In Situ Growth of Exsolved Nanoparticles under Varying rWGS Reaction Conditions—A Catalysis and Near Ambient Pressure-XPS Study

Abstract

Perovskite-type oxides are highly flexible materials that show properties that are beneficial for application in reverse water-gas shift processes (rWGS). Due to their stable nature, the ability to incorporate catalytically active dopants in their lattice structure, and the corresponding feature of nanoparticle exsolution, they are promising candidates for a materials design approach. On an industrial level, the rWGS has proven to be an excellent choice for the efficient utilisation of CO2 as an abundant and renewable carbon source, reflected by the current research on novel and improved catalyst materials. In the current study, a correlation between rWGS reaction environments (CO2 to H2 ratios and temperature), surface morphology, and catalytic activity of three perovskite catalysts (Nd0.6Ca0.4Fe0.9Co0.1O3-δ, Nd0.6Ca0.4Fe0.97Co0.03O3-δ, and Nd0.6Ca0.4Fe0.97Ni0.03O3-δ) is investigated, combining catalytic measurements with SEM and NAP-XPS. The materials were found to react dynamically to the conditions showing both activation due to in situ nanoparticle exsolution and deactivation via CaCO3 formation. This phenomenon could be influenced by choice of material and conditions: less reductive conditions (larger CO2 to H2 or lower temperature) lead to smaller exsolved particles and reduced carbonate formation. However, the B-site doping was also important; only with 10% Co-doping, a predominant activation could be achieved.

Exsolution on perovskite oxides: morphology and anchorage of nanoparticles

“…[13][14][15][16][17][18][19] Depending on the conditions, it reversibly 20 leads to the decoration of a material's surface with finely dispersed nanoparticles. 16 The formation of such nanoparticles can be achieved before reactions (in reducing gas atmospheres or electrically by applying a bias to the sample 21,22 ) or in situ, provided the reaction conditions are sufficiently reducing: 14,23 The perovskite material is partially reduced causing reducible B-site cations to migrate to the surface, where they exsolve and form metallic or oxidic nanoparticles (depending on the conditions) -as can be seen in Fig. 2.…”

“…Among the many advantages of nanoparticles exsolved via exsolution are the fine dispersion of the resulting particles, 16 their high stability and sinter resistance due to ''anchoring'' in the surface, 15,16,19 and their tunability: either by varying the composition of the perovskite 16,17,24 or adapting the conditions during the exsolution process. 18,21,23 Another feature of exsolution catalysts (provided they exhibit reversible exsolution) is the possible regeneration of the active nanoparticles via re-oxidation, thus improving catalyst lifetime. 1,25 (c) Scope of this work Exsolution in perovskite oxides as well as its applications in catalysis and related fields have been the topic of extensive studies, since catalysts with nanoparticles formed via exsolution offer a great number of advantages and chances for new and improved catalysts.…”

“…Another way of classifying the publications review for this article is to distinguish between exsolution triggered during a pre-treatment before further tests or applications (for instance electrochemical measurements 65 or NH 3 sensing 95 ) and in situ exsolution, i.e. nanoparticles are exsolve during tests, applications, or chemical reactions (like the aforementioned methane 13,17,23, Fe-Ni, 35,[48][49][50][51][52][53][54][55][56][57][58][59][60][61][62][63][64] Co-Ni, 65 Cu-Ni, 66 Fe-Ni-Re, 49 Fe-Ni-Mo 58 Fe 41 Fe, 13,24,[67][68][69][70][71][72][73][74] Fe-Ni, 35,[48][49][50][51][52][53][54][55][56][57][58]…”

“…13–19 Depending on the conditions, it reversibly 20 leads to the decoration of a material's surface with finely dispersed nanoparticles. 16 The formation of such nanoparticles can be achieved before reactions (in reducing gas atmospheres or electrically by applying a bias to the sample 21,22 ) or in situ , provided the reaction conditions are sufficiently reducing: 14,23…”

Dynamic Surface Evolution of Metal Oxides for Autonomous Adaptation to Catalytic Reaction Environments

“…In addition, Lindenthal et al also reported the emergence of nano-sized particles from the host material of Nd 0.6 Ca 0.4 FeO 3 with undesirable CaCO 3 during a reverse water-gas shift reaction. [12] Lorenz and co-workers investigated the relationship between the surface structures and catalytic activities as a function of the reaction environment (i.e., various CO 2 -to-H 2 ratios and temperatures) and material compositions, finding that the formation of Ni particles can improve the catalytic performance, while undesired CaCO 3 caused catalytic deactivation. With ceramic-based (specifically, oxide-based) fuel cells, Guo and co-workers found the in situ formation of Ni-Ru alloy NPs from an A-site-deficient La 2−x NiRuO 6−δ double perovskite oxide when it acts as the anode during an electrochemical measurement.…”