The team of Zhang Tao and the researcher Li Weizhen, an academician of the Chinese Academy of Sciences and a researcher Li Weizhen from the Chinese Academy of Sciences and the Institute of Aerospace Catalysis and New Materials Research Center of the Chinese Academy of Sciences, reported the research work of nano-gold catalysts resistant to high temperature of 1100 ° C. Express (Nano Letters) and was selected as the current supplementary cover.
The thermal stability and catalytic activity of metal nanocatalysts usually show this seesaw relationship, which is particularly significant on nanogold catalysts. More than 30 years ago, the discovery of small-sized (1-5nm) gold nanoparticles with excellent catalytic CO oxidation activity at low temperature was once amazing, refreshing people's understanding of the catalytic properties of chemically inert gold and even nanocatalysis. And a gold-catalyzed "gold rush" has arisen. So far, researchers have found that nano-gold catalysts have good catalytic effects on many reaction processes such as oxidation, hydrogenation, hydrochlorination, and carbon-carbon coupling. However, since the melting point of small-sized gold nanoparticles is about 330-380 ° C, even if they are supported on a carrier, their thermal stability is poor, and it is easy to agglomerate and deactivate, which seriously hinders the industrial application of nano-gold catalysts. Similar to the high chemical inertness of gold nanoparticles, which hinders people's understanding of their catalytic properties, the relatively low melting point of bulk gold (1064 ° C) also significantly affects people's confidence in stabilizing small-sized gold nanoparticles.
The team's previous work found that, since the oxygen and noble metal atoms of the spinel oxide are close-packed structures, the aluminum spinel support can effectively stabilize the lattice parameters smaller than the oxygen sublattice parameters in the spinel. Precious metal and alloy nanoparticles (such as Rh, Pd, Ir, and Pt, etc.), but Au and Ag (Nat. Commun., Chem. Mater., Appl. Catal. B-Environ., J Catal.). Therefore, the use of MgGa2O4 spinel carriers with larger oxygen sublattice parameters is expected to achieve stability to gold nanoparticles. In this work, the researchers confirmed through theoretical calculations that Au is indeed more stable on the MgGa2O4 (111) plane than that on the MgAl2O4 (111) plane; the MgGa2O4 supported gold nanoparticles with a size of about 1.5 nm were prepared by a simple dipping method. After 5 hours or 28 days of 800 ° C high-temperature roasting, it was found that, except for a few gold particles, which are larger in size, most of the gold particles are small 2-3 nm nanoparticles. The study found that even after firing at a high temperature of 1100 ° C for 5 hours, small-sized gold nanoparticles of about 3.6 nm still exist stably. Spherical aberration electron microscopy analysis of this unusually stable structure revealed that the contact interface between the two was Au (112) and MgGa2O4 (111); further in-situ heating and high-resolution electron microscopy observation at 1100 ° C found that the melting of a large particle of gold At temperature, small-sized gold nanoparticles still exist in the form of grains that give clear lattice fringes. The change of the melting point indicates that the nano-gold crystal phase has changed, that is, Au-MgGa2O4 grows together to form a new crystal phase. Researchers have named it metal-oxide "heterogeneous twins" and used the "" symbol to refer to this difference Special "growing on" structure for conventional load-type structures. By measuring the heat of fusion per unit mass of gold, it is known that more than 80% of the gold still exists as a hetero-twin structure after firing at 1100 ° C. The AuMgGa2O4 catalyst has the size effect and carrier effect of a supported nano-gold catalyst. After aging at 800 ° C, it still maintains the high catalytic activity of nano-gold. The light-off temperatures for catalyzing the combustion of CO and propylene are about 150 ° C and 300 °, respectively. C, which is expected to be used as an active component in catalysts for oxidation of diesel engine exhaust gas, and solves the problem of inefficient removal of CO by platinum group metals at low temperature during cold start.
Nano-gold particles can stably exist at a temperature higher than the melting point of bulk gold. Like its low-temperature catalysis of CO oxidation, it is surprising on the one hand, and also shows the commonality of gold as a metal: gold is actually like other platinum Like group metals, they can be constructed as stable, highly active catalysts. The concept of metal-oxide heterogeneous twins proposed in this work can be used to understand the mechanism of spinel group oxides stabilizing precious metal nanoparticles, and it is also expected to be used to guide the preparation of other high temperature resistant nano precious metals and alloy catalysts. The composition and structure of the metal-oxide heterotwin interface still need to be further revealed at the atomic scale.