“Single” Pd(0) Atom Encapsulated in Multiporphyrin Arrays as a Highly Efficient Heterogeneous Catalyst

Abstract

Well-defined "single" Pd(0) atoms were encapsulated in Pd-mediated multiporphyrin arrays and used as a catalyst. Compared with the Pd(0) nanoparticles prepared from dilute K 2 PdCl 4 solution, the encapsulated Pd(0) atoms showed much higher catalytic activity for photoinduced hydrogen evolution.Palladium-catalyzed reactions such as hydrogen production, hydrogenation, and the Heck reaction have attracted much attention because of their importance to clear energy development and organic synthesis applications. 1,2 Although many Pd(0)-based catalysts are well established, catalyst separation and enhancement of catalytic activity are still large challenges and continue to be the focus of intense research. Over the last three decades, several elegant approaches have been explored to overcome the limitation of catalyst separation, for example, aqueous and fluorous biphase catalysis, 3 reactions in supercritical media, 4 and catalyst immobilization onto solid supports. 5 However, for biphase catalysis including immobilized Pd(0) particles, particle size often plays an important role in catalytic activity since hydrogen or organic molecules interacting with small palladium clusters is a strong function of cluster size. 6 To achieve a highly active palladium catalyst, recently many palladium nanoparticles have been prepared and used as an efficient catalyst. 7 We present here a novel methodology to prepare an encapsulated Pd(0) single atom in multiporphyrin arrays to be used in catalyzing hydrogen production. We have previously reported the preparation of Pd-mediated multiporphyrin arrays at the airwater interface and directly on solid supports. 8 Multilayers of the multiporphyrin arrays, especially those directly assembled on the solid supports, showed a well-defined chemical structure and high thermal and chemical stability and were expected to a possibility for practical applications. In the present work, the linkage, the Pd 2+ ion, in the multiporphyrin arrays was reduced by H 2 in water, which results in well-separated Pd(0) atoms in the organized ultrathin films (Scheme 1). The porphyrin that was used for the assembly was zinc-5,10,15,20-tetra(4-pyridyl)-21H,23H-porphine (ZnTPyP). Experimental details on the preparation of multilayers can be found in our previous paper. 8d The encapsulated Pd(0) atoms were used as a catalyst for photoinduced hydrogen evolution with zinc-5,10,15,20-tetra(4-methylpyridyl)-21H,23H-porphine tetrakis(methochloride) (ZnTMPyP) as a photosensitizer, ethylenediamine-N,N,N′,N′-tetraacetic acid, disodium salt (EDTA) as an electron donor, and methyl viologen (MV 2+ ) as an electron carrier. 9 To evaluate the catalytic activity of the present Pd(0) atoms, we compared the hydrogen evolution rate with the use of Pd(0) nanoparticles under the same experimental conditions. These Pd(0) nanoparticles were prepared by reducing 0.02 mM and 0.2 mM K 2 -PdCl 4 aqueous solutions with H 2 gas. 10 Figure 1 shows two photographs obtained from field emission scan electron microscopy (FESEM) that indicate t...

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