Lefu Yang

Pt-Au alloying at the nanoscale.

The formation of nanosized alloys between a pair of elements, which are largely immiscible in bulk, is examined in the archetypical case of Pt and Au. Element specific resonant high-energy X-ray diffraction experiments coupled to atomic pair distribution functions analysis and computer simulations prove the formation of Pt-Au alloys in particles less than 10 nm in size. In the alloys, Au-Au and Pt-Pt bond lengths differing in 0.1 Å are present leading to extra structural distortions as compared to pure Pt and Au particles. The alloys are found to be stable over a wide range of Pt-Au compositions and temperatures contrary to what current theory predicts. The alloy-type structure of Pt-Au nanoparticles comes along with a high catalytic activity for electrooxidation of methanol making an excellent example of the synergistic effect of alloying at the nanoscale on functional properties.

Atomic-Structural Synergy for Catalytic CO Oxidation over Palladium-Nickel Nanoalloys

Alloying palladium (Pd) with other transition metals at the nanoscale has become an important pathway for preparation of low-cost, highly active and stable catalysts. However, the lack of understanding of how the alloying phase state, chemical composition and atomic-scale structure of the alloys at the nanoscale influence their catalytic activity impedes the rational design of Pd-nanoalloy catalysts. This work addresses this challenge by a novel approach to investigating the catalytic oxidation of carbon monoxide (CO) over palladium-nickel (PdNi) nanoalloys with well-defined bimetallic composition, which reveals a remarkable maximal catalytic activity at Pd:Ni ratio of ~50:50. Key to understanding the structural-catalytic synergy is the use of high-energy synchrotron X-ray diffraction coupled to atomic pair distribution function (HE-XRD/PDF) analysis to probe the atomic structure of PdNi nanoalloys under controlled thermochemical treatments and CO reaction conditions. Three-dimensional (3D) models of the atomic structure of the nanoalloy particles were generated by reverse Monte Carlo simulations (RMC) guided by the experimental HE-XRD/PDF data. Structural details of the PdNi nanoalloys were extracted from the respective 3D models and compared with the measured catalytic properties. The comparison revealed a strong correlation between the phase state, chemical composition and atomic-scale structure of PdNi nanoalloys and their catalytic activity for CO oxidation. This correlation is further substantiated by analyzing the first atomic neighbor distances and coordination numbers inside the nanoalloy particles and at their surfaces. These findings have provided new insights into the structural synergy of nanoalloy catalysts by controlling the phase state, composition and atomic structure, complementing findings of traditional density functional theory studies.

Role of Support-Nanoalloy Interactions in the Atomic-Scale Structural and Chemical Ordering for Tuning Catalytic Sites

The understanding of the atomic-scale structural and chemical ordering in supported nanosized alloy particles is fundamental for achieving active catalysts by design. This report shows how such knowledge can be obtained by a combination of techniques including X-ray photoelectron spectroscopy and synchrotron radiation based X-ray fine structure absorption spectroscopy and high-energy X-ray diffraction coupled to atomic pair distribution function analysis, and how the support-nanoalloy interaction influences the catalytic activity of ternary nanoalloy (platinum-nickel-cobalt) particles on three different supports: carbon, silica, and titania. The reaction of carbon monoxide with oxygen is employed as a probe to the catalytic activity. The thermochemical processing of this ternary composition, in combination with the different support materials, is demonstrated to be capable of fine-tuning the catalytic activity and stability. The support-nanoalloy interaction is shown to influence structural and chemical ordering in the nanoparticles, leading to support-tunable active sites on the nanoalloys for oxygen activation in the catalytic oxidation of carbon monoxide. A nickel/cobalt-tuned catalytic site on the surface of nanoalloy is revealed for oxygen activation, which differs from the traditional oxygen-activation sites known for oxide-supported noble metal catalysts. The discovery of such support-nanoalloy interaction-enabled oxygen-activation sites introduces a very promising strategy for designing active catalysts in heterogeneous catalysis.

