Catalytic remediation of automobile exhaust has relied on precious metals (PMs) including platinum (Pt). Herein, we report that an intermetallic phase of Ni and niobium (Nb) (i.e., Ni3Nb) exhibits a significantly higher activity than that of Pt for the remediation of the most toxic gas in exhaust (i.e., nitrogen monoxide (NO)) in the presence of carbon monoxide (CO). When subjected to the exhaust-remediation atmosphere, Ni3Nb spontaneously evolves into a catalytically active nanophase-separated structure consisting of filamentous Ni networks (thickness < 10 nm) that are incorporated in a niobium oxide matrix (i.e., NbOx (x < 5/2)). The exposure of the filamentous Ni promotes NO dissociation, CO oxidation and N2 generation, and the NbOx matrix absorbs excessive nitrogen adatoms to retain the active Ni0 sites at the metal/oxide interface. Furthermore, the NbOx matrix immobilizes the filamentous Ni at elevated temperatures to produce long-term and stable catalytic performance over hundreds of hours.

A copper-and-zinc (Cu–Zn) alloy material was synthesized using a vacuum sealing method, in which evaporated zinc was reacted with copper film or nanoparticles to form a homogeneous Cu–Zn alloy. This alloy was evaluated as an electrocatalyst and/or cocatalyst for photocatalysis to selectively reduce carbon dioxide to formic acid. Based on the optimised alloy composition, the Cu5Zn8 catalyst exhibited efficient electrochemical CO2 reduction. Furthermore, we constructed a photoelectrochemical (PEC) three-electrode system, in which the Cu5Zn8 film functioned as the cathode for CO2 reduction in the dark and strontium titanate (SrTiO3) served as the photoanode for water oxidation. The PEC system also selectively reduced CO2 to formic acid with a faradaic efficiency of 79.11% under UV-light and the absence of an applied bias potential. SrTiO3 particles decorated with nanoparticles of the Cu–Zn alloy also photocatalytically reduced CO2 to formic acid under UV-light. Isotope trace analysis demonstrated that water served as the electron donor to produce oxygen and organic molecules under UV light, similar to photosynthesis in plants. The Cu–Zn alloy material developed in the present study is composed of ubiquitous and safe materials, and can catalyse CO2 conversion by means of various kinds of renewable energies.

Nanocrystals of sodium antimony oxide, NaSb3O7 (pyrochlore structure, a = 1.030 nm), act as an efficient catalyst support for the electrocatalytic oxygen-reduction reaction (ORR) in acidic media. The NaSb3O7 nanocrystals (edge length ∼ 150 nm) were synthesized by hydrothermal decomposition of SbCl5 in aqueous solution of NaOH. The NaSb3O7 nanocrystals were then decorated with Pt nanoparticles by chemical reduction of H2PtCl6 in water to yield an ORR catalyst, Pt/NaSb3O7. The Pt/NaSb3O7 exhibited higher ORR performance than the state-of-the-art Pt/TiO2- or Pt/C catalysts in terms of the +40 mV higher half-wave reduction potential and the retained electrochemical surface area than the Pt/TiO2 after 10 000-times repeated ORR in an acidic electrolyte. Unlike NaSb3O7, Pt-decorated Sb2O5 (Pt/Sb2O5) was much less active than the Pt/TiO2 or Pt/C. The enhanced ORR activity of the Pt/NaSb3O7 may be attributed to the promoted electron hopping between the Sb3+ and Sb5+ ions in mixed-valence Na1+(Sb3+Sb25+)O7, which is inhibited in single-valence Sb25+O5.

Environmentally compatible energy management is one of the biggest challenges of the 21st century. Low-temperature conversion of chemical to electrical energy is of particular importance to minimize the impact to the environment while sustaining the consumptive economy. In this review, we shed light on one of the most versatile energy-conversion technologies: heterogeneous catalysts. We establish the integrity of structural tailoring in heterogeneous catalysts at different scales in the context of an emerging paradigm in materials science: catalytic nanoarchitectonics. Fundamental backgrounds of energy-conversion catalysis are first provided together with a perspective through state-of-the-art energy-conversion catalysis including catalytic exhaust remediation, fuel-cell electrocatalysis and photosynthesis of solar fuels. Finally, the future evolution of catalytic nanoarchitectonics is overviewed: possible combinations of heterogeneous catalysts, organic molecules and even enzymes to realize reaction-selective, highly efficient and long-life energy conversion technologies which will meet the challenge we face.

