziegler-Natta polymerization catalyst.1-2 In consideration of the Pd expense and scarcity, substantial efforts have been devoted to improving the utilization efficiency of the catalysts by tuning their geometric and electronic properties, including introducing promoters, e.g., TiO2,3 La2O34 and Ga2O3,5 and alloying Pd with other metals, such as Ag,6-9 Ga,10-12 Cu,13-15 Au,16-18 Zn
19-20
and In.21-22 On the other hand,
some parallel and consecutive side reactions, e.g., over-hydrogenation and oligomerization of acetylene, inevitably occur during the catalytic process, but it is still challenging to rationally tailor the selectivity to targeted product at the catalyst atomic level, such as catalyst active site. The hydrogenation of acetylene on the Pd catalyst has been shown to be a structure-sensitive reaction,23-24 i.e., the dependence of the activity on the Pd particle size, but it remains open whether and why the product selectivity and kinetic behavior are also size-dependent. In our previous work,2526
we have proposed a new kinetics-assisted method to understand the size-dependent activity in Pt-
catalyzed hydrolytic dehydrogenation of ammonia borane as a probe reaction and then to discriminate the dominant active sites. It is highly desirable to employ this method for the full exploration of the structure sensitivity in the Pd-catalyzed hydrogenation of acetylene and thus the discrimination of the dominant active sites for the acetylene conversion and each product formation. Moreover, as the Pd particle size varies, the type, fraction and number of the exposed crystal facets/sites over the Pd nanoparticles also change, which in principle give rise to different catalytic performance. Meanwhile, differently sized metal nanoparticles usually exhibit different electronic properties. These call for
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better understanding on how the size-dependent metal properties contribute to their activity and selectivity, aiming to guide the rational design and optimization of the Pd catalysts. The nature of the Pd active sites for the reaction is a longstanding scientific question. Up to now, there is no consensus of the dominant active site for the reaction. Yarulin et al.27 reported that the Pd(111) is more active than the Pd(100), while Kim et al.28 suggested that the Pd(100) is more active than the Pd(111). These apparent controversial findings might be arising from the presence of other influencing factors. For example, Kim et al. thought that the UV/O3 treatment is not complete for the removal of residual surfactant on the metal nanoparticles, and some others indicated that the oxidationreduction process, e.g., cyclic O2-H2 treatment, may change the electronic properties and/or shape of metal nanoparticles.29-30 Moreover, DFT calculations suggested that the activities of the clean Pd(100), Pd(111) and Pd(211) surfaces decrease in the order of Pd(211) > Pd(111) > Pd(100).31 Normally, the fresh catalyst usually undergoes decreased activity at the initial reaction stage followed by relatively stable activity, i.e., a so-called steady state,32 which could be attributed to the deposition of oligomers on the active sites, most likely on the low-coordinated sites.33 In addition, to our best knowledge, discrimination of the dominant active sites for each product formation is rarely studied. The objective of this work is to discriminate the dominant active sites for the reaction and each product formation. Semi-hydrogenation of acetylene over differently sized Pd/CNT catalysts prepared by an impregnation followed by hydrogen reduction method was carried out, and the multi-faceted kinetics analysis combined with the model calculations was employed for the full exploration of the structure sensitivity including the size-sensitive activity, selectivity and kinetic behavior. Further combining the characterization results, the Pd/CNT catalyst sized in
