CHEMICAL SCIENCE

Single-atom Cu/ZrO2 catalysts are used for CO2 hydrogenation to synthesize methanol


Recently, Tan Li’s team of Fuzhou University and Liu Zhipan’s team of Fudan University designed and prepared a low-temperature and stable single-atom copper-based catalyst (Cu1/amorphous-ZrO2), and its clear active site structure and single product distribution clearly revealed the relationship between catalyst structure and performance.

On September 15, 2022, the study was published in the journal Nature Catalysis under the title “The role of Cu1–O3 species in single-atom Cu/ZrO2 catalyst for CO2 hydrogenation.” Liu Zhipan and Tan Li are the corresponding authors of the paper, and Zhao Huibo of Fuzhou University is the first author.

Background

Copper-based catalysts have attracted much attention due to their excellent catalytic properties and low price during the synthesis of methanol by CO2 hydrogenation, but their true active sites have been controversial. Under real reaction conditions, due to the variable valence state of copper species, particle size effects and interaction with carriers, the local coordination structure of copper species is changed, making it extremely difficult to identify its true active center. Although a large number of research efforts have been carried out in depth on the site of activity, there are still many different views on the relationship between catalyst structure and activity. Therefore, it is particularly important to build a stable and well-structured copper-based catalyst under the real reaction conditions of methanol synthesis.

Highlights of this article

Aiming at the complex active sites and vague structure-activity relationships in the traditional copper-based methanol synthesis catalysts, the low-temperature and stable single-atom copper-based catalysts (Cu1/amorphous-ZrO2) were designed and prepared, and the clear active site structure and single product distribution clearly revealed the relationship between the catalyst structure and performance, and the specific results were as follows:

1. Successfully constructed a monodisperse copper-based catalyst on the surface of amorphous zirconia by optimized co-precipitation method, which has a quasiplanar structure of Cu1-O3 during the reaction;

2. For the first time, the single-atom Cu catalyst was used to realize the CO2 synthesis process containing oxides in the mobile phase reaction system, which expanded the application scope of single-atom catalysts. Compared with the traditional copper zirconium catalyst, it has the highest TOF value and 100% selectivity for methanol at low temperature, and there is no obvious inactivation during the 100h reaction.

3. The inducing effect of the reaction gas can lead to the migration of copper species to the surface of the carrier during the reaction;

4. In situ high-voltage infrared and theoretical calculations show that the reaction process follows the reaction mechanism of CO2→ HCOO* →CH3O* → CH3OH, of which HCOO* →CH3O* are the rapid steps of the reaction.

5. For the Cu/ZrO2 catalyst, the isolated copper site (Cu1-O3) is the active site of methanol synthesis, cu nanoclusters or small nanoparticles are the active centers of CO generated by the RWGS reaction, while the larger copper particles do not have the ability to activate CO2 at lower temperatures.

Graphic and text analysis

Figure 1: Characterization of different Cu/ZrO2 catalysts.

Figure 2: Catalytic properties of different copper-based catalysts.

Figure 3: Morphology and crystal structure of different Cu/ZrO2 catalysts.

Essentials:

1. Significant copper particle aggregation (Figures 1a, b, d, e) could not be observed even if the load of copper (≤15%) (CAZ-x) was increased on an amorphous zirconia carrier, indicating that copper species were uniformly dispersed on the surface of amorphous zirconia. Copper species, on the other hand, have significant agglomeration on the surface of monoclinic zirconia (CMZ-15) (Figure 1c, f), mainly in the form of copper oxide. X-ray absorption proximal structure spectroscopy (EXAFS, Figure 1g) and wavelet transform (WT, Figure 1h) further indicate that copper species in CAZ-1 and CAZ-15 exist in single-atom form, and Cu-(O)-Cu signals can be clearly seen in CMZ-15, further verifying the presence of copper oxide.

Under the reaction conditions of 180 °C, CO2/H2=1:3,3 MPa, the CAZ-1 sample had the highest TOF value (1.37h-1), spatiotemporal yield (2.06 *10-4 mol·h-1·gCu-1) and 100% selectivity of the target product for methanol (Figure 2a, b). With the increase of copper loading, the CO2 conversion rate increased first and then stabilized, and the methanol selectivity also decreased first and then remained unchanged, indicating that different copper loading amounts formed different active sites (Figure 2c). In addition, the single-atom catalyst has excellent stability, and no significant inactivation is seen after 100h of operation (Figure 2d).

3. Significant aggregation and reduction of copper species in the CAZ-1 catalyst after the reaction was still unable to see significant agglomeration, while significant reduction aggregation of copper species occurred in CAZ-15, CMZ-15 and CS-15 (15wt%Cu/SiO2) samples (Figure 3a-c). Combined with in situ XRD, TPR, XPS and other characterizations, it can be proved that copper species in CAZ-1 are difficult to reduce under reaction conditions, while CAZ-15 is a state in which copper single atoms coexist with particles (Figure 3d).

Figure 4: Electronic characteristics and local structure of a single-atom (CAZ-1) catalyst.