Tin-dioxide nanofacets (SnO2 NFs) are crystal-engineered so that oxygen defects on the maximal {113} surface are long-range ordered to give rise to a non-occupied defect band (DB) in the bandgap. SnO2 NFs-supported platinum-nanoparticles exhibit an enhanced ethanol-electrooxidation activity due to the promoted charge-transport via the DB at the metal–semiconductor interface.

Novel intermetallic TaPt3 nanoparticles (NPs) are materialized, which exhibit much higher catalytic performance than state-of-the-art Pt3Sn NPs for electrooxidation of ethanol. In situ infrared-reflection-absorption spectroscopy (IRRAS) elucidates that the TaPt3 NPs cleave the C–C bond in ethanol at lower potentials than Pt NPs, efficiently promoting complete conversion of ethanol to CO2. Single-cell tests demonstrate the feasibility of the TaPt3 NPs as a practical energy-extraction catalyst for ethanol fuels, with more than two times higher output currents than Pt-based cells at high discharge currents.

Naturally occurring clay nanotubes, halloysite (Al2Si2O5(OH)4·2H2O), with exterior and interior surfaces, respectively, composed of SiOx and AlOx layers, act as an agglomeration-tolerant exhaust catalyst when copper–nickel alloy nanoparticles (Cu–Ni NPs, 2–3 nm) are immobilized at the AlOx interior. Co-reduction of Cu2+ and Ni2+ (respectively derived from CuCl2 and NiCl2) in the presence of sodium citrate (Na3C6H5O7·2H2O) and halloysite yielded the required nanocomposite, Cu–Ni@halloysite. Cu–Ni@halloysite efficiently catalyzes the purification of simulated motor vehicle exhaust comprising nitrogen monoxide (NO) and carbon monoxide (CO) near the activation temperature of Pt-based exhaust catalysts, ≤400 °C, showing its potential as an alternative to Pt-based catalysts. In contrast, a different halloysite nanocomposite with the SiOx exterior decorated with Cu–Ni NPs, Cu–Ni/halloysite, is poorly active even at >400 °C because of particle agglomeration. The enhanced exhaust-purification activity of Cu–Ni@halloysite can ultimately be attributed to the topology of the material, where the alloy NPs are immobilized at the tubular AlOx interior and protected from particle agglomeration by the tubular form and SiOx exterior.

Two vanadates, Ag2Sr(VO3)4 and Sr(VO3)2, have been studied as visible-light-driven water oxidation photocatalysts with the help of density-functional theory calculations. Our computational results for the density of states and partial charge densities implied that Ag2Sr(VO3)4 and Sr(VO3)2 possess desirable electronic structures for the water oxidation reaction, i.e., the valence band (VB) maximum of Ag2Sr(VO3)4 consists of multiple orbitals of Ag d and O p, while Sr(VO3)2 has a broad VB associated with oxygen non-bonding states. We have experimentally demonstrated that these vanadates efficiently oxidize water to O2 under irradiation of visible light in the presence of the sacrificial agent.

The surface electronic structure and CO-oxidation activity of Pt and Pt alloys, Pt3T (T = Ti, Hf, Ta, Pt), were investigated. At temperatures below 538 K, the CO-oxidation activities of Pt and Pt3T increased in the order Pt < Pt3Ti < Pt3hHf < Pt3Ta. The center-of-gravity of the Pt d-band (the d-band center) of Pt and Pt3T was theoretically calculated to follow the trend Pt3Ti < Pt3Ta < Pt3Hf < Pt. The CO-oxidation activity showed a volcano-type dependence on the d-band center, where Pt3Ta exhibited a maximum in activity. Theoretical calculations demonstrated that the adsorption energy of CO on the catalyst surface monotonically decreases with the lowering of the d-band center because of diminished hybridization of the surface d-band and the lowest-unoccupied molecular orbital (LUMO) of CO. The observed volcano-type correlation between the d-band center and the CO oxidation activity is rationalized in terms of the CO adsorption energy, which counterbalances the surface coverage by CO and the rate of CO oxidation.

Atomically ordered nickel carbide, Ni3C, was synthesized by reduction of nickel cyclopentadienyl (NiCp2) with sodium naphthalide to form Ni clusters coordinated by Cp (Ni–Cp clusters). Ni–Cp clusters were thermally decomposed to Ni3C nanoparticles smaller than 10 nm. The Ni3C nanoparticles showed better performance than Ni nanoparticles and Au nanoparticles in the electrooxidation of sodium borohydride.

