Switching Co/N/C Catalysts for Heterogeneous Catalysis and Electrocatalysis by Controllable Pyrolysis of Cobalt Porphyrin

摘要

class="head no_bottom_margin" id="sec1title">IntroductionCatalysis, which can accelerate the speed of chemical reactions by changing the reaction pathway and lowering the activation energy, plays a crucial role in diverse fields, such as electrochemical energy conversion (, , , , ), petrochemical industry (), organic synthesis chemistry (, , ), and living systems (). Particularly, electrocatalysis for energy conversion and heterogeneous catalysis for molecule transformation have received extensive attention, owing to their great application potential and high fundamental research value (, , , ). Currently, the most effective catalysts for these reactions are generally precious metals and their alloys or complexes (, href="#bib27" rid="bib27" class=" bibr popnode">Suzuki, 2011, href="#bib22" rid="bib22" class=" bibr popnode">Lyons and Sanford, 2010). However, the prohibitive cost and scarcity of precious metals hinder their widespread technological use and therefore have spurred many interests in developing non-precious metal catalysts (NPMCs) with earth-abundant elements for these catalytic processes (href="#bib14" rid="bib14" class=" bibr popnode">Jiao et al., 2015, href="#bib8" rid="bib8" class=" bibr popnode">He et al., 2016).Pyrolyzed metalitrogen/carbon (M/N/C, M = Fe, Co, Ni, etc.) compounds, typical NPMCs, exhibit excellent catalytic performance for various electrochemical processes, particularly for the oxygen reduction reaction (ORR) (href="#bib4" rid="bib4" class=" bibr popnode">Bezerra et al., 2008, href="#bib5" rid="bib5" class=" bibr popnode">Chen et al., 2011, href="#bib12" rid="bib12" class=" bibr popnode">Jaouen et al., 2011, href="#bib15" rid="bib15" class=" bibr popnode">Lefèvre et al., 2009, href="#bib30" rid="bib30" class=" bibr popnode">Wu et al., 2011a, href="#bib29" rid="bib29" class=" bibr popnode">Wang et al., 2017, href="#bib6" rid="bib6" class=" bibr popnode">Chung et al., 2017). It is found that the electrocatalytic performance of M/N/C catalysts is strongly dependent on the intrinsic nature of M-Nx active sites, numbers of exposed active sites, porous structure, and electrical conductivity (href="#bib12" rid="bib12" class=" bibr popnode">Jaouen et al., 2011, href="#bib16" rid="bib16" class=" bibr popnode">Liang et al., 2013, href="#bib32" rid="bib32" class=" bibr popnode">Wu et al., 2015). Based on these in-depth understanding, now the rationally prepared M/N/C catalysts can compete with platinum-based catalysts in acidic medium and even surpass platinum-based counterparts in alkaline medium for ORR (href="#bib29" rid="bib29" class=" bibr popnode">Wang et al., 2017, href="#bib6" rid="bib6" class=" bibr popnode">Chung et al., 2017, href="#bib16" rid="bib16" class=" bibr popnode">Liang et al., 2013, href="#bib35" rid="bib35" class=" bibr popnode">Yin et al., 2016). The great success of M/N/C catalysts in eletrocatalysis encourages considerable research works in employing these NPMCs for heterogeneous catalysis in various organic reactions (href="#bib8" rid="bib8" class=" bibr popnode">He et al., 2016, href="#bib11" rid="bib11" class=" bibr popnode">Jagadeesh et al., 2017, href="#bib18" rid="bib18" class=" bibr popnode">Liu et al., 2016a, href="#bib19" rid="bib19" class=" bibr popnode">Liu et al., 2016b, href="#bib20" rid="bib20" class=" bibr popnode">Liu et al., 2017, href="#bib37" rid="bib37" class=" bibr popnode">Zhou et al., 2017), including selective oxidation (href="#bib9" rid="bib9" class=" bibr popnode">Jagadeesh et al., 2013a), selective hydrogenation (href="#bib10" rid="bib10" class=" bibr popnode">Jagadeesh et al., 2013b), epoxidation (href="#bib1" rid="bib1" class=" bibr popnode">Banerjee et al., 2014), oxidative dehydrogenation (href="#bib7" rid="bib7" class=" bibr popnode">Cui et al., 2015), reductive amination (href="#bib11" rid="bib11" class=" bibr popnode">Jagadeesh et al., 2017), hydrogenative coupling of nitroarenes, and C-C coupling reactions (href="#bib18" rid="bib18" class=" bibr popnode">Liu et al., 2016a, href="#bib36" rid="bib36" class=" bibr popnode">Zhang et al., 2015). In spite of these progresses, the understanding of structure-performance relationship of M/N/C catalysts for heterogeneous catalysis is very limited, compared with that for electrocatalysis. The identification of the difference in designing and optimizing M/N/C materials for electrocatalysis and heterogeneous catalysis is therefore urgently necessary for developing high-performance M/N/C catalysts for various organic reactions.Generally, to boost electrocatalysis, high pyrolysis temperature is needed to endow the M/N/C materials with high electrical conductivity for efficient electron transport from the active centers within the catalysts to the electrode (href="#bib4" rid="bib4" class=" bibr popnode">Bezerra et al., 2008, href="#bib12" rid="bib12" class=" bibr popnode">Jaouen et al., 2011). Nevertheless, such long-range electron transport is absent in the heterogeneous catalysis for organic reactions (href="#bib8" rid="bib8" class=" bibr popnode">He et al., 2016). This seems to suggest that the optimal M/N/C materials for heterogeneous catalysis should be prepared at a lower temperature than that for electrocatalysis, as high-temperature pyrolysis would inevitably destroy the highly active single-atom M-Nx active sites to form less-active metallic nanoparticles. To identify the difference in optimizing the M/N/C catalysts for electrocatalysis and heterogeneous catalysis, herein we develop a group of mesoporous Co/N/C materials, ranging from polymerized cobalt porphyrin to Co/N-doped carbons, which are prepared by the pyrolysis of cobalt porphyrin with silica nanoparticle templates under different temperatures and studied for both heterogeneous catalysis in organic reactions and electrocatalysis in a comparative perspective.