Essentials:

By studying the electronic states of copper species in the CAZ-1 sample, we found that copper species existed in a state close to divalent before and after the reaction (Figures 4a, b). In situ high-pressure X-ray absorption spectra showed that the copper species would be slightly reduced during the reaction, and the fitting of the standard card showed that the valence state of the copper species was about 1.4+ (Figure 4c, d), which was consistent with the theoretical calculation of Bader charge results;

2. By fitting the fine structure of the CAZ-1 sample, we can know that the local structure of the copper species is the quasi-planar configuration of Cu1-O3, and the copper-oxygen coordination number will be slightly reduced under the reaction conditions, corresponding to the theoretically calculated structure (Figure 4e-f);

3. The slightly decreased copper valence state and copper oxygen coordination number during the reaction process indicate that the copper activity site is in the activated state;

4. Combined with in situ high-pressure X-ray absorption spectroscopy, in situ XRD, XPS, HAADF-STEM and TPR results can determine that the local structure of Cu1-O3 is very stable during the reaction process, which also determines that it has excellent reaction stability.

Figure 5: Migration of Cuδ+ species to the surface of the carrier.

Essentials:

1. Many research works have found that the atmosphere during pretreatment or reaction will significantly affect the structure of the catalyst, including the migration of active sites and surface reconstruction. Similar phenomena also occur in our reaction system, such as the copper atom mapping (Figure 5a, b) of time-flight secondary mass spectrometry (TOF-SIMS) and the semi-quantitative results (Figure 5c), the surface of the reacted CAZ-1-U catalyst can capture more copper ion fragments, indicating that the copper species in the carrier will significantly migrate to the surface of the support during the reaction;

The Cu/Zr ratio of CAZ-1 on the surface of CAZ-15 samples before and after the reaction increased from 0.0012 and 0.24 to 0.0018 and 0.38, respectively, indicating that more copper species were enriched on the surface of the catalyst after the reaction. At the same time, compared with the fresh sample, the chemical adsorption capacity of the sample on CO after the reaction is stronger, which also shows that more copper species are exposed to the surface of the carrier during the reaction;

3. Combined with the experimental results, we believe that the migration phenomenon of this catalyst is roughly divided into three categories. When the copper load was less than 2wt%, the migrating copper species were evenly dispersed on the surface and did not aggregate, so only methanol was generated and the CO2 conversion rate increased linearly with the copper load. When the load was 4-8wt%, the copper species underwent partial agglomeration and reduction during migration, which was reflected in the activity as the CO2 conversion rate increased with the increase of the load, but it no longer followed a good linear relationship, while the CO selectivity increased and the methanol selectivity decreased; When the metal load is higher than 8wt%, some copper species continue to aggregate into larger particles, and this large particle has no catalytic activity against CO2, so the CO2 conversion rate and product selectivity no longer change with the copper load. The schematic diagram is shown in Figure 5d.

Figure 6: Characterization and evolution of active intermediates.

Figure 7: Mechanism analysis of CO2 hydrogenation to methanol at isolated Cuδ+ (1 < δ < 2) sites.

Essentials:

1. In situ high-pressure infrared and theoretical calculations show that the process of CO2 hydrogenation to methanol in this catalytic system follows the reaction mechanism of CO2-HCOO*-CH3O*-CH3OH, and the gradually increasing HCOO*/CH3O* ratio and the higher hydrogenation barrier of formate indicate that HCOO*→CH3O* is the rapid step of the reaction (Figure 6a-c, Figure 7a);

2. Compared with the HCOO*/CH3O* ratio trend on the CAZ-1 sample, the HCOO*/CH3O* of CAZ-15 was more gentle, indicating that the rate of contract hydro-hydrogenation to methoxy on the surface of the CAZ-15 sample was faster (Figure 6c), which also explained why CAZ-15 had a higher spatio-temporal yield of methanol.

3. Microdynamic simulation shows that the reaction rate of CO2 hydrogenation to methanol on the surface of CAZ-1 is much higher than that of co-product CO, which microly explains the reason for the high selectivity of methanol synthesis.

Conclusions and Outlook

This work reveals the strong dependence of carbon dioxide hydrogenation activity and methanol selectivity on the structure of copper species on the surface of Cu/ZrO2 catalysts. The unsaturated cationic Cu1-O3 species dynamically accumulated on the surface of the catalyst and formed a stable catalytic activity site during the catalytic process, and 100% selectivity of CH3OH was achieved in the CO2 hydrogenation reaction at 180 °C. Copper single-atom catalysts can easily dissociate hydrogen and activate CO2 to generate HCOO*, a key intermediate in ch3OH synthesis with the help of nearby oxygen atoms. The methanol synthesis pathway via HCOO* is the only viable way to hydrogenate CO2 at isolated Cu1-O3 active sites. In contrast, small copper clusters or nanoparticles form the active site of CO through the RWGS pathway, while large copper particles hardly activate carbon dioxide at low temperatures. The intrinsic relationship between the special geometric configuration and the unique activity revealed by the copper single atom catalyst provides a new entry point for the deep understanding of the role of the copper-based catalyst in the process of CO2 hydrogenation, and the first use of the single atom Cu catalyst to achieve the CO2 synthesis process in the mobile phase reaction system further expands the application scope of the single atom catalyst, and provides theoretical guidance and support for the further application of the single atom catalyst in the thermal catalytic CO2 conversion reaction. (Source: Science Network)

Correlation theoryText Information:https://doi.org/10.1038/s41929-022-00840-0



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