Comprehensive Experimental and Theoretical Study of the CO + NO Reaction Catalyzed by Au/Ni Nanoparticles

Insight into the mechanism of direct N-C coupling in selective catalytic reduction of NO by CO over Ni(111)-supported graphene

Selective catalytic reduction (SCR) of NO using CO as a reducing agent is a straightforward and promising approach to the simultaneous removal of NO and CO. Herein, a novel mechanism of N-C direct coupling of gaseous NO and CO into ONCO and subsequent hydrogenation of *ONCO to nitrogen-containing compounds over Ni(111)-supported graphene ((Gr/Ni(111)) is reported. The results indicate that Gr/Ni(111) can not only trigger direct N-C coupling of NO and CO to form ONCO with a low activation energy barrier of 0.11 eV, but also enable the key intermediate of *ONCO to be stable. The *ONCO chemisorbed on Gr/Ni(111) exhibits negative univalent [ONCO]- and is more stable than neutral ONCO. The hydrogenation pathways show that HNCO preferably forms through a kinetically favorable initial N-C coupling due to the lowest free-energy barrier of 0.18 eV, while NH2CH3 is a considerably competitive product because its free-energy barrier is only 0.20 eV higher than that of HNCO. Our results provide a fundamental insight into the novel reaction mechanism of the SCR of NO and also suggest that nickel-supported graphene is a potential and high-efficient catalyst for eliminating CO and NO harmful gases.

Insight into the Mechanism of Selective Catalytic Reduction of NO by CO over a Bimetallic IrRu/ZSM-5 Catalyst in the Absence/Presence of O2 by Isotopic C13O Tracing Methods

The development of efficient catalysts for the selective catalytic reduction of NO by CO (CO-SCR) in the presence of O₂ is highly desirable for controlling the emission of toxic gases from tailpipes. Here, a bimetallic IrRu/ZSM-5 catalyst was prepared for the selective catalytic reduction of NO by CO in the presence of O₂ (5%) for the low-temperature treatment of exhaust gas. IrRu/ZSM-5 afforded 90% NO_x conversion in the range of 225-250 °C and maintained 90% NO_x conversion after 12 h of reaction. Ru addition inhibited agglomeration of the Ir particles during the reduction process and provided more active sites for NO adsorption. Isotopic C¹³O tracing and in situ diffuse reflectance infrared Fourier-transform spectroscopy experiments were used to elucidate the CO-SCR mechanism in the absence or presence of O₂. NCO could easily form on the surface of catalysts in the absence of O₂, whereas NCO formation has been inhibited owing to the quick consumption of CO in the presence of O₂. Moreover, some byproducts such as N₂O and NO₂ are generated in the presence of O₂. Finally, a possible mechanism for CO-SCR under different conditions was proposed based on in situ experiments and physicochemical analyses.

Nickel-Laden Dendritic Plasmonic Colloidosomes of Black Gold: Forced Plasmon Mediated Photocatalytic CO2 Hydrogenation

In this work, we have designed and synthesized nickel-laden dendritic plasmonic colloidosomes of Au (black gold-Ni). The photocatalytic CO₂ hydrogenation activities of black gold-Ni increased dramatically to the extent that measurable photoactivity was only observed with the black gold-Ni catalyst, with a very high photocatalytic CO production rate (2464 ± 40 mmol g_Ni^-1 h^-1) and 95% selectivity. Notably, the reaction was carried out in a flow reactor at low temperature and atmospheric pressure without external heating. The catalyst was stable for at least 100 h. Ultrafast transient absorption spectroscopy studies indicated indirect hot-electron transfer from the black gold to Ni in less than 100 fs, corroborated by a reduction in Au-plasmon electron-phonon lifetime and a bleach signal associated with Ni d-band filling. Photocatalytic reaction rates on excited black gold-Ni showed a superlinear power law dependence on the light intensity, with a power law exponent of 5.6, while photocatalytic quantum efficiencies increased with an increase in light intensity and reaction temperature, which indicated the hot-electron-mediated mechanism. The kinetic isotope effect (KIE) in light (1.91) was higher than that in the dark (∼1), which further indicated the electron-driven plasmonic CO₂ hydrogenation. Black gold-Ni catalyzed CO₂ hydrogenation in the presence of an electron-accepting molecule, methyl-p-benzoquinone, reduced the CO production rate, asserting the hot-electron-mediated mechanism. Operando diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) showed that CO₂ hydrogenation took place by a direct dissociation path via linearly bonded Ni-CO intermediates. The outstanding catalytic performance of black gold-Ni may provide a way to develop plasmonic catalysts for CO₂ reduction and other catalytic processes using black gold.

