Aiqin Wang

Single-atom catalysis of CO oxidation using Pt1/FeOx.

Platinum-based heterogeneous catalysts are critical to many important commercial chemical processes, but their efficiency is extremely low on a per metal atom basis, because only the surface active-site atoms are used. Catalysts with single-atom dispersions are thus highly desirable to maximize atom efficiency, but making them is challenging. Here we report the synthesis of a single-atom catalyst that consists of only isolated single Pt atoms anchored to the surfaces of iron oxide nanocrystallites. This single-atom catalyst has extremely high atom efficiency and shows excellent stability and high activity for both CO oxidation and preferential oxidation of CO in H2. Density functional theory calculations show that the high catalytic activity correlates with the partially vacant 5d orbitals of the positively charged, high-valent Pt atoms, which help to reduce both the CO adsorption energy and the activation barriers for CO oxidation.

Single-Atom Catalysts: A New Frontier in Heterogeneous Catalysis

Supported metal nanostructures are the most widely used type of heterogeneous catalyst in industrial processes. The size of metal particles is a key factor in determining the performance of such catalysts. In particular, because low-coordinated metal atoms often function as the catalytically active sites, the specific activity per metal atom usually increases with decreasing size of the metal particles. However, the surface free energy of metals increases significantly with decreasing particle size, promoting aggregation of small clusters. Using an appropriate support material that strongly interacts with the metal species prevents this aggregation, creating stable, finely dispersed metal clusters with a high catalytic activity, an approach industry has used for a long time. Nevertheless, practical supported metal catalysts are inhomogeneous and usually consist of a mixture of sizes from nanoparticles to subnanometer clusters. Such heterogeneity not only reduces the metal atom efficiency but also frequently leads to undesired side reactions. It also makes it extremely difficult, if not impossible, to uniquely identify and control the active sites of interest. The ultimate small-size limit for metal particles is the single-atom catalyst (SAC), which contains isolated metal atoms singly dispersed on supports. SACs maximize the efficiency of metal atom use, which is particularly important for supported noble metal catalysts. Moreover, with well-defined and uniform single-atom dispersion, SACs offer great potential for achieving high activity and selectivity. In this Account, we highlight recent advances in preparation, characterization, and catalytic performance of SACs, with a focus on single atoms anchored to metal oxides, metal surfaces, and graphene. We discuss experimental and theoretical studies for a variety of reactions, including oxidation, water gas shift, and hydrogenation. We describe advances in understanding the spatial arrangements and electronic properties of single atoms, as well as their interactions with the support. Single metal atoms on support surfaces provide a unique opportunity to tune active sites and optimize the activity, selectivity, and stability of heterogeneous catalysts, offering the potential for applications in a variety of industrial chemical reactions.

Direct Catalytic Conversion of Cellulose into Ethylene Glycol Using Nickel-Promoted Tungsten Carbide Catalysts

