Ru Catalysts For Ammonia Synthesis Research Articles

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.

Zirconia prepared from UIO-66 as a support of Ru catalyst for ammonia synthesis

The development of effective Ru catalyst for ammonia synthesis is of important practical value and scientific significance because of the wide application of ammonia as a fertilizer and its promising applications in the renewable energy. Generally, ZrO2 was regarded as an inferior support for Ru catalyst used in ammonia synthesis. Here we prepare ZrO2 with monoclinic phase and carbon species from ZrCl4 following the preparation route of UiO-66 as well as ammonia treatment. Owing to the presence of a larger amount of hydrogen adsorption as well as the easier desorption of hydrogen species, the ill effect of hydrogen species on the nitrogen adsorption-desorption and ammonia synthesis can be effectively alleviated. The resulting ZrO2-supported Ru catalyst showed 4 times higher ammonia synthesis activity than the conventional Ru/ZrO2 obtained from zirconium nitrate.

Titanium modified Ru/CeO2 catalysts for ammonia synthesis

In catalytic reaction that involves hydrogen species, the adsorption/desorption property of hydrogen species would greatly affect the catalytic performance. In this work, we introduce titanium into ceria to tune the properties of ceria-supported Ru catalysts for ammonia synthesis, and the result shows that the ammonia synthesis rate of Ru catalysts exhibits a volcanic trend with an increase of titanium content. XRD and Raman results reveal that an appropriate amount of Ti addition would be incorporated into CeO2 lattice to form CeTiO2-x solid solution, while the presence of excess TiO2 leads to the formation of TiO2 particles. Raman and UV-vis results of Ru catalysts confirm that CeTiO2-x serves as preferential platform for the dispersion of Ru species. Moreover, the O 1s spectra, CO-TPR and 1H NMR indicate that the introduction of titanium decreases the number of the active oxygen species. As a result, the introduction of an appropriate amount of Ti species in Ru/CeO2 catalysts leads to enhancement of the hydrogen species desorbed at low temperature, contributing to N2 dissociation or ammonia synthesis as well as the alleviation of hydrogen poisoning. The sample with 3 mL titanium butoxide (19.4 wt% of TiO2) exhibits the optimal ammonia synthesis rates. This work demonstrates a general approach of the decoration of oxide supports to tune the adsorption property and the catalytic activity for the catalytic reaction that involves hydrogen species.

Size sensitivity of supported Ru catalysts for ammonia synthesis: From nanoparticles to subnanometric clusters and atomic clusters

<h2>Summary</h2> 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 <i>d</i>-orbitals with the <i>σ</i> and <i>π</i> 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.

Effect of texture and physical properties of catalysts on ammonia synthesis

The catalysts with higher efficiencies at mild reaction conditions are of great importance for development of ammonia synthesis with zero CO2 emissions. Here, we report, highly efficient mixed oxide supported Ru catalysts for ammonia synthesis. MgO-CeO2 supports with different Mg and Ce contents were fabricated and loaded with 1% Ru using impregnation technique. MgO-CeO2 supported Ru catalysts demonstrated efficient ammonia synthesis. Mixed oxide MgO-CeO2 supported catalysts even with higher MgO content were established with comparable ammonia synthesis yields to that obtained from pure CeO2 supported catalyst under same reaction conditions. The catalytic activity was highly dependent on the texture and physical properties of the catalysts. Presence of highly dispersed Ce and Ru has been suggested as a possible explanation for the higher yield of ammonia obtained from Ru/MgO-CeO2 catalysts having Ru selectively deposited on CeO2.