Single atom alloys vs. phase separated alloys in Cu, Ag, and Au atoms with Ni(111) and Ni, Pd, and Pt atoms with Cu(111): a theoretical exploration

A single-atom alloy (SAA) consisting of an abundant metal host and a precious metal guest is a promising catalyst to reduce the cost without a loss of activity. DFT calculations of Ni- and Cu-based alloys nX/M(111) (X = Cu, Ag, or Au for M = Ni; X = Ni, Pd, or Pt for M = Cu; n = 1-4) reveal that a phase-separated alloy (PSA) is produced by Cu atoms with Ni(111) but an SAA is produced by Au atoms with Ni(111) and Pd and Pt atoms with Cu(111). In the Ni(111)-based Ag alloy and Cu(111)-based Ni alloy, the relative stabilities of the SAA and PSA depend on coverages of Ag on Ni(111) and Ni on Cu(111). The interaction energy (E_int) between the X_n cluster and M(111) host is larger than that between one X atom and the M(111) host, because the X_n cluster forms more bonding interactions with the M(111) host than does one X atom. When going from one X atom to the X₄ cluster, the E_int values of Au and Pt clusters respectively with Ni(111) and Cu(111) increase to a lesser extent than those of Cu and Ni clusters respectively with Ni(111) and Cu(111). Consequently, Au and Pt atoms tend to form SAAs respectively with Ni(111) and Cu(111) hosts compared to Cu and Ni atoms. This trend in the E_int value is determined by the valence orbital energies of the X atom and the X_n cluster. Cu atoms in nCu/Ni(111) have a slightly positive charge but Ag atoms in nAg/Ni(111), Au atoms in nAu/Ni(111), and Ni, Pd, and Pt atoms in nX/Cu(111) (X = Ni, Pd, or Pt) have a negative charge. The negative charge increases in the order Ag < Au in nX/Ni(111) and Ni < Pd < Pt in nX/Cu(111). The Fermi level decreases in energy in the order nCu/Ni(111) ≥ Ni(111) > nAg/Ni(111) > nAu/Ni(111) and Cu(111) ≥ nNi/Cu(111) > nPd/Cu(111) > nPt/Cu(111). The d valence band center decreases in energy in almost the same order. The CO adsorption energy decreases in the order Ni(111) ∼ nCu/Ni(111) > nAg/Ni(111) ∼ nAu/Ni(111) and Cu(111) > nNi/Cu(111) > nPd/Cu(111) > nPt/Cu(111). These properties are explained based on the electronic structures.

Tracking the redox reaction-induced reconstruction of NiAu nanoparticles via environmental scanning transmission electron microscopy

Atmosphere-related atom migration and phase reconstruction are an easy way for optimizing the catalytic activity of a bimetallic catalyst. Herein, the structure evolutions of NiAu nanoparticles under oxidative and reductive environments are investigated via combining identical location and in situ environmental scanning transmission electron microscopy. During oxidation, a NiO layer first forms and the redispersion of Ni and Au atoms yields a Ni@Au@NiO multi-shell structure at 350 °C. Further, Ni and Au segregate into an Au-NiO hybrid structure at 600 °C. During reduction, Au atoms disperse over the particle surface forming a NiAu alloy shell with scattered Au atoms/clusters. In situ observation further discloses that the reduction changes the local structural ordering from Ni₃Au to NiAu alloy. Very interestingly, the reduced NiAu exhibits promoted activity over oxidized ones for the CO-NO reaction. Density functional theory calculations further reveal the structure-property relationships of CO, NO, and O adsorbates on NiAu alloy surfaces. This study is beneficial for understanding the atmosphere-related evolution behaviors of bimetallic systems, thereby inspiring the catalytic surface optimization.

Nano-Sized NiO Immobilized on Au/CNT for Benzyl Alcohol Oxidation: Influences of Hybrid Structure and Interface

Tiny gold nanoparticles were successfully anchored on carbon nanotubes (CNT) with NiO decoration by a two-step synthesis. Characterizations suggested that Ni species in an oxidative state preferred to be highly dispersed on CNT. During the synthesis, in situ reduction by NaBH₄ and thermal treatment in oxidation atmosphere were consequently carried out, causing the formation of Au-Ni-O_x interfaces and bimetal hybrid structure depending on the Ni/Au atomic ratios. With an appropriate Ni/Au atomic ratio of 8:1, Ni atoms migrated into the sub-layers of Au particles and induced the lattice contraction of Au particles, whilst a higher Ni/Au atomic ratio led to the accumulation of NiO fractions surrounding Au particles. Both contributed to the well-defined Au-Ni-O_x interface and accelerated reaction rates. Nickel species acted as structure promoters with essential Au-Ni-O_x hybrid structure as well as the active oxygen supplier, accounting for the enhanced activity for benzyl alcohol oxidation. However, the over-layer of unsaturated gold sites easily occured under a high Ni/Au ratio, resulting in a lower reaction rate. With an Au/Ni atomic ratio of 8:1, the specific rate of AuNi₈/CNT reached 185 μmol/g/s at only 50 °C in O₂ at ordinary pressure.

Synthesis of small Ni-core-Au-shell catalytic nanoparticles on TiO2 by galvanic replacement reaction

We report the first preparation of small gold-nickel (AuNi) bimetallic nanoparticles (<5 nm) supported on titania by the method of galvanic replacement reaction (GRR), evidenced by the replacement of Ni atoms by Au atoms according to the stoichiometry of the reaction. We showed that this preparation method allowed not only the control of the gold and nickel contents in the samples, but also the formation of small bimetallic nanoparticles with strained core-shell structures, as revealed by aberration-corrected scanning transmission electron microscopy in combination with energy-dispersive X-ray spectroscopy mapping. The catalytic characterization by the probe reaction of semi-hydrogenation of butadiene showed that the resulting nickel-based nanocatalysts containing a small amount of gold exhibited higher selectivity to butenes than pure nickel catalysts and a high level of activity, closer to that of pure nickel catalysts than to that of pure gold catalysts. These improved catalytic performances could not be explained by a mere structural model of simple core-shell structure of the nanoparticles. Instead, they could come from the incorporation of Ni within the gold surface and/or from surface lattice relaxation and subsurface misfit defects.