High Activity of Au/γ-Fe₂O₃ for CO Oxidation: Effect of Support Crystal Phase in Catalyst Design

Au/γ-Fe₂O₃ and Au/α-Fe₂O₃ catalysts with identical size of Au nanoparticles, chemical state of Au species, and amount of surface OH^– group were prepared. The Au/γ-Fe₂O₃ catalyst exhibited exceptionally high activity, regardless of the heat treatments. The CO-TPR, sequential pulse reaction, and in situ Raman spectra demonstrate that the much higher activity of Au/γ-Fe₂O₃ originated from its higher redox property at low temperature. Systematic study shows that this higher-redox-property-based higher activity could be extended to γ-Fe₂O₃-supported Pt-group metals and to other reactions that follow Mars–Van Krevelen mechanism. This finding may provide a new avenue for catalyst improvement or development by choosing the suitable crystal phase of the oxide support

Ferric Oxide-Supported Pt Subnano Clusters for Preferential Oxidation of CO in H₂‑Rich Gas at Room Temperature

Pt single atoms and small clusters were dispersed on iron oxides by a facile coprecipitation method. These catalysts, with or without calcination at elevated temperatures, show excellent activity and selectivity for preferential oxidation of CO in the H₂-rich gas. They can completely remove CO from H₂-rich gas at a wide temperature range of 20–70 °C, which renders them suitable for low-temperature applications. The reaction followed a mixture of competitive mechanism and a noncompetitive/redox mechanism. The weakened CO adsorption on small Pt clusters and atoms makes the competitive adsorption of O₂ feasible, which ensures a high activity of Pt/Fe catalysts, even calcined at elevated temperature

Identifying Size Effects of Pt as Single Atoms and Nanoparticles Supported on FeO_<i>x</i> for the Water-Gas Shift Reaction

Identification of size effects at an atomic level is essential for designing high-performance metal-based catalysts. Here, the performance of a series of FeO_<i>x</i>-supported Pt catalysts with Pt as nanoparticles (Pt-NP) or single atoms (Pt-SAC) are compared for the low-temperature water-gas shift (WGS) reaction. A variety of characterization methods such as adsorption microcalorimetry, H₂-TPR, in situ DRIFTS, and transient analysis of product tests were used to demonstrate that Pt nanoparticles exhibit much higher adsorption strength of CO; the adsorbed CO reacts with the OH groups, which are generated from activated H₂O, to form intermediate formates that subsequently decompose to produce CO₂ and H₂ simultaneously. On the other hand, Pt single atoms promote the formation of oxygen vacancies on FeO_<i>x</i> which dissociate H₂O to H₂ and adsorbed O that then combines with the weakly adsorbed CO on these Pt sites to produce CO₂. The activation energy for the WGS reaction decreases with the downsizing of Pt species, and Pt-SAC possesses the lowest value of 33 kJ/mol. As a result, Pt-SAC exhibits 1 order of magnitude higher specific activity in comparison to Pt-NP. With a loading of only 0.05 wt % the Pt-SAC can achieve ∼65% CO conversion at 300 °C, representing one of the most active catalysts reported so far

Titanium-catalyzed hydrosilylation of olefins: A comparison study on Cp₂TiCl₂/Sm and Cp₂TiCl₂/LiAlH₄ catalyst system

<p>Hydrosilylation of olefins catalyzed by Cp₂TiCl₂/Sm (Cp = cyclopentadienyl) under solvent free conditions have been investigated. By using Cp₂TiCl₂/Sm as catalyst system, β-adducts and hydrogenation products were detected. Hydrosilylation of olefins catalyzed by Cp₂TiCl₂/LiAlH₄ under room temperature has also been studied. The influence of TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) on Cp₂TiCl₂/Sm and Cp₂TiCl₂/LiAlH₄, respectively, indicated that hydrosilylation of olefins catalyzed with Cp₂TiCl₂/Sm went through a free radical reaction pathway while a coordination mechanism was applied for Cp₂TiCl₂/LiAlH₄ catalyst system.</p

Highly Active and Sintering-Resistant Pt Clusters Supported on FeO_<i>x</i>–Hydroxyapatite Achieved by Tailoring Strong Metal–Support Interactions

The catalytic performance of supported metal catalysts is closely related to their structure. While Pt-based catalysts are widely used in many catalytic reactions because of their exceptional intrinsic activity, they tend to deactivate in high-temperature reactions, requiring a tedious and expensive regeneration process. The strong metal–support interaction (SMSI) is a promising strategy to improve the stability of supported metal nanoparticles, but often at the price of the activity due to either the coverage of the active sites by support overlay and/or the too-strong metal–support bonding. Herein, we newly constructed a supported Pt cluster catalyst by introducing FeOx into hydroxyapatite (HAP) support to fine-tune the SMSIs. The catalyst exhibited not only high catalytic activity but also sintering resistance, without deactivation in a 100 h test for catalytic CO oxidation. Detailed characterizations reveal that FeOx introduced into HAP weaken the strong covalent metal–support interaction (CMSI) between Pt and FeOx while simultaneously inhibiting the oxidative strong metal–support interaction (OMSI) between Pt and HAP, giving rise to both high activity and thermal stability of the supported Pt clusters

