Bimetallic catalytic materials are in widespread use for numerous reactions, as the properties of a monometallic catalyst are often improved upon addition of a second metal. In studies with bimetallic catalysts, it remains challenging to establish clear structure–property relationships using traditional impregnation techniques, due to the presence of multiple coexisting active phases of different sizes, shapes, and compositions. In this work, a convenient approach to prepare small and uniform Pt/Pd bimetallic nanocrystals with tailorable composition is demonstrated, despite the metals being immiscible in the bulk. By depositing this set of controlled nanocrystals onto a high-surface-area alumina support, we systematically investigate the effect of adding platinum to palladium catalysts for methane combustion. At low temperatures and in the absence of steam, all bimetallic catalysts show activity nearly identical with that of Pt/Al2O3, with much lower rates in comparison to that of the Pd/Al2O3 sample. However, unlike Pd/Al2O3, which experiences severe low-temperature steam poisoning, all Pt/Pd bimetallic catalysts maintain combustion activity on exposure to excess steam. These features are due to the influence of Pt on the Pd oxidation state, which prevents the formation of a bulk-type PdO phase. Despite lower initial combustion rates, hydrothermal aging of the Pd-rich bimetallic catalyst induces segregation of a PdO phase in close contact to a Pd/Pt alloy phase, forming more active and highly stable sites for methane combustion. Overall, this work unambiguously clarifies the activity and stability attributes of Pt/Pd phases which often coexist in traditionally synthesized bimetallic catalysts and demonstrates how well-controlled bimetallic catalysts elucidate structure–property relationships.

This paper reports on the synthesis and stability of a polymorphic system of a metal–organic framework (MOF) composed of zinc(ii) and 2-methylimidazole, as well as its potential applicability in gas storage.

Templated encapsulation of platinum-based catalysts promotes high-temperature stability to 1,100 °C.

A common expectation in heterogeneous catalysis is that the optimal activity will occur for the particle size with the highest concentration of undercoordinated step, edge, or corner sites, expectedly in the <5 nm range. However, many metal-catalyzed reactions follow a different trend, where the turnover frequency (TOF, here rate per surface atom) is instead lower for these smaller particles and increases strongly with increasing size toward a stabilized level with a size-independent TOF. Here, we use one of these reactions, the Rh-catalyzed CO hydrogenation to hydrocarbons and C2-oxygenates, to illuminate the origin of this effect. Studying Rh/SiO2 catalysts, we show that smaller (<4 nm) Rh particles are richer in undercoordinated edge, corner, and step sites, but are nevertheless of lower activity because the entire surface, including the planar facets, is shifted to a prohibitively high adsorbate coveragein this case of CO. In transient experiments, where the inhibiting adsorbates are allowed to desorb, smaller 1.7 nm Rh particles and larger 3.7 nm Rh particles reach similar rates of CO activation despite the steady-state TOF being an order of magnitude higher on the larger particles. This shows that it is a prohibitive adsorbate coverage under reaction conditions rather than a lower number of active sites or a lower intrinsic activity of the sites that causes the lower activity of the smaller particles. In steady-state experiments at 20 bar, the TOF for CO hydrogenation increases by 55% from 3.7 nm Rh particles to 5.3 nm Rh particles even though the measured concentration of step sites decreases by 30% in this size range. This indicates that such undercoordinated sites are not necessarily the primary active centers and that the reaction is instead focused on the planar facets. The reaction kinetics show that the reaction becomes increasingly pressure-dependent with increasing particle size, implying that the surface becomes increasingly free of adsorbates on larger particles. Taken together with the indications that the reaction may be focused on the planar facets, this leads to the new insight that it is a prohibitively high adsorbate coverage on the entire surface (and not just on a minority of undercoordinated sites) that is the primary reason for the low activity of small nanoparticles. The identification of a detrimental high-coverage state for small particles is expected to be of general relevance to the many industrially important reactions sharing the same behavior. The high-coverage state is not exclusively negative, but can also facilitate different reaction pathways. It is the higher CO coverage on small particles that drives the C2-oxygenate formation and is the reason for the high selectivity of rhodium to such complex products, which is at its highest for the smallest (∼2 nm) investigated particles.

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Low-temperature removal of noxious environmental emissions plays a critical role in minimizing the harmful effects of hydrocarbon fuels. Emission-control catalysts typically consist of large quantities of rare, noble metals (e.g., platinum and palladium), which are expensive and environmentally damaging metals to extract. Alloying with cheaper base metals offers the potential to boost catalytic activity while optimizing the use of noble metals. In this work, we show that Pt x Cu 100−x catalysts prepared from colloidal nanocrystals are more active than the corresponding Pt catalysts for complete propene oxidation. By carefully controlling their composition while maintaining nanocrystal size, alloys with dilute Cu concentrations (15−30% atomic fraction) demonstrate promoted activity compared to pure Pt. Complete propene oxidation was observed at temperatures as low as 150 °C in the presence of steam, and five to ten times higher turnover frequencies were found compared to monometallic Pt catalysts. Through DFT studies and structural and catalytic characterization, the remarkable activity of dilute Pt x Cu 100−x alloys was related to the tuning of the electronic structure of Pt to reach optimal binding energies of C* and O* intermediates. This work provides a general approach toward investigation of structure−property relationships of alloyed catalysts with efficient and optimized use of noble metals.

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The development of inexpensive and abundant catalysts with high activity, selectivity, and stability for the oxygen reduction reaction (ORR) is imperative for the widespread implementation of fuel cell devices. Herein, we present a combined theoretical−experimental approach to discover and design first-row transition metal antimonates as excellent electrocatalytic materials for the ORR. Theoretically, we identify first-row transition metal antimonatesMSb 2 O 6 , where M = Mn, Fe, Co, and Nias nonprecious metal catalysts with good oxygen binding energetics, conductivity, thermodynamic phase stability, and aqueous stability. Among the considered antimonates, MnSb 2 O 6 shows the highest theoretical ORR activity based on the 4e − ORR kinetic volcano. Experimentally, nanoparticulate transition metal antimonate catalysts are found to have a minimum of a 2.5-fold enhancement in intrinsic mass activity (on transition metal mass basis) relative to the corresponding transition metal oxide at 0.7 V vs RHE in 0.1 M KOH. MnSb 2 O 6 is the most active catalyst under these conditions, with a 3.5-fold enhancement on a per Mn mass activity basis and 25-fold enhancement on a surface area basis over its antimony-free counterpart. Electrocatalytic and material stability are demonstrated over a 5 h chronopotentiometry experiment in the stability window identified by theoretical Pourbaix analysis. This study further highlights the stable and electrically conductive antimonate structure as a framework to tune the activity and selectivity of nonprecious metal oxide active sites for ORR catalysis.