Earth-abundant catalysts

Reece SY, Hamel JA, Sung K, et al. Wireless solar water splitting using silicon-based semiconductors and earth-abundant catalysts. Science. 2011 334 645-8. 

It is vital that we seek to maximise the metals catalytic activity and recover 100% of elements from catalytic processes at both the end of reaction and end of life (the only exception may be carbon that can be burnt for energy production at end of life). Development and application of Earth-abundant catalysts for a wider range of catalytic applications is possible in the midterm. However, the long-term and ideal scenario would be that even critical elements can be used as sustainable catalysts if total recoveiy from anthropogenic cycles is guaranteed. The concept of elemental sustainability for catalysis is likely to become increasingly important in the future. Now is the time for producers and users alike to progress to circular economies and embrace sustainable catalysis. 

Contained within this book are various chapters that review the possibilities for the sustainable use of catalysts in our chemical industiy. Earth abundant metals are discussed in Sustainable Catalysis With Non-endangered Metals, Parts 1 and 2, while the options for organocatalysis are discussed in Sustainable Catalysis Without Metals or Other Endangered Elements, Parts 1 and 2. The future chemical industiy cannot survive by the use of just one of the above catalyst classes, but will require the flexibility and versatility of both. An important aspect of sustainable catalysis that is also vital for the long-term security of elements is ensuring that we establish improved methods of catalyst recovery and reuse. 

Enzymes have long been known to be efficient catalysts [1-3]. In particular, they are admired for selectivity, fast rates, and low activation energies. When considering applications that require sustainability and scalability, enzymes also have the advantage of being constructed exclusively from earth-abundant, bioavailable materials. Thus metalloenzymes typically employ only earth-abundant metals... 

The search for earth abundant substitutes for ruthenium and iridium is a fundamental target towards WOC sustainability and hydrogen economy. Recently, an increasing interest grew towards cobalt-based electrode coatings and molecular cobalt complexes to promote water oxidation. Cobalt oxide and related aquo or hydroxo complexes have been known as water oxidation catalysts since the early SOs. ... 

Once electrochemically or chemically activated, this complex undergoes a stepwise loss of four electrons and four protons, producing an intermediate reactive species able to oxidizes water. Unfortunately, the blue dimer loses its catalytic efficiency after few cycles, due to the degradation of the organic ligands. However, its discovery paved the way to the development of a variety of molecular water oxidation catalysts, most of them still based on ruthenium centers, but also on iridium, as well as on earth abundant and cheap metals, such as manganese, iron, and cobalt. " ... 

Most recently, Pt-based electrocatalysts with novel nanostructures such as nanowire, nanotube, hollow, core-shell, and nanodendrite structures have been investigated [71-74, 101]. One-dimensional ternary PtRuM (M = Ni, Co, and W) nanowire catalysts were synthesized, and these catalysts outperform Pt-Ru commercial catalyst and have a low noble-metal content due to the incorporation of an Earth-abundant element [101]. 

Heat-treated non-precious metal catalysts, synthesized from earth-abundant elements, are capable of catalyzing the ORR and efficiently generating electricity from fuels via a direct electrochemical conversion. Carbon-nitrogen precursors, supports, and in situ formed graphitized carbon play an important role in the catalyst performance. 

The use of nickel catalysts as a replacement for palladium is attractive since it would be far more cost-effective as it much cheaper and more earth abundant than palladium. But because nickel usually displays Ni(0)/Ni(ii) as well as Ni(i)/Ni(iii) oxidation states and is more nucleophilic than palladium, nickel cannot be simply considered as a direct substitute for palladium— it possesses distinctive catalytic properties that palladium does not have. One particular advantage to using nickel is its propensity to insert into C-Cl bonds more readily than palladium this is advantageous sinee aryl ehlorides are generally much cheaper than the corresponding aryl bromides or iodides. 

Dudnik, A. S. Weidner, V. L. Motta, A. Delferro, M. Marks, T. J. Atom-efficient regioselective 1,2-dearomatization of functionalized pyridines by an earth-abundant organolanthanide catalyst. Nat. Chem. 2014, 6, 1100-1107. 

Nonetheless, single-site catalysts did a very significant contribution to the field by involving other late transition metals. In addition to cobalt, nickel , and copper, iron and iridium have yielded a larger number of robust and active molecular catalysts for water oxidation, especially with the latter metal. Iron catalysts are of high relevance, due to the low toxicity and earth abundance of this metal. 

Until recently, C—H functionalization reactions have relied heavily on expensive noble transition-metal catalysts. In the past 10 years, there has been a rise in the interest of low-valent cobalt catalysts for C—H functionalization. These catalysts are earth-abundant, green, and can generally be used under milder reaction conditions than their noble transition-metal catalyst counterparts. 

Du P, Eisenberg R (2012) Catalysts made of earth-abundant elements (Co, Ni, Fe) for water splitting recent progress and future challenges. Energy Environ Sci 5 6012-6021... 

On the basis of the aforementioned analyses, we had previously [25] hypothesized that a catalyst with a combination of earth-abundant transition metal iron... 

On the basis of these analyses, we hypothesized the use of an earth-abundant transition-metal complex containing a non-redox-active metal center and redox-active (non-innocent) ligand as a catalyst. We demonstrated our prediction on [(PDl)Ca(THF)3], where PDI is a non-innocent pyridine-2,6-diimine ligand, and the catalyzed benzyhc (of the MeCH2Ph substrate) C-H bond alkylation by unsubstituted and diphenyl (termed the iio or-(io or)-substituted diazocarbene precursors, N2CH2 and N2CPh2. 

There are many challenges to the science of catalysis that need to be met over the coming years. A sustainable future calls for the development of catalytic processes that do not rely on a net input of fossil resources. This can only be achieved if we discover new catalysts that can efficiently utilize the energy input from the sun or other sustainable sources to synthesize fuels as well as base chemicals for the production of everything from plastics to fertilizers. It also requires more selective processes with fewer waste products and catalysts made from Earth-abundant elements. This represents a formidable challenge. This textbook describes some of the fundamental concepts that will be needed to address this challenge. 

The landscape of research in solar photocatalysis has been rapidly changing in recent years, with a flurry of activity in the development and analysis of catalysts for water oxidation and fundamental studies of photocatalysis based on semiconductor surfaces. Significant effort is currently focused on the development of more efficient catalysts based on earth-abundant materials and various strategies for the design of molecular assemblies that efficiently couple multielectron photoanodic processes to fuel production. The outstanding challenge is to identify robust materials that could catalyze the necessary multielectron transformations at energies and rates consistent with solar irradiance. 

However, the scarcity and high cost of noble metal-based H2-evolution co-catalysts limit their practical applications in photocatalytic water reduction. Thereby, much attention has focused on the development and application of earth-abundant metal-based H2-evolution co-catalysts. For instance, the earth-abundant Mo-, Co-, Ni-, and Cu-based co-catalysts have been successfully developed and... 

In addition, the earth-abundant hydrogenases, biomimetic complexes, and graphene have also been extensively utilized as H2-evolution co-catalysts of nanoscale metal sulfides. For example, the Clostridium acetobutylicum [FeFe]-hydrogenase I (Cal) could greatly enhance the H2-evolution activity of CdS NRs... 

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