H2 tends to be a crucial medium in the foreseeable future as it is not only a green and renewable energy source for vehicles but also a fundamental feedstock for the chemical industry. For instance, selective hydrogenation, one of the catalytic processes used to produce fine compounds, is of vital importance because it enables the selective and efficient conversion of a variety of functional groups under mild reaction conditions. Catalytic hydrogenation of liquid organic hydrogen carriers (LOHCs; eg, methylbenzene) is a safe and economic approach for H2 storage. At present, H2 is mainly produced from the reforming of fossil fuels and industrial waste H2. Notably, both crude H2 from reforming and industrial waste H2 contain a notable amount of CO as an impurity, which can poison the noble–metal-based catalysts due to the strong and preferential adsorption of CO, inducing the blockage of active sites. Crude H2 and waste H2 need to be purified through various routes such as pressure swing adsorption, palladium, or polymeric membrane stack before being used; however, the purification processes are estimated to cause a significant extra cost of about 10%–20%. Consequently, developing CO-tolerant selective hydrogenation catalysts makes it possible for the direct utilization of crude H2 and waste H2 in the production of fine chemicals. What’s more, if the LOHCs can be hydrogenated with crude H2 or even industrial waste H2, the purification of H2 can be realized through a catalytic process, as the pure H2 can be obtained from the dehydrogenation of the corresponding organic compound of LOHCs (eg, methyl-cyclohexane). The cycle of “hydrogenation by crude/waste H2 – dehydrogenation to produce pure H2” can bring promising economic and environmental benefits (Figure 1), though other impurities like chlorine hydrogen sulfide and oxygen in waste H2 may be an obstacle at the current stage. The strong adsorption of CO on noble metal (eg, Pt) is resulted from the electron transfer of the d orbital electrons from metal to the 2π∗ anti-bonding orbital of CO molecules. Decreasing the d electron density of noble metal via the confinement of noble-metal species to an atomically dispersed motif has been demonstrated to be a useful method to reduce the strong adsorption of CO.1Qiao B. Wang A. Yang X. et al.Single-atom catalysis of CO oxidation using Pt1/FeOx.Nat. Chem. 2011; 3: 634-641Google Scholar Although the trade-off between CO-tolerant property and hydrogenation capability has brought harsh requirements for designing noble-metal catalysts, there have been some successful catalysts for the challenging process. Single-atom alloy (SAA) catalysts are stable under hydrogenation reaction conditions. The Pt–M bond maintains the metallic property of noble-metal single atoms, making it possible to reduce the adsorption of CO and catalyze the hydrogenation reaction simultaneously. Flytzani–Stephanopoulos et al. reported the Pt0.008Cu–SAA catalyst for CO-tolerant hydrogenation of acetylene.2Liu J. Lucci F.R. Yang M. et al.Tackling CO poisoning with single-atom alloy catalysts.J. Am. Chem. Soc. 2016; 138: 6396-6399Google Scholar The authors demonstrated that the adsorption of CO on the Pt0.008Cu–SAA was weaker than that on the Pt nanoparticles. Moreover, they uncovered that the activation of H2 was less affected by CO over the SAA catalysts through the H2–D2 exchange experiment. In the authors’ view, weak adsorption of CO on the Pt0.008Cu–SAA catalyst led to more “CO-free” Pt sites under H2/CO atmosphere and further superior ability for H2 activation. As a result, when the two catalysts were applied to catalyze selective hydrogenation of acetylene with simulated crude H2 containing 200 ppm CO, the Pt0.008Cu–SAA catalyst exhibited 15 times higher activity than the Pt nanoparticles did. The existence of Pt–M is the key for the CO-tolerant hydrogenation activity of the SAA catalyst. Therefore, tuning the coordination environment of atomically dispersed Pt catalysts is crucial for CO-tolerant hydrogenation. As is discussed above, Pt SAA catalysts with a Pt–M bond can be potential catalysts for CO-tolerant hydrogenation reaction. Our group discovered that Pt could be confined into single atoms with a Pt–Mo bond because of the strong interaction between noble metal and α-MoC.3Lin L. Yao S. Gao R. et al.A highly CO-tolerant atomically dispersed Pt catalyst for chemoselective hydrogenation.Nat. Nanotechnol. 2019; 14: 354-361Google Scholar Moreover, the Pt–Mo coordination endows the Pt single atom on α-MoC with metallic properties, leading to the partially positive-charge Pt species (Ptδ+). The atomically dispersed Pt/α-MoC (Pt1/α-MoC) was developed as a successful CO-tolerant catalyst for selective hydrogenation of nitrobenzene and its derivatives. The commercial Pt/C catalyst could catalyze the hydrogenation of nitrobenzene in pure H2 at room temperature. However, once 0.1% CO (1000 ppm) was added, the commercial Pt/C catalyst was poisoned directly. By contrast, the Pt1/α-MoC catalyst showed unprecedented CO tolerance for this reaction, whose turnover frequency remained 50%, reaching about ∼3500 h−1. Moreover, the catalyst could keep the activity and selectivity unchanged after eight cycles. Spectroscopic studies and theoretical calculations indicated the weakened adsorption of CO on a Ptδ+ single atom by decreasing the back donation of the Pt to CO, which was resulted from strong interaction between the α-MoC and supported Pt species. The interface structure of the Pt1/α-MoC catalyst enabled a new reaction pathway for the hydrogenation of a nitro group by involving both Pt1 and α-MoC sites. Water served as an H donor and thus facilitated the hydrogenation reaction, then the adsorption of CO on Pt sites reacted with the abundant ∗OH on the surface to help extract H from water and regenerate active sites at room temperature in the assistance of the hydrogenation reaction. From this work, the choice of support of the atomically