Despite significant recent developments in the field of high entropy oxides, previously reported HEOs are overwhelmingly stoichiometric structures containing a single cationic site and are stabilized solely by intermixing increasing numbers of cations. For the first time, we demonstrate here that cationic vacancies can significantly increase configurational entropy and stabilize phase-pure HEOs. Aluminate spinel HEOs with AB2O4 stoichiometry are used as a model crystal structure. These spinels tolerate large divalent cation deficiencies without changing phase, allowing for high concentrations of cationic vacancies. Stoichiometric and sub-stoichiometric spinels (with A:B molar ratios <0.5), which contained various mixtures of Co, Cu, Mg, Mn, Ni, and cationic vacancies in nominal equimolar concentration, were systematically compared as a function of heat treatment temperature and number of unique cationic species. We found that the same number of cationic species were needed to stabilize both stoichiometric and sub-stoichiometric nickel-containing spinels at 800 °C calcination, as exemplified by (CoCuMgNi)Al2O4 and (CoMgNi)0.75Al2Ox samples, signifying that vacancies stabilize phase-pure spinels similarly to cations. The chromatic, structural, and chemical properties of these complex spinels were highly tunable via incorporation of cationic vacancies and multiple divalent metals, promoting their potential application as unique pigments, catalysts, and thermal coatings.

The addition of Pd to Pt-based diesel oxidation catalysts is known to enhance performance and restrict the anomalous growth of Pt nanoparticles when subjected to aging at high temperatures in oxidative environments. To gain a mechanistic understanding, we studied the transport of the mobile Pt and Pd species to the vapor phase, since vapor phase transport is the primary route for sintering in these catalysts. The results are surprising: there is a 30-fold drop in the effective vapor pressure of Pt in the Pt−Pd catalysts compared to monometallic Pt. At the same time, there is a significant enhancement in the vapor pressure of Pd, compared to PdO, which otherwise has a negligible vapor pressure at the aging temperature. Such behavior cannot be explained simply by alloying Pt and Pd in the metallic phase, or a core−shell morphology where a PdO shell covers a Pt core. Transmission electron microscopic examination of catalysts aged up to 50 h in air at 800 °C shows that the particles exhibit a biphasic "Janus"-like structure. The metal and oxide phases are conjoined, exposing a metal and an oxide face to the gas phase. The high mobility of the Pt and Pd allows them to be partitioned into the metal and oxide phases, in apparent thermodynamic equilibrium. The PdO helps to trap mobile PtO 2 and as a result contains high concentrations of Pt oxide, consistent with its role in mitigating the transport of Pt to the vapor phase and preventing the growth of anomalously large particles. In turn, Pt allows Pd to remain metallic, allowing the catalyst to retain both metal and oxide functionality for catalysis. The regeneration of deactivated catalysts typically requires an external input, such as a change in the working environment from reducing to oxidizing or vice-versa. Here, we show that the mobile species, which are primary contributors to catalyst sintering are effectively returned to the active site, hence our use of the term "selfhealing". The detailed insights into the inner workings of the Pt−Pd diesel oxidation catalysts can help provide clues to the design of robust and durable heterogeneous catalysts.

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