Morphology, structure and compositions analysis
The XRD patterns of the synthesized Cu-MOFs and ZIF-9(Co) are shown in Figure S1(a, b), which are consistent with the results reported in the literature [27, 28, 29]. After Cu-MOF and Co-MOFs are respectively phosphated, Cu-P and Co-P samples can be obtained. After fully phosphating ZIF-9,there exist typical diffraction peaks in 2θ = 31.60º, 35.18º, 36.34º, 46.19º, 48.18º, and 56.27º, as shown in Figure 2(a). The analysis can be indexed to the JCPDS#65-1474 standard card of CoP. The corresponding crystal planes are (011), (200), (111), (112), (211), and (202). The weakening of the intensity of all the peaks of CoP indicates relatively low degree of crystallization [30]. In the XRD pattern of Cu-P (Figure 2(b)), the corresponding characteristic peaks of Cu3P (JCPDS#74-1067) and Cu element (JCPDS#70-3038) can be corresponded. The characteristic peaks of Cu3P appear at 28.43º, 35.86º, 38.90º, 41.38º, 44.91º, 46.02º, 47.08º, 53.30º, 56.74º, 58.14º, 66.34º and 73.01°, corresponding to (111), (112), (022), (211), (300), (113), (122), (104), (311), (222), (223), and (322) crystal planes. The obvious diffraction peaks of (111) and (200) plane diffraction of elemental Cu are located at 43.17° and 50.29°. Figure 2(c) shows the results of Cu-Co-xP-2 (x = 1, 2, 3, 4.) composite catalysts obtained by phosphating at different degrees. The outer layer of ZIF-9 was gradually phosphorylated to produce diffraction peaks related to CoP. With the increasing of phosphating degree in the composite samples, the content of phosphide (Cu3P, CoP) gradually increases, and the intensity of the related diffraction peaks gradually increases. Because the crystallinity of the outer phosphide is relatively low, the related diffraction peaks of the internal unphosphorized Cu-MOFs is enhanced. The spectrums of Cu-Co-x (x = 1, 2, 3, 4) obtained by adjusting the mass of nucleated Cu-MOFs and maintaining the quality of the outer layer ZIF-9 are shown in Figure S1(c). There are both Cu-MOFs and ZIF-9 peaks, and the diffraction peaks related to Cu-MOFs increase as the mass of Cu-MOFs increases. Cu-Co-x (x = 1, 2, 3, 4) was phosphorylated to obtain Cu-Co-2P-x (x = 1, 2, 3, 4.) composite catalyst (Figure 2(d)). With the increasing of the mass of Cu-MOFs under the same phosphatization degree, the diffraction peaks related to Cu-MOFs gradually decrease. Therefore, the Cu-Co-2P-4 sample only contains the diffraction peaks of Cu3P, Cu and CoP, and there are no peaks of MOFs.