NH3 Synthesis Rate Research Articles

Kinetic evaluation of ammonia synthesis mechanism over molybdenum-based catalysts: Effects of oxygen and nitrogen vacancies

The synthesis of NH3 from N2 and H2 is a significant chemical process, because essential products such as fertilizers and industrial chemicals can be obtained from NH3. In the catalytic synthesis of NH3, N2 activation is generally considered a kinetically relevant step (KRS). Therefore, advanced heterogeneous catalysts that promote N2 conversion have been extensively investigated. In this study, the roles of O and N vacancies (Ov and Nv) of Mo oxide formed during reduction were kinetically evaluated and compared using structural characterization, isotope experiments, kinetic studies, and density functional theory calculations. On the Mo oxide catalyst with Ov, NH3 was formed by a dissociative mechanism, and the surface reaction involving the adsorbed N* and H* was considered the KRS. However, on the catalyst with Nv, NH3 was formed by an associative mechanism, and N–N dissociation of N2H2* species by H* to produce NH3 was considered the KRS. The catalyst with Ov resulted in an NH3 synthesis rate of 5.6 mmol gMo−1 h−1 under relatively mild conditions (1.0 MPa and 400 °C), thus outperforming the catalyst with Nv (0.3 mmol gMo−1 h−1). These results provide insight into the importance of anionic vacancies in reducible metal oxides for NH3 synthesis.

Tuning clusters-metal oxide promoters electronic interaction of Ru-based catalyst for ammonia synthesis under mild conditions

Ammonia (NH3) is an excellent candidate for hydrogen storage and transport. However, producing NH3 under mild conditions is a long-term, arduous task. Atomic cluster catalysts (ACCs) have been shown to be effective for catalytic N2-to-NH3 conversion, opening the door to the development of efficient catalysts under mild conditions. Still, ACC formation with thermally stable catalytic sites remains a challenge because of their high surface free energy. Herein, we report anchoring Ba and/or Ce onto Ru ACCs (2 wt% Ru atomic clusters supported on N-doped carbon) to form so-called clusters–metal oxide promoters electronic interaction (CMEI) to stabilize the Ru atomic clusters. The resulting Ba/Ce/Ru ACCs significantly boost the NH3 synthesis rate to 56.2 mmolNH3 gcat−1 h−1 at 400 °C and 1 MPa, which is 7.5-fold higher than that of Ru ACC. The strengthened CMEI between the Ba/Ce and Ru atomic clusters across the Ba/Ce/Ru ACC enables electron transfer from Ba and/or Ce to Ru atomic clusters. As such, the electron-enriched Ru atom could facilitate electron transfer to N≡N bond π* orbitals, which would weaken the N≡N bond and drive the eventual conversion of N2 to NH3. This study offers insight into the role of CMEI in Ru ACCs and provides an effective approach for designing stable atomic cluster catalysts for NH3 synthesis.

Potassium promoter regulates electronic structure and hydrogen spillover of ultrasmall Ru nanoclusters catalyst for ammonia synthesis

The effects of promoters on Ru nanoparticles (≥2 nm) catalysts for NH3 synthesis have been extensively elaborated, but their roles on ultrasmall Ru nanoclusters (NCs, 1–2 nm) remain largely unknown and need to be further uncovered. Herein, a series of K-promoted MgO supported Ru NCs were synthesized and investigated for NH3 synthesis. The addition of 5wt.%K onto Ru/MgO NCs leads to a significantly high NH3 synthesis rate of 21.7 mmolNH3 gcat−1h−1 at 400 °C and 0.2 MPa, close to the thermodynamic equilibrium. Out studies reveal that anchoring K onto Ru NCs can increase the electron density and cause an upshift of the d-band center of Ru entities. Moreover, the addition of K regulates the hydrogen affinity and accelerates the migration of hydrogen from the Ru NCs surface to MgO support, which is crucial in avoiding the hydrogen poisoning effect on Ru NCs. With the synergistic effect of the Ru NCs sites bridged by H-spillover, makes the K-mediated Ru/MgO NCs catalysts efficient for NH3 synthesis at mild conditions.

