AgPd and CuPd Catalysts for Selective Hydrogenation of Acetylene

Abstract

The selective hydrogenation of acetylene has been studied over AgPd and CuPd catalysts. Controlled surface reactions were used to synthesize these bimetallic nanoparticles on both TiO2 and SiO2 supports. Chemisorption measurements of the bimetallic catalysts indicate that Pd prefers to be on the nanoparticle surface with a Cu parent catalyst, while Pd prefers to be subsurface with a Ag parent catalyst. From energy-dispersive X-ray spectroscopy analysis, the composition of the nanoparticles is determined to be more uniform on the SiO2 support compared to that on the TiO2 support. X-ray absorption spectroscopy results indicate that, after reduction, the CuPd bimetallic catalysts have some Pd–Pd bonds, but the average number of Pd–Pd bonds decreases after reaction. Infrared spectra of the adsorbed CO show that an increased fraction of isolated Pd species are present on the bimetallic catalysts compared to those on the monometallic catalysts. Adsorption of acetylene and ethylene, however, indicates adsorbed surface species that require contiguous Pd ensembles. These results suggest that the surface structure of the catalyst is highly dynamic and influenced by the gas environment, as well as the support. The catalysts are active for the selective hydrogenation of acetylene in an ethylene-rich environment under mild conditions. Over all catalysts, the ethylene selectivity is greater than 92%; however, improved selectivity is observed over the bimetallic catalysts compared to that over the monometallic Pd catalysts. An ethylene selectivity of 100% is observed over the CuPd0.08/TiO2 catalyst. The highest acetylene conversion rate per gram of Pd is observed over the CuPd0.02/TiO2 catalyst, while the highest turnover frequency is found over the AgPd0.64/TiO2 catalyst. The bimetallic SiO2-supported catalysts have lower rates than Pd/SiO2 but still show improved selectivity. The combined characterization measurements and reaction kinetics studies indicate that the performance improvements of the bimetallic catalysts may be attributed to both electronic and geometric modifications of Pd by the parent Cu or Ag metal.

Oxygenate Reactions over PdCu and PdAg Catalysts: Distinguishing Electronic and Geometric Effects on Reactivity and Selectivity

“…The presence of Cu in the surface is attested in the XAS results, which show a lower total coordination number on the Cu edge, compared to the Pd edge for the Pd3Cu and Pd1Cu1 samples. This result is in seeming contrast with the recent results of Ball et al [43], which show Pd preferentially segregated to the surface; we believe that this difference can be explained by the different particle size of the two catalysts. In the work of Ball et al [43], the catalyst particles are much smaller (~2 nm) than ours; we have shown in our previous work [19] that higher index planes, such as those found in smaller nanoparticles tend to stabilize surface Pd atoms, as opposed to surface Cu atoms.…”

Oxygenate Reactions over PdCu and PdAg Catalysts: Distinguishing Electronic and Geometric Effects on Reactivity and Selectivity

“…The presence of Cu in the surface is attested in the XAS results, which show a lower total coordination number on the Cu edge, compared to the Pd edge for the Pd3Cu and Pd1Cu1 samples. This result is in seeming contrast with the recent results of Ball et al [43], which show Pd preferentially segregated to the surface; we believe that this difference can be explained by the different particle size of the two catalysts.…”

“…In the work of Ball et al [43], the catalyst particles are much smaller (~2 nm) than ours; we have shown in our previous work [19] that higher index planes, such as those found in smaller nanoparticles tend to stabilize surface Pd atoms, as opposed to surface Cu atoms. This may account for the observed difference in surface segregation.…”

Integration of Bimetallic Electronic Synergy with Oxide Site Isolation Improves the Selective Hydrogenation of Acetylene

“…Meanwhile, for comparison, a Pd/SiO 2 catalyst with a particle size of 5.5±1.0 nm was also synthesized using the wet impregnation method (Figure S10), and then coated with different cycles of Ga 2 O 3 ALD (denoted as x cGa‐Pd/SiO 2 , Figure S11). We also synthesized the benchmark Pd 1 Ag/SiO 2 SAA catalyst by performing 3 cycles of Pd ALD on the Ag/SiO 2 catalyst followed by reduction at 500 °C in 10 % H 2 in Ar for 3 h. The particle size of Pd 1 Ag SAAs was about 4.0±0.6 nm (Figure S12a), and the SAA structure was confirmed by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) CO chemisorption, where a peak at 2052 cm −1 , assigned to linear CO on isolated Pd atoms was only observed after the high temperature reduction (Figure S12b) [24] …”

“…To get an insight of the vast selectivity difference between Ga 2 O 3 ‐coated Ag@Pd and Pd samples as well as the much improved activity of Ga 2 O 3 ‐coated Ag@Pd than Pd 1 Ag SAA, in situ DRIFTS CO chemisorption measurements were carried out, since CO is a well‐known sensitive probe of Pd ensemble surface structures [24] . As shown in Figure 3 a, Pd/SiO 2 exhibited three CO vibrational peaks at 2095, 1967 and 1942 cm −1 , assigned to linear CO, bridge‐bonded CO on low‐coordination sites (eg.…”