Active sites and deactivation of room temperature CO oxidation on Co₃O₄ catalysts: combined experimental and computational investigations

Abstract

Co3O4 is a well-known low temperature CO oxidation catalyst, but it suffers from rapid deactivation. We have thus examined room temperature CO oxidation on Co3O4 catalysts by combined DSC, TGA and MS measurements, as well as by pulsed chemisorption to differentiate the contributions of CO adsorption and reaction to CO2. Catalysts pretreated in oxygen at 400 ⁰C are most active, with the initial interaction of CO and Co3O4 being strongly exothermic and with maximum amounts of CO adsorption and reaction. The initially high room temperature activity levels-off within tens of minutes, suggesting that the oxidative pretreatment creates an oxygen-rich reactive Co3O4 surface that upon reaction onset loses its active oxygen. This specific active oxygen is not reestablished by gas phase O2 during the room temperature reaction. When the reaction temperature is increased to 150 ⁰C, full conversion can be maintained for 100 hours, and even after cooling back to room temperature. Apparently, deactivating species are avoided this way, whereas exposing the active surface even briefly to pure CO leads to immediate deactivation. Computational studies using DFT helped to identify the CO adsorption sites, determine oxygen vacancy formation energies and the origin of deactivation. A new species of CO bonded to oxygen vacancies at room temperature was identified, which only reacts with an O2 moiety filling a vacancy site at higher temperature (around 100⁰C). The interaction between oxygen vacancies was found to be small, so that in the active state most lattice oxygen species are available for reaction in parallel.

Microalgae-derived Co3O4 nanomaterials for catalytic CO oxidation

“…The results suggest a complex network of different interactions due to CO and O 2 interacting with different sites, as discussed previously. 8,26…”

“…5,6 Apart from well-established noble metal catalysts, cobalt(II,III) oxide nanomaterials (Co 3 O 4 NMs) have received considerable attention due to their abundance, low production costs, and excellent low-temperature catalytic performance. [7][8][9][10][11][12] Despite progress and improvements in Co 3 O 4 catalyst design, the synthesis methods are usually chemical or physical, while a biological approach has not yet been studied extensively. The typical synthesis routes oen involve expensive and hazardous chemicals and procedures that counterproductively may impose negative effects on the environment.…”

“…As mentioned, current research typically focused on Co 3 O 4 NMs synthesized via chemical or physical approaches, combined with characterization and catalytic activity testing. [7][8][9][10][11][12][18][19][20] Several spectroscopic and theoretical studies aimed at identifying the CO oxidation reaction mechanism on Co 3 O 4 NMs. 8,[21][22][23][24][25][26][27] While Pollard et al suggested CO adsorption on Co 2+ , other (theoretical) studies suggested CO adsorption on Co 3+ ions as active sites.…”

“…[7][8][9][10][11][12][18][19][20] Several spectroscopic and theoretical studies aimed at identifying the CO oxidation reaction mechanism on Co 3 O 4 NMs. 8,[21][22][23][24][25][26][27] While Pollard et al suggested CO adsorption on Co 2+ , other (theoretical) studies suggested CO adsorption on Co 3+ ions as active sites. 21 The abundance of surface Co 3+ ions was also made responsible for cryogenic (−77 °C) CO oxidation on Co 3 O 4 nanorods, mainly exposing (110) planes.…”

“…5,6 Apart from well-established noble metal catalysts, cobalt( ii , iii ) oxide nanomaterials (Co 3 O 4 NMs) have received considerable attention due to their abundance, low production costs, and excellent low-temperature catalytic performance. 7–12…”