Electrochemical Synthesis of Ammonia via Nitrogen Reduction and Oxygen Evolution Reactions—A Comprehensive Review on Electrolyte-Supported Cells

The application of protonic ceramic electrolysis cells (PCECs) for ammonia (NH3) synthesis has been evaluated over the past 14 years. While nitrogen (N2) is the conventional fuel on the cathode side, various fuels such as methane (CH4), hydrogen (H2), and steam (H2O) have been investigated for the oxygen evolution reaction (OER) on the anode side. Because H2 is predominantly produced through CO2-emitting methane reforming, H2O has been the conventional carbon-free option thus far. Although the potential of utilizing H2O and N2 as fuels is considerable, studies exploring this specific combination remain limited. PCEC fabrication technologies are being developed extensively, thus necessitating a comprehensive review. Several strategies for electrode fabrication, deposition, and electrolyte design are discussed herein. The progress in electrode development for PCECs has also been delineated. Finally, the existing challenges and prospective outlook of PCEC for NH3 synthesis are analyzed and discussed. The most significant finding is the lack of past research involving PCEC with H2O and N2 as fuel configurations and the diversity of nitrogen reduction reaction catalysts. This review indicates that the maximum NH3 synthesis rate is 14 × 10−9 mol cm−2 s−1, and the maximum current density for the OER catalyst is 1.241 A cm−2. Moreover, the pellet electrolyte thickness must be maintained at approximately 0.8–1.5 mm, and the stability of thin-film electrolytes must be improved.

Boosting N2 Conversion into NH3 over Ru Catalysts via Modulating the Ru-Promoter Interface.

Promoters are indispensable components of Ru-based catalysts to promote N2 activation in ammonia (NH3) synthesis. The rational addition and regulation of promoters play a critical role in affecting the NH3 synthesis rate. In this work, we report a simple method by altering the loading sequence of Ba and Ru species to modulate the Ru-promoter interface, thus significantly boosting the NH3 synthesis rate. The Ba-Ru/GC BM catalyst via the prior loading of Ba rather than Ru over graphitic carbon (GC) exhibits a high NH3 synthesis rate of 18.7 mmol gcat-1 h-1 at 400 °C and 1 MPa, which is 2.5 times that of the Ru-Ba/GC BM catalyst via the conventional prior loading of Ru rather than Ba on GC. Our studies reveal that the prior loading of Ba benefits the high dispersion of the basic Ba promoter over an electron-withdrawing GC support, and then Ba species serve as structural promoters to stabilize Ru with small particle sizes, which exposes more active sites for N2 activation. Additionally, the intimate Ba and Ru interface enables facile electron donation from Ba to Ru sites, thus accelerating N2 dissociation to realize efficient NH3 synthesis. This work provides a simple approach to modulating the Ru-promoter interface and maximizing promoter utilization to enhance NH3 synthesis performance.

BaCeO3 perovskite-incorporated Co catalyst for efficient NH3 synthesis under mild conditions

The growing demand for ammonia (NH3) as a carbon-free fuel and hydrogen carrier for decarbonization has increased interest in its synthesis under mild conditions to cope with the intermittent renewable supply of electricity and green hydrogen. Developing active and cost-efficient catalysts is crucial for achieving this goal. Herein, we investigated Ba and Ce incorporated Co (Ba/CoCe) as an efficient catalyst for ammonia synthesis. Our findings show that the optimal amount of Ba and Ce incorporation onto Co resulted in a drastic increase in catalytic activity compared to both pure Co and Ba or Ce singly promoted Co catalysts. Moreover, it exhibits superior catalytic performance and cost efficiency when compared to state-of-the-art benchmark catalysts. Co-incorporation of Ba and Ce forms BaCeO3 perovskite during catalyst preparation. Co–BaCeO3 interface bears a moderate adsorption strength of nitrogen, showing high adsorption–desorption reversibility of N atoms. Reaction studies coupled with structural characterizations give an insight into the presence of the kinetically favored associative mechanism, thereby significantly reducing the activation energy and boosting the NH3 synthesis rate. This work provides insight into the design of non-noble metal catalysts for ammonia synthesis under mild conditions, and also contributes to the potential design of the perovskite family for catalytic purposes.

Plasma-enabled sustainable ammonia synthesis at atmospheric pressure: The role of catalysts on synergistic effect