Skeletal gold nanocages (Au NCs) are synthesized and coated with TiO2 layers (TiO2–Au NCs). The TiO2–Au NCs exhibit enhanced photodecomposition activity toward acetaldehyde under visible light (>400 nm) illumination because hot electrons are generated over the Au NCs by local surface plasmon resonance (LSPR) and efficiently transported across the metal/semiconductor interface via the defect states of TiO2.

Although compositional tuning of metal nanoparticles (NPs) has been extensively investigated, possible control of the catalytic performance through bulk-structure tuning is surprisingly overlooked. Here we report that the bulk structure of intermetallic ZrPt3 NPs can be engineered by controlled annealing and their catalytic performance is significantly enhanced as the result of bulk-structural transformation. Chemical reduction of organometallic precursors yielded the desired ZrPt3 NPs with a cubic FCC-type structure (c-ZrPt3 NPs). The c-ZrPt3 NPs were then transformed to a different phase of ZrPt3 with a hexagonal structure (h-ZrPt3 NPs) by annealing at temperatures between 900 and 1000 °C. The h-ZrPt3 NPs exhibited higher catalytic activity and long-term stability than either the c-ZrPt3 NPs or commercial Pt/C NPs toward the electro-oxidation of ethanol. Theoretical calculations have elucidated that the enhanced activity of the h-ZrPt3 NPs is attributed to the increased surface energy, whereas the stability of the catalyst is retained by the lowered bulk-free-energy.Keywords: bulk structural transformation; catalysts; intermetallic compounds; polymer membrane electrolyte fuel cells; surface energy

A mixed-valence tin oxide, (Sn2+)2(Sn4+)O4, was synthesized via a hydrothermal route. The Sn3O4 material consisted of highly crystalline {110} flexes. The Sn3O4 material, when pure platinum (Pt) was used as a co-catalyst, significantly catalyzed water-splitting in aqueous solution under illumination of visible light (λ > 400 nm), whereas neither Sn2+O nor Sn4+O2 was active toward the reaction. Theoretical calculations have demonstrated that the co-existence of Sn2+ and Sn4+ in Sn3O4 leads to a desirable band structure for photocatalytic hydrogen evolution from water solution. Sn3O4 has great potential as an abundant, cheap, and environmentally benign solar-energy conversion catalyst.Keywords: mixed valence; photocatalyst; tin oxide; visible light; water splitting;

The global environment has been variously compromised leading to problems such as global warming and radioactive contamination. In this feature article, we will focus especially on materials for environmental remediation based on the concept of materials nanoarchitectonics. The topics are classified into three categories: removal and degradation of toxic substances including waste due to fossil fuel usage and organic pollutants (continuously arising problems), current emerging topics concerning oil spills and nuclear waste (current urgent problems), and advanced methods based on supramolecular chemistry and nanotechnology (including breakthroughs for future development).

Intermetallic Pt3Ti nanoparticles are solubilized in water by using a generation-five, hydroxyl-terminated, poly(amidoamine) dendrimer, G5OH, as a post-synthesis surfactant. Pt3Ti nanoparticles, encapsulated in G5OH and dispersed over the electrode surface, exhibited a superior catalytic activity toward the electro-reduction reaction of oxygen compared to as-prepared, highly agglomerated nanoparticles.

