1. The Field of the Invention
The invention is in the field of nanoparticles and/or catalysts that incorporate such nanoparticles. More particularly, the present invention relates to multi-component nanoparticles made using a dispersing agent that helps bring together and distribute different (e.g., dissimilar) components within the nanoparticles.
2. The Relevant Technology
Nanoparticles are becoming increasingly more important in many industrial processes and products. Nanoparticles find use in a variety of applications, including catalysis and nanomaterials. Catalytic applications include uses for both supported and unsupported nanoparticles of various components, including precious metals, base metals, and oxides. Nanomaterial applications include uses for light blocking, pigmentation, UV absorption, antimicrobial activity, chemical mechanical polishing, and others.
While useful nanoparticles may include only a single component (element or compound), it may be the case that advantageous properties can be achieved if the nanoparticles were to contain two or more distinct components to form a multicomponent nanoparticle. In general, combinations of two or more metals can have a variety of beneficial effects. In the case of catalysts, the use of different elements can modify the catalytic activity to improve an important performance parameter such as activity or selectivity, or they may make the catalyst particle or crystal more resistant to some deleterious effect, such as chemical poisoning or mechanical attrition. In the case of nanomaterials, the inclusion of two or more components would be expected to add additional functionality to the particles, such as combining light blocking function with UV absorption or anti-microbial activity. Alternatively, additional components might be expected to stabilize or strengthen the nanoparticles.
While there is a strong motivation for producing multicomponent nanoparticles, it is difficult, if not impossible, to manufacture particles that contain two or more dissimilar components. This problem is particularly true of small nanoparticles. Recently, academia and industry have made significant advancements toward making very small particles. In some cases, the sizes of the particles are near or below 1 nanometer.
While nanometer sized particles are very advantageous for producing desired properties such as increased catalytic activity and unique material properties, the very smallness of such particles makes it difficult, if not impossible, to create multicomponent nanoparticles that include dissimilar components or elements within the same nanoparticle. One reason for this difficulty is that similar or like elements or compounds have a greater affinity for each other than to dissimilar materials. This same-component attraction means each component has a propensity to combine and form particles with itself rather than forming a mixture with other, dissimilar components. As a result, multicomponent nanoparticle mixtures are largely heterogeneous, composed of two or more distinct particle compositions, each relatively rich in one component and largely depleted or devoid of the other dissimilar components.
In general, the composition of particles, including the distribution of different components among and between the particles, is driven by thermodynamics. The chance of finding multiple components in any given particle depends to a large extent on the size of the particles being formed. When the particles are relatively large, the probability is higher that two dissimilar components can be compounded within a single particle and/or form an alloy. As the size of the particles decreases, however, the likelihood of finding multiple components within a single particle decreases dramatically. At the nanometer scale, it is virtually impossible to consistently and predictably compound two or more dissimilar elements within a single nanoparticle using known procedures. Small nanoparticles tend to be all of one component or another.
Part of the problem with forming multicomponent nano-sized particles is that conventional methods used to form nano-sized particles are performed at relatively low temperatures since high temperatures can causes nanoparticles to undesirably sinter or agglomerate together to form larger particles. Unfortunately, at such low temperatures, the thermodynamics of nanoparticle formation favors formation of single-component particles, as described above. On the other hand, raising the temperature sufficiently to overcome thermodynamic barriers to multicomponent formation causes agglomeration of smaller to larger particles. Consequently, conventional particle formation methods are not able to form nano-sized particles in which a substantial portion of the nanoparticles contain two or more components in each particle.
Another factor that significantly affects the uniformity of multicomponent particles is the dissimilarity of the components. For example, two noble metals such as palladium and platinum are typically more easily combined together within particles because their electronic and chemical properties are similar. In contrast, a noble metal such as platinum and a base metal such as iron have different electronic and chemical properties and are thus much more difficult, if not impossible, to compound together in a single nanoparticle using conventional manufacturing methods. In many cases, compounding dissimilar components does not produce a viable nanoparticle system because of the lack of uniformity in the distribution of the components throughout the nanoparticles. This is particularly true in the case of catalyst particles that require both catalyst components to be in close proximity and/or to be alloyed together to generate the desired catalytic activity.
R. W. J. Scott et al., JACS Communications, 125 (2003) 3708, state: “. . . at present there are no methods for preparing nearly monodisperse, bimetallic nanoparticles that are catalytically active . . . .” X. Zhang and K. Y. Chan, Chem. Mater., 15 (2003) 451, teach: “A number of techniques have been used for producing nanoparticles, including vapor phase techniques, sol-gel methods, sputtering, and coprecipitation. The synthesis of mixed metal nanoparticles is attracting a lot of recent interest for their catalytic properties . . . . The synthesis of mixed metal nanoparticles is a complex problem because of the composition control in addition to size and size distribution control. Platinum-ruthenium bimetallic catalysts have been prepared by co-impregnation methods but without good control of particle size, particle size distribution, and chemical composition.” R. W. J. Scott et al., JACS Communications 127 (2005),1380, disclose: “Most other methods for preparing supported bimetallic nanoparticles in the <5 nm size range lead to phase segregation of the two metals and thus poor control over the composition of individual particles.” K. Hiroshima et al., Fuel Cells, 2 (2002) 31, teach: “The preparation of a highly dispersed alloy catalyst typically requires heat treatment, which is necessary to form an alloy but promotes particle aggregation. As a result, alloy catalysts usually have lower surface areas.”
Therefore, what are needed are multicomponent nanoparticles that include different components that are more evenly dispersed among the particles. Furthermore, what is needed are compositions and processes that can be used to bring together and compound different (e.g., dissimilar) components together in individual nanoparticles without destroying the nanometer size of the particles.