Ammonia (NH3) has received remarkable interest for the synthesis of many nitrogenous product and as storage hydrogen media. However, the synthesis of NH3 remains challenging due to the high pressure and temperature from the dissociation of the stable N≡N triple bond. Here, we report an environmentally friendly plasma-thermal catalytic process for the sustainable synthesis of NH3 over M/CeO2 and M/MgO catalysts (M = Ru, Fe) with dielectric barrier discharge (DBD) reactor at atmospheric pressure, which showed excellent activity and high temperature stability. The synergistic effect strongly depended on the support properties, that CeO2-supported catalysts were more obvious below 300 °C compared with the MgO-supported catalysts regardless of the active metals (Ru or Fe). The highest NH3 synthesis rate of 6.8 mmol g−1 h−1 was achieved over Ru/CeO2 at 400 °C under a ratio of N2:H2 = 1:3. This value was much higher compared with that using DBD reactor reported in the literature. The structure-activity relation of catalysts was characterized through X-ray photoelectron spectroscopy, temperature programmed reduction by hydrogen, Raman spectra and the in-situ diffuse reflectance infrared Fourier transform spectroscopy (Iin-situ DRIFTS). In addition to the gas-phase reaction mediated by plasma and the catalyst surface reaction, the increased NH3 concentration under plasma-catalysis condition can be attributed the synergistic effect between plasma and catalyst, where plasma irradiation promoted the desorption of NH3 and NHx species. These results are helpful for the design of highly efficient catalysts and the understanding for the role of plasma in plasma-enabled green chemistry.

Colloid carbonization-stabilized Ru nanoparticle catalyst for efficient ammonia synthesis at mild conditions

Ruthenium-based catalysts are widely used in ammonia (NH3) synthesis because of a high affinity for N2 activation, while confronting the challenge of facile deactivation due to sintering. Herein, for the first time, we develop a highly active and sintering-resistant Ru2.1-CC catalyst via colloid carbonization (CC) for NH3 synthesis. It exhibits a superior NH3 synthesis rate around 9.4-fold that of Ru catalyst synthesized by impregnation method, displaying extremely high stability without aggregation of Ru nanoparticle after 100 h stability test. Our studies show that Ru2.1-CC possesses the average Ru particle size of 2.1 nm with narrow size distribution and thus exposes more active sites for N2 activation. Moreover, the formation of Ru-C interaction via colloid carbonization over Ru2.1-CC can stabilize Ru particle size to realize high stability, meanwhile reduce work function and upshift the d-band center of Ru metal to accelerate its electron transfer for N2 activation and NH3 synthesis. This study provides a new strategy for simple synthesis of noble metal-based catalysts that are highly active and stable for NH3 synthesis.

Nanoporous NiBi catalyst for efficient electrochemical N2 fixation

Electrochemical nitrogen reduction reaction (NRR) in ambient condition provides a sustainable avenue for large-scale NH3 production. However, the development of effective and durable NRR catalysts to promote NH3 synthesis rate and Faraday efficiency (FE) remains a challenge due to the inertness of the N≡N bond. Herein, porous NiBi catalysts which provides abunant active sites were prepared by chemical dealloying of the precursor. Combining theoretical calculations and experimental verification, it is demonstrated that the synergistic effect of Ni and NiBi facilitates the adsorption of N2, inhibits the competition reaction, and reduces the free energy of intermediates (*NNH). Porous Ni91.5Bi8.5 exhibits high catalytic activity for NH3 formation with a yield of 18.35 μg h−1 mg−1 and a FE of 51.12% at −0.3 V vs. RHE, outperforming most reported NRR electrocatalysts under ambient conditions. This strategy is an effective and universal technique for synthesizing catalysts that shows great potential in energy conversion applications.

Plasma-catalytic ammonia synthesis in a dielectric barrier discharge reactor: A combined experimental study and kinetic modeling

Plasma-catalytic ammonia synthesis in a dielectric barrier discharge reactor has emerged as a possible route for electrification of nitrogen fixation. In this study, we use a combination of experiments and a plasma kinetic model to investigate the ammonia synthesis from N2 and H2, both with and without a solid packing material in the plasma zone. The effect of plasma power, feed flow rate, N2:H2 feed ratio, gas residence time, temperature, and packing material (MgAl2O4 alone or impregnated with Co or Ru) on the ammonia synthesis rate were examined in the experiments. The kinetic model was employed to improve our understanding of the ammonia formation pathways and identify possible changes in these pathways when altering the N2:H2 feed ratio. A higher NH3 synthesis rate was achieved when increasing the feed flow rate, as well as when increasing the gas temperature from 100 to 200 °C when a packing material was present in the plasma. At the elevated temperature of 200 °C, an optimum in the NH3 synthesis rate was observed at an equimolar feed ratio (N2:H2 = 1:1) for the plasma alone and MgAl2O4, while a N2-rich feed was favored for Ru/MgAl2O4 and Co/MgAl2O4. The optimum in the synthesis rate with the N2-rich feed, where high energy electrons are more likely to collide with N2, suggests that the rate-limiting step is the dissociation of N2 in the gas phase. This is supported by the kinetic model when packing material was used. However, for the plasma alone, the model found that the N2 dissociation is only rate limiting in H2-rich feeds, whereas the limited access to H in N2-rich feeds makes the hydrogenation of N species limiting.

