Metal-support interactions in the design of heterogeneous catalysts for redox processes

The effect of the metal-support interaction (MSI) has been discussed for several types of catalytic systems comprising metal nanoparticles (Ni, Pd, Au, Fe) on oxide and carbon supports, showing promising catalytic properties in hydrogenation of unsaturated C–C bonds, hydrodechlorination (HDC) of chlorinated organic molecules and CO total oxidation. The MSI of a different strength, from the redistribution of the electron density of nanoparticles (NPs) to the chemical interactions, is determined by the composition of the support and the active site, the method of active metal deposition, calcination temperature, particle size etc. The types of MSI considered in this review include: (1) the interaction of the active metal (Me) NPs with alumina and modified zirconia to form several oxidation states of Me in the composition of surface or bulk chemical compounds with a support; (2) the influence of oxide (alumina, silica) or carbon (highly oriented pyrolytic graphite, Sibunit) supports on the formation of active sites in the catalysts with ultra-low Me loading prepared by deposition of pre-formed metal NPs produced by laser electrodispersion (LED) or as colloidal dispersion; (3) the anchoring of Me NPs on the surface of carbon supports (nanodiamonds and carbon nanotubes) directly with a support surface, e.g. through surface defects, or through surface functional groups; (4) ‘reverse’ MSI in the Me@C composites, consisting of metal NPs, covered with the defected graphene layers or immersed into carbon matrix. It is demonstrated on the example of LED systems, that oxidation of metal under MSI is less significant in carbon-supported systems than in oxide-supported ones, but charge effects can play a noticeable role for both types of supports. Different ways of MSI tuning provide the possibilities to achieve the optimal Men+/Me0 ratio in the catalysts for HDC of mono- and polychlorinated organic molecules, including persistent organic pollutants. One of these ways is tuning the composition of functional groups on the surface of nanodiamonds and carbon nanotubes by additional treatments to achieve the desirable metal anchoring, the optimal metal NPs size and the improved catalytic properties. Unusual type of MSI is represented by the activation of thin graphene shell of Me@C composites by the presence of defects in the shell and a transition metal (Ni, Fe) in subsurface layer. This effect allows H2 activation that is a significant step in many industrially important reactions. The selectivity and activity of such systems can be intentionally changed by varying the nature of metal and reaction temperature. Significant attention has been given in the review to the novel catalytic systems described in the previous works of the authors.