Improving the Catalytic Performance of Cobalt for CO Preferential Oxidation by Stabilizing the Active Phase through Vanadium Promotion

Abstract

Preferential oxidation of CO (COPrOx) is a catalytic reaction targeting the removal of trace amounts of CO from hydrogen-rich gas mixtures. Non-noble metal catalysts, such as Cu and Co, can be equally active to Pt for the reaction; however, their commercialization is limited by their poor stability. We have recently shown that CoO is the most active state of cobalt for COPrOx, but under certain reaction conditions, it is readily oxidized to Co3O4 and deactivates. Here, we report a simple method to stabilize the Co 2+ state by vanadium addition. The V promoted cobalt catalyst exhibits considerably higher activity and stability than pure cobalt. The nature of the catalytic active sites during COPrOx was established by operando NAP-XPS and NEXAFS, while the stability of the Co 2+ state on the surface was verified by in situ NEXAFS at 1 bar pressure. The active phase consists of an ultra-thin cobalt-vanadate surface layer, containing tetrahedral V 5+ and octahedral Co 2+ cations, with an electronic and geometric structure that is deviating from the standard mixed bulk oxides. In addition, V addition helps to maintain the population of Co 2+ species involved in the reaction, inhibiting carbonate species formation that are responsible for the deactivation. The promoting effect of V is discussed in terms of enhancement of CoO redox stability on the surface induced by electronic and structural modifications. These results demonstrate that V-promoted cobalt is a promising COPrOx catalyst and validate the application of in situ spectroscopy to provide the concept for designing better performing catalysts.

Structural and Valence State Modification of Cobalt in CoPt Nanocatalysts in Redox Conditions

“…The specimen was exposed to H 2 and O 2 to simulate reductive and oxidative conditions during reactions. Indeed, it is known that catalysts can be either reduced or oxidized during the catalytic process. , Even though some reactions such as ORR and HER are performed at room temperature (RT), the sample was studied at elevated temperature to accelerate and amplify the effects of exposure to O 2 and H 2 . Hence, the experiment provides an insight into degradation mechanisms occurring after long usage of the nanocatalysts .…”

Isotopic evidence for the tangled mechanism of the CO-PROX reaction over mixed and bare cobalt spinel catalysts

“…The fact that p % (C 16 O 18 O) > p % (C 16 O 2 ) ≈ p % (C 18 O 2 ) until 210 °C can be explained by the predominant operation of the LH/ER-α 1 mechanism (0.69) with a persistently small value of the z parameter in this temperature window. The observed lower selectivity in comparison to the carbonate model prediction, leading to the corresponding larger "experimental" z-values, may probably be associated with an enhanced hydrogen oxidation (see eqn (15) and ( 16)) on the segregated CuO phase of the significant 17.7% abundance (see Table S1 †). Indeed, the apparent activation energy of 14.5 kcal mol −1 for the EP oxidation is favorable for the occurrence of this reaction, 64 and full conversion of H 2 oxidation was actually observed for CuO below 250 °C in a separate measurement.…”

“…[4][5][6][7] Transition metal oxides are widely used for the development of model and applied catalytic materials with promising total CO oxidation [8][9][10][11][12][13] and CO-PROX activities. 5,14,15 Among many oxide systems, supported CuO x /CeO 2 catalysts are undoubtedly the most explored, due to the beneficial effect of combining the active CuO phase with the CeO 2 support, which is characterized by oxygen release and storage capacity. 16,17 Another attractive group of CO-PROX catalysts that have been recently widely investigated are perovskites (ABO 3 ) [18][19][20] and spinels (AB 2 O 4 ).…”

Effect of manganese promotion on the activity and selectivity of cobalt catalysts for CO preferential oxidation

“…Therefore, the effort to enhance the stability of CoO in cobaltbased COPrOx catalysts should be pursued, possibly by testing other promoters with higher synergy with cobalt. As we have recently shown vanadium might be a very promising choice in this direction [85].…”