The influence of pH on the dendritic structure of strongly fluorescent ammonium persulfate (APS)-treated poly(amidoamine) (PAMAM) dendrimers was examined. The APS-treated PAMAM dendrimers were prepared by aging of 0.2 mM aqueous solutions of a hydroxyl groups-terminated, generation-five PAMAM (G5OH) dendrimer together with 200 μl of 0.1 M APS solutions. The G5OH dendrimer showed an absorbance at 280 nm, which was red-shifted to 360 nm after APS treatment. The APS-treated G5OH dendrimer solutions emitted much stronger fluorescence than the pristine G5OH dendrimer solutions when irradiated at 360 nm. The pH of the G5OH dendrimer solution is 7.6 while that of the APS-treated G5OH dendrimer solution is 5.2. The pore surface of both pristine and fluorescent G5OH dendrimers was altered more significantly under acidic than basic conditions. The tertiary amine groups of the pore surface of the fluorescent G5OH dendrimers were protonated by APS treatment as well as under the acidic condition; therefore, the pore surface of the G5OH dendrimer was filled with tertiary ammonium cations, which were, however, further deprotonated under basic conditions. The sulfur anions (VI and II) were generated during the hydrolysis of APS, which were interacted with the G5OH dendrimers under both acidic and basic conditions.Graphical abstractAmmoniumpersulfate (APS)-treated, hydroxyl groups-terminated generation-five poly(amidoamine) (G5OH) dendrimer solutions emitted much stronger fluorescence than the pristine G5OH dendrimer solutions when irradiated at 360 nm. The pH of the pristine G5OH dendrimer solution is 7.6, while that of the APS-treated G5OH dendrimer solution is 5.2. The pore surface of both pristine and fluorescent G5OH dendrimers was altered more significantly under acidic than basic conditions. The tertiary amine groups of the pore surface of the fluorescent G5OH dendrimers were protonated by APS treatment as well as under the acidic condition. Thus, the pore surface of the APS-treated G5OH dendrimer was filled with tertiary ammonium cations, which were, however, further deprotonated under basic conditions.Highlights► The aging of poly(amidoamine) (PAMAM) dendrimer in the presence of a persulfate solution emitted much stronger fluorescence than the pristine dendrimer. ► The persulfate-treated PAMAM dendrimer showed a larger red shift in wavelength. ► The pore surface of persulfate-treated PAMAM dendrimer contains tertiary ammonium cations. ► Pristine PAMAM dendrimer were solely filled with tertiary amine groups on the pore surface. ► The pore surface of both pristine and fluorescent PAMAM dendrimers was altered more significantly under acidic than basic conditions.

Crystal structure of a series of mixed-metal oxides, T2Mo3O8 (T=Mg, Co, Zn and Mn; P63mc; a=5.7628(1) Å, c=9.8770(3) Å for Mg2Mo3O8; a=5.7693(3) Å, c=9.9070(7) Å for Co2Mo3O8; a=5.7835(2) Å, c=9.8996(5) Å for Zn2Mo3O8; a=5.8003(2) Å, c=10.2425(5) Å for Mn2Mo3O8) was investigated by X-ray diffraction on single crystals. Structural analysis, magnetization measurements, X-ray photoemission spectroscopy and cyclic voltammetry showed that the Mn ions at the tetrahedral and octahedral sites in Mn2Mo3O8 adopt different valences of +2 and 2+δ (δ>0), respectively. The formal valence of the Mo3 in Mn2Mo3O8 is 12−δ to retain electric neutrality of the compound. In contrast, the T ions and Mo3 in T2Mo3O8 (T=Mg, Co and Zn) adopt the valences of +2 and +12, respectively.Trinuclear Mo3 clusters in Mn2Mo3O8 adopt an anomalous valence of 12−≅ (≅>0) unlike the Mo312+ clusters that are usually recognized for Mo3-containing inorganic compounds including T2Mo3O8 (T=Mg, Co or Zn).

A platinum-based intermetallic phase with an early d-metal, Pt3Ti, has been synthesized in the form of nanoparticles (NPs) dispersed on silica (SiO2) supports. The organometallic Pt and Ti precursors, Pt(1,5-cyclooctadiene)Cl2 and TiCl4(tetrahydrofuran)2, were mixed with SiO2 and reduced by sodium naphthalide in tetrahydrofuran. Stoichiometric Pt3Ti NPs with an average particle size of 2.5 nm were formed on SiO2 (particle size: 20−200 nm) with an atomically disordered FCC-type structure (Fm3̅m; a = 0.39 nm). A high dispersivity of Pt3Ti NPs was achieved by adding excessive amounts of SiO2 relative to the Pt precursor. A 50-fold excess of SiO2 resulted in finely dispersed, SiO2-supported Pt3Ti NPs that contained 0.5 wt % Pt. The SiO2-supported Pt3Ti NPs showed a lower onset temperature of catalysis by 75 °C toward the oxidation reaction of CO than did SiO2-supported pure Pt NPs with the same particle size and Pt fraction, 0.5 wt %. The SiO2-supported Pt3Ti NPs also showed higher CO conversion than SiO2-supported pure Pt NPs even containing a 2-fold higher weight fraction of Pt. The SiO2-supported Pt3Ti NPs retained their stoichiometric composition after catalytic oxidation of CO at elevated temperatures, 325 °C. Pt3Ti NPs show promise as a catalytic center of purification catalysts for automobile exhaust due to their high catalytic activity toward CO oxidation with a low content of precious metals.

