Schuster Ralf, Bertram Manon, Runge Henning, Geile Simon, Chung Simon, Vonk Vedran, Noei Heshmat, Poulain Agnieszka, Lykhach Yaroslava, Stierle Andreas, Libuda Jörg
Interface Research and Catalysis, ECRC, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstraße 3, 91058 Erlangen, Germany.
Deutsches Elektronen-Synchrotron DESY, Notkestrasse 85, 22607 Hamburg, Germany and Fachbereich Physik, Universität Hamburg, Jungiusstrasse 11, 20355 Hamburg, Germany.
Phys Chem Chem Phys. 2021 Jan 21;23(2):1371-1380. doi: 10.1039/d0cp05606e.
Efficient hydrogen release from liquid organic hydrogen carriers (LOHCs) requires a high level of control over the catalytic properties of supported noble metal nanoparticles. Here, the formation of carbon-containing phases under operation conditions has a direct influence on the activity and selectivity of the catalyst. We studied the formation and stability of carbide phases using well-defined Pd/α-Al2O3(0001) model catalysts during dehydrogenation of a model LOHC, methylcyclohexane, in a flow reactor by in situ high-energy grazing incidence X-ray diffraction. The phase composition of supported Pd nanoparticles was investigated as a function of particle size and reaction conditions. Under operating conditions, we detected the formation of a PdxC phase followed by its conversion to Pd6C. The dynamic stability of the Pd6C phase results from the balance between uptake and release of carbon by the supported Pd nanoparticles in combination with the thermodynamically favorable growth of carbon deposits in the form of graphene. For small Pd nanoparticles (6 nm), the Pd6C phase is dynamically stable under low flow rate of reactants. At the high reactant flow, the Pd6C phase decomposes shortly after its formation due to the growth of graphene. Structural analysis of larger Pd nanoparticles (15 nm) reveals the formation and simultaneous presence of two types of carbides, PdxC and Pd6C. Formation and decomposition of Pd6C proceeds via a PdxC phase. After an incubation period, growth of graphene triggers the decomposition of carbides. The process is accompanied by segregation of carbon from the bulk of the nanoparticles to the graphene phase. Notably, nucleation of graphene is more favorable on bigger Pd nanoparticles. Our studies demonstrate that metastability of palladium carbides associated with dynamic formation and decomposition of the Pd6C and PdxC phases is an intrinsic phenomenon in LOHC dehydrogenation on Pd-based catalysts and strongly depends on particle size and reaction conditions.
从液态有机氢载体（LOHCs）中高效释放氢气需要对负载型贵金属纳米颗粒的催化性能进行高度控制。在此，操作条件下含碳相的形成对催化剂的活性和选择性有直接影响。我们使用定义明确的Pd/α-Al2O3(0001)模型催化剂，通过原位高能掠入射X射线衍射，在流动反应器中对模型LOHC甲基环己烷脱氢过程中碳化物相的形成和稳定性进行了研究。研究了负载型Pd纳米颗粒的相组成与粒径和反应条件的关系。在操作条件下，我们检测到PdxC相的形成，随后其转化为Pd6C。Pd6C相的动态稳定性源于负载型Pd纳米颗粒对碳的吸收和释放之间的平衡，以及石墨烯形式的碳沉积物在热力学上有利的生长。对于小的Pd纳米颗粒（6 nm），在低反应物流量下，Pd6C相是动态稳定的。在高反应物流量下，由于石墨烯的生长，Pd6C相在形成后不久就会分解。对较大的Pd纳米颗粒（15 nm）的结构分析表明，两种类型的碳化物PdxC和Pd6C同时形成并存在。Pd6C的形成和分解通过PdxC相进行。经过一段潜伏期后，石墨烯的生长引发碳化物的分解。该过程伴随着碳从纳米颗粒本体向石墨烯相的偏析。值得注意的是，石墨烯在较大的Pd纳米颗粒上更容易成核。我们的研究表明，与Pd6C和PdxC相的动态形成和分解相关的钯碳化物的亚稳性是基于Pd的催化剂上LOHC脱氢中的固有现象，并且强烈依赖于粒径和反应条件。