Why Ca2NH works as an efficient and stable support of Ru catalyst in ammonia synthesis

Hydride-based materials have recently attracted attention because of their significant promotion effect on transition metal catalysts in ammonia synthesis under mild conditions. Here, we clarify the effect of hydride-nitride, Ca2NH, on the activity and stability of Ru catalyst as a catalyst support for ammonia synthesis. The anionic electrons formed at H‒ ion vacancy sites in Ca2NH effectively promote the N2 dissociation over Ru surface, which accounts for the high catalytic performance with a low apparent activation energy. The catalytic activity of Ru/Ca2NH is much superior to those of Ru/C12A7:e‒, Ru/Sr2NH, and Ru/CaNH. The simple metal hydride, CaH2, with Ru exhibits higher catalytic performance than Ru/Ca2NH, but its stability is poor because weak Ru-CaH2 interaction causes aggregation of Ru nanoparticles during the reaction. On the other hand, Ru nanoparticles are anchored on Ca2NH surface through a strong Ru–N interaction, which leads to excellent stability of Ru/Ca2NH catalyst.

Investigation on Deactivation of K‐promoted Ru Catalyst for Ammonia Synthesis by CO Formation

AbstractThe 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.

Pure Siliceous Zeolite-Supported Ru Single-Atom Active Sites for Ammonia Synthesis

As the most active new frontier, a single-atom catalyst (SAC) combining the merits of heterogeneous and homogeneous catalysts would have a significant effect on the 100 year history of ammonia synthesis research. Here, the Ru SAC has been demonstrated to be active and efficient for ammonia synthesis. To this end, an ideal model catalyst, pure siliceous zeolite-supported Ru SAC (Ru SAs/S-1), which shows surprising catalytic ammonia synthesis activity compared to that of a conventional Ru catalyst, was designed. Both atomic-resolution scanning transmission electron microscopy, X-ray absorption spectrometric analysis, and in situ diffuse reflectance infrared Fourier transform spectroscopy identify the single-Ru-atom nature of Ru SAs/S-1 before and after the reaction. Further DFT calculations reveal that the reaction mechanism is different from traditional mechanisms. Therefore, this paper provides an alternative SAC strategy to design high-performance Ru catalysts for ammonia synthesis.

Effects of Using Carbon-Coated Alumina as Support for Ba-Promoted Ru Catalyst in Ammonia Synthesis

Ru catalysts using alumina (Al2O3) as support are inferior in ammonia synthesis. In the present study, we coated alumina with carbon to enhance the activity of Ba-promoted Ru/Al2O3 catalyst. The su...

Acid-durable electride with layered ruthenium for ammonia synthesis: boosting the activity via selective etching.

Ruthenium (Ru) loaded catalysts are of significant interest for ammonia synthesis under mild reaction conditions. The B5 sites have been reported as the active sites for ammonia formation, i.e., Ru with other coordinations were inactive, which has limited the utilization efficiency of Ru metal. The implantation of Ru into intermetallic compounds is considered to be a promising approach to tune the catalytic activity and utilization efficiency of Ru. Here we report an acid-durable electride, LnRuSi (Ln = La, Ce, Pr and Nd), as a B5-site-free Ru catalyst. The active Ru plane with a negative charge is selectively exposed by chemical etching using disodium dihydrogen ethylenediaminetetraacetate (EDTA-2Na) acid, which leads to 2-4-fold enhancement in the ammonia formation rate compared with that of the original catalyst. The turnover frequency (TOF) of LnRuSi is estimated to be approximately 0.06 s-1, which is 600 times higher than that of pure Ru powder. Density functional theory (DFT) calculations revealed that the dissociation of N2 occurs easily on the exposed Ru plane of LaRuSi. This systematic study provides firm evidence that layered Ru with a negative charge in LnRuSi is a new type of active site that differs significantly from B5 sites.

Carbon support surface effects in the catalytic performance of Ba-promoted Ru catalyst for ammonia synthesis

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.

Anchoring Bond between Ru and N Atoms of Ru/Ca2NH Catalyst: Crucial for the High Ammonia Synthesis Activity

Key functions for the stability and activity of Ru catalysts for ammonia synthesis were examined by x-ray absorption fine structure (XAFS) experiments. Ru K-edge XAFS measurements were carried out for two Ru catalysts, Ru/Ca2NH and Ru/CaNH, which include N atoms in the supports. The stable Ru-N bond was observed in the Ru/Ca2NH catalyst, but not in the Ru/CaNH catalyst, which was also supported by theoretical calculations in terms of Ru--anion bond strength. The bond observed in the Ru/Ca2NH works to anchor the Ru nanoparticles to the Ca2NH support, and the activity has been maintained. Formation of anchoring bonds between catalyst particles and supports is essential to create highly active and long lasting catalysts.

