Editorial: Cobalt and Iron Catalysis

Cobalt and iron have long history of importance in the field of catalysis that continues to this day [...]

are involved, how promoters and poisons influence the active site, and which elementary steps are involved in the complex surface reaction mechanisms [4,[47][48][49][50][51][52]. A quick search from Google Scholar on the "Fischer Tropsch mechanism" resulted in over 6500 publications published in the past two years on this topic alone. The routes are not fully understood, and much more work is needed to bridge the gap between, on the one hand, density functional theory and microkinetic modeling, and, on the other hand, actual data (i.e., product workup, catalyst characterization) generated from the testing of industriallyand academically-relevant catalyst formulations. A multitude of unanswered questions come to the fore. (1) What is the rate limiting step of the mechanism and how can activity be improved? [53] (2) Which metals and promoters are most suitable for a specific process, and how do they work together to bring about the desired selectivity? [54][55][56] (3) Which specific reactor configurations and process conditions are superior for targeting specific product ranges? [57][58][59][60] (4) Which processes are involved in catalyst deactivation, which ones dominate, and how can they be managed? (5) What are the relationships between homogeneous and heterogeneous catalysts? [61].
Much of the importance of these metals lies in their ability to manage carbon, including the activation of carbon monoxide, the coupling of carbon with carbon, hydrogen, or oxygen [62], and the scission of the same, as well as their ability remove products, once formed [62]. The capacity for iron and cobalt to manage carbon creates an avenue for useful products to be constructed from much simpler molecules. This is a significant advantage over alternative approaches such as direct liquefaction and the upgrading of bio-oil produced from flash pyrolysis. For example, Fischer-Tropsch synthesis produce a plethora of paraffins, 1-olefins, and oxygenates from simply carbon monoxide and hydrogen. These products are then readily hydro-processed to produce alternative fuels (i.e., especially diesel and aviation fuels), as well as lubricants and waxes. The olefins and oxygenates produced can be used as feedstocks for various applications, including the manufacturing of plastics. Thus, deepening our understanding of how the properties of the catalyst and process conditions can be tuned will give rise to greater control of the product distribution [62].
This special series highlights the work of scientists and engineers from all over the world who embody the leading edge of catalyst research on cobalt and iron. This collection provides a broad overview of a multitude of processing routes. In addition to the aforementioned topics, global concerns also drive: environmentally-attractive catalysts through specific homogeneous asymmetric iron complexes [63], the development of consumer products (preservatives, cosmetics, and flavors) [64] and materials (inks, coating, and paints) [65], and effective waste water treatment [66]. Furthermore, environmental concerns necessitate not just the capture of CO 2 , but also its utilization as a means of mitigation [46,57,67]. Yet, alternative means of energy, such as hydrogen, may help limit harmful emissions and could also pave the way for change. The importance of cobalt, as a less expensive metal relative to the precious metals, for light-driven water oxidation [68] and electrochemical processes for hydrogen production [69] further highlights the potential of harnessing these metals in a way that benefits society. Regarding current methods that utilize carbon, such as the FTS process, a series of studies included here examine the effect of reaction conditions and promoters [70], investigate the kinetics [53,59,60], and examine the reaction route through computational modeling [71], with the aim of tailoring new formulations in a scientifically-driven manner [55,56,72]. This offers the potential to more effectively utilize the routes that are already available. Our aim is to tune [54] catalysts to efficiently produce fuels and chemicals in a more environmentally-benign manner.
The guest editors wish to thank all of the authors, reviewers, and editorial staff who took the time to contribute to and shape this special issue in a meaningful way. We dedicate this special issue to the memory of our mentor and friend, Professor Burtron H. Davis.

Conflicts of Interest:
The authors declare no conflict of interest.