State of Pd on H-ZSM-5 and other acidic supports during the selective reduction of NO by CH4 studied by EXAFS/XANES

Ali, Ahmed K.; Alvarez, Walter E.; Loughran, Christopher J.; Resasco, Daniel E.

doi:10.1016/s0926-3373(97)00008-8

Cited by 103 publications

(80 citation statements)

References 24 publications

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“…The Pd K-edge XANES plots of the impregnated samples are very similar to that of the Pd-foil, suggesting that the reduction treatment had successfully produced metallic Pd species. For one of the samples (Pd-Pt*/Beta) the rising absorption edge was slightly higher than for the other samples, which could indicate that Pd particles were still bonded to oxygen since the maximum in the rising absorption edge (due to a 1s-5p dipole transition) for PdO is more intense than for metallic Pd [56]. The reason for this increased intensity is not clear but is probably due to a change in 5p-4d hybridization and a decrease in the density of the d population at the Pd site in the oxidic sample [57,58].…”

Section: Dr-uv-visible Spectroscopymentioning

confidence: 93%

Effect of the impregnation order on the nature of metal particles of bi-functional Pt/Pd-supported zeolite Beta materials and on their catalytic activity for the hydroisomerization of alkanes

Roldán

Beale²,

Sánchez‐Sánchez

et al. 2008

Journal of Catalysis

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A series of mono-and bi-metallic (Pt and/or Pd) impregnated zeolite Beta samples have been prepared, characterized using a number of experimental techniques and catalytically tested for the hydroisomerization of a mixture of light paraffins. In particular, the effect of the order of Pt and Pd impregnation on the type/structure of the metallic species of the zeolite support and on the catalytic activity has been studied. Studied samples included a zeolite Beta in which both Pt and Pd (0.5 wt% of each) were simultaneously impregnated (and subsequently calcined and reduced) and two equally metal-loaded samples where the metals were sequentially impregnated (samples Pt*-Pd/Beta and Pd*-Pt/Beta, in which Pt and Pd were impregnated first, respectively). Mono-metallic samples were also prepared for comparison. TPR, DR-UV-visible, TEM, and XAS studies confirm that the order of impregnation plays a key role in the formation of metallic particles, influencing aspects as decisive in their catalytic behavior as their size, their dispersion and their composition (e.g., mono-or bi-metallic). The initial impregnation of Pd and subsequently of Pt produces a higher number of hetero-metallic (Pt-Pd) bonds than the simultaneous co-impregnation and especially for the impregnation in reverse order, which does not produce Pt-Pd bonds in a detectable amount. Based on the results from the different characterization techniques, a model of the metal distribution and composition of the particles was proposed for each bi-metallic sample. In good agreement, the order of the catalytic activity was found to be Pd*-Pt/Beta > Pd-Pt/Beta > Pt/Beta > Pt*-Pd/Beta > Pd/Beta, making clear that not only the presence of both metals, but also an adequate preparation method generating a high number of hetero-metallic bonds must be taken into account to improve the catalytic properties in relation to the mono-metallic samples.

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Section: Dr-uv-visible Spectroscopymentioning

confidence: 93%

Effect of the impregnation order on the nature of metal particles of bi-functional Pt/Pd-supported zeolite Beta materials and on their catalytic activity for the hydroisomerization of alkanes

Roldán

Beale²,

Sánchez‐Sánchez

et al. 2008

Journal of Catalysis

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“…Nishizaka and Misono [8,9] first reported a high activity of Pd-exchanged H-ZSM-5 catalysts and indicated the direct involvement of protonic acid sites in both palladium dispersion and the mechanism of the CH 4 -SCR of NO. Nowadays, it is widely accepted that the Brønsted acid sites of the zeolite are needed to keep palladium(II) highly dispersed and active [4,5,[10][11][12][13][14][15][16][17][18][19][20] although the structure of the palladium species (isolated Pd 2+ ions [10][11][12][13][14][15] versus highly dispersed PdO [16][17][18][19][20]) is still matter of discussion.…”