Dynamic surface-coverage alteration based on microkinetic analysis for enhanced ammonia synthesis over ruthenium catalysts at low temperatures

The ammonia (NH3) synthesis reaction has been well established via Harbor-Bosch process, but recent demands of NH3 as a hydrogen carrier necessitates the synthesis operations at much milder conditions for renewable energy utilization. Catalytic ammonia synthesis is among the most investigated heterogeneous catalytic reactions but the detailed kinetic studies especially at low temperatures are still missing. This study investigated thorough microkinetic analysis at a wide range of reaction temperatures from 200 to 400 °C for NH3 synthesis. Although the united kinetic expression for the NH3 synthesis was established at all temperature range, the rate at low temperatures was found to be strongly reduced not only by hydrogen but also NH3 (NHx) surface intermediates that become more significant as the conversion increases. Knowing these strong inhibitions by both the reactant (H2) and product (NH3), transient perturbation of the surface intermediates by alternating between pure H2 and N2 gas flow was tested, which may lead to rate jumps by controlling the most abundant surface intermediates. The results show dramatic enhancement of the overall NH3 synthesis rate of as high as > 1 mmol g−1 h−1 over 5 wt% Ru/CeO2 catalysts at 200 °C and 0.1 MPa. The results not only give insight into improving existing catalysts and catalytic reaction operations, but also cast doubts on the consistency of how to report the rates of ammonia synthesis due to the strong dependence of the catalysts on ammonia yield, especially at low temperatures.

Mechanochemical Synthesis of RuCo/MgTiO3 Catalysts for Nonthermal Plasma-Assisted Ammonia Synthesis

The development of robust yet effective catalysts is vital for promoting nonthermal plasma (NTP)-assisted ammonia (NH3) synthesis toward practical applications. Herein, we report a simple mechanochemical synthesis method (via ball milling) for preparing bimetallic RuCo supported on MgTiO3 catalyst (i.e., RuCo/MgTiO3-6-500) to be used in NTP-assisted catalytic NH3 synthesis. The physicochemical properties of the developed catalysts were investigated systematically with respect to the relevant process parameters of ball milling. Catalytic tests showed that, at a low specific energy input of 15 kJ L–1, the developed RuCo/MgTiO3-6-500 catalyst exhibited a high NH3 synthesis rate and an energy efficiency of ∼50 μmol min–1 gcat–1 and 1.54 gNH3kWh–1, respectively, which outperformed the monometallic Ru or Co control catalysts. Additionally, the energy barriers were calculated as a function of N2/H2 ratio for the NTP-catalytic NH3 synthesis over RuCo/MgTiO3-6-500, and the results were interpreted together with the activity data measured during the catalysis. It was found that the energy barrier approach may not be appropriate for discussing the energy required for the dynamics of interest in the NTP-catalytic system. Compared to MgTiO3-packed and plasma-alone systems, results by in situ optical emission spectroscopic (OES) diagnosis of the plasma-catalytic systems showed that the presence of active metal sites (i.e., Ru and/or Co) could intensify the activation and dissociation rates of N2 and H2 molecules, showing the relatively higher peak intensity of NH, N2*, N2+, and Hα species. However, based on the obtained information, the possible roles of Ru and/or Co in NTP-assisted ammonia synthesis system are difficult to be identified.

Efficient ammonia synthesis over Ru/CeO2-PrOx catalysts with controlled Ru dispersion by Ru-Pr interaction

Ru/CeO2-PrOx catalysts for NH3 synthesis were prepared using the urea-based homogeneous co-precipitation method, and the effect of Pr-doped CeO2 support on the catalytic activity was investigated. An optimal range of Pr content (25–75 %) improved NH3 synthesis activity. CO pulse adsorption and operando X-ray absorption fine structure (XAFS) measurements revealed that the increase in NH3 synthesis activity was due to the improvement in Ru dispersion by the interaction between PrOx and Ru, rather than the generally discussed Ce3+/Ce4+ ratio. However, excess Pr doping prevented the Ru reduction and formation of active sites. In this Ru/CeO2-PrOx catalyst system, the activity is governed by the number of active sites that can be controlled by the Pr content. In the high-pressure evaluation (5 MPaG), the NH3 synthesis rate of the Ru/CeO2-PrOx catalyst (Ce/Pr = 1) was 9.0 mmol g-1h−1, which is 1.4 times higher than that of the Ru/CeO2 catalyst.

