Ammonia synthesis is structure sensitive, and a minute change in the catalyst structure would cause a dramatic change in activity. To date, none of the studies reveal the metal size effect at a ...subnanometer scale on NH3 synthesis, and such investigation remains a challenge. Here, we report the synthesis of Ru catalysts with sizes ranging from single atoms, atomic clusters (ACCs), sub-nanometric clusters, to nanoparticles (NPs) by adjusting precursor and/or loading of Ru. Sub-nanometric Ru catalysts not only exhibit performance different from that of NPs but also follow a different route for N2 activation. The strong intra-cluster interaction of Ru atomic clusters enables the formation of strong interactions of Ru d-orbitals with the σ and π orbitals of N2 molecules, resulting in N2 activation over Ru ACCs to occur more easily than that over Ru NPs. Consequently, Ru ACCs display an unprecedentedly high NH3 synthesis rate and large turnover frequency at mild conditions.
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•Ru size effect at a subnanometer scale on NH3 synthesis was studied•NH3 synthesis rate varies with the Ru size decreasing to sub-nanometric level•Ru atomic clusters could effectively tune the N2 activation pathway
Ammonia synthesis is a structure-sensitive reaction, and its activity depends on the metal size. With the fast development of subnanometer catalysis, it is of great importance to investigate the metal size effect at a subnanometer scale on NH3 synthesis. Hence, we report multiscale Ru catalysts on NH3 synthesis with sizes ranging from single atoms, atomic clusters, sub-nanometric clusters, to nanoparticles. Our studies show that as the size of Ru entities decreases to sub-nanometric level, B5 and/or terrace sites decrease, which leads to a change of N2 activation route. The strong intra-cluster interaction in Ru atomic clusters catalyst induces strong interaction of Ru d-orbitals with the σ and π orbitals of N2 molecules, which results in N2 activation over Ru atomic clusters to occur much more easily than that over Ru nanoparticles. The findings have significant implications for the design of structure-sensitive catalysts for NH3 synthesis under mild conditions.
A series of Ru catalysts were prepared with Ru sizes ranging from single atoms, atomic clusters, sub-nanometric clusters, to nanoparticles. Among them, the sub-nanometric Ru catalysts not only exhibit performance different from that of Ru nanoparticles but also follow a different route for N2 conversion to NH3.
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•N-doped carbon supported sub-nano Ru clusters (Ru-SNCs) catalyst was prepared.•Ru-SNCs catalyst showed a higher NH3 synthesis rate than nanoparticle Ru catalyst.•Ru-SNCs catalyst can ...follow an associative route for NH3 synthesis.
The industrial manufacture of ammonia (NH3) using Fe-based catalyst works under rigorous conditions. For the goal of carbon-neutrality, it is highly desired to develop advanced catalyst for NH3 synthesis at mild conditions to reduce energy consumption and CO2 emissions. However, the main challenge of NH3 synthesis at mild conditions lies in the dissociation of steady NN triple bond. In this work, we report the design of subnanometer Ru clusters (0.8 nm) anchored on the hollow N-doped carbon spheres catalyst (Ru-SNCs), which effectively promotes the NH3 synthesis at mild conditions via an associative route. The NH3 synthesis rate over Ru-SNCs (0.49% (mass) Ru) reaches up to 11.7 mmol NH3·(g cat)−1·h−1 at 400 °C and 3 MPa, which is superior to that of 8.3 mmol NH3·(g cat)−1·h−1 over Ru nanoparticle catalyst (1.20% (mass) Ru). Various characterizations show that the N2H4 species are the main intermediates for NH3 synthesis on Ru-SNCs catalyst. It demonstrates that Ru-SNCs catalyst can follow an associative route for N2 activation, which circumvents the direct dissociation of N2 and results in highly efficient NH3 synthesis at mild conditions.
