Page 1 - The Effects of Confinement inside Carbon Nanotubes on Catalysis
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The Effects of Confinement inside Carbon
                                          Nanotubes on Catalysis

                                               XIULIAN PAN* AND XINHE BAO*
                                 State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese
                                     Academy of Sciences, Zhongshan Road 457, Dalian 116023, China
                                                   RECEIVED ON DECEMBER 13, 2010


                    CONSPECTUS

                       he unique tubular morphology of carbon nanotubes (CNTs) has trig-
                   T gered wide research interest. These structures can be used as nano-
                   reactors and to create novel composites through the encapsulation of guest
                   materials in their well-defined channels. The rigid nanotubes restrict the
                   size of the encapsulated materials down to the nanometer and even the sub-
                   nanometer scale. In addition, interactions may develop between the
                   encapsulated molecules and nanomaterials and the CNT surfaces. The
                   curvature of CNT walls causes the π electron density of the graphene
                   layers to shift from the concave inner to the convex outer surface, which
                   results in an electric potential difference. As a result, the molecules and
                   nanomaterials on the exterior walls of CNTs likely display different
                   properties and chemical reactivities from those confined within CNTs.
                   Catalysis that utilizes the interior surface of CNTs was only explored
                   recently. An increasing number of studies have demonstrated that confining metal or metal oxide nanoparticles inside CNTs
                   often leads to a different catalytic activity with respect to the same metals deposited on the CNT exterior surface.
                   Furthermore, this inside and outside activity difference varies based on the metals used and the reactions catalyzed.
                      In this Account, we describe the efforts toward understanding the fundamental effects of confining metal nanoparticles inside
                   the CNT channels. This research may provide a novel approach to modulate their catalytic performance and promote rational
                   design of catalysts. To achieve this, we have developed strategies for homogeneous dispersion of nanoparticles inside nanotubes.
                   Because researchers have previously demonstrated the insertion of nanoparticles within larger nanotubes, we focused specifically
                   on multiwalled carbon nanotubes (MWCNTs) with an inner diameter (i.d.) smaller than 10 nm and double-walled carbon
                   nanotubes (DWCNTs) with 1.01.5 nm i.d. The results show that CNTs with well-defined morphology and unique electronic
                   structure of CNTs provide an intriguing confinement environment for catalysis.



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          Introduction                                          synthesis, respectively. Inspired by the successful applica-
          CNTs can be envisioned as rolled-up graphene layers   tions of activated carbon (AC) as a catalyst support in
          forming a tubular structure. According to the number of  industrial processes, CNTs were widely studied as an alter-
          layers, single-walled CNTs (SWCNTs), double-walled CNTs  native for dispersion of transition metals on their exterior
          (DWCNTs), and multiwalled CNTs (MWCNTs) are distin-   walls for hydrogen involving reactions and electrocatalytic
          guished. Their high electron and thermal conductivity, high  reactions. A general conclusion can be drawn from those
          surface area, and functionalizable surfaces have evoked  studies that the activity or product selectivities are improved
          wide interest for catalytic applications. 1,2  For example,  in comparison to the metals supported on AC and conven-
          MWCNTs were reported to catalyze oxidative dehydro-   tional oxides. 1,2
          genation of ethylbenzene to styrene due to the presence  However, catalysis utilizing the interior surface of CNTs
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          of surface oxygen functional groups. As a catalyst additive,  has been less explored, 5,6  although theoretical studies
          MWCNTs promoted substantially the catalytic activities of  predicted that chemical reactions (without additional
          CuZnAl and CoMoCu for methanol and mixed alcohol      catalysts) may be influenced inside such a significantly
                                                                   Vol. 44, No. 8 ’ 2011 ’ 553–562 ’ ACCOUNTS OF CHEMICAL RESEARCH ’ 553
          Published on the Web 06/27/2011 www.pubs.acs.org/accounts
          10.1021/ar100160t  & 2011 American Chemical Society
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