The development of a disposal technique for the radiotoxic 137Cs in nuclear wastes is one of the most urgent issues in nuclear fuel technology. An effective disposal method of 137Cs is to immobilize it in a synthetic rock (SYNROC) material: cesium titanate hollandite, 137CsxTi8O16 (I4/m  , a=10.2866(3)Å, c=2.9669(1)Å). Practical applications of 137CsxTi8O16 have been restricted so far because the conventional synthetic method requires strong chemical reducers and reaction temperatures higher than 1250 °C. In this report, we present a milder preparation method of CsxTi8O16 by electrolysis of a mixture of Cs2MoO4 and TiO2 in ambient atmosphere at 900 °C. The Cs content in the resultant single-crystalline Cs1.35Ti8O16 is competitive with the highest value in polycrystalline Cs1.36±0.03Ti8O16 prepared by the conventional synthetic method. The electrochemical preparation of Cs1.35Ti8O16 is a promising way to immobilize a high quantity of 137Cs ions in a stable form of single-crystalline SYNROC.Single crystals of cesium titanate hollandite, Cs1.35Ti8O16, were prepared electrochemically in molten MoO3 flux. The electrochemical preparation of Cs1.35Ti8O16 is applicable to the practical disposal of the radiotoxic 137Cs in a highly stable form of single-crystalline SYNROC material.

A copper-and-zinc (Cu–Zn) alloy material was synthesized using a vacuum sealing method, in which evaporated zinc was reacted with copper film or nanoparticles to form a homogeneous Cu–Zn alloy. This alloy was evaluated as an electrocatalyst and/or cocatalyst for photocatalysis to selectively reduce carbon dioxide to formic acid. Based on the optimised alloy composition, the Cu5Zn8 catalyst exhibited efficient electrochemical CO2 reduction. Furthermore, we constructed a photoelectrochemical (PEC) three-electrode system, in which the Cu5Zn8 film functioned as the cathode for CO2 reduction in the dark and strontium titanate (SrTiO3) served as the photoanode for water oxidation. The PEC system also selectively reduced CO2 to formic acid with a faradaic efficiency of 79.11% under UV-light and the absence of an applied bias potential. SrTiO3 particles decorated with nanoparticles of the Cu–Zn alloy also photocatalytically reduced CO2 to formic acid under UV-light. Isotope trace analysis demonstrated that water served as the electron donor to produce oxygen and organic molecules under UV light, similar to photosynthesis in plants. The Cu–Zn alloy material developed in the present study is composed of ubiquitous and safe materials, and can catalyse CO2 conversion by means of various kinds of renewable energies.

Nanocrystals of sodium antimony oxide, NaSb3O7 (pyrochlore structure, a = 1.030 nm), act as an efficient catalyst support for the electrocatalytic oxygen-reduction reaction (ORR) in acidic media. The NaSb3O7 nanocrystals (edge length ∼ 150 nm) were synthesized by hydrothermal decomposition of SbCl5 in aqueous solution of NaOH. The NaSb3O7 nanocrystals were then decorated with Pt nanoparticles by chemical reduction of H2PtCl6 in water to yield an ORR catalyst, Pt/NaSb3O7. The Pt/NaSb3O7 exhibited higher ORR performance than the state-of-the-art Pt/TiO2- or Pt/C catalysts in terms of the +40 mV higher half-wave reduction potential and the retained electrochemical surface area than the Pt/TiO2 after 10000-times repeated ORR in an acidic electrolyte. Unlike NaSb3O7, Pt-decorated Sb2O5 (Pt/Sb2O5) was much less active than the Pt/TiO2 or Pt/C. The enhanced ORR activity of the Pt/NaSb3O7 may be attributed to the promoted electron hopping between the Sb3+ and Sb5+ ions in mixed-valence Na1+(Sb3+Sb25+)O7, which is inhibited in single-valence Sb25+O5.

Atomically ordered nickel carbide, Ni3C, was synthesized by reduction of nickel cyclopentadienyl (NiCp2) with sodium naphthalide to form Ni clusters coordinated by Cp (Ni–Cp clusters). Ni–Cp clusters were thermally decomposed to Ni3C nanoparticles smaller than 10 nm. The Ni3C nanoparticles showed better performance than Ni nanoparticles and Au nanoparticles in the electrooxidation of sodium borohydride.

