Colloidal Bimetallic Catalysts: PtAu

Abstract

Two distinct but related topics constitute the subject of this communication: preparation and characterization of bimetallic colloidal sols which have not hitherto been reported; and the catalytic performance of these bimetallic particles of colloidal dimension after supporting them on graphite.Monometallic colloidal sols are of ancient lineage1" with those of Ptl'l and Ad3' receiving particular attention; and it has long been recognized[41 that they are potentially powerful catalysts. Effort is currently being expended in devising new ways of preparing and characterizingi5] such sols. However, as a result of their greater catalytic selectivity and resistance to poisoning, metal alloys are of increasing importance.[61 We have therefore explored sol routes to the preparation of bimetallic particles as they could have advantages over the traditional methods of co-impregnation, co-adsorption or co-precipitation. This approach to the production of uniform bimetallic particles of colloidal size has not, to our knowledge, been reported.We chose PtToo-,Au, (O<x< 100) because of the wealth of catalytic knowledge on the respective monometallic sols" 31 and also because, as x increases in bulk alloys of Pt and Au, the activity for hydrocarbon isomerization and hydrogenolysis decreases but activity for dehydrogenation increases.['] These bimetallic sols were prepared by reducing mixed aqueous solutions of H2PtCI, and HAuCI, (30 ~n g . d m -~ of each) with a 1% trisodium citrate solution at 373 K in a manner similar to that used for monometallic ~0 1 s .~'~~ Their average particle sizes were found by electron microscopy to be larger than those predicted on the basis of differences in crystallographic lattice energies and surface energies. High resolution electron microscopy was used to examine the internal and surface structurefg1 of these sols; the JOEL 200CX electron microscope employed having been fitted with an improved pole-piece and stagef5] with which a point-to-point resolution of 0.196 nm was achievable. The composition of the alloys was determined in two ways: globally by means o f atomic absorption (AA) analysis of the digested sols and locally (2 nm diameter spot) from the intensities of the L, X-ray emission peaks in a scanning transmission electron microscope (STEM). The catalytic activity of the colloidal particles, after they had been adsorbed onto graphite (Fluka, 99.9% purity) in a manner similar to that used recentlyl'ol for Pt sols, was tested at 633 K in the hydrogenolysis, isomerization and dehydrogenation of n-butane and compared with a standard silica-supported Pt sample EuroPt."'] Several noteworthy features emerged. First, the individual colloidal particles possess a high degree of crystalline order (see Fig. la, b), with well-defined (1 11) and (200) planes present in sols of all compositions. Second, the size distribution of the sols at each composition is narrow (2.3-t0.9nm, 4.8+1.8nm, 12.3+1.4nm, 14.3f4.0nm, and 24.7 f 6 . 4 nm for x=O, 10, 50, 90, and 100, respectively) and the average size in...

In situ investigation of Pt100−xAuxand Pt100−ySnynanoalloys

“…Previously 25 nanoalloy particle sizes were shown to increase as x increased. Table 2 confirms that as x increased the average size of particles seen in transmission electron microscopy (d TEM ) increased, 25 but in addition as x increased the extent of hydrogen chemisorption decreased by 67% at 9 at% Au (and d chem increased). Although both the Pt and the Au were zero-valent at all compositions, the Au may reduce faster than Pt and lead to some surface enrichment by Pt.…”

“…Extents of hydrogen chemisorption (n) at 298 K were measured volumetrically for Pt 100Àx Au x /C and Pt 100Ày Sn y /Al 2 O 3 (pre-calcined in air at 673 K for 2 h, reduced in situ at 673 K in H 2 for 2 h and finally outgassed to 0.1 mPa for 1 h) by extrapolation of n-p isothermal data to p = 0 Pa. High-resolution transmission electron microscopy (TEM) [54][55][56] and electron diffraction 57 have characterised the structure, size and morphology of unsupported nanoparticles, including those of Pt-Sn; 32 here a Jeol 200CX 25 was used. 32 X-Ray photoelectron spectroscopy (XPS; Kratos E3500 using AlK a (1486.6 eV) normalised to Al 2p at 74.7 eV) was applied to Pt 100Ày Sn y / Al 2 O 3 samples (pre-calcined in air at 673 K for 2 h and reduced in situ at 673 K in H 2 for 2 h).…”

“…Here we have chosen Pt-Au 21 to test because of their potential immiscibility and previous work. 25 Monodisperse PtRu nanoalloy can be prepared from acac precursors using 1,2hexadecanediol reducing agent deposited on carbon. 26 Here PtSn nanoalloys have also been chosen because they have greater electro-oxidation activity towards ethanol than Pt-Ru/C 27 which have a resistance to CO poisoning that is important in a fuel cell context and is not possessed by Pt alone.…”

Chemical catalysis by colloids and clusters

“…Only the Pt-Cd-containing colloids were able to effect hydrogenation of the C=0 group, while Pt-only colloids catalyzed hydrogenation of the C=C groups .198 In one bimetallic colloid of Pt-Au, several members of the citrate-stabilized series Ptioo_xAux (0 < x < 100) were prepared and analyzed by HREM (Figure 14). 109 The catalyzed rates of dehydrogenation, isomerization, and hydrogenolysis of n-butane were a function of Pt-Au composition.109 Other studies employed the polymer-stabilization technique to prepare mixed metal colloids and investigate their catalytic activity. While monometallic colloids of either Pd or Pt were small, they had a tendency to agglomerate, whereas excellent uniformity of particle size was observed for PVPstabilized Pd-Pt bimetallic colloids over a wide Pd/Pt ratio (average diameter 15 A).…”

Catalyst particle sizes from Rutherford scattered intensities

“…It is found that, under practical operating conditions, detection sensitivity is limited primarily by signal-to-noise (S/N) rather than by the probe size. This technique could be used, in principle, to distinguish between quasi-spherical and raft-like clusters, an issue of considerable importance in some catalyst (Prestridge et al, 1977) and colloid (Sermon et al, 1987) studies.…”