Essential Role of Ru–Anion Interaction in Ru-Based Ammonia Synthesis Catalysts

The metal–anion interaction (MAI) widely exists over supported metal catalysts, which has a significant effect on tuning metal sites and reaction performance. However, the effect of MAI on NH3 synthesis and the reaction mechanism is still elusive. Here, we report that the strength of Ru–anion interaction gradually increases from Ru–N to Ru–H and Ru–O when the anion of the support is changed from H (ZrH2) to N (ZrN) and O (ZrO2). Moreover, the Ru–anion interaction induced by different support compositions can affect the hydrogen spillover and N2 activation route. Due to the weak Ru–N interaction, the aggregation of Ru particles is observed over Ru/ZrN. Although Ru particles are highly dispersed over ZrO2 owing to the strong Ru–O interaction, hydrogen poisoning is inevitable over Ru/ZrO2. Comparatively, hydrogen poisoning can be alleviated over Ru/ZrH2 via hydrogen spillover to its support. The results of NEXAFS, in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and isotope-labeling experiments show that N2 can be activated via both dissociative and associative routes over Ru/ZrH2. Consequently, the developed Ba-promoted Ru/ZrH2 catalyst displays a high NH3 synthesis rate of 27.5 mmol g–1 h–1 and robust stability during 300 h time-on-stream at 400 °C and 1 MPa.

Free-Standing Nanoarrays with Energetic Electrons and Active Sites for Efficient Plasmon-Driven Ammonia Synthesis.

Direct ammonia (NH3 ) synthesis from water and atmospheric nitrogen using sunlight provides an energy-sustainable and carbon-neutral alternative to the Haber-Bosch process. However, the development of such a route with high performance is impeded by the lack of effective charge transfer and abundant active sites to initiate the nitrogen reduction reaction (NRR). Here, the authors report efficient plasmon-induced photoelectrochemical (PEC) NH3 synthesis on the hierarchical free-standing Au/Kx MoO3 /Mo/Kx MoO3 /Au nanoarrays. Endowed with energetically hot electrons and catalytically active sites, the plasmonic nanoarrays exhibit an efficient PEC NH3 synthesis rate of 9.6µg cm-2 h-1 under visible light irradiation, which is among the highest PEC NRR systems. This work demonstrates the rationally designed plasmonic nanoarrays for highly efficient NH3 synthesis, which paves a new path for PEC catalytic reactions driven by surface plasmons and future monolithic PEC devices for direct artificial photosynthesis.

Insight into the critical role of strong interaction between Ru and Co in RuCo single-atom alloy structure for significant enhancement of ammonia synthesis performance

Rational design of efficient catalysts for ammonia (NH3) synthesis at mild conditions is of paramount importance for the development of electrolysis driven Haber-Bosch (eHB) process. Ammonia synthesis is a structure sensitive reaction, and any small change of electronic structure could result in dramatic change of catalytic activity, especially for the Ru-based NH3 synthesis catalysts. Electronic structure modulation of the Ru-based catalysts could provide desirable catalytic activity. Herein, we explore a class of Ru-Co single-atom alloy catalysts (marked as RuxCo1 SAA, x stands for Ru/Co molar ratio), in which Co single atoms are located on Ru nanoclusters to form Co-Ru coordination, and then electronic structure of RuxCo1 SAA could be effectively tuned by regulating the Ru/Co molar ratio. Our studies show that a moderate ratio of Ru/Co (1.7:1) favors superior NH3 synthesis rate and low activation energy. Using a suite of elaborate characterizations, we have observed that the outstanding performance over Ru1.7Co1 SAA can be attributed to the electron redistribution between Ru and Co atom, resulting in an upshift of d band center and lowering of work function. However, there is decrease of NH3 synthesis rate when Ru/Co molar ratio is 2, due to the partial detachment of Ru entities from SAA structure, and the formed individual Ru nanoparticles (NPs) overlay the surface of catalyst. In such a case, N2 activation behavior over Ru2Co1 is alike that of Ru NPs catalysts. The present study demonstrates that the Ru/Co ratio in SAA structure can effectively regulate the electronic structure of catalyst for NH3 synthesis rate enhancement.