Section: Introductionmentioning

confidence: 99%

“…It has been shown that simple oxides with strong acidity, such as sulfated and tungstated zirconia, possess the ability to stabilize highly dispersed Pd(II) species analogous to the acidic zeolites [11,12,[21][22][23]. Lowloading palladium catalysts supported on sulfated and tungstated zirconia exhibit catalytic properties in the CH 4 -SCR of NO under dry conditions comparable to those of zeolitic catalysts and they show improved selectivity in the presence of H 2 O and SO 2 [21][22][23][24].…”

Section: Introductionmentioning

confidence: 99%

Routes of formation and composition of NOx complexes adsorbed on palladium-promoted tungstated zirconia

Kantcheva

Çayırtepe

2006

Journal of Molecular Catalysis A: Chemical

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Surface species obtained during the adsorption of NO and NO/O 2 coadsorption at room temperature on Pd-free (WZ) and Pd-promoted tungstated zirconia (Pd/WZ) are identified by means of in situ FT-IR spectroscopy. The WZ and Pd/WZ samples have a tetragonal structure and contain randomly distributed mesoporous phase. Dispersed palladium(II) species are present in two different environments: (i) Pd 2+ ions, which have only Zr 4+ ions in their second coordination sphere and (ii) Pd 2+ ions, which are linked to both zirconium and tungsten ions via oxygen bridges. On the Pd/WZ sample, NO undergoes oxidation to various NO x species depending on the temperature. The compounds formed at room-temperature oxidation are adsorbed N 2 O 3 and products of its self-ionization, NO + and NO 2 − . In this process W(VI) is involved, being reduced to W(IV). At high temperature N 2 O 3 decomposes, restoring the W O species. Under these conditions, NO undergoes oxidation to NO 2 by the Pd(II) ions, which are reduced to Pd(I). The nitrosyls of Pd(I) display high thermal stability and do not disappear upon evacuation at 623 K. During NO/O 2 coadsorption on the Pd/WZ catalyst at room temperature, the amounts of surface nitrates and NO 2 /N 2 O 4 formed in the gas phase are significantly lower than those observed under identical conditions in the presence of tungstated zirconia. It is concluded that promotion of tungstated zirconia with palladium(II) suppresses the oxidation of NO by molecular oxygen at room temperature due to the elimination of acidic protons involved in the process.

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“…Loughran and Resasco [14] also observed activity over other acidic supports, including sulfated zirconia. Further work by Resasco et al showed that acidic supports served to stabilize Pd as active 2+ ions, while over non-acidic supports PdO clusters were formed and were responsible for CH 4 combustion [15]. Sulfated zirconia was further shown to maintain activity as well or better than zeolitic supports in the presence of H 2 O and SO 2 [16][17][18].…”

Section: Introductionmentioning

confidence: 99%

Pd-supported on sulfated monoclinic zirconia for the reduction of NO2 with methane under lean conditions

2006

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NO 2 reduction with methane under lean conditions is studied as a part of a two-stage scheme where NO is first oxidized to NO 2 over an oxidation catalyst and NO 2 is subsequently reduced to N 2 over a reduction catalyst. Pd supported on a sulfated monoclinic zirconia support was observed to have good activity for the reduction of NO 2 with CH 4 under lean conditions, giving N 2 yields over 60% over a broad temperature range below 450°C. Sulfate groups are shown to be thermally stable below the calcination temperature.

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State of Pd on H-ZSM-5 and other acidic supports during the selective reduction of NO by CH4 studied by EXAFS/XANES

Cited by 103 publications

References 24 publications

Effect of the impregnation order on the nature of metal particles of bi-functional Pt/Pd-supported zeolite Beta materials and on their catalytic activity for the hydroisomerization of alkanes

Effect of the impregnation order on the nature of metal particles of bi-functional Pt/Pd-supported zeolite Beta materials and on their catalytic activity for the hydroisomerization of alkanes

Routes of formation and composition of NOx complexes adsorbed on palladium-promoted tungstated zirconia

Pd-supported on sulfated monoclinic zirconia for the reduction of NO2 with methane under lean conditions

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