**4. Conclusions**

In this study, Mn-MOF-74 catalysts were modified by introducing surfactants P123 and PVP and by ligand methyl functionalization to enhance their water resistance. The structure and morphology of the samples were investigated by PXRD, FT-IR, SEM, TEM and TG-MA while low-temperature SCR performance was tested in a fix-bed reactor. It was concluded that the modified samples were successfully synthesized and that the water resistance of the samples was significantly enhanced after modification. Moreover, all the catalysts were able to remain stable for at least 12 h at their own optimal temperature in an SCR atmosphere. After the addition of 5% H2O in the feed gas, the NO conversion of P123-Mn-MOF-74 remained at 92.1% at 250 ◦C while that of unmodified Mn-MOF-74 dropped by 44%. In summary, P123-Mn-MOF-74 has superior low temperature SCR activity, SCR stability, and

water resistance, and its NO conversion rate can be maintained above 90%, which proves it to be a potential low-temperature NH3-SCR catalyst.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2073-4344/9/12/1004/s1, Figure S1: FT-IR spectra of P123/PVP-Mn-MOF-74 andMn-MOF-74-CH3 at (**a**) 4000~400 cm<sup>−</sup><sup>1</sup> and (**b**) 4000–2500 cm<sup>−</sup>1; Figure S2: SEM images of (**a**) Mn-MOF-74, (**b**) P123-Mn-MOF-74, (**c**) PVP-Mn-MOF-74, and (**d**) Mn-MOF-74-CH3; Figure S3: TG curves of Mn-MOF-74, P123-Mn-MOF-74, PVP-Mn-MOF-74, and Mn-MOF-74-CH3 under atmospheres of (**a**) air and (b) N2. MS results of Mn-MOF-74-CH3 under atmospheres of (**c**) air and (**d**) N2.

**Author Contributions:** Conceptualization and methodology, S.W. and Q.G.; writing—original draft preparation, Q.W. and Y.N.; writing—review and editing, X.D. and Y.C.; supervision, H.J.; S.W., and Q.G. (these authors made equal contributions).

**Funding:** This research was funded by the National Natural Science Foundation of China, gran<sup>t</sup> number 21506150.

**Acknowledgments:** The authors are grateful for financial support from the National Natural Science Foundation of China (no. 21506150). This project was sponsored by the Scientific Research Foundation for Returned Overseas Chinese Scholars, State Education Ministry.

**Conflicts of Interest:** The authors declare no conflict of interest.
