English

• Ammonia (NH₃) has attracted worldwide interest because of its successful applications in as a fertilizer in crop production as well as its promising applications in the renewable energy [1-3]. However, the high energy-consumption and high emission of ammonia production (Haber–Bosch process using Fe catalyst), which represents 1%–2% world's energy every year and more than 1% CO₂ emission [4, 5], impedes the widely application of ammonia in the future. Diverse strategies [6-11], including electrocatalysis, photocatalysis, plasma catalysis, have been used to develop the environmentally-benign ammonia synthesis process. Unfortunately, the efficiency of these methods is far below commercially viable levels [6, 7], and Haber-Bosch process is still the only method for the large-scale ammonia production, and thus the continuing improvement in catalyst is necessary to lower the energy consumption [7, 12-15]. The replacement of Fe catalyst by carbon-supported Ru catalyst can decrease significantly the energy consumption of ammonia synthesis [16], but the application of Ru catalyst in ammonia synthesis is greatly inhibited by carbon loss, including carbon methanation and carbon oxidation [17, 18]. It is highly desirable to develop an effective Ru catalyst for ammonia synthesis, and oxide-supported Ru catalysts are supposed to be the promising candidates.

  It has been reported that ZrO₂ was inferior supports for Ru catalysts of ammonia synthesis [19]. However, the catalytic activity of oxide-supported metal catalyst could be improved by tuning the metal–support interaction [20-24]. The N₂ dissociation is proposed to be the rate-determining step of ammonia synthesis [25-27], but other steps, including the hydrogen activation, the reactions of H and N atoms, as well as the desorption of ammonia or other unreacted gases (N₂ and H₂), also can strongly affect the ammonia synthesis activity of Ru catalysts. Wu et al. found that owing to the higher mobility of hydrogen species, the polar MgO(111) supportported Ru catalysts showed higher ammonia synthesis activities than those supported on nonpolar MgO [28]. Lin et al. reported that the change in the alumina phase affected the desorption/desorption property of hydrogen species, leading to enhancement of ammonia synthesis activities for alumina-supported Ru catalysts [29]. In the meantime, it has been found that various pre-treatment methods, including of N₂H₄ reduction [30], NaBH₄ treatment [31], CO activation [32] and sacrificial sucrose strategy [22], would affect the adsorption-desorption property and the desorption pathway of hydrogen species for ceria-supported Ru catalysts, resulting in enhancement of the ammonia synthesis rates. As a result, it is envisaged that ZrO₂ or other oxides might be able to be support candidate for Ru catalyst used in ammonia synthesis by optimizing of hydrogen adsorption-desorption property.

  In this work, we prepared ZrO₂ from ZrCl₄ following the preparation route of UiO-66. The results showed that the ammonia-treated ZrO₂ prepared from ZrCl₄ contained carbon species and monoclinic ZrO₂, the resulting Ru/ZrO₂ catalyst showed 4 times higher ammonia synthesis activity than the counterpart obtained from zirconium nitrate. The superior performance can be attributed to a larger amount of hydrogen adsorption as well as the easier desorption of hydrogen species, which resulted in the alleviation ill effect of hydrogen species on the nitrogen adsorption-desorption and ammonia synthesis. This work confirms the possibility that the design of high active Ru catalyst for ammonia synthesis by the use of those oxides, which are traditionally not considered to be an ideal support for Ru catalyst.

  ZrO_2-O and ZrO_2-N were prepared from ZrCl₄ following a preparation route of UiO-66 [33], the resulting UiO-66 was calcined in air or NH₃ at 600 ℃ for 3 h to obtain ZrO_2-O (air) or ZrO_2-N (NH₃). The traditional zirconia (ZrO₂) was obtained by decomposing of ZrO₂ sol–gel solution [34]. Ru was introduced by incipient wetness impregnation of zirconia with ruthenium(Ⅲ) nitrosyl nitrate solution. The diffraction peaks of tetragonal zirconia at 2θ = 30.3, 34.6, 35.3, 50.3, 50.7, 59.4, 60.2, 62.9, 74.6 and 81.8° can be found in the XRD patterns of Ru/ZrO₂ and Ru/ZrO_2-O (Fig. 1). However, besides the peaks of tetragonal zirconia, the new diffraction peaks at about 24.5, 28.2 and 31.6°, which are assigned to the monoclinic zirconia (PDF #81–1314) [35-37], can be found in the XRD patterns of Ru/ZrO_2-N. It can conclude that the tetragonal ZrO₂ is dominant phase in Ru/ZrO₂ and Ru/ZrO_2-O, while there are the mixed-phases of tetragonal and monoclinic ZrO₂ in Ru/ZrO_2-N. A similar conclusion on the difference in zirconia phases could be drawn from Rietveld analysis based on the as-prepared ZrO₂ supports (Fig. S1 in Supporting information). On the other hand, no characteristic diffraction peaks corresponding to Ru metal or Ru oxide are found, indicating that Ru species is well dispersed on support material. This result is in line with the observations of EDS mappings and TEM images (Figs. S2 and S3 in Supporting information). The results of temperature-programmed oxidation study of ZrO_2-O and ZrO_2-N show that there is a larger amount of carbon species on ZrO_2-O than that on ZrO_2-N (Fig. S4 in Supporting information). However, the differences in the amount of carbon species between Ru catalysts are slight because most unstable carbon would be removed during the heat treatment of Ru catalysts. As a consequence, elemental analysis shows that the carbon contents of Ru/ZrO_2-O and Ru/ZrO_2-N are 3.67 and 3.50 wt%, respectively (Table S1). A similar amount of carbon species remained on Ru/ZrO_2-O and Ru/ZrO_2-N is also confirmed by the result of Raman spectra (Fig. S5 in Supporting information), and two bands at 1345 (D band) and 1595 cm⁻¹ (G band), which are characteristic peaks of carbon [38-41], can be found in Raman spectra of Ru catalysts. The presence of carbon species leads to enhancement of the surface area for Ru catalysts, but no significant difference in the surface area can be found between Ru/ZrO_2-O and Ru/ZrO_2-N catalysts (Table S1 in Supporting information). Moreover, the Ru loadings are 5.55, 5.28 and 5.37 wt% for Ru/ZrO₂, Ru/ZrO_2-O and Ru/ZrO_2-H (Table S1), indicating that the variation in catalytic performances of Ru catalysts cannot be fully attributed to the change in Ru content.