Cellulose, the most abundant source of biomass, is currently regarded as a promising alternative for fossil fuels as it cannot be digested by human beings and thus its use, unlike corn and starch, will not impose a negative impact on food supplies. One of the most attractive routes for the reaction of cellulose utilization is its direct conversion into useful organic compounds. A recent example of the catalytic conversion of cellulose has been demonstrated by Fukuoka and Dhepe, who utilized Pt/Al2O3 as an effective catalyst to convert cellulose into sugar alcohols (Scheme 1, Route A). The product sugar alcohols can be used as chemicals in their own right or as new starting materials for the production of fuels, as demonstrated by Dumesic and co-workers. 6] Recently, Luo et al. have studied this process further. In their work, the reaction was conducted at elevated temperatures so that water could generate H ions to catalyze the hydrolysis reactions. The subsequent hydrogenation reaction was catalyzed by Ru/C. An increased sugar alcohol yield was obtained, which was attributed to the higher reaction temperatures and the wellknown high efficiency of Ru/C in the hydrogenation reaction. . A disadvantage of the above two studies is the use of precious-metal catalysts. The amount of precious metals needed for the degradation of cellulose was relatively high, 4– 10 mg per gram of cellulose. This is too expensive for the conversion of large quantities of cellulose, even though the solid catalyst could be reused. Therefore, it is highly desirable to develop a less expensive but efficient catalyst to replace precious-metal catalysts in this cellulose degradation process. The carbides of Groups 4–6 metals show catalytic performances similar to those of platinum-group metals in a variety of reactions involving hydrogen. In our previous work, tungsten and molybdenum carbides were found to exhibit excellent performances in the catalytic decomposition of hydrazine, which were comparable with those of expensive iridium catalysts. Tungsten carbides have been used as electrocatalysts because of their platinum-like catalytic behavior, stability in acidic solutions, and resistance to CO poisoning. 18] However, to the best of our knowledge, there have been no attempts so far to utilize metal carbides as catalysts for cellulose conversion. Herein we report the first observation that carbonsupported tungsten carbide (W2C/AC; AC = activated carbon) can effectively catalyze cellulose conversion into polyols (Scheme 1, Route B). More interestingly, when the catalyst is promoted with a small amount of nickel, the yield of polyols, especially ethylene glycol (EG) and sorbitol, can be significantly increased. These Ni-W2C/AC catalysts showed a remarkably higher selectivity for EG formation than Pt/Al2O3 [4] and Ru/C. After 30 minutes at 518 K and 6 MPa H2, the cellulose could be completely converted into polyols and the yield of EG was as high as 61 wt % with a 2% Ni-30% W2C/AC-973 catalyst. This value is the highest yield reported to date. Currently in the petrochemical industry, EG is mainly produced from ethylene via the intermediate ethylene oxide. The global production of EG in 2007 is estimated to be 17.8 million tonnes, an increase of Scheme 1. Catalytic conversion of cellulose into polyols.

Catalytic Transformation of Lignin for the Production of Chemicals and Fuels.

and Fuels Changzhi Li,† Xiaochen Zhao,† Aiqin Wang,† George W. Huber,†,‡ and Tao Zhang*,† †State Key Laborotary of Catalysis, iChEM (Collaborative Innovation Center of Chemistry for Energy Materials), Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China ‡Department of Chemical and Biological Engineering, University of WisconsinMadison, Madison, Wisconsin 53706, United States

Remarkable performance of Ir1/FeOx 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(x) (Ir1/FeO(x)) 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(x) support and generation of oxygen vacancies, leading to the excellent performance of the Ir1/FeO(x) single-atom catalyst. The results have broad implications on designing supported metal catalysts with better performance and lower cost.

FeOx-supported platinum single-atom and pseudo-single-atom catalysts for chemoselective hydrogenation of functionalized nitroarenes

The catalytic hydrogenation of nitroarenes is an environmentally benign technology for the production of anilines, which are key intermediates for manufacturing agrochemicals, pharmaceuticals and dyes. Most of the precious metal catalysts, however, suffer from low chemoselectivity when one or more reducible groups are present in a nitroarene molecule. Herein we report FeOx-supported platinum single-atom and pseudo-single-atom structures as highly active, chemoselective and reusable catalysts for hydrogenation of a variety of substituted nitroarenes. For hydrogenation of 3-nitrostyrene, the catalyst yields a TOF of ~1,500 h(-1), 20-fold higher than the best result reported in literature, and a selectivity to 3-aminostyrene close to 99%, the best ever achieved over platinum group metals. The superior performance can be attributed to the presence of positively charged platinum centres and the absence of Pt-Pt metallic bonding, both of which favour the preferential adsorption of nitro groups.

One-pot conversion of cellulose to ethylene glycol with multifunctional tungsten-based catalysts.

