The reduction of the solid solutions Mn_xZr_1−xO_2−δproceedsviatwo stages. During the first stage, the Mn cations incorporated into the solid solutions Mn_xZr_1−xO_2−δundergo partial reduction. At the second stage, Mn cations segregate on the surface.

The reduction of nanocrystalline cobalt oxide samples (single-phase and supported on γ-Al₂O₃) was studied using in situ X-ray diffraction (XRD) analysis. The atomic structures of single-phase and supported Co₃O₄ samples were refined, and the occurrence of cationic vacancies was demonstrated. A set of methods (XRD, temperature-programmed reduction, and differential dissolution) was used to find that the reduction of supported and unsupported model cobalt oxide was considerably different. The single-phase sample was reduced in undiluted hydrogen to cobalt metal with a hexagonal closely packed structure. The reduction of the supported sample (unlike the single-phase sample) occurred through the formation of a crystalline CoO phase to the formation of cobalt metal with a face-centered cubic structure. Interaction of cobalt oxide with the γ-Al₂O₃ support, which hinders the reduction to cobalt metal, was detected.

The work reported here was aimed at determining differences in redox properties of simple and double oxides. Comparison between the reduction of double oxides (Mn,Co)₃O₄ and simple oxides Co₃O₄ and Mn₃O₄ was performed using in situ X-ray diffraction (XRD), temperature-programmed reduction (TPR) and transmission electron microscopy (TEM). The double oxides with a ratio of cations Mn : Co = 1 : 1 were prepared by the coprecipitation method and contained a mixture of 50% MnCo₂O₄ and 50% CoMn₂O₄. It was shown that the mechanism of reduction of double oxides with hydrogen differs significantly from the processes occurring on simple oxides. For simple cobalt and manganese oxides, transformations Co₃O₄ → CoO → Co and Mn₃O₄ → MnO are observed under a hydrogen atmosphere. The reduction of mixed-metal oxides occurs in two steps. In the first step, at 300–450 °C, (Mn,Co)₃O₄ transforms to (Mn,Co)O solid solutions. In situ XRD under isothermal conditions illustrates that Co-rich Co₂MnO₄ oxide starts to be reduced to Co_0.6Mn_0.4O first, and then Mn-rich Mn₂CoO₄ passes into Mn_0.6Co_0.4O. In the second step, at 450–700 °C, the reduction of solid solutions (Mn,Co)O to metallic cobalt Co and MnO proceeds. Again, the reduction begins with transformation of Co-rich oxide with the Co_0.6Mn_0.4O structure. The temperature of appearance of the intermediate phase (Mn,Co)O shifts to the higher values as compared to those observed for CoO, and to lower temperatures as compared to MnO during simple oxide reduction.

The hydrogen reduction of Mn_1.7Ga_1.3O_4+δproceeds in two steps. In the first step, excess oxygen is removed, Mn_1.7Ga_1.3O_4+δ→ Mn_1.7Ga_1.3O₄. In the second step, Mn³⁺cations are reduced to Mn²⁺in the spinel structure and formation of MnO, Mn_1.7Ga_1.3O₄→ Mn₂Ga₁O₄+ MnO.

X-ray diffraction (XRD) is a standard technique that is widely applied in heterogeneous catalysis to determine phase composition, atomic structure, and size of crystallites. This review is focused on the application of in situ XRD for studying the catalysts during their “lifetime” (under synthesis, activation, operation, and deactivation conditions), limiting the objects of research to oxide and metal oxide catalysts. Also included is a brief overview of modern techniques and instruments and the latest works illustrating different aspects of this technique in catalyst research. The main conclusion is that the field of heterogeneous catalysis research would benefit substantially from the application of in situ XRD for the structural, phase, and morphological characterization of solid catalysts. Even more useful information can be obtained if XRD is combined with other techniques that are more sensitive at length scales different from that of XRD.