  Figure 1

  

  Figure 1.  XRD patterns of the reduced Ru catalysts.

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  As shown in the XPS spectra of Ru catalysts (Fig. 2), the ratio of C 1s to Ru 3d are 0.29, 0.68 and 0.59 for Ru/ZrO₂, Ru/ZrO_2-O and Ru/ZrO_2-N catalysts, confirming that there is a larger amount of carbon species for the samples prepared from ZrCl₄. The binding energy of Ru metal appears at about 279.8 eV for Ru/ZrO₂, while the values can be found at 280.0 eV for Ru/ZrO_2-O and Ru/ZrO_2-N. It is well known that electronic metal support interaction would affect the electronic property of metal−support interfacial sites [42, 43]. The lower Zr binding energy and Ru binding energy of Ru/ZrO₂ indicates that there is an electronic metal support interaction for ZrO₂ supported Ru catalysts, which is accordance with the findings on Au@TiO_2−x/ZnO [42]. On the other hand, the Zr 3d peak and Ru 3d peak appear at higher binding energy for the samples with carbon species, indicating that the electronic metal support interaction would be lessened by the presence of carbon species. Besides metallic Ru, two other Ru 3d_5/2 peaks at 280.8 and 281.7 eV, which are characteristic XPS peaks of Ru oxides (RuO₂ and RuO_x/Ru) [29, 44-48], can be found in the XPS spectra of Ru catalysts. The slight difference in the ratio of Ru⁰/(Ru⁰ + Ruⁿ⁺) (0.70, 0.75 and 0.77 for Ru/ZrO₂, Ru/ZrO_2-O and Ru/ZrO_2-N) suggests that the presence of carbon species has a negligible influence on the proportion of metallic Ru.

  Figure 2

  

  Figure 2.  XPS spectra of Ru catalysts (a) Ru 3d and (b) Zr 3d.

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  The catalytic performances of Ru/ZrO₂, Ru/ZrO_2-O and Ru/ZrO_2-N are shown in Fig. 3. The ammonia synthesis rates remain stable for 120 h at 400 ℃ and 1.0 MPa, indicating that the long-term stability of all Ru catalysts. The ammonia synthesis rate of Ru/ZrO₂ is 2.0 mmol g_cat⁻¹ h ⁻¹. The rates of Ru/ZrO_2-O are 4 times higher than those of Ru/ZrO₂, and Ru/ZrO_2-N shows highest ammonia synthesis rates (10.3 mmol g_cat⁻¹ h ⁻¹), which are comparable to other oxide- or carbon-supported Ru catalysts reported in the literatures (Table S1) [10, 11, 14, 27-31]. The rates of Ru/ZrO₂ are independent of reaction pressures (from 1 to 10 MPa), demonstrating that there is hydrogen poisoning for Ru/ZrO₂, which is in line with the findings over Ru catalysts supported on electride, calcium amide or La_0.5Ce_0.5O_1.75 [13, 49-51]. By contrast, the catalytic activities increase greatly with the increase of reaction pressure from 1 MPa to 10 MPa for Ru/ZrO_2-O and Ru/ZrO_2-N, indicating that the ill effect of hydrogen poisoning on ammonia synthesis rates is effectively alleviated for the samples obtained from UIO-66 which might be due to the change in the hydrogen adsorption/desorption property.

  Figure 3

  

  Figure 3.  (a) Time dependence of ammonia synthesis rates and (b) pressure dependence of ammonia synthesis activities at 400 ℃, 36,000 mL g⁻¹ h⁻¹ and 3:1 of H₂/N₂ ratio.

