Promotion Effect of Ceria on Pd/Al2O3 Catalyst for the Hydrogenation of Carbon Monoxide to Dimethyl Ether.

Abstract

Hydrogenation of CO was carried out over Pd catalyst supported on Al2O3, MgO, silica alumina, and silica gel supports.Addition of CeO2 increased the activity of Pd/Al2O3 and the main product was dimethyl ether (DME). DME formation proceeded via dehydration of methanol on the acid sites of the supports. However, the yield of DME was much higher over Al2O3 than silica alumina.Therefore, Al2O3 has an inherently high activity for methanol production.Temperature programmed reduction experiments showed that CeO2 on Al2O3 was reduced with H2, whereas CeO2 on MgO underwent little reduction as confirmed by a XAFS measurement.XAFS analysis indicated that Pd/CeO2/MgO contained Pd0 plus Pd2+ owing to the presence of the reduction-resistant CeO2 on MgO and the Pd on CeO2/Al2O3 was in a more reduced state although Pd2+ still remained.The results of CO adsorption measurement and XAFS and TEM observation revealed that an addition of CeO2 increased the dispersion of Pd metal particles on Al2O3. The intermediate strength interactions between Pd, CeO2, and Al2O3 seems to account for the higher activity of the Pd/CeO2/Al2O3 catalyst for O-containing compounds formation than Pd/CeO2/MgO.

XAFS and XRD Studies of PdOCeO2 Catalysts on Al2O3

“…These results indicate that PdO was reduced and is present in its metallic state on alumina. In a detailed comparison of the XANES spectra, the difference in the ratio of two peak intensities above the absorption edge was observed between catalysts for PdO and PdO-CeO 2 system on alumina after the catalytic activity test at 573 K. That is to say, the intensity ratio of the second peak around 24.37 keV to the first peak around 24.34 keV of PdO-CeO 2 /Al 2 O 3 catalyst after the test at 573 K is smaller than that of CeO 2 -free catalyst after the test at 573 K. This characteristic of the spectral shape has been observed on a catalyst, which has high dispersiveness and activities [10]. This suggests that the addition of CeO 2 improves the dispersiveness of PdO on supports.…”

Comparative Density Functional Study of Methanol Decomposition on Cu4 and Co4 Clusters

“…To probe support effects on the methanol decomposition chemistry, we have studied the thermodynamics and kinetics of C−O, C−H, and O−H bond scission steps on Co 4 and Cu 4 clusters supported on a θ-Al 2 O 3 (010) surface. We chose this surface because alumina has been used as a support in several studies of catalysis on subnanometer clusters. − Using the results from the gas-phase clusters as a starting point, we have calculated adsorption geometries and energies for the first bonding breaking steps on supported Cu 4 and Co 4 clusters. We considered both planar and tetrahedral structures of the clusters on θ-Al 2 O 3 (010).…”

“…We chose this surface because alumina has been used as a support in several studies of catalysis on subnanometer clusters. [54][55][56][57] Using the results from the gas-phase clusters as a starting point, we have calculated adsorption geometries and energies for the first bonding breaking steps on supported Cu 4 and Co 4 clusters. We considered both planar and tetrahedral structures of the clusters on θ-Al 2 O 3 (010).…”

Density Functional Studies of Methanol Decomposition on Subnanometer Pd Clusters

“…A variety of experimental studies have established that methanol, catalyzed by supported metal nanoparticles, dehydrates to dimethyl ether (DME). ,, Most of these studies suggested that this reaction occurs on acid sites of the support materials, but relatively little mechanistic support for this proposal has been provided, and the role of the metal catalyst in the reaction, if any, has not been carefully analyzed. In this work, we study multiple reaction pathways for DME formation.…”

“…The adsorption of carbon monoxide has been widely investigated on single crystal Pd and other transition metal surfaces, together with Pd nanoparticles, using both experiment and theory. 10,12,[48][49][50][51][52][53][54][55] On Pd 4 , we find that CO has a clear preference for adsorption in 3-fold-hollow sites with an adsorption energy of -2.54 eV. We note that there is a well-known difficulty with density functional theory calculations that consistently predict a preference for CO adsorption at 3-fold-hollow sites over experimentally observed top or bridge sites, particularly on transition metal surfaces.…”