Interaction of Alkali Acetates with silica supported PdAu

The temperature dependence of the interaction of PdAu alloy particles with alkali acetate promoters (MOAc; M+ = Li+, Na+, K+, Cs+) was studied by X-ray absorption and infrared spectroscopy as well as in situ X-ray diffraction. Alkali acetate promotion on PdAu particles results in irreversible enlargements of the Pd–Pd distances upon heating compared to promoter free PdAu/SiO2. The promoter induces the migration of Pd to the metal particle surface and the formation of a mixed Pd/M+–O layer on the surface of the bimetallic PdAu particles. This structure enhances catalytic reactions by electronically and geometrically modifying the PdAu surface.


Particle size distribution [%]
Particle size [nm] Electronic Supplementary Material (ESI) for Catalysis Science & Technology.This journal is © The Royal Society of Chemistry 2016

S2
The particle size distribution of PdAu/SiO 2 and PdAu/KOAc/SiO 2 revealed an average particle size of ~3.6 nm and 3.7 nm, respectively.The promotion with KOAc does not affect the PdAu particle size but their mobility to form particle groups which may undergo sintering during subsequent reaction to form agglomerates.

by IR spectroscopy
In order to determine the effect of temperature 1 on CO band positions, CO was adsorbed on Au/SiO 2 , Pd/SiO 2 and PdAu/SiO 2 at -89, -50, 0, 50 and 100 °C (Figure S6).On Au/SiO 2 , CO linearly adsorbed at 2104 cm -1 while the broad bands at 2020 and 1890 cm -1 derived from subtraction errors with the spectrum of the activated sample measured at 50 °C (Figure S6a).CO linearly adsorbed on Pd at 2109 and 1991 cm -1 at -89 °C (Figure S6b).2][3] In the bimetallic case (Figure S6c), CO adsorbed on Au in contact to Pd at 2109 cm -1 and on Pd close to Au at 2083 cm -1 at -89 °C.Thus, the contact of Au with Pd leads to an increase and the contact of Pd to Au to a decrease of CO band frequencies according to the electronic Pd-Au interactions described in the two band model. 4,5 owever, an increase in temperature leads to a slight upshift of CO on Au (~ 3 cm -1 ) like observed from Rainer 6 and France et al. 7 and a downshift of linear CO on Pd (~7 cm -1 ).CO/Au shows a large negative shift which offsets the positive dipolar shift as CO coverage increases.The upshift can be explained by the weakening of the Au-C bond upon temperature increase.The downshift of CO on Pd might be due to the reduction in CO dipole couplings (transition from a compressed to an uncompressed CO overlayer).At 100 °C, linear and bridged adsorption modes were observed at 2090 cm −1 and 1984 cm -1 on Pd/SiO 2 , whereas on Au/SiO 2 , CO adsorbed only in a linear mode at 2118 cm −1 (50 °C). 8The temperature dependent spectra revealed that CO adsorbed exclusively on Pd above 50 °C.

Figure S2 .
Figure S2.In situ XRD temperature programs.(a) The temperature was increased from 30 to 300 °C and cooled to 50 °C with a hetaing/cooling rate of 3 °C/min.At each temperature level, a 20min XRD pattern was recorded after equlibrating the temperature for 20 min.(b) The temperature was rised from 30 °C to 100 °C and then in steps of 10 °C to 140 °C (heating rate 3 °C/min).In contrast to program (a), the temperature was equilibrated for 2h prior to a 20 min XRD measurement.

FigureFigure S5 .
Figure S5 depicts the peak position of Pd(111) of (a) unpromoted as well as alkali acetate promoted PdAu/SiO 2 and (b) of KOH, K 2 CO 3 , K 2 C 2 O 4 and KOAc impregnated PdAu/SiO 2 as function of the temperature for the in situ XRD program in Figure S2a.