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  As shown in Fig. S6 (Supporting information), the results of H₂ temperature-programmed desorption show that a larger amount of hydrogen would be adsorbed on Ru catalysts using UIO-66 as precursor. Most hydrogen would desorb below 200 ℃ for Ru/ZrO_2-N, in contrast, a larger proportion of hydrogen species desorbs above 300 ℃ for Ru/ZrO_2-O. It is well known that H₂ molecules mainly adsorb and dissociate on metal sites for oxide-supported metal catalyst, but the resulting H atoms might migrate into the oxide support. Thus a D/H exchange reaction was performed to further study the hydrogen adsorption property of various Ru catalysts (Fig. 4). After Ar purging and 3.3% N₂-10% H₂-Ar mixture introducing, the release of HD and D₂ can be found for all Ru catalysts, indicating that there is an exchange of the adsorbed deuterium species and the hydrogen species from the gaseous phase. The amount of the deuterium-containing species desorbed at 50 ℃ decreases in the order of Ru/ZrO_2-N > Ru/ZrO_2-O > Ru/ZrO₂. It should be noted there is no hydrogen desorption peak for Ru/ZrO₂ with the rise of temperature in Ar (Fig. S6), and a strong hydrogen desorption peak appears at above 320 ℃ in the H₂-TPD profile of Ru/ZrO_2-O. On the contrary, strong HD signals appear at 255 ℃ for Ru/ZrO₂ with the rise of temperature during the TPSR study in the N₂−H₂−Ar mixture over the catalyst preadsorbed with D₂ (Fig. 4c). Moreover, a larger amount of HD species would release from Ru/ZrO_2-O below 200 ℃ during TPSR study. There results indicate that the presence of nitrogen species facilitates the desorption of hydrogen species, which is in line with previous work [23]. On the other hand, besides the strongest HD signals below 200 ℃, there are strong HD signals in the temperature range of 220–460 ℃ for Ru/ZrO_2-N. There results suggest that the preparation of ZrO₂-supported Ru catalysts from UIO-66 not only enhances the amount of the adsorbed hydrogen or deuterium species, but also facilitates the desorption of these species, which might be due to the presence of carbon species, and the formation of monoclinic ZrO₂ leads to the enhancement of these effects.

  Figure 4

  

  Figure 4.  Mass signals of (a) HD, (b) D₂ at 50 ℃ during the D/H exchange reaction and (c) HD, (d) D₂ during the TPSR study.

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  It is well known that there is competition between hydrogen adsorption and nitrogen adsorption on the same active sties [27, 52]. For a certain Ru catalyst, the presence of a larger amount of hydrogen species indicates that there are a larger number of active sites, which are responsible for nitrogen activation and ammonia synthesis reaction. In the meantime, a higher proportion of hydrogen species desorbed at low temperature leads to enhancement of active sites available for nitrogen activation and ammonia synthesis under the reaction condition. As a result, a larger amount of nitrogen deuterium species would be observed during the TPSR study (Fig. 4d), as well as the higher ammonia synthesis rates for the samples obtained from UIO-66 (Fig. 3).

  In summary, we have successfully developed an effective ZrO₂-supported Ru catalyst containing carbon species and monoclinic ZrO₂ from ZrCl₄ following the preparation route of UiO-66. The introduction of carbon species leads to lowering of the electronic metal support interaction between Ru species and ZrO₂ (Fig. 2). Moreover, the presence of carbon not only enhances the amount of the adsorbed hydrogen species, but also facilitates the desorption of these species (Fig. 4). In such a case, the ill effect of hydrogen species on the nitrogen adsorption-desorption and ammonia synthesis would be alleviated. As a result, Ru catalyst supported on the NH₃-treated ZrO₂ obtained from ZrCl₄ following a synthetic route of UiO-66 shows 4 times higher ammonia synthesis activity than that prepared from zirconium nitrate. This work not only provides us an effective ZrO₂-supported Ru catalyst, but also highlights a promising strategy to develop the effective oxide-supported metal catalysts used in ammonia synthesis or other involved-hydrogen reactions.

  Declaration of competing interest

  The authors report no declarations of interest.

  Acknowledgment

  This work was supported by the National Natural Science Foundation of China (Nos. 22178061, 21776047, 21825801, and 21978051).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2022.02.042.

  
  
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  Figure 1  XRD patterns of the reduced Ru catalysts.

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  Figure 2  XPS spectra of Ru catalysts (a) Ru 3d and (b) Zr 3d.

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  Figure 3  (a) Time dependence of ammonia synthesis rates and (b) pressure dependence of ammonia synthesis activities at 400 ℃, 36,000 mL g⁻¹ h⁻¹ and 3:1 of H₂/N₂ ratio.

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  Figure 4  Mass signals of (a) HD, (b) D₂ at 50 ℃ during the D/H exchange reaction and (c) HD, (d) D₂ during the TPSR study.

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