Carbon covered Mg–Al hydrotalcite supported nanosized Ru catalysts for ammonia synthesis

Carbon covered hydrotalcite (CCHT) supported Ru catalysts with single (Cs or Ba) and double (Cs and Ba) promoters were prepared by two different methods viz., impregnation (IMP) and polyol reduction (PR) methods using ethylene glycol as solvent. The performance of the catalysts for ammonia synthesis was carried out in the temperature range of 523–698K at atmospheric pressure. The following sequence with respect to the % NH3 (volume/volume) formation was observed; BaRuCCHT-IMP<BaRuCCHT-PR<CsBaRuCCHT-IMP<CsRuCCHT-IMP<CsRuCCHT-PR<CsBaRuCCHT-PR. XRD, TEM, N2-adsorption, H2-TPR, CO chemisorption and TPD of H2 and CO2 techniques were used to investigate the physico-chemical properties of the catalysts and also to establish the reasons responsible for superior catalytic activity of CsBaRuCCHT-PR catalyst. The CO chemisorption and TEM studies revealed that CsBaRuCCHT-PR catalyst possessed the high active Ru metal surface area and small particle size. The highest activity of this catalyst could be attributed to high surface area, low particle size, high active surface metal (Ru) area, and strong Ru–CCHT interaction.

Influence of chemical treatments of activated carbon support on the performance and deactivation behavior of promoted Ru catalyst in ammonia synthesis

A series of Ru/activated carbon (AC) catalysts promoted with potassium and barium were prepared by impregnation method for ammonia synthesis. The chemical treatments effects of the support on catalytic performance were studied by modifying AC with H2 (ACH) and H2 + nitric acid (ACHN). The catalysts were characterized by using nitrogen adsorption/desorption, TEM, SEM, EDS and XRD techniques. The catalytic performance was carried out in a fixed-bed reactor at 400–450 °C, 30 bar and N2:H2 (1:3). The order for both activity and stability was ACHN > ACH > AC according to the support type. The excellent performance of K–Ba–Ru/ACHN catalyst was attributed to efficient chlorine and sulfur removal from AC and high distribution of Ru and promoters. The reaction was found not to be intra-particle diffusion limited. A second-order independent decay law was proposed for catalyst deactivation. Deactivation rate constants of 0.016h−1 and 0.0018h−1 were obtained for AC and ACH supported samples, respectively.

Highly Dispersed Ru on Electride [Ca24Al28O64]4+(e–)4 as a Catalyst for Ammonia Synthesis

A high surface area 12CaO·7Al2O3 electride (HT-C12A7:e–) was synthesized by the hydrothermal process and subsequent reduction with CaH2 as a reducing agent, and was then examined as a support for a Ru catalyst in ammonia synthesis. Electrons are successfully incorporated into the cages of C12A7 powder by reduction with CaH2 at 973 K. The concentration of encaged electrons increases with the dehydration temperature of the sample before the CaH2 treatment. The surface area of the resulting material (ca. 20 m2 g–1) is higher than that of the C12A7 electride prepared by conventional solid-phase reaction (SP-C12A7:e–, ca. 1 m2 g–1). The rate of ammonia synthesis over Ru/HT-C12A7:e– is twice that of Ru/SP-C12A7:e–, and there is no significant difference in the apparent activation energy between both catalysts under the optimal conditions. Transmission electron microscopy (TEM) and CO chemisorption measurements revealed that Ru nanoparticles are more highly dispersed on HT-C12A7:e– than that on SP-C12A7:e–, whic...

