Reaction mechanism of preferential oxidation of carbon monoxide on Pt, Fe, and Pt–Fe/mordenite catalysts

Confinement chemistry of FeOx centers for activating molecular oxygen under ambient conditions

Activating molecular oxygen under mild conditions is highly important for developing advanced green technologies and for understanding the origin and running of life as well, which still remains a challenge. In this work, we report on the confinement chemistry for activating molecular oxygen over oxides under mild conditions by presenting the synthesis and characterization of FeO_x species confined to the pores of support CeO₂ nanospheres. Active catalytic materials are obtained by a controllable three-step method via the formation of porous CeO₂ nanospheres that have an average diameter of 120 nm and exhibit a large surface area of 168 m² g^-1 and a pore size of 18.7 nm, confining FeO_x in intimate contact with ultra-small Pt particles in pores. The optimized PtO_y-FeO_x/CeO₂-H catalyst showed an excellent performance in the preferential oxidation of CO reactions, as featured by 100% CO conversion at room temperature with almost no attenuation in a prolonged operation, which could not be accessible without pore-confined FeO_x centers. Mechanical studies prove that the reaction progresses via abnormal non-competitive adsorption associated with synergistic roles from uniform loading, stabilization of divalent Fe species, surface oxygen activation on CeO₂ supports, and the reduced H₂ spillover effect on Pt⁰, making the CO species adsorbed on Pt^δ+ easier to be desorbed. The methodology demonstrated here may inspire one to explore more advanced catalysts with high activity at room temperature essential for a wide range of applications.

Recent advances in synergistic effect promoted catalysts for preferential oxidation of carbon monoxide

We reviewed recent advances in catalysts for PROX with emphasis on synergistic effects that contribute to enhanced catalytic performance.

Neighboring Pt Atom Sites in an Ultrathin FePt Nanosheet for the Efficient and Highly CO-Tolerant Oxygen Reduction Reaction

Single atom catalyst and ultrathin two-dimensional (2D) nanostructures exhibit improved properties because of the improved exposure of more active atomic sites and optimized electronic structures. However, the oxygen reduction reaction (ORR) in fuel cells via a fast four-electron path usually uses at least two Pt atoms, which cannot be realized in highly isolated single Pt atoms. The synthesis of a densely dispersed single atom catalyst with adjacent atoms accessible at the same time on a matrix with a high surface area provides a feasible way and, however, is challenging. Here, we synthesize ultrathin FePt nanosheets (NSs) with 6.7 wt % neighboring dispersed Pt atoms. Different from the reported isolated Pt single atom catalysts, these ultrathin wrinkled FePt NSs with neighboring Pt sites adopt a four-electron reduction pathway, a high electrochemical active surface area (ECSA) of 545.54 m² g_Pt^-1, and an improved mass activity 7 times as high as Pt/C in the ORR. The improved performance results from the optimal use of neighboring Pt atoms dispersed in a more packed spacing and exposed on the surface of ultrathin sheets. The Pt atoms can interact synergistically to catalyze a fast ORR process. Furthermore, both the experiment and density functional theory (DFT) calculation indicated an outstanding CO-tolerance performance of this catalyst in the ORR.

Improved CO-PROX Performance of CuO/CeO2 Catalysts by Using Nanometric Ceria as Support

Despite of the huge number of papers about the catalytic preferential oxidation of CO (CO-PROX) for the purification of H2 streams, there is still a need for more effective catalysts in order to reduce the large required catalyst volume of CO-PROX unity. In this work, large surface area nanometric ceria was used as support for CuO/CeO2 catalysts with CuO load up to 10 wt % easily dispersed by wet impregnation. Catalysts were characterized by ICP-MS, XRD, SEM/EDS, N2 physisorption, H2 temperature programmed reduction (TPR), and CO2 temperature programmed desorption (TPD) and tested under different reaction conditions (including under feed containing inhibiting species such as CO2 and H2O). Catalytic tests revealed that our samples show high activity and selectivity even under stringent reaction conditions; moreover, they result among the most active catalysts when compared to those reported in the scientific literature. The high activity can be related to the enhanced amount of highly dispersed copper sites in strong interaction with ceria related to the nature of the nanometric support, as evidenced by the characterization techniques. Despite the high concentration of active copper sites, catalytic performance is limited by CO2 desorption from ceria in the neighborhood of copper sites, which prevents a further improvement. This suggests that new catalyst formulations should also provide a lower affinity towards CO2.

Facets Matching of Platinum and Ferric Oxide in Highly Efficient Catalyst Design for Low-Temperature CO Oxidation

Rational design of supported noble metal is of great importance for highly efficient heterogeneous catalysts. On the basis of the distinct adsorption characteristics of noble metal and transition-metal oxides toward O₂ and CO, the overall catalytic performance of CO oxidation reaction could be further modified by controlling the surface property of the materials to achieve optimal adsorption activity. Here, we studied the influence of facets matching between both platinum and ferric oxide support on CO conversion efficiency. It shows that the activities of four catalysts rank following the order of Pt{100}/α-Fe₂O₃{104} > Pt{100}/α-Fe₂O₃{001} > Pt{111}/α-Fe₂O₃{001} > Pt{111}/α-Fe₂O₃{104}. The strong metal-support interaction and adsorption energy varying with matched enclosed surface are demonstrated by density functional theory based on the projected d-band density of states. Compared with the other three cases, the combination of Pt{100} and α-Fe₂O₃{104} successfully weakens CO poisoning and provides proper active sites for O₂ adsorption. It reveals that the facets matching could be a practicable approach to design catalysts with enhanced catalytic performance.

