The most stable binding sites for H on Pt10 and Pt5Ge5 deposited on the rest of the 2D materials are shown in Figure . Strikingly, in some of these supports the surface atoms become competing active sites for H*. Specifically, in Pt10/5-8-5-DV, Pt5Ge5/5-8-5-DV and Pt5Ge5/silicene H* prefers to bind to the surface instead to Pt. This may be indicative of hydrogen spillover phenomena. In the rest of the Pt(Ge)/2D systems, the most stable H* adsorption sites are found to be the Pt sites on the cluster-support interface. Our results reveal that in Pt10 all supports except SmSI present a negative ΔGH*, whereas in Pt5Ge5 all supports except gr-B and silicene present a positive ΔGH* (see Figure and). It is thus clear that Ge introduces a repulsive interaction with H*. The phosphorene-deposited PtGe clusters show that the Ge content can be used to adjust ΔGH* to the optimal value. The clusters that fall inside the optimal ΔGH* energy range are, in order of decreasing absolute |ΔGH*| value: Pt10/silicene, Pt5Ge5/germanene, Pt10/SmSI, Pt5Ge5/2H-MoS2 and Pt5Ge5/silicene. Surprisingly, on most of the 2D surfaces, including the inert 2H-MoS2 basal plane, alloyed clusters exhibit smaller ΔGH* values, indicating higher catalytic activity. This scenario is unexpected because usually doping or alloying Pt with a second element improves sintering or poisoning resistance, selectivity, etc., but at the expense of the catalytic activity. In this case, except in 585-DV and SmSI equimolar Pt5Ge5 clusters are predicted to be more active for the HOR.