ReviewToward efficient single-atom catalysts for renewable fuels and chemicals production from biomass and CO2
Graphical abstract
Introduction
Single-atom catalysts (SACs) are defined as supported metal catalysts with isolated single metal atoms as the primary active centers [[1], [2], [3]] and have gained significant research attention over the past decade. Apart from nanoparticle catalysts (NCs) which contain bulk metals, SACs expose all metal atoms on the surface with a 100 % metal utilization rate [3]. This is a unique feature and important in terms of industrial application because the maximized metal dispersion on SACs could potentially reduce the high demand for precious noble metals [4,5]. The maximum metal utilization rate on SACs lead to even higher reactivity per surface atom than that on nanoparticles (NPs) [2,[6], [7], [8]]. The strong interactions between the metal center and the support enable the fine-tuning of intrinsic electronic properties of the metal center by adjusting the local environment [9,10]. At the same time, the isolated metal center could strongly affect the properties (e.g., redox or acid/base properties) of its neighboring atoms [[11], [12], [13], [14], [15]]. SACs recently have been identified as a bridge between homogeneous and heterogeneous catalysis [16,17], since they present uniform and tunable active sites as well as unique interactions with different ligands similar to the homogeneous catalyst, while maintaining high stability and easy separation as a heterogeneous catalyst [[18], [19], [20]]. Because of these unique characteristics, SACs are attracting growing research attention for use in the many catalytic processes of chemical transformations, including biomass and CO2 conversion to produce renewable chemicals and fuels.
Modern industry depends on fossil resources for producing chemicals and fuels, contributing to increasing greenhouse gas emissions—it is predicted that about 200 million tons of CO2 will be produced from these feedstocks by 2050 if there is no effective reduction measure put in place [21,22]. Considering this, production of renewable chemicals and fuels from alternative resources is urgently desirable, and technologies for renewable biomass valorization and CO2 utilization have attracted growing interest [23,24]. As the only carbon-containing sustainable resource for producing renewable biofuels and chemicals, biomass is not only a key resource for supplementing growing energy demand but also generates less greenhouse gas emissions [[25], [26], [27], [28], [29], [30]]. A recent report has estimated that biomass-derived transportation fuels and chemicals will occupy 20 % and 25 % of the market by 2030, respectively [31]. There are two main approaches for upgrading biomass feedstocks into fuels and high-value chemicals: (1) thermochemical pyrolysis, liquefaction, and gasification (often coupled with upgrading technologies such as hydrotreating or Fischer–Tropsch synthesis); and (2) biomass deconstruction to carbohydrate and phenolic compounds followed by selective upgrading [25,26,[32], [33], [34], [35], [36], [37], [38], [39], [40]]. The selective upgrading pathway (also referred to as the hydrolysis pathway) and its sequential steps usually allow for highly selective production toward biomass platforms under the relatively gentle reaction conditions [39,[41], [42], [43]]. In addition, CO2 utilization as carbon feedstock is a valuable complement to biomass valorization to not only mitigate the greenhouse effect but also provide renewable chemicals and fuels [24]. Moreover, the CO2-to-chemicals/fuels pathway, which has three energy input forms (heat, electricity, and photons), offers a nearly carbon-neutral or even carbon-negative way during energy use; it is therefore considered to be the “two birds, one stone” approach for simultaneously reducing CO2 emissions and generating useful commodities [24,[44], [45], [46], [47]]. Currently, the main thermochemical routes of CO2 conversion produce CO, methanol, dimethyl ether (DME), etc [48]. The advantage of using an electrochemical process is that it is able to produce higher-value or energy-dense chemicals such as ethanol and formic acid [49]. The photochemical CO2 conversion process that is driven by solar energy requires semiconductor photocatalysts to facilitate light absorption, charge separation and migration, and surface reactions [47]. Fig. 1 illustrates an integrated carbon cycle to convert both biomass and CO2 into renewable fuels and chemicals simultaneously. In this cycle, the plants participate in converting a portion of CO2 into biomass via photosynthesis. Besides the fuels and chemicals generated, CO2 could also be produced during biomass catalytic conversion as a byproduct of certain biological processes (denoted as biomass CO2 in Fig. 1). This biomass-derived, enriched CO2 is an ideal resource for catalytic CO2 reduction to produce fuels and chemicals. Additionally, the CO2 generated from exhaust emissions and combustion of fossil/biofuels could again be fed into the CO2 reduction process. Ideally, renewable energy such as wind, solar, and hydropower can be applied as energy sources during CO2 reduction [22].
Catalysis plays a central role in biomass and CO2 conversion. A variety of catalysts have been designed and demonstrated promising performances for both biomass and CO2 valorization, but developing a technology that is commercially mature is still hindered by several common issues: (1) how to reduce the catalyst cost to the maximum extent possible; (2) how to increase the activity of active sites by modifying the local environment; (3) how to control selectivity toward the target products; and (4) how to maintain the stability of the catalysts. These issues call for innovations in catalytic systems through cost-efficient, high-activity, and specific selective catalysts. SACs with isolated metal atoms on the support could reduce the catalytic cost by maximizing the atom utilization efficiency, providing a more active catalytic site by tuning their geometric and electronic configurations, and contributing to higher selectivity due to the presence of a very uniform active center. As a result, SACs have been considered as the new frontier in heterogeneous catalysis of biomass and CO2 upgrading [3]. For instance, SACs could potentially and dramatically increase selectivity toward hydrodeoxygenation (HDO) over ring saturation during HDO of biomass-derived aromatic compounds [12], as well as significantly boost Faradaic efficiency toward CO and other valuable chemicals, such as CH3OH, during electrochemical CO2 reduction (CO2RR) [50,51].
A comprehensive introduction to SACs [1,3,13,14,52], biomass conversion [26,39,[53], [54], [55], [56], [57], [58], [59], [60], [61], [62], [63], [64], [65], [66]], as well as thermal conversion [[67], [68], [69]], electrochemical reduction [[70], [71], [72]], and photocatalytic reduction [73,74] of CO2 can be found in the excellent reviews published recently. In this review, we summarize the recent advances of the applications of SACs in converting biomass and CO2 into various renewable fuels and chemicals. We focus on the general principles of catalyst design and the structure–performance relationship, with an attempt to reveal the key factors in determining catalyst performance. We hope this collection will be a valuable reference for researchers seeking to study more efficient heterogeneous catalysts with the goal of lowering the cost of renewable fuel and chemical production so that these products can be a viable replacement for depleting fossil-based resources.
Section snippets
Understanding SACs
This section is to introduce the catalytic consequence of metal particle size of supported metal catalysts and unique geometric and electronic properties of SACs.
Converting biomass to renewable fuels and chemicals with SACs
This section summarizes the recent advances of applying SACs for different types of reactions during biomass valorization.
Converting CO2 to renewable fuels and chemicals with SACs
This section summarizes the recent advances of applying SACs for converting CO2 into renewable fuels and chemicals via thermochemical, electrochemical and photochemical processes.
Conclusions and perspectives
The maximized atomic efficiency, highly uniform active center, and unique and tunable electronic properties make SACs a new frontier for converting biomass and CO2 into versatile renewable fuels and chemicals. In this review, we highlight both the principle of catalyst design and the applications of SACs for improving the catalytic performance of biomass and CO2 valorizations.
The application of SACs in typical reactions for biomass valorizations, including hydrodeoxygenation, hydrogenation,
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors acknowledge the U.S. Department of Energy (DOE), Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences for the funding. PNNL is a multiprogram national laboratory operated for the DOE by Battelle.
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