Ligand-Assisted, One-Pot Synthesis of Rh-on-Cu Nanoscale Sea Urchins with High-Density Interfaces for Boosting CO Oxidation

Predictable synthesis of bimetallic nanocrystals with spatially controlled metal distributions offers a versatile route to the development of highly efficient nanocatalysts. Here we report a one-pot synthesis of super branched Rh-on-Cu nanoscale sea urchins (Rh-Cu NSUrs) with a high density of Cu-Rh interfaces by manipulating the ligand coordination chemistry. Structural analysis and UV-vis spectra reveal that ascorbic acid can serve as a Rh-selective coordination ligand in the nonaqueous synthesis to reverse the reduction potentials of Rh³⁺ and Cu²⁺ cations. The sequential reduction of Cu²⁺ and then Rh³⁺ cations, as well as the island epitaxial growth of Rh atoms on Cu cores, leads to the formation of Rh-on-Cu nanostructures mimicking sea urchin. The size of the Cu cores and the density of Rh branches can both be facilely regulated by tuning the mole ratio of Cu to Rh. The Cu-Rh NSUrs show enhanced activity and stability in catalyzing CO oxidation, as the intrinsic Cu-Rh interfaces can act as catalytic hot spots through a bifunctional mechanism. The Cu-Rh two-component system can separate the adsorption and activation of CO and O₂ on the Rh and Cu surfaces, respectively, accelerating the generation of CO₂ at the interfaces.

Unique reaction mechanism of preferential oxidation of CO over intermetallic Pt3Co catalysts: surface-OH-mediated formation of a bicarbonate intermediate

A unique and novel reaction mechanism for the preferential oxidation of CO involving surface-OH-derived bicarbonate as an intermediate is reported.

High catalytic performance of mesoporous zirconia supported nickel catalysts for selective CO methanation

Ni/mesoporous ZrO₂, without precious metals, exhibits high performance for selective CO methanation at a wide range of working temperatures and superior long-term stability.

Interface-confined oxide nanostructures for catalytic oxidation reactions

Heterogeneous catalysts, often consisting of metal nanoparticles supported on high-surface-area oxide solids, are common in industrial chemical reactions. Researchers have increasingly recognized the importance of oxides in heterogeneous catalysts: that they are not just a support to help the dispersion of supported metal nanoparticles, but rather interact with supported metal nanoparticles and affect the catalysis. The critical role of oxides in catalytic reactions can become very prominent when oxides cover metal surfaces forming the inverse catalysts. The source of the catalytic activity in homogeneous catalysts and metalloenzymes is often coordinatively unsaturated (CUS) transition metal (TM) cations, which can undergo facile electron transfer and promote catalytic reactions. Organic ligands and proteins confine these CUS cations, making them highly active and stable. In heterogeneous catalysis, however, confining these highly active CUS centers on an inorganic solid so that they are robust enough to endure the reaction environment while staying flexible enough to perform their catalysis remains a challenge. In this Account, we describe a strategy to confine the active CUS centers on the solid surface at the interface between a TM oxide (TMO) and a noble metal (NM). Among metals, NMs have high electron negativity and low oxygen affinity. This means that TM cations of the oxide bind strongly to NM atoms at the interface, forming oxygen-terminated-bilayer TMO nanostructures. The resulting CUS sites at the edges of the TMO nanostructure are highly active for catalytic oxidation reactions. Meanwhile, the strong interactions between TMOs and NMs prevent further oxidation of the bilayer TMO phases, which would otherwise result in the saturation of oxygen coordination and the deactivation of the CUS cations. We report that we can also tune the oxide-metal interactions to modulate the bonding of reactants with CUS centers, optimizing their catalytic performance. We review our recent progress on oxide-on-metal inverse catalysts, mainly the TMO-on-Pt (TM = Fe, Co, and Ni) systems and discuss the interface-confinement effect, an important factor in the behavior of these catalytic systems. We have studied both model catalyst systems and real supported nanocatalysts. Surface science studies and density functional theory calculations in model systems illustrate the importance of the oxide-metal interfaces in the creation and stabilization of surface active centers, and reveal the reaction mechanism at these active sites. In real catalysts, we describe facile preparation processes for fabricating the oxide-on-metal nanostructures. We have demonstrated excellent performance of the inverse catalysts in oxidation reactions such as CO oxidation. We believe that the interface confinement effect can be employed to design highly efficient novel catalysts and that the inverse oxide-on-metal catalysts may find wide applications in heterogeneous catalysis.

Synergetic effect of surface and subsurface Ni species at Pt-Ni bimetallic catalysts for CO oxidation

Various well-defined Ni-Pt(111) model catalysts are constructed at atomic-level precision under ultra-high-vacuum conditions and characterized by X-ray photoelectron spectroscopy and scanning tunneling microscopy. Subsequent studies of CO oxidation over the surfaces show that a sandwich surface (NiO(1-x)/Pt/Ni/Pt(111)) consisting of both surface Ni oxide nanoislands and subsurface Ni atoms at a Pt(111) surface presents the highest reactivity. A similar sandwich structure has been obtained in supported Pt-Ni nanoparticles via activation in H(2) at an intermediate temperature and established by techniques including acid leaching, inductively coupled plasma, and X-ray adsorption near-edge structure. Among the supported Pt-Ni catalysts studied, the sandwich bimetallic catalysts demonstrate the highest activity to CO oxidation, where 100% CO conversion occurs near room temperature. Both surface science studies of model catalysts and catalytic reaction experiments on supported catalysts illustrate the synergetic effect of the surface and subsurface Ni species on the CO oxidation, in which the surface Ni oxide nanoislands activate O(2), producing atomic O species, while the subsurface Ni atoms further enhance the elementary reaction of CO oxidation with O.