Carbon supports with a low ratio of micropore volume to total pore volume can be prepared by KOH activation of thermally modified carbon. KOH treatment of carbons in nitrogen led to the creation of ...basic sites as well as acidic surface groups, whereas KOH treatment in air resulted in more acidic groups. Steam treatment quickly increased the surface area and pore volume, but did not create surface‐reactive groups. The increase in surface areas, micropores, and surface functional groups of carbon supports led to a decrease of the particle sizes, which not only improved the ability of hydrogen to migrate from Ru metal to the remote promoter precursors, but also increased the amount of adsorbed hydrogen and nitrogen. The properties of carbon combined with the size of the Ru particles affected the ammonia‐synthesis activity.
Activated carbon: Thermally modified carbon can be activated by KOH treatment and the quantity of the surface groups and the textural properties can be controlled by changing the treatment conditions, which improved the dispersion of Ru particles. The difference in the amount of surface functional groups and the size of Ru particles affected the catalytic activity of Ru catalysts in the synthesis of ammonia.
A series of MgAl-layered double oxides (LDO) doped with different rare-earth elements (Y, La, and Ce) were synthesized by the calcination of Mg–Al layered double hydroxides, and Ru, which were used ...to prepare ammonia synthesis catalysts. The as-obtained oxides and catalysts were characterized by XRD, TEM, TPD, TPR and XPS to understand their catalytic performances in ammonia synthesis. The H2-TPR and HRTEM studies reveal that Ru/Y-LDO catalyst possesses more active Ru metal and small particle size. The XPS demonstrates that the electronic interaction between Y and Ru metals is stronger, which can be tentatively explained by most of Y inserted into the hydrotalcites structure. CO2-TPD demonstrates that Ru/Y-LDO catalyst shows stronger basic site densities than catalysts doped with Ce and La. Higher activity of the Ru/Y-LDO catalyst can be attributed to smaller particle size, more active metal (Ru) and strong Ru–support interaction.
The activity enhanced remarkably for the Ru/MgAl-LDO catalysts doped with different rare earth elements (Y, La, Ce). Y3+ could improve strong metal–support interaction by forming more active surface Ru metal and optimum Ru particles sizes. The strong basic sites were responsible for enhancing electron donation ability of ammonia synthesis catalysts. REE doping is an effective way to improve the performance of Ru-based ammonia synthesis catalysts supported on MgAl-LDO. Display omitted
•Ru catalysts supported on post-hydrotalcites with large pore volume were prepared.•The activity of ammonia synthesis was remarkably improved for the Ru/Y-LDO catalyst doped with Y.•Rare earth doping is an effective way to improve the performance of the Ru/MgAl-LDO catalysts for ammonia synthesis.
Ru/AC catalysts with K promoter were prepared by impregnation of the samples with different amounts of KOH. The catalysts were characterized using a variety of techniques, including X-ray ...diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy, CO chemisorption and temperature-programmed desorption. The presence of K promoter had a limited effect on the size of Ru particles, but significantly enhanced the amount of adsorbed hydrogen, nitrogen and ammonia. A new desorption peak of nitrogen and ammonia at low temperature can be observed in their TPD profiles for Ru catalysts with K promoter. We propose that the presence of K promoter not only improved the adsorption properties of hydrogen, nitrogen and ammonia for Ru catalyst supported on activated carbon, but also weakened the adsorption strength of nitrogen and ammonia. The change in the chemisorptive properties of reactant gases including hydrogen, nitrogen and ammonia may be responsible for K promoter enhanced ammonia synthesis activity of Ru/AC catalyst, more than just the electronic properties of K promoter.
The industrial synthesis of ammonia (NH3) using Fe-based derived catalysts requires harsh reaction conditions (400–600 °C, 20–40 MPa). It is desirable to develop catalysts that perform well at low ...temperature and pressure (<400 °C, <2 MPa). The main challenge of low-temperature NH3 synthesis is the dissociation of the extremely stable NN triple bond (945 kJ/mol). Herein, the N-doped C-supported Co catalyst was demonstrated to be active and efficient for NH3 synthesis under mild conditions, resulting from the hybridization of the d orbitals of Co with p orbitals of nitrogen for Co–N coordination. The doping of N has three relevant effects. The first is the size decrease in Co nanoparticles. It is proven by in situ X-ray photoelectron spectroscopy and synchrotron-based X-ray absorption near-edge structure and extended X-ray absorption fine structure analyses coupled with density functional theory calculation that there is strong electron transfer from the doped N to Co. Hence, the second effect of N doping is the increase in electronic state of Co d orbitals which promotes the donation of 3d electrons from Co to the π* orbital of N2, leading to a decrease in activation energy for the N2 molecule. The third is the involvement of nitrogen vacancies. The pyridine N weakly coordinated with highly dispersed Co reacts with adsorbed H2 to form NH3, simultaneously generating N vacancies; then, the consumed N species can be replenished via N2 adsorption on vacancy sites. These factors contribute to the superiority of the N-doped carbon-supported Co-based catalyst, which achieves an NH3 production rate of 1.59 mmolNH3 ·gcat –1·h–1 at even 250 °C and 1 MPa.
