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On the nature of active phases and sites in CO or CO2 hydrogenation catalysts


The production of hydrocarbons or oxygenates via the hydrogenation-polymerisation of either CO (Fischer-Tropsch) or CO2 as the carbon sources bear significant similarities from a catalytic perspective. Strategically, though, both processes differ. Fischer-Tropsch technologies, commercially implemented for almost a century, rely mostly on fossil resources such as coal or natural gas after previous gasification to obtain syngas. Conversely, the direct transformation of waste CO2 using renewable hydrogen is called to partially compensate for the enormous emissions of such gas, and to alleviate their impact on global climate. The development of catalysts for Fischer-Tropsch processes is mature, and has resulted in valuable knowledge, also representing a valid starting point for the use of CO2 as a feedstock. In general terms, catalysts require active phases promoting: (i) CO-CO2 interconversion via the water-gas shift (WGS) and its reverse (rWGS) reactions; (ii) CO activation via dissociation; (iii) hydrogenation; (iv) polymerisation leading to hydrocarbon chain growth; and (v) non-dissociative CO insertion for the formation of oxygenated moieties. Characterisation techniques have traditionally pointed to iron oxides as active for rWGS, to metallic cobalt or iron carbides as the active phases for hydrogenation-polymerisation into hydrocarbons, and to copper, mixed cobalt-copper or rhodium phases as selective for oxygenate formation. However, the nature of active phases and sites is still debated. Novel, improved in situ and operando investigations are challenging some of the previously accepted concepts by allowing the observation of metastable phases, most likely in the form of few-layers surface domains, which might play key roles. For example, the direct implication of cobalt carbides either as precursors of highly active Co(hcp), or as genuinely active phases, is being proposed. Thus, the simplistic view of unique catalytically active sites is progressively being replaced with a more dynamic scenario whereby surface interconversion appears as a usual phenomenon. Herein, a perspective on the most enlightening of recent discoveries on the nature of active COx hydrogenation phases and sites, providing valuable knowledge for future catalyst design, is presented.

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Publication details

The article was received on 11 Jun 2018, accepted on 09 Sep 2018 and first published on 10 Sep 2018

Article type: Perspective
DOI: 10.1039/C8CY01216D
Citation: Catal. Sci. Technol., 2018, Accepted Manuscript
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    On the nature of active phases and sites in CO or CO2 hydrogenation catalysts

    A. PUGA VACA, Catal. Sci. Technol., 2018, Accepted Manuscript , DOI: 10.1039/C8CY01216D

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