Skeletal gold nanocages (Au NCs) are synthesized and coated with TiO2 layers (TiO2–Au NCs). The TiO2–Au NCs exhibit enhanced photodecomposition activity toward acetaldehyde under visible light (>400 nm) illumination because hot electrons are generated over the Au NCs by local surface plasmon resonance (LSPR) and efficiently transported across the metal/semiconductor interface via the defect states of TiO2.

Intermetallic Pt3Ti nanoparticles are solubilized in water by using a generation-five, hydroxyl-terminated, poly(amidoamine) dendrimer, G5OH, as a post-synthesis surfactant. Pt3Ti nanoparticles, encapsulated in G5OH and dispersed over the electrode surface, exhibited a superior catalytic activity toward the electro-reduction reaction of oxygen compared to as-prepared, highly agglomerated nanoparticles.

Naturally occurring clay nanotubes, halloysite (Al2Si2O5(OH)4·2H2O), with exterior and interior surfaces, respectively, composed of SiOx and AlOx layers, act as an agglomeration-tolerant exhaust catalyst when copper–nickel alloy nanoparticles (Cu–Ni NPs, 2–3 nm) are immobilized at the AlOx interior. Co-reduction of Cu2+ and Ni2+ (respectively derived from CuCl2 and NiCl2) in the presence of sodium citrate (Na3C6H5O7·2H2O) and halloysite yielded the required nanocomposite, Cu–Ni@halloysite. Cu–Ni@halloysite efficiently catalyzes the purification of simulated motor vehicle exhaust comprising nitrogen monoxide (NO) and carbon monoxide (CO) near the activation temperature of Pt-based exhaust catalysts, ≤400 °C, showing its potential as an alternative to Pt-based catalysts. In contrast, a different halloysite nanocomposite with the SiOx exterior decorated with Cu–Ni NPs, Cu–Ni/halloysite, is poorly active even at >400 °C because of particle agglomeration. The enhanced exhaust-purification activity of Cu–Ni@halloysite can ultimately be attributed to the topology of the material, where the alloy NPs are immobilized at the tubular AlOx interior and protected from particle agglomeration by the tubular form and SiOx exterior.

Two vanadates, Ag2Sr(VO3)4 and Sr(VO3)2, have been studied as visible-light-driven water oxidation photocatalysts with the help of density-functional theory calculations. Our computational results for the density of states and partial charge densities implied that Ag2Sr(VO3)4 and Sr(VO3)2 possess desirable electronic structures for the water oxidation reaction, i.e., the valence band (VB) maximum of Ag2Sr(VO3)4 consists of multiple orbitals of Ag d and O p, while Sr(VO3)2 has a broad VB associated with oxygen non-bonding states. We have experimentally demonstrated that these vanadates efficiently oxidize water to O2 under irradiation of visible light in the presence of the sacrificial agent.

Tin-dioxide nanofacets (SnO2 NFs) are crystal-engineered so that oxygen defects on the maximal {113} surface are long-range ordered to give rise to a non-occupied defect band (DB) in the bandgap. SnO2 NFs-supported platinum-nanoparticles exhibit an enhanced ethanol-electrooxidation activity due to the promoted charge-transport via the DB at the metal–semiconductor interface.

The surface electronic structure and CO-oxidation activity of Pt and Pt alloys, Pt3T (T = Ti, Hf, Ta, Pt), were investigated. At temperatures below 538 K, the CO-oxidation activities of Pt and Pt3T increased in the order Pt < Pt3Ti < Pt3hHf < Pt3Ta. The center-of-gravity of the Pt d-band (the d-band center) of Pt and Pt3T was theoretically calculated to follow the trend Pt3Ti < Pt3Ta < Pt3Hf < Pt. The CO-oxidation activity showed a volcano-type dependence on the d-band center, where Pt3Ta exhibited a maximum in activity. Theoretical calculations demonstrated that the adsorption energy of CO on the catalyst surface monotonically decreases with the lowering of the d-band center because of diminished hybridization of the surface d-band and the lowest-unoccupied molecular orbital (LUMO) of CO. The observed volcano-type correlation between the d-band center and the CO oxidation activity is rationalized in terms of the CO adsorption energy, which counterbalances the surface coverage by CO and the rate of CO oxidation.