Enhancing Catalytic CO Oxidation over Co₃O₄ Nanowires by Substituting Co²⁺ with Cu²⁺

Abstract

Co 3 O 4 is an attractive earth-abundant catalyst for CO oxidation, and its high catalytic activity has been attributed to Co 3+ cations surrounded by Co 2+ ions. Hence, the majority of efforts for enhancing the activity of Co 3 O 4 have been focused on exposing more Co 3+ cations on the surface. Herein, we enhance the catalytic activity of Co 3 O 4 by replacing the Co 2+ ions in the lattice with Cu 2+ . Polycrystalline Co 3 O 4 nanowires for which Co 2+ is substituted with Cu 2+ are synthesized using a modified hydrothermal method. The Cusubstituted Co 3 O 4 _Cux polycrystalline nanowires exhibit much higher catalytic activity for CO oxidation than pure Co 3 O 4 polycrystalline nanowires and catalytic activity similar to those single crystalline Co 3 O 4 nanobelts with predominantly exposed most active {110} planes. Our computational simulations reveal that Cu 2+ substitution for Co 2+ is preferred over Co 3+ both in the Co 3 O 4 bulk and at the surface. The presence of Cu dopants changes the CO adsorption on the Co 3+ surface sites only slightly, but the oxygen vacancy is more favorably formed in the bonding of Co 3+ −O−Cu 2+ than in Co 3+ −O−Co 2+ . This study provides a general approach for rational optimization of nanostructured metal oxide catalysts by substituting inactive cations near the active sites and thereby increasing the overall activity of the exposed surfaces. ■ INTRODUCTIONCarbon monoxide (CO) emission from transportation and industrial activities is harmful to both human health and the environment. Currently, CO emission is effectively reduced, mainly through catalytic oxidation over catalysts. 1−4 The most active catalysts for CO oxidation are noble metals, but they are expensive and are of limited supply. Co 3 O 4 has emerged as an attractive alternative catalyst for CO oxidation because of its optimal CO adsorption strength, low barrier for CO reaction with lattice O, and excellent redox capacity. 1,5−8 A breakthrough on Co 3 O 4 for catalytic CO oxidation showing that Co 3 O 4 nanorods with predominantly exposed {110} planes exhibit a much higher catalytic activity for CO oxidation and larger resistance to deactivation by water than Co 3 O 4 nanoparticles was reported by Xie et al. 9 The high catalytic activity of Co 3 O 4 {110} planes is attributed to its higher concentration of Co 3+ cations (correspondingly fewer Co 2+ cations) than other crystal planes, since only Co 3+ cations surrounded by Co 2+ ions are active for catalytic oxidation of CO. 10,11 Subsequently, a number of Co 3 O 4 nanostructures, ranging from nanobelts, nanospheres, nanocubes, and nanotubes to nanowires, have been synthesized with the purpose of preferentially exposing Co 3+ cations. 11−14 Nevertheless, regardless of the morphology of the Co 3 O 4 nanostructures, even the highly active Co 3 O 4 {110} planes still contain Co 2+ cations, which have been assumed to be inactive for catalytic oxidation of CO, 9−11 and ultimately limits the catalytic activity of Co 3 O 4 for CO oxidation. Therefore, substituting Co 2+ with...

Integrated Cobalt Oxide Based Nanoarray Catalysts with Hierarchical Architectures: In Situ Raman Spectroscopy Investigation on the Carbon Monoxide Reaction Mechanism

“…[10,15,50,51] In contrast, some researchers revealed that the CO oxidation reactionm echanism can follow the LH mechanism, which includes the main four steps in CO oxidation:1 )COa dsorption on the cationic metal sites, 2) the formation of active oxygen that resultsf rom O 2 molecules adsorbed on oxygen vacancies, 3) the reaction of CO with active oxygen, and 4) CO 2 desorption from the catalyst surface. [10,15,50,51] In contrast, some researchers revealed that the CO oxidation reactionm echanism can follow the LH mechanism, which includes the main four steps in CO oxidation:1 )COa dsorption on the cationic metal sites, 2) the formation of active oxygen that resultsf rom O 2 molecules adsorbed on oxygen vacancies, 3) the reaction of CO with active oxygen, and 4) CO 2 desorption from the catalyst surface.…”

“…[10,15,50,51] In contrast, some researchers revealed that the CO oxidation reactionm echanism can follow the LH mechanism, which includes the main four steps in CO oxidation:1 )COa dsorption on the cationic metal sites, 2) the formation of active oxygen that resultsf rom O 2 molecules adsorbed on oxygen vacancies, 3) the reaction of CO with active oxygen, and 4) CO 2 desorption from the catalyst surface. According to previous studies, [9,10,15] the choice of as uitable doping cation can create structurald efects and residual stress as well as oxygen vacancies. These previous studies suggested that there are no clear reaction mechanisms on certain catalysts for CO oxidation and that tuningt he surface structurald efects (CO adsorption and the formation of active oxygen) of am aterial should be an efficient method to design new catalysts.…”

“…This strategy to prepare grafted ZIF-67@Co 3 O 4 and MOF-199@Co 3 O 4 precursor structures is based on as imple hydrothermals ynthesis methodt oo btain the Co 3 O 4 precursor and the subsequent in situ growth of ZIF-67 and MOF-199, respectively.T he morphologies of the Co 3 O 4 products can be tailoredb yc ontrolling the solvent polarity and concentration of precipitants. Zhou et al [15] revealed that Cu-substituted Co 3 O 4 -Cu x polycrystalline nanowires on 2in. These analysis resultsd emonstrate that irislike Co 3 O 4 exhibits ah igh catalytic activity for CO oxidation and contains an abundance of surface defect sites (Co 3+ + species) to result in an excellent low-temperature reducibility,o xygen vacancies and unsaturated chemicalb onds on the surface.…”

Active Species of Sulfated Metal Oxide Catalyst for Propane Dehydrogenation

“…In the S2p spectrum, the peak for sulfate ion of SO4 2-was confirmed at 169.7 eV. Compared to the peak positions between the Co/Al2O3 and Co/SO4 2-/Al2O3 catalysts before the de hydrogenation, the peak of cobalt species was slightly 20) . This phenomenon might derive from the interaction between sulfated ion (SO4 2-) and cobalt element to form CoSO4-like species.…”

Mesoporous Co‐Al oxide nanosheets as highly efficient catalysts for CO oxidation

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] Up to now, many highly efficient catalysts for CO oxidation, mainly including supported noble metals (such as Au and Pt) 8,9,11,12,[19][20][21][22][23][24][25] and metal-oxides (such as Co 3 O 4 and MnO 2 ) have been successfully developed. 7,[13][14][15][16][17][18][26][27][28][29][30][31][32][33] For example, TiO 2 -supported Au catalyst is very active for CO oxidation at the temperature of −70 C. 19-21 SiO 2 -supported Pt-Fe catalysts give almost 100% CO conversion and 100% CO selectivity at room temperature. 8 Metal oxides are less expensive compared with supported noble metals, which enhances their potential for practical applications.…”