With diminishing fossil resources and increasing concerns about environmental issues, searching for alternative fuels has gained interest in recent years. Cellulose, as the most abundant nonfood biomass on earth, is a promising renewable feedstock for production of fuels and chemicals. In principle, the ample hydroxyl groups in the structure of cellulose make it an ideal feedstock for the production of industrially important polyols such as ethylene glycol (EG), according to the atom economy rule. However, effectively depolymerizing cellulose under mild conditions presents a challenge, due to the intra- and intermolecular hydrogen bonding network. In addition, control of product selectivity is complicated by the thermal instabilities of cellulose-derived sugars. A one-pot catalytic process that combines hydrolysis of cellulose and hydrogenation/hydrogenolysis of cellulose-derived sugars proves to be an efficient way toward the selective production of polyols from cellulose. In this Account, we describe our efforts toward the one-pot catalytic conversion of cellulose to EG, a typical petroleum-dependent bulk chemical widely applied in the polyester industry whose annual consumption reaches about 20 million metric tons. This reaction opens a novel route for the sustainable production of bulk chemicals from biomass and will greatly decrease the dependence on petroleum resources and the associated CO₂ emission. It has attracted much attention from both industrial and academic societies since we first described the reaction in 2008. The mechanism involves a cascade reaction. First, acid catalyzes the hydrolysis of cellulose to water-soluble oligosaccharides and glucose (R1). Then, oligosaccharides and glucose undergo C-C bond cleavage to form glycolaldehyde with catalysis of tungsten species (R2). Finally, hydrogenation of glycolaldehyde by a transition metal catalyst produces the end product EG (R3). Due to the instabilities of glycolaldehyde and cellulose-derived sugars, the reaction rates should be r₁ << r₂ << r₃ in order to achieve a high yield of EG. Tuning the molar ratio of tungsten to transition metal and changing the reaction temperature successfully optimizes this reaction. No matter what tungsten compounds are used in the beginning reaction, tungsten bronze (HxWO₃) is always formed. It is then partially dissolved in hot water and acts as the active species to homogeneously catalyze C-C bond cleavage of cellulose-derived sugars. Upon cooling and exposure to air, the dissolved HxWO₃ is transformed to insoluble tungsten acid and precipitated from the solution to facilitate the separation and recovery of the catalyst. On the basis of this temperature-dependent phase-transfer behavior, we have developed a highly active, selective, and reusable catalyst composed of tungsten acid and Ru/C. Our work has unearthed new understanding of this reaction, including how different catalysts perform and the underlying mechanism. It has also guided researchers to the rational design of catalysts for other reactions involved in cellulose conversion.

One-pot catalytic hydrocracking of raw woody biomass into chemicals over supported carbide catalysts: simultaneous conversion of cellulose, hemicellulose and lignin.

Using raw lignocellulosic biomass as feedstock for sustainable production of chemicals is of great significance. Herein, we report the direct catalytic conversion of raw woody biomass into two groups of chemicals over a carbon supported Ni-W2C catalyst. The carbohydrate fraction in the woody biomass, i.e., cellulose and hemicellulose, were converted to ethylene glycol and other diols with a total yield of up to 75.6% (based on the amount of cellulose & hemicellulose), while the lignin component was converted selectively into monophenols with a yield of 46.5% (based on lignin). It was found that the chemical compositions and structures of different sources of lignocellulose exerted notable influence on the catalytic activity. The employment of small molecule alcohol as a solvent could increase the yields of phenols due to the high solubilities of lignin and hydrogen. Remarkably, synergistic effect in Ni-W2C/AC existed not only in the conversion of carbohydrate fractions, but also in lignin component degradation. For this reason, the cheap Ni-W2C/AC exhibited competitive activity in comparison with noble metal catalysts for the degradation of the wood lignin. Furthermore, the catalyst could be reused at least three times without the loss of activity. The direct conversion of the untreated lignocellulose drives our technology nearer to large-scale application for cost-efficient production of chemicals from biomass.