Size-dependent activity of supported Ru catalysts for ammonia synthesis at mild conditions

Ammonia (NH3) synthesis is a structure-sensitive reaction, where a minute variation of catalyst structure would lead to a dramatic change of activity. To date, the roles of Ru size on NH3 synthesis still remain elusive, and it remains a major challenge. Herein, a series of Ru catalysts with sizes ranging from 1.4 to 5.0 nm were prepared by colloidal Ru deposition, which allows the investigation of size influence on NH3 synthesis. Through analyses of the geometric and electronic properties of variable-sized Ru particles, it shows that the reduction of Ru particle size could increase the proportion of corner sites while decrease that of terrace sites, which can decrease the work function and thus promote N2 for its activation. Isotopic investigations show that the decrease of dissociative contribution but the increase of associative contribution as Ru particle size was decreased from 5.0 to 1.4 nm. Meanwhile, the NH3 synthesis rate increases when the size of Ru catalysts decreases from 5.0 to 1.4 nm. Using a suite of elaborate characterizations, we have observed that 1.4 nm Ru nanoclusters (NCs) with ample corner sites are conducive to activate N2 via an associative route. As a result, the 1.4 nm Ru NCs catalyst shows the highest NH3 synthesis rate (up to 17.13 mmolNH3 gcat−1h−1) at 400 ℃ and 1 MPa.

Catalytic photo-redox of simulated air into ammonia over bimetallic MOFs nanosheets with oxygen vacancies

Solar-driven conversion of nitrogen (N2) into ammonia (NH3) is a promising alternative to the Haber-Bosch process, while still suffers from low conversion efficiency due to inactive NN bond. Herein, a novel pathway of photocatalytic air redox reaction (ARR) to ammonia via NO is proposed and tested over an effective catalyst of oxygen-vacancy-rich bimetallic Cu-Co organic framework ultrathin nanosheets (OVR-CuCo-MOFs NS) under visible light. The catalyst with unique oxygen defective sites shows an excellent NH3 synthesis rate from air (287.76 ± 7.02 μmol g-1·h-1), which is 5.4-fold higher than that from pure N2. Moreover, experiments and theoretical calculations indicate that the transformation of air mainly follows a redox pathway, in which N2 and O2 can be trapped at the oxygen vacancies to generate nitric oxide (*NO) and further be reduced to ammonia by visible light. The ARR process shows a lower barrier of free energies in the onset activation step (*N2 → *N-NO, −0.08 eV) and rate-limiting step (*NO → *NHO, 1.23 eV) compared with those of traditional nitrogen reduction (*N2 → *N-NH, 1.48 eV and H2N-NH2 → *NH2, 1.29 eV, respectively). This work provides a new and sustainable pathway for photo-driven ammonia synthesis.

Ru alloying with La or Y for ammonia synthesis via integrated dissociative and associative mechanism with superior operational stability

The alloying of Ru with rare-earth elements (REEs) is an effective approach for the modification of the electronic structure of Ru. Here, for the first time, we report the formation of Ru-M (M = La or Y) alloys that are endowed with a completely new type of active sites. Over the Ru-M (M = La or Y) catalysts, N2 activation in ammonia synthesis follows neither the well-known dissociative nor the associative route but rather a co-operation of both, leading to outstanding performance. The developed RuLa/HZ catalyst exhibits a NH3 synthesis rate of 14.73 mmol gcat−1 h−1 at 400 °C and 1 MPa without obvious deactivation in a run of 1756 h (ca.73 days).

Unraveling the size-dependent effect of Ru-based catalysts on Ammonia synthesis at mild conditions

Identifying optimal Ru size in NH3 synthesis can improve reaction activity and maximize the utilization of Ru to reduce catalyst cost. However, previous researches are focused on large Ru particle size (≥2 nm) while that below 2 nm in NH3 synthesis is unclear. Here we synthesized a series of Rux/BaCeO3 with different Ru sizes (x = 1.1–3.0 nm) through size-controlled Ru colloid. With the decrease of Ru size, NH3 synthesis rate over Ru1.1/BaCeO3 increases to 19.4 mmol gcat−1h−1 at 400 °C and 1 MPa, which is 5.7 times that of Ru3.0/BaCeO3 and superior to most of Ru-based catalysts previously reported. It reveals that the reduction of Ru size enhances the generation of Ce3+ and oxygen vacancies in BaCeO3, which can donate electron to Ru centers and promote N2 dissociation. Moreover, the small Ru size enhances hydrogen spillover from Ru to BaCeO3 to alleviate hydrogen poisoning, resulting in efficient NH3 synthesis.