Effect of pore structure of mesoporous carbon on its supported Ru catalysts for ammonia synthesis

Mesoporous carbon (MC) was prepared by a hard-template method and used as support for the preparation of a Ru-based ammonia synthesis catalyst, Ba-Ru-K/MC. N 2 adsorption-desorption, scanning electron microscopy, and transmission electron microscopy were used to characterize the mesoporous carbon and its supported Ru catalysts. The effects of pore structure of the Ba-Ru-K/MC catalyst on its performance for ammonia synthesis were studied. The results show that the surface area of the mesoporous carbon material varies with the SiO 2 /C mass ratio and reaches the largest at 1.0 of SiO 2 /C. The catalytic activity of Ba-Ru-K/MC for ammonia synthesis increases with increased mesoporous surface area of the mesoporous carbon. The reaction rate of ammonia synthesis is 139 mmol/(g cat ·h) at 425 °C, 10 MPa, and a gas hourly space velocity of 10000 h −1 . The pore structure of the Ba-Ru-K catalyst supported on mesoporous carbon was controlled by changing the particle size of SiO 2 and the SiO 2 /C ratio. The catalyst activity for ammonia synthesis increased with an increased mesoporous surface area.

Highly effective perovskite-type BaZrO3 supported Ru catalyst for ammonia synthesis

The ammonia synthesis was performed over Ru catalyst supported on perovskite-type BaZrO3. Physiochemical properties of this catalyst were also characterized by means of various techniques to show the relationships between catalyst structure and catalytic performance. It was found that the ammonia concentration in the effluent of Ru/BaZrO3 catalyst was 6.36% at 673K under 3.0MPa, which was much higher than that of Ru catalyst supported on other supports including carbon material systems under the similar conditions, and the rate did not decrease even if the reaction was executed for 120h. This might be attributed to the facts that the optimum size of Ru particles has more B5-type sites, strong electron-donating ability of the BaZrO3 support and the strong metal–support interaction provides an efficient channel for electron transfer. On the other hand, the adsorption of H2 was inhibited over the surface of this catalyst and the number of active sites for the activation of N2 was also increased.

KOH Activation of Thermally Modified Carbon as a Support of Ru Catalysts for Ammonia Synthesis

AbstractCarbon 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.

Preparation of Ru/Ba-ZrO2Catalyst and Its Performance for Ammonia Synthesis

分别采用柠檬酸络合法、改性共沉淀法和湿浸渍法制备了掺Ba纳米ZrO2材料,负载Ru后用于催化氨合成反应.采用X射线衍射、CO2程序升温脱附（CO2-TPD）、N2物理低温吸附、H2程序升温还原技术（H2-TPR）、扫描电镜（SEM）、透射电镜（TEM）、X射线光电子能谱（XPS）和CO化学吸附对载体材料和催化剂进行了表征.结果表明,不同方法制备载体的物相结构和织构性能均有明显差别,负载Ru后催化剂的氨合成性能差别也较大.其中,以柠檬酸络合法制备的载体材料中Ba以BaZrO3的形式存在,钙钛矿型BaZrO3具有较强的供电子能力,电子可以通过Ru与载体间强相互作用传递到Ru表面,有效地促进N≡N的断裂,使催化剂的低温活性显著提高.在425℃,3 MPa,空速为10000 h-1条件下,出口氨浓度为5.72%.其氨合成活性分别是改性共沉淀法和湿浸渍法制备催化剂的3.8倍和14.3倍.

Effect of Chlorine on the Chemisorptive Properties and Ammonia Synthesis Activity of Alumina-Supported Ru Catalysts

A series of Ru/Al2O3 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 RuCl3 would not influence on Ru 3d5/2 binding energy, but chlorine from HCl solution significantly increased Ru 3d5/2 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 RuCl3 precursor. With a similar amount of residual chlorine, the catalysts with chlorine originated from the RuCl3 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. Chlorine affects on the sites at the metal/support interface for Ru/Al2O3 catalysts, and then hydrogen adsorption and ammonia synthesis both are suppressed.