dispersed catalyst is drastically important for the CO-tolerant hydrogenation reaction because the support can not only tune the coordination environment of atomic noble-metal species but can also participate in catalyzing the hydrogenation reaction. Very recently, Yang et al. reported that intermetallic Pt2Mo nanocrystals with twin boundaries on mesoporous carbon (Pt2Mo/C) were also active and stable for CO-tolerant hydrogenation of nitro-benzene and its derivatives.4Wang K. Wang L. Yao Z. et al.Kinetic diffusion–controlled synthesis of twinned intermetallic nanocrystals for CO-resistant catalysis.Sci. Adv. 2022; 8 (eabo4599)Google Scholar The Pt2Mo/C catalyst also had a Pt–Mo bond, and the twin boundaries were demonstrated to be the key factor to weaken the adsorption of CO. CO-tolerant hydrogen storage is an appealing process, but it is even more challenging than the above-reviewed CO-tolerant selective hydrogenation. Specifically, the hydrogenation of a benzene ring requires a noble-metal catalyst with large particles. It means that atomically dispersed catalysts are less active for hydrogenation of benzene ring, so the above-mentioned strategy may be not suitable. Alternatively, if crude hydrogen can be purified in situ by means of parallel methanation reaction over a bifunctional catalyst, CO-tolerant hydrogenation of LOHCs with benzene ring may be achieved. Very recently, our group reported the synthesis of RuNi/TiO2 catalysts with bifunctional active sites for CO-tolerant hydrogenation of methylbenzene to methyl-cyclohexane.5Wang Z. Dong C. Tang X. et al.CO-tolerant RuNi/TiO2 catalyst for crude hydrogen storage and purification.Nat. Commun. 2022; 13: 4404Google Scholar Commercial noble-metal catalysts and 2Ru/TiO2 all exhibited drastically high activity toward methylbenzene hydrogenation in pure H2, but once 0.1% CO was incorporated in the gas feed, the activity decreased by more than an order of magnitude. By contrast, although the reaction rate over the 5Ni/TiO2 was much lower than that of 2Ru/TiO2, it only decreased slightly with the incorporation of 0.1% CO. Significantly, the 2Ru5Ni/TiO2 catalyst took the advantages of the 2Ru/TiO2 and 5 Ni/TiO2 catalysts, reaching a reaction rate of 18.2 moltoluene/molmetal/h in H2 and 12.5 moltoluene/molmetal/h in 0.1% CO/H2, respectively. Besides, high conversion of methylbenzene and CO were kept over 2Ru5Ni/TiO2 during 24 h long-term tests. In other words, a crude H2 storage process was realized over the 2Ru5Ni/TiO2 catalyst. It was demonstrated that Ru was the active site for CO methanation, while Ni was the active site for methylbenzene hydrogenation. Moreover, the strong interaction between the RuO2 and TiO2 and Ni species led to the high dispersion of Ni sites. Therefore, the 2Ru5Ni/TiO2 catalyst exhibited much better activity than the 5Ni/TiO2 catalyst due to its small particle size. Overall, by rapid purification of CO through the methanation reaction, CO-tolerant crude H2 storage in methylbenzene/methyl-cyclohexane was realized over the RuNi/TiO2 catalyst. It is believed that the strategy of fabricating bifunctional catalysts can be generalized to other LOHC systems. To sum up, direct utilization and storage of crude H2 and industrial waste H2 is a drastically attractive process in both academic and industrial fields, but there is only limited research focusing on this topic. Still, some successful strategies for the design of CO-tolerant hydrogenation catalysts have been raised as below. Construction of Pt-based SAAs leads to atomic Pt species with metallic property thanks to the Pt–M coordination environment, which can decrease the adsorption of CO but keep the ability for hydrogenation reaction, leading to the feasibility for CO-tolerant hydrogenation. Moreover, support can also play an important role in CO-tolerant hydrogenation as it not only determines the coordination environment of supported noble-metal species but also participates in catalyzing the CO-tolerant hydrogenation. Finally, fabrication of bifunctional catalyst for the simultaneous hydrogenation of CO and organic substrate has been demonstrated a successful strategy for CO-tolerant crude H2 storage process. Until now, CO-tolerant hydrogenation has still been at the initial stage, where plenty of opportunities and challenges exist. To achieve a deeper understanding and progress in this field, there are several aspects to be improved. Firstly, many more catalysts should be designed for CO-tolerant hydrogenation and H2 storage processes so that the underlying trends and mechanisms can be summarized. Some reported noble-metal catalysts based on strong metal-support interaction may be possible for this challenging process. Secondly, more kinds of organic substrates should be considered for CO-tolerant hydrogenation to meet the practical demand in the chemical industry. Last but not least, the industrialization of the above-reviewed CO-tolerant hydrogenation or H2 storage processes should be pushed forward. The use and storage of crude H2 that only contains a small amount of CO poison has been achieved at the laboratory scale, which may be realized in the industry in the near future. For industrial by-product H2 containing other impurities, it is highly desired but challenging to develop multiple-poisons-tolerant catalysts. Motivated by the economic benefits of crude H2/waste H2 utilization, we believe more research will be devoted to this challenging area for extended hydrogen development. This work was financially supported by the National Natural Science Foundation of China (21725301, 21932002, 21821004, 22209129, and 22278367); the National Key R&D Program of China (2021YFA1501100); “Young Talent Support Plan” of Xi’an Jiaotong University (HG6J024) and the High-Level Innovation and Entrepreneurship Talent Project of Qinchuangyuan (QCYRCXM-2022-123). D.M. acknowledges support from the Tencent Foundation through the EXPLORER PRIZE. The authors declare no competing interests.