Pyrolyzed phthalocyanines as surrogate carbon catalysts: Initial insights into oxygen-transfer mechanisms

Abstract

a b s t r a c tDeposited and heat-treated phthalocyanines are promising electrocatalysts for replacing platinum in the oxygen reduction reaction (ORR), the most important process in energy conversion systems such as fuel cells; and yet its key mechanistic features are not well understood. To optimize their use, it is necessary to understand their behavior in the absence of an electric field. In the pursuit of this goal, we pyrolyzed metal-free, cobalt and copper phthalocyanines between 550 and 1000°C and studied their structural and chemical changes by elemental analysis, N 2 and CO 2 adsorption, X-ray diffraction (XRD), Raman spectroscopy, X-ray analysis fine structure (XAFS) and X-ray photoelectron spectroscopy (XPS). Their catalytic activity was assessed by non-isothermal O 2 gasification and NO reduction reactions. A comparison of these results with their other properties allowed us to reach the following conclusions: (i) the loss of reactivity of metal-free phthalocyanine with heat treatment is attributed to its structural annealing and heteroatom loss, with the porosity changes having no effect; (ii) for metal phthalocyanines at intermediate heat treatment temperatures, the optimum in reactivity correlates with the micropore surface area and the presence of metal particles, with no influence of nitrogen content; (iii) the coordination metal increases phthalocyanine thermal stability in an inert atmosphere, but in an oxidizing atmosphere it acts as a gasification catalyst even below decomposition temperatures. The implications of these findings for catalytic oxygen-transfer mechanisms are discussed.

The Active Center of Co–N–C Electrocatalysts for the Selective Reduction of CO2 to CO Using a Nafion-H Electrolyte in the Gas Phase

“…Though the electrolysis conditions such as reaction phase, supporting electrolyte, pH value, etc., were very different from our conditions, it has been already reported that the Co–N 4 center in Co–TPP and Co–Pc was effective for the CO 2 reduction. 21 , 23 , 29 , 39 , 46 − 50 We have also reported that Co–phthalocyanine supported on a carbon support (vapor-grown carbon fiber, Showa Denko Co.) showed a lower electrocatalytic activity for the reduction of CO 2 to CO in the gas phase. 50 These facts proposed that a Co–N 4 structure was effective for the CO 2 reduction.…”

Understanding of oxygen reduction reaction by examining carbon-oxygen gasification reaction and carbon active sites on metal and heteroatoms free carbon materials of different porosities and structures

“…Meanwhile, extensive investigation regarding carbon-oxygen gasification were carried out from which clear evidences about the involved active sites and the reaction mechanism were collected [21,22]. Oxygen-transfer reactions of carbon materials are at the heart of fuel combustion and gasification processes and much progress has been made in achieving their fundamental understanding [23]. As described by reaction (1), in carbon gasification or combustion the oxygen molecule reacts with the carbon reactive site (Cf1) from the carbon structure forming an intermediate that desorbs as CO or CO2.…”

Electrocatalysis of oxygen reduction on multi-walled carbon nanotube supported copper and manganese phthalocyanines in alkaline media

“…There are numerous reports on the electrocatalytic properties of carbon materials modified with Co and Fe phthalocyanines and porphyrins and there is a continuously growing research activity in this area [37][38][39]. Whereas the ORR on Cu phthalocyanine (CuPc) and especially on Mn phthalocyanine (MnPc) modified carbon materials is rather poorly investigated compared to aforementioned other transition metal macrocyclic catalysts [21,34,[40][41][42][43][44][45][46][47]. MnPc-based catalysts may reportedly have similar electrochemical behaviour to that of Fe phthalocyanine [41].…”