Catalytic and Electrocatalytic Oxidation of Ethanol over Palladium-Based Nanoalloy Catalysts

The control of the nanoscale composition and structure of alloy catalysts plays an important role in heterogeneous catalysis. This paper describes novel findings of an investigation for Pd-based nanoalloy catalysts (PdCo and PdCu) for ethanol oxidation reaction (EOR) in gas phase and alkaline electrolyte. Although the PdCo catalyst exhibits a mass activity similar to Pd, the PdCu catalyst is shown to display a much higher mass activity than Pd for the electrocatalytic EOR in alkaline electrolyte. This finding is consistent with the finding on the surface enrichment of Pd on the alloyed PdCu surface, in contrast to the surface enrichment of Co in the alloyed PdCo surface. The viability of C-C bond cleavage was also probed for the PdCu catalysts in both gas-phase and electrolyte-phase EOR. In the gas-phase reaction, although the catalytic conversion rate for CO2 product is higher over Pd than PdCu, the nanoalloy PdCu catalyst appears to suppress the formation of acetic acid, which is a significant portion of the product in the case of pure Pd catalyst. In the alkaline electrolyte, CO2 was detected from the gas phase above the electrolyte upon acid treatment following the electrolysis, along with traces of aldehyde and acetic acid. An analysis of the electrochemical properties indicates that the oxophilicity of the base metal alloyed with Pd, in addition to the surface enrichment of metals, may have played an important role in the observed difference of the catalytic and electrocatalytic activities. In comparison with Pd alloyed with Co, the results for Pd alloyed with Cu showed a more significant positive shift of the reduction potential of the oxygenated Pd species on the surface. These findings have important implications for further fine-tuning of the Pd nanoalloys in terms of base metal composition toward highly active and selective catalysts for EOR.

Nanoalloy catalysts: structural and catalytic properties

Noble metals alloyed with certain transition metals in the form of a nanoalloy exhibit enhanced catalytic or electrocatalytic activities for various reactions, especially when oxygen activation is involved in the reactions. Recent studies have gained important insights into how the interatomic distances and structures of the nanoalloy catalysts operate synergistically in activating oxygen and maneuvering surface oxygenated species. This article highlights some of these insights into nanoalloy catalysts in which Pt is alloyed with a second and/or third transition metal (M/M′ = Co, Fe, V, Ni, Ir, etc.), for catalytic oxidation of carbon monoxide in the gas phase and electrocatalytic oxygen reduction reaction in fuel cell reaction conditions. One important emphasis is placed on understanding of atomic-scale chemical/structural ordering and coordination in correlation with the catalytic or electrocatalytic properties based on findings from ex and in situ synchrotron X-ray techniques such as high energy X-ray diffraction coupled to atomic pair distribution function and X-ray absorption fine structure spectroscopic analysis. The understanding of the details of active sites of the nanoalloys has significant implications for the design of low-cost, active, and durable catalysts for sustainable energy production and conversion reactions.

Insight into the effect of non-stoichiometric sulfur on a NiMoS hydrodesulfurization catalyst

The findings in this study provide new insight into the NiMoS model, revealing that there is a sulfur dynamic equilibrium between the NiMoS edge and the gas phase. Since the evolution of non-stoichiometric sulfur proceeds rapidly at the initial stage of hydrodesulfurization reaction, the sulfur dynamic equilibrium does not draw so much attention. The results indicate that excess sulfur on the Ni–Mo–S edge can be reduced by hydrogen to form SH groups and release H2S, which lead to a low sulfur-covered NiMoS edge and a significant increase in coordinatively unsaturated sites (CUS), resulting in an outstanding HDS activity. Apart from elucidating the effect of non-stoichiometric sulfur on the NiMoS structure, the relationship among H2S partial pressure, Sx and HDS activity has been quantitatively studied. It is expected that these results will be used to deepen the understanding of HDS reaction over promoted MoS2 catalysts, as well as to guide the research on ultra-deep HDS of fuel oils.

CO oxidation on supported platinum group metal (PGM) based nanoalloys

The oxidation of carbon monoxide is widely investigated for realistic and potential uses in energy production and environmental processes. As a probe reaction to the surface properties, it gives an insight into the relationship between the structure of active phase and catalytic performance. Noble metals alloyed with certain transition metals in the form of a nanoalloy exhibit enhanced catalytic activity for various reactions, especially when simultaneous activation of oxygen and CO is involved. This article highlights some of these insights into nanoalloy catalysts in which platinum group metal (PGM) is alloyed with a second and/or third transition metal (M/M′=Co, Fe, V, Ni, Ir, etc.), for catalytic oxidation of carbon monoxide in a gas phase. Recent studies have provided important insights into how the atomic-scale structures of the nanoalloy catalysts operate synergistically in activating oxygen and maneuvering surface oxygenated species. The exploration of atomic-scale chemical/structural ordering and coordination in correlation with the catalytic oxidation properties based on findings from ex- and in-situ synchrotron X-ray techniques is emphasized; for example, high-energy X-ray diffraction coupled to atomic-pair distribution function and X-ray absorption fine-structure spectroscopic analysis. The understanding of the detailed active sites of the nanoalloys has significant implications for the design of low-cost, active, and durable catalysts for sustainable energy production and environmental processes.

