Two-dimensional (2D) materials attract enormous attention and show promising applications in many fields of science and technologies (nanodevices, energy storage/harvest and catalytic processes, etc.). Pentagonal compounds emerge as a new family in 2D materials along with classic trigonal transition metal dichalcogenides and MXenes, which have been intensively investigated to date. Encouraged by the successful synthesis of pentagonal PdSe using CVD method, we explore nine pentagonal monolayers, MX (M = Ni, Pd & Pt, and X = S, Se & Te), based on the first-principles calculations. We find that all MX are dynamically and thermodynamically stable, and intrinsic semiconductors. Our results show that PdTe exhibits excellent potential application in solar water splitting due to optimal band gap and suitable band-edge positions matching with both the water reduction and oxidation potentials (0 and 1.23 V vs. NHE). We further find that the majority of MX monolayers (except NiTe) are applicable in photocatalytic oxygen production. Our findings are expected to shed light on the possible synthesis of pentagonal MX and their application in photocatalytic water splitting.