A Noble-Metal-Free Catalyst Derived from Ni-Al Hydrotalcite for Hydrogen Generation from N2H4⋅H2O Decomposition†

Storing hydrogen safely and efficiently is one of the major technological barriers preventing the widespread application of hydrogen-fueled cells, such as proton exchange membrane fuel cells (PEMFCs). Hydrous hydrazine (N2H4·H2O) is considered as a promising liquid hydrogen storage material owing to the high content of hydrogen (7.9%) and the advantage of CO-free H2 produced. [1] In particular, hydrous hydrazine offers great potential as a hydrogen storage material for some special applications, such as unmanned space vehicles and submarine power sources, where hydrazine is usually used as a propellant. The decomposition of hydrazine proceeds by two typical reaction routes:

Design of a Highly Active Ir/Fe(OH)x Catalyst: Versatile Application of Pt‐Group Metals for the Preferential Oxidation of Carbon Monoxide

The proton-exchange membrane fuel cell (PEMFC) has been regarded as one of the most promising candidates for the efficient use of hydrogen energy. However, small amounts of CO (0.3–1%) in the H2 stream from reforming processes must be selectively removed because CO is highly poisonous to the Pt anode of a PEMFC. The preferential oxidation of CO in a H2-rich gas (PROX) is presently the most effective approach to address this problem. Oxide-supported Au catalysts are highly active for the PROX reaction even at room temperature, but the lower stability and sensitivity to CO2 constrain their practical applications. Supported Pt catalysts, on the other hand, are less active and only a few have shown reasonable activity for conversion of CO at temperatures lower than 60 8C. Therefore, it is highly desirable to develop improved catalysts with better catalytic performance for the PROX reaction at lower temperatures. Ir has a higher melting point and surface energy than other metals with 5f orbitals, such as Pt and Au, and Ir can be well-dispersed on and strongly interact with the support. However, compared to Ptand Au-based catalysts, Ir-based catalysts have limited applications in heterogeneous catalysis and are rarely investigated for the PROX reaction, most probably because of its inferior activity. Although much effort has been made to improve the activity of Ir-based catalysts and remarkable progress has been achieved, their activities for the PROX reaction are still low at low temperatures. In fact, there is no report so far claiming that Ir-based catalysts can show high activity at temperatures below 80 8C; thus it remains a formidable challenge to utilize Ir-based catalysts for the PROX reaction at ambient temperatures. One basic task of modern catalysis is to rationally design catalysts based on the fundamental understanding of their reaction mechanisms. Especially, the contribution of support materials to the performance of the final catalysts should be taken into account. For the PROX reaction, the strong binding of CO to Ir poisons the surface so that O2 cannot competitively adsorb on the Ir surface and be activated at low temperatures, thereby prohibiting the conversion of CO to CO2. Therefore, weakening the adsorption strength of CO and/or promoting the activation of O2 at lower temperatures have become the crucial steps. Ferric oxide has proven effective for O2 activation and has been used extensively as an additive to Pt-based catalysts. Recently, we have designed a bimetallic catalyst by adding FeOx to a supported Ir catalyst, and the activity for the PROX reaction was improved. Further study of the catalytic reactions showed that the reaction rate of CO oxidation correlated well with the presence and amount of Fe, suggesting that Fe sites were indeed the active sites for O2 activation. [13] The coordinatively unsaturated Fe center was also recently identified as the site to activate O2, which helped the design of a highly active FeOx/Pt/SiO2 catalyst to totally convert CO at room temperature. All of these studies suggest that the presence of low-valent Fe (Fe) played a decisive role in improving the PROX activity, thus providing a clue for obtaining a highly effective Ir-based catalyst by incorporating materials containing, or easily forming, Fe species. Ferric hydroxide (Fe(OH)x) is a novel support material which has recently been adopted to stabilize various types of metal species for CO oxidation. It possesses a large surface area and a large amount of OH groups; these unique properties make Fe(OH)x a good candidate for generating highly dispersed metal clusters or even single-atom catalysts. Furthermore, the longer Fe O bonds in Fe(OH)x (compared to those in Fe2O3) make it easier to form Fe 2+