Ru/CeO2 catalysts with different amounts of surface oxygen vacancies were prepared by changing the morphology of CeO2. The conversion of Ce4+ to Ce3+ and the formation of Ru–O–Ce bonds led to ...enhancement of the amount of oxygen vacancies. Ru species of low crystallinity enriched with Ru4+ ions exist on the surface of CeO2 nanorods, while metallic Ru particles exist on CeO2 nanocubes. The low crystallinity of Ru species and high concentration of oxygen vacancies enhanced the adsorption of hydrogen and nitrogen and also led to desorption of surface hydrogen in the form of H2. Therefore, Ru/CeO2 nanorods showed high ammonia synthesis activities. On the contrary, lower catalytic activity was observed over Ru/CeO2 nanocubes catalyst because H2 and N2 adsorption was less favorable plausibly due to the large particle size of Ru species and low concentration of oxygen vacancies, and most of the hydrogen species were consumed in H2O formation.
Ammonia is an important chemical for synthesized fertilizers traditionally and for a potential energy vector increasingly. Industrial ammonia synthesis through the Haber–Bosch process is ...energy-intensive and requires advanced materials to catalyze ammonia synthesis under mild conditions, whereas the utilization of ammonia as a hydrogen carrier via ammonia decomposition is facing a similar situation as well. The developed second-generation Ru-based catalysts performs superior activities over commercial Fe- or Ni-based catalysts; however, it remains challenging to construct effective Ru catalysts to achieve the usage of Ru metal affordably as well as further understanding the nature of Ru catalysis. This Perspective summarizes the recent contributions in engineering Ru-based catalysts via various strategies and related catalysis in ammonia synthesis and decomposition, and it discusses the similarities and differences of Ru catalysis in these reactions. Finally, an overview of this area and opportunities for further investigation are also provided.
The increase of alumina calcination temperature from 800 °C to 1300 °C results in the transformation of γ-Al2O3 to α-Al2O3 phase accompanying a decrease of specific surface area and the amount of ...tetrahedral Al3+ sites. Over Ru–Ba/alumina catalysts, an increase in alumina calcination temperature would broaden the size distribution of Ru particles, enlarge the metal-to-oxide ratio of Ru, decrease the amount of surface hydroxyl groups, as well as lower the temperature for N2 desorption. As a result, the increase of alumina calcination temperature lessens the effect of hydrogen poisoning and decreases the activation energy for ammonia synthesis. The Ru–Ba/Al2O3 catalyst with alumina calcined at 980 °C having both θ-Al2O3 and α-Al2O3 shows ammonia synthesis rate three times higher than that with alumina calcined at 800 °C having a γ-Al2O3 phase.
A series of Co/CeO2 catalysts with different morphology of CeO2 support were prepared. It was shown that part of Co species could be introduced into the CeO2 lattice and thus a new CoO bonding would ...be created for the ceria-supported Co catalysts. The morphology of CeO2 was found to affect the reduction of both the cobalt oxides and CeO2. A larger amount of CeO2 was reduced in the case of Co supported on polyhedral CeO2. Catalytic tests showed that the higher concentration of Ce3+ and the lower binding energy of Co species present in the catalyst are key factors that led to higher ammonia synthesis rates for the Co/CeO2 catalysts.
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•Co catalysts supported on polyhedral CeO2, CeO2 nanorods and hexagonal CeO2 were prepared.•The morphology of CeO2 affected the reduction of cobalt oxides and CeO2.•Co/CeO2 catalyst with polyhedral CeO2 had highest Ce3+ concentration and lowest Co 3p3/2 value.•Higher concentration of Ce3+ and lower binding energy of Co species led to higher rates.
In the present work, LaCo1–x Ru x O3 perovskites (x = 0, 0.01, and 0.02) were synthesized, and the influences of the partial substitution of Co by Ru on the reducibility and structural properties of ...LaCoO3 perovskite were investigated by XRD, TPR, XPS, STEM, and XAFS techniques. The results indicate that Ru is incorporated into the LaCoO3 lattice, giving rise to a decrease of mean crystallite size as well as promotion of perovskite reducibility. At 450 °C and 1 MPa, LaCo0.98Ru0.02O3 shows an ammonia synthesis rate of 10.5 mmol/(gNH3·h) and insignificant decline of activity in a run of 50 h. On contrast, the ammonia synthesis rate over the Ru/LaCoO3 counterpart prepared by wet impregnation method gradually drops from 8.05 to 7.15 mmol/(gNH3·h). Moreover, there is strong synergy between Ru and Co species in LaCo0.98Ru0.02O3, affording enrichment of Ruδ+ clusters and surface Co2+ species that are beneficial to N2 adsorption and activation.
