Low to near-zero CO production of hydrogen from fossil fuels: critical role of microwave-initiated catalysis.

Presently, there is no single, clear route for the near-term production of the huge volumes of CO-free hydrogen necessary for the global transition to any type of hydrogen economy. All conventional routes to produce hydrogen from hydrocarbon fossil fuels (notably natural gas) involve the production-and hence the emission-of CO, most notably in the steam methane reforming (SMR) process. Our recent studies have highlighted another route; namely, the critical role played by the microwave-initiated catalytic pyrolysis, decomposition or deconstruction of fossil hydrocarbon fuels to produce hydrogen with low to near-zero CO emissions together with high-value solid nanoscale carbonaceous materials. These innovations have been applied, firstly to wax, then methane, crude oil, diesel, then biomass and most recently Saudi Arabian light crude oil, as well as plastics waste. Microwave catalysis has therefore now emerged as a highly effective route for the rapid and effective production of hydrogen and high-value carbon nanomaterials co-products, in many cases accompanied by low to near-zero CO emissions. Underpinning all of these advances has been the important concept from solid state physics of the so-called Size-Induced-Metal-Insulator Transition (SIMIT) in mesoscale or mesoscopic particles of catalysts. The mesoscale refers to a range of physical scale in-between the micro- and the macro-scale of matter (Huang W, Li J and Edwards PP, 2018, Mesoscience: exploring the common principle at mesoscale, . , 321-326 (doi:10.1093/nsr/nwx083)). We highlight here that the actual physical size of the mesoscopic catalyst particles, located close to the SIMIT, is the primary cause of their enhanced microwave absorption and rapid heating of particles to initiate the catalytic-and highly selective-breaking of carbon-hydrogen bonds in fossil hydrocarbons and plastics to produce clean hydrogen and nanoscale carbonaceous materials. Importantly, also, since the surrounding 'bath' of hydrocarbons is cooler than the microwave-heated catalytic particles themselves, the produced neutral hydrogen molecule can quickly diffuse from the active sites. This important feature of microwave heating thereby minimizes undesirable side reactions, a common feature of conventional thermal heating in heterogeneous catalysis. The low to near-zero CO production of hydrogen via microwave-initiated decomposition or cracking of abundant hydrocarbon fossil fuels may be an interim, viable alternative to the conventional, widely-used SMR, that a highly efficient process, but unfortunately associated with the emission of vast quantities of CO. Microwave-initiated catalytic decomposition also opens up the intriguing possibility of using distributed methane in the current natural gas structure to produce hydrogen and high-value solid carbon at either central or distributed sites. That approach will lessen many of the safety and environmental concerns associated with transporting hydrogen using the existing natural gas infrastructure. When completely optimized, microwave-initiated catalytic decomposition of methane (and indeed all hydrocarbon sources) will produce no aerial carbon (CO), and only solid carbon as a co-product. Furthermore, reaction conditions can surely be optimized to target the production of high-quality synthetic graphite as the major carbon-product; that material of considerable importance as the anode material for lithium-ion batteries. Even without aiming for such products derived from the solid carbon co-product, it is of course far easier to capture solid carbon rather than capturing gaseous CO at either the central or distributed sites. Through microwave-initiated catalytic pyrolysis, this decarbonization of fossil fuels can now become the potent source of sustainable hydrogen and high-value carbon nanomaterials.This article is part of the discussion meeting issue 'Microwave science in sustainability'.