Strong Metal–Support Interactions between Gold Nanoparticles and Nonoxides

The strong metal–support interaction (SMSI) is of great importance for supported catalysts in heterogeneous catalysis. We report the first example of SMSI between Au nanoparticles (NPs) and hydroxyapatite (HAP), a nonoxide. The reversible encapsulation of Au NPs by HAP support, electron transfer, and changes in CO adsorption are identical to the classic SMSI except that the SMSI of Au/HAP occurred under oxidative condition; the opposite condition for the classical SMSI. The SMSI of Au/HAP not only enhanced the sintering resistance of Au NPs upon calcination but also improved their selectivity and reusability in liquid-phase reaction. It was found that the SMSI between Au and HAP is general and could be extended to other phosphate-supported Au systems such as Au/LaPO₄. This new discovery may open a new way to design and develop highly stable supported Au catalysts with controllable activity and selectivity

Decoration of Gold and Platinum Nanoparticle Catalysts by 1 nm Thick Metal Oxide Overlayer and Its Effect on the CO Oxidation Activity

Exfoliated M–Al layered double hydroxide (M–Al LDH; M = Mg, Co, Ni, and Zn) nanosheets were adsorbed on Au/SiO2 and calcined to transform LDH into mixed metal oxides (MMOs) and yield Au/SiO2 coated with a thin MMO overlayer. These catalysts showed a higher catalytic activity than pristine Au/SiO2. In particular, the 50% CO conversion temperature decreased by more than 250 °C for Co–Al MMO-coated Au/SiO2. In contrast, the deposition of CoAlOx on Au/SiO2 by impregnation or the deposition of Au on Co–Al MMO-coated SiO2 resulted in a worse catalytic activity. Moreover, the presence of a thick MMO overlayer decreased the catalytic activity, suggesting that the control of the overlayer thickness to less than 1 nm is a requisite for obtaining a high catalytic activity. Moreover, the thin Co–Al MMO overlayer on Au/SiO2 possessed abundant oxygen vacancies, which would play an important role in O2 activation, resulting in a highly active interface between Au and the defect-rich MMO on the Au NP surface. Finally, this can be applied to Pt/SiO2, and the obtained Co–Al MMO-coated Pt/SiO2 also exhibited a much improved catalytic activity for CO oxidation

Highly Efficient Catalysis of Preferential Oxidation of CO in H₂‑Rich Stream by Gold Single-Atom Catalysts

Preferential oxidation of CO (PROX) in H₂-rich stream is critical to the production of clean H₂ for the H₂-based fuel cells, which provide clean and efficient energy conversion. Development of highly active and selective PROX catalysts is highly desirable but proved to be extremely challenging. Here we report that CeO₂-supported Au single atoms (Au₁/CeO₂) are highly active, selective, and extremely stable for PROX at the PEMFC working temperature (∼80 °C) with >99.5% CO conversion over a wide temperature window, 70–120 °C (or 50–100 °C, depending on the Au loading). The high CO conversion realized at high temperatures is attributed to the unique property of single-atom catalysts that is unable to dissociatively adsorb H₂ and thus has a low reactivity toward H₂ oxidation. This strategy is proven in general and can be extended to other oxide-supported Au atoms (e.g., Au₁/FeO_<i>x</i>), which may open a new window for the efficient catalysis of the PROX reaction

Supported Single Pt₁/Au₁ Atoms for Methanol Steam Reforming

The single Pt₁ and Au₁ atoms stabilized by lattice oxygen on ZnO­{1010} surface for methanol steam reforming is reported. Density functional theory calculations reveal that the catalysis of the single precious metal atoms together with coordinated lattice oxygen stems from its stronger binding toward the intermediates, lowering reaction barriers, changing on the reaction pathway, enhancing greatly the activity. The measured turnover frequency of single Pt₁ sites was more than 1000 times higher than the pristine ZnO. The results provide valuable insights for the catalysis of the atomically dispersed precious metals on oxide supports

Remarkable Performance of Ir₁/FeO_<i>x</i> Single-Atom Catalyst in Water Gas Shift Reaction

High specific activity and cost effectiveness of single-atom catalysts hold practical value for water gas shift (WGS) reaction toward hydrogen energy. We reported the preparation and characterization of Ir single atoms supported on FeO_<i>x</i> (Ir₁/FeO_<i>x</i>) catalysts, the activity of which is 1 order of magnitude higher than its cluster or nanoparticle counterparts and is even higher than those of the most active Au- or Pt-based catalysts. Extensive studies reveal that the single atoms accounted for ∼70% of the total activity of catalysts containing single atoms, subnano clusters, and nanoparticles, thus serving as the most important active sites. The Ir single atoms seem to greatly enhance the reducibility of the FeO_<i>x</i> support and generation of oxygen vacancies, leading to the excellent performance of the Ir₁/FeO_<i>x</i> single-atom catalyst. The results have broad implications on designing supported metal catalysts with better performance and lower cost