The spatial arrangements of Ti species would affect the electronic metal-support interactions and the proportion of Ce3+ sites for ceria-supported Ru catalysts. Ti-Surface-loaded CeO2 supported Ru ...catalysts exhibited excellent ammonia synthesis activity, which is attributed to a larger proportion of Ru metal, more electrons of Ru species and better adsorption ability of hydrogen and nitrogen.
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Carbon-supported Ru catalyst is arguably the most promising applications of Ru catalysts in ammonia synthesis. However, the insufficient stability of Ru/C catalyst greatly limits its ...applications, and the elimination of surface oxygen species by carbon preactivation has been imagined to be an efficient method for preparing Ru/C catalyst with high stability. However, herein we demonstrate that the presence of some oxygen functional groups in carbon support of Ba-promoted Ru/C catalyst was beneficial not only to the dispersion of Ru particles, but also to the low-temperature hydrogen adsorption. Furthermore, the formation of CO during heat treatment, which was closely related to oxygen groups, enhanced the ammonia synthesis activities of carbon-supported Ru catalyst with Ba promoter. Therefore, Ru catalyst supported on carbon with a large number of surface oxygen groups showed higher activity. Understanding the effect of carbon support surface on the activity and stability of Ru/C provides a basis for the rational design of the optimal carbon-supported metal catalyst for ammonia synthesis and related reactions.
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•A dynamic N- and H-involved cyclic reaction pathway proposed for NH3 production.•A dual-path mechanism including chemical looping and thermal catalysis illustrated.•Bottleneck of ...direct NN dissociation bypassed.
The synthesis of ammonia via the Haber-Bosch process using Fe-derived catalysts requires harsh reaction conditions. It is hence meaningful to develop catalysts for low-temperature synthesis of ammonia for industrial application. Herein for the first time, we report that with the synergy between LaN and Ru/ZrH2, NH3 can be synthesized via a dual-path mechanism. The LaN-promoted Ru/ZrH2 catalyst shows exceptionally high NH3 synthesis rate (up to 305 mmolNH3 gRu−1 h−1) at 350 °C under 1 MPa and remarkable durability (tested for 200 h). The outcomes of isothermal surface reaction and a suite of 15N2 and D2 isotopic labeling experiments reveal that the N3− of LaN reacts with H− ions to produce NH3, leaving behind N and H vacancies. The initial state of Ru/xLaN/ZrH2 can be restored by having the N and H entities replenished under the atmosphere adopted for NH3 synthesis. Moreover, based on the results of N2 reaction order, nitrogen K-edge NEXAFS, in situ XPS as well as in situ DRIFTS analyses under a 25%N2-75%D2 atmosphere, it is reckoned that the direct dissociation of N2 does not occur on the LaN-promoted Ru/ZrH2 catalyst while N2 hydrogenation takes place via an associative pathway under mild conditions. For the hydrogenation of N2, an appropriate amount of LaN would induce a synergic effect on the Ru active sites, leading to facile activation and hydrogenation of N2 to *N2H2. Nonetheless, an excess amount of LaN would result in blocking of Ru sites, consequently hindering the formation of *N2H2 and decreasing the catalytic activity. Following the associative and chemical looping pathways, the LaN-promoted Ru/ZrH2 catalyst bypasses the bottleneck of N2 direct dissociation, making the synthesis of NH3 at mild conditions possible.
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•M single atom (M = Fe, Co, Ni) anchored on Ru nanoclusters were prepared to form M1Ru SAA.•Co1Ru SAA shows the highest NH3 synthesis rate and TOFRu value.•Compared with Co-Ru ...nanoparticle alloy, Co1Ru SAA exhibits lower work function and up-shift d band center.•SAA structure effectively tune N2 activation pathway.
The desire of green ammonia (NH3) production requires efficient catalysts that could operate at mild conditions. However, the activity of catalysts is restricted by scaling relation that low activation energy of N2 is in conjunction with the over-strong affinity of intermediates, which blocks adsorption sites of catalyst and hinders NH3 production. Single atom alloy (SAA) is an efficient approach to circumvent this relation. In the present work, we offer a feasible strategy of preparing M1Ru (M = Fe, Co, Ni) SAA. Our studies show that Co1Ru SAA has the highest NH3 synthesis rate and the largest TOFRu value among M1Ru SAA. Compared with CoRu nanoparticle alloy (NPA), Co1Ru SAA has stronger electronic interaction between Co and Ru, which induces lower work function and up-shift d band center toward Fermi level for Co1Ru SAA, thus promoting N2 activation. Meanwhile, the unique SAA structure could effectively tune N2 activation pathway. Those findings make a contribution to the development of advanced catalysts for efficient NH3 synthesis at mild conditions.