From a Au-rich core/PtNi-rich shell to a Ni-rich core/PtAu-rich shell: an effective thermochemical pathway to nanoengineering catalysts for fuel cells

A major challenge in the design of nanocatalysts containing noble metals is the ability to engineer their relative surface composition and structure so that their catalytic activity and stability can be enhanced with minimum use of the noble metals. We demonstrate here this ability by an effective thermochemical pathway enabling us to control the structural evolution of fuel cell nanocatalysts from an Au-rich core/PtNi-rich shell to a Ni-rich core/PtAu-rich shell. The synthesis starts from the introduction of a third low-cost transition metal (nickel) into AuPt nanoparticles through a facile one-pot synthesis followed by thermochemical and electrochemical treatments. By exploiting the surface free energy differences among Au, Pt and Ni, the as-synthesized Au-rich core/PtNi-rich shell structure is transformed into a Ni-rich core/PtAu-rich shell structure, producing a significant multifunctional synergy in comparison with bimetallic PtAu nanoparticles. The surface enrichment of PtAu with slightly segregated Au, along with shrinking of Pt–Pt distances, is shown to enhance the dehydrogenation of methanol and effectively remove the surface carbonaceous species. The surface Au atoms facilitate maneuvering of the electrons in the oxidation reactions, whereas the positively charged PtAu rich surface resulted from electrochemical treatment enhances the oxidation activity. The mass activity of the nanocatalysts is shown to maximize as a function of Ni doping. The result is further supported by computational analysis of the adsorption energy of methanol on the nanoclusters, revealing that the increased catalytic activity correlates well with the decreased adsorption energy. These findings demonstrate an unprecedented ability to invert the core–shell structure of the as-synthesized nanocatalysts for electrocatalytic enhancement, which has significant implications for the design of noble metal containing nanocatalysts for fuel cells.

Quantitative relationship model between support properties and dibenzothiophene hydrodesulfurization conversion over NiMo/Al2O3

A “property index” has been developed, which is a supergeometric average of the support properties. Based on the relationship between the catalytic reaction conversion and the apparent rate coefficient (k), a quantitative relationship model was established between the support properties and the conversion of dibenzothiophene (DBT) hydrodesulfurization (HDS) reaction by building a correlation between “property index” and the apparent rate coefficient. Moreover, an analysis method about the contribution rate of the property variables to catalytic activity was provided. The model parameters were estimated by fitting a series of the experimental data of the DBT HDS conversion over the catalysts with different support properties. Finally it has been shown that for the proposed model, the total average relative deviation was 1.43% and the prediction deviation was 2.21%.

CaO as a Solid Base Catalyst for Transesterification of Soybean Oil

Three different calcium oxide catalysts were synthesized from different precursors and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and temperature-programmed desorption (TPD). They were used as catalysts in the transesterification of soybean oil (SBO) for the production of fatty acid methyl esters (FAME), namely biodiesel. Calcium oxide from calcite (Cal(N)) showed the highest activity towards the transesterification of SBO. The transesterification activity of CaO was found to be highly related to the basicity of the catalysts. The catalytic activity of CaO greatly decreased when CaO was exposed to CO2. (Raman spectroscopic studies demonstrated that the formation of CaCO3 and Ca(OH)2 on the surface of CaO when CaO was exposed to room air prevented CaO from participating in the transesterification of SBO). The degree of poisoning was highly dependent on the type of precursors with Cal(N) more resistant to CO2 poisoning than CaO from aragonite (Ara(N)). Deactivated CaO catalysts could be partially regenerated. A mechanism was proposed to explain the poisoning and regenerating processes. Furthermore, whether the solid phase of CaO or dissolved CaO was the active species in the transesterification of SBO was also investigated.