A series of CeO
2
-La
2
O
3
supported ruthenium catalysts were prepared by co-precipitation method and the as-obtained samples were characterized by N
2
physisorption, X-ray diffraction, CO ...chemisorption, H
2
-TPR, H
2
-TPD and XPS. The activity test shows that ammonia concentration of the catalyst with 10% La is 13.9% at 10 MPa, 10,000 h
−1
, 450 °C, which is 17% higher than that of Ru/CeO
2
. La doping can improve the activity of Ru-ceria catalyst for ammonia synthesis by facilitating the reduction of oxygen which subsists in the cerium oxide surface. In addition, it can be realized that the test of catalyst stability proves the stability performance of Ru/CeO
2
-La
2
O
3
catalyst within the reaction time of 55 h.
The prevailing view of the deactivation of carbon‐supported Ru catalyst in ammonia synthesis is carbon methanation, but those strategies focusing on inhibiting carbon methanation cannot effectively ...solve the challenge of Ru/C catalysts. Herein we prepared four K‐promoted Ru catalysts with different Ru loadings and treated the samples at 450 or 500 °C under 1 MPa during ammonia synthesis reaction. The result showed that the presence of a larger amount of Ru species enhanced methane formation at the reaction temperature, which was irrelevant to the loss in activity of ammonia synthesis. On the contrary, CO formation along with a great decline in catalytic activity can be observed when Ru catalysts are heated at 500 °C. The characterization results indicated that the severe depletion of surface oxygen groups following the CO‐formation pathway, resulting in the notable decrease in the amount of gas adsorption, contributed to the deactivation of carbon‐supported Ru catalyst.
The prevailing view of the deactivation of carbon‐supported Ru catalyst in ammonia synthesis is carbon methanation. However, the change in the Ru loading affected methane formation at the reaction temperature, which was irrelevant to the loss in activity of ammonia synthesis. On the contrary, the severe depletion of surface oxygen groups following the CO‐formation pathway, resulting in the notable decrease in the amount of gas adsorption, contributed to the deactivation of carbon‐supported Ru catalyst.
A series of Ru/Al
2
O
3
catalysts were prepared to study the effect of the amount and the origin of residual chlorine on chemisorptive property and the ammonia synthesis activity. The catalysts were ...characterized by X-ray fluorescence, CO chemisorption, transmission Electron Microscopy, X-ray photoelectron spectroscopy, hydrogen temperature-programmed desorption, hydrogen temperature-programmed reduction. It is found that the presence of chlorine had a limited impact on Ru particle size. Residual chlorine originated from RuCl
3
would not influence on Ru 3d
5/2
binding energy, but chlorine from HCl solution significantly increased Ru 3d
5/2
binding energy. Regardless of their source, the presence of chlorine severely reduced the amount of hydrogen species corresponding to the desorption peak at medium temperature. The inhibition effect of chlorine on hydrogen adsorption was more strong for Ru catalyst with residual chlorine from the RuCl
3
precursor. With a similar amount of residual chlorine, the catalysts with chlorine originated from the RuCl
3
precursor showed much lower catalytic activity than those prepared by impregnation of HCl. These results suggest that chlorine mainly affects the catalytic properties of alumina supported Ru catalysts for ammonia synthesis by selective site blocking.
Graphical Abstract
Chlorine affects on the sites at the metal/support interface for Ru/Al
2
O
3
catalysts, and then hydrogen adsorption and ammonia synthesis both are suppressed.
