Photocatalytic hydrogen evolution may provide one of the solutions to the shift to a sustainable energy society, but the quantum efficiency of the process still needs to be improved. Precise control of the composition and structure of the metal nanoparticle cocatalysts is essential, and we show that fine-tuning the Au-Pd nanoparticle structure modifies the electronic properties of the cocatalyst significantly. Specifically, Pd(shell)-Au(core) nanoparticles immobilized on TiO2 exhibit extremely high quantum efficiencies for H2 production using a wide range of alcohols, implying that chemical byproducts from the biorefinery industry can be used as feedstocks. In addition, the excellent recyclability of our photocatalyst material indicates a high potential in industrial applications. We demonstrate that this particular elemental segregation provides optimal positioning of the unoccupied d-orbital states, which results in an enhanced utilization of the photoexcited electrons in redox reactions. We consider that the enhanced activity observed on TiO2 is generic in nature and can be transferred to other narrow band gap semiconductor supports for visible light photocatalysis.

We investigate the adsorption of CO2 over an MgO(001) terrace, as calculated using an embedded cluster method. We find adsorbed geometries for CO2 on the perfect surface with energies which differ appreciably from previous studies, and observe that it is polarization of the surface rather than the inclusion of electron correlation which leads to this discrepancy. Our results suggest that both monodentate and tridentate carbonate formation on the MgO(001) surface are favourable processes, with the monodentate structure being of lower energy. Adsorption of CO2 is found to be favourable at both F(0) and F(+) terrace sites, but not at F(2+). We also find that chemisorption at oxygen vacancy sites with a single localized electron (F(+)) could provide a route for the conversion of CO2 to other products, and that this system may be a useful model for other, more effective catalysts.

We have investigated the energetic properties of Mn-doped MgO bulk and (100) surfaces using a QM/MM embedding computational method, calculating the formation energy for doped systems, as well as for surface defects, and the subsequent effect on chemical reactivity. Low-concentration Mn doping is endothermic for isovalent species in the bulk but exothermic for higher oxidation states under p-type conditions, and compensated by electrons going to the Fermi level rather than cation vacancies. The highest occupied dopant Mn 3d states are positioned in the MgO band gap, about 4.2 eV below the vacuum level. Surface Mn-doping is more favourable than subsurface doping, and marginally exothermic on a (100) surface at high O pressures. For both types of isovalent Mn-doped (100) surfaces, the formation energy for catalytically important oxygen defects is less than for pristine MgO, with F and F-centres favoured in n- and p-type conditions, respectively. In addition, F-centres are stabilised by favourable exchange coupling between the Mn 3d states and the vacancy-localised electrons, as verified through calculation of the vertical ionisation potential. The adsorption of CO on to the pristine and defective (100) surface is used as a probe of chemical reactivity, with isovalent subsurface Mn dopants mildly affecting reactivity, whereas isovalent surface-positioned Mn strongly alters the chemical interactions between the substrate and adsorbate. The differing chemical reactivity, when compared to pristine MgO, justifies further detailed investigations for more varied oxidation states and dopant species.