Adsorption, desorption and surface reactions of CO and NO on Pd{320} (original) (raw)
Related papers
The Journal of Physical Chemistry B, 2005
The kinetics of NO adsorption and dissociation on Pd(111) surfaces and the NO sticking coefficient (s NO) were probed by isothermal kinetic measurements between 300 and 525 K using a molecular beam instrument. NO dissociation and N 2 productions were observed in the transient state from 425 K and above on Pd(111) surfaces with selective nitrogen production. Maximum nitrogen production was observed between 475 and 500 K. It was found that, at low temperatures, between 300 and 350 K, molecular adsorption occurs with a constant initial s NO of 0.5 until the Pd(111) surface is covered to about 70-80% by NO. Then s NO rapidly decreases with further increasing NO coverage, indicating typical precursor kinetics. The dynamic adsorption desorption equilibrium on Pd(111) was probed in modulated beam experiments below 500 K. CO titration experiments after NO dosing indicate the diffusion of oxygen into the subsurface regions and beginning surface oxidation at g475 K. Finally, we discuss the results with respect to the rate-limiting character of the different elementary steps of the reaction system.
Dynamic Behavior of Adsorbed NO and CO under Transient Conditions on Pd/Al2O3☆
Journal of Catalysis, 1999
The dynamic behavior of adsorbed NO and CO under transient NO-CO reaction conditions on Pd/Al 2 O 3 has been studied by in situ infrared (IR) spectroscopy coupled with TPR and pulse reaction techniques in the 303-673 K range. Below the light-off temperature (i.e., 561 K), Pd 0 -NO and Pd 0 -CO are the dominant adsorbates on the Pd surface. Pd 0 -NO competes favorably over Pd 0 -CO for the same reduced Pd 0 site when the temperature is increased. Pulse reaction studies at 473 K suggest that Pd 0 -NO dissociates to form adsorbed nitrogen and adsorbed oxygen. Adsorbed oxygen further reacts with Pd 0 -CO to produce CO 2 . Concentration profiles of CO 2 and Pd 0 -CO during the pulse reaction studies indicate that removal of adsorbed oxygen from the Pd surface to produce CO 2 is the ratelimiting step. Prolonged exposure of the catalyst to the NO flow at 473 K results in oxidation of Pd 0 to Pd + and produces Pd-NO + ; the presence of gaseous CO reduces Pd + to Pd 0 and increases the surface coverage of Pd 0 -NO. Above the light-off temperature, Pd-NO + , Al-NCO, nitrate, and carbonate species are the dominant adsorbates. The presence of Pd-NO + indicates that the process for Pd 0 oxidation to Pd + by NO is faster than that of Pd + reduction to Pd 0 by CO. This study demonstrates that careful selection of transient IR techniques allows (i) determination of the modes of adsorbed NO and CO participating in the reaction and (ii) development of a comprehensive mechanism for the NO-CO reaction on Pd/Al 2 O 3 catalyst.
Physical Chemistry Chemical Physics, 2002
UV-laser induced desorption of NO from nanostructured palladium aggregates on an epitaxial alumina support has been studied by means of resonance enhanced multiphoton ionisation (REMPI) to detect desorbing molecules quantum state resolved by Fourier-transform infrared reflection absorption spectroscopy (FT-IRAS), X-ray photoemission spectroscopy (XPS) and thermal desorption spectroscopy (TPD). The size of the Pd-aggregates was systematically varied between 5 Å and 80 Å. Different morphologies were chosen depending on the growth conditions of the aggregates by Pd-atom deposition on the support. The aggregates were either amorphous (deposition at 100 K) or ordered (deposition at 300 K) with the aggregates having a cubooctahedral shape with dominating (111) terraces and a minority of (100) terraces. Adsorption is only similar to single crystal data for aggregate sizes beyond 75 Å. For smaller aggregates NO is bound on on-top sites of palladium. On small amorphous aggregates a substantial amount of NO is weakly bound, which has only been observed for stepped single Pd-crystals. Dissociation of NO occurs at elevated temperatures above 350 K. The system was excited with nanosecond laser pulses at 6.4 eV. In contrast to single crystals, desorption of intact NO molecules has been observed for small aggregates with increasing efficiencies with decreasing aggregate size for aggregate sizes of 80 Å and below. Desorption cross sections vary by at least one order of magnitude. Dominantly the weakly bound species desorbs. REMPI data do not show a strong size dependence. Different models are discussed to explain the data, including the role of local effects of the adsorption site, spill-over to the alumina support or formation of oscillations in electron densities. y Dedicated to Professor Jü rgen Troe on the occasion of his 60th birthday.
Journal of Physical Chemistry C, 2013
Interactive surface chemistry of CO 2 and NO 2 on BaO x /Pt(111) model catalyst surfaces were investigated via X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD) techniques with a particular emphasis on the competition between different adsorbates for the catalytic adsorption sites and adsorbateinduced morphological changes. After NO 2 adsorption, nitrated BaO x /Pt(111) surfaces do not reveal available adsorption sites for CO 2 at 323 K, irrespective of the presence/absence of exposed Pt sites on the surface. Although NO 2 adsorption on thick BaO x (>10MLE)/ Pt(111) overlayers at 323 K leads to the formation of predominantly nitrate species, NO 2 adsorption on the corresponding carbonated surface leads to the formation of coexisting nitrates and nitrites. The presence of carbonates on BaO x /Pt(111) overlayers does not prevent NO 2 uptake. Carbonated BaO x (1.5 MLE)/Pt(111) surfaces (with exposed Pt sites) obtained via CO 2 adsorption can also further interact with NO 2 , forming surface nitrate/nitrite species, accompanied by the transformation of surface carbonates into bulk carbonate species. These results suggest that the nitrate formation process requires the presence of two adjacent unoccupied adsorption sites. It is apparent that in the presence of both NO 2 and CO 2 , carbonate species formed on Lewis base (O 2−) sites enable the formation of nitrites on Lewis acid (Ba 2+) sites. Thermal aging, nitration, and carbonation have a direct impact on the morphology of the two-/three-dimensional (2D/3D) BaO x aggregates on Pt(111). While thermal aging in vacuum leads to the sintering of the BaO x domains, nitration and carbonation results in redispersion and spreading of the BaO x domains on the Pt(111) substrate.
The Journal of Physical Chemistry C, 2013
Interactive surface chemistry of CO 2 and NO 2 on BaO x /Pt(111) model catalyst surfaces were investigated via X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD) techniques with a particular emphasis on the competition between different adsorbates for the catalytic adsorption sites and adsorbateinduced morphological changes. After NO 2 adsorption, nitrated BaO x /Pt(111) surfaces do not reveal available adsorption sites for CO 2 at 323 K, irrespective of the presence/absence of exposed Pt sites on the surface. Although NO 2 adsorption on thick BaO x (>10MLE)/ Pt(111) overlayers at 323 K leads to the formation of predominantly nitrate species, NO 2 adsorption on the corresponding carbonated surface leads to the formation of coexisting nitrates and nitrites. The presence of carbonates on BaO x /Pt(111) overlayers does not prevent NO 2 uptake. Carbonated BaO x (1.5 MLE)/Pt(111) surfaces (with exposed Pt sites) obtained via CO 2 adsorption can also further interact with NO 2 , forming surface nitrate/nitrite species, accompanied by the transformation of surface carbonates into bulk carbonate species. These results suggest that the nitrate formation process requires the presence of two adjacent unoccupied adsorption sites. It is apparent that in the presence of both NO 2 and CO 2 , carbonate species formed on Lewis base (O 2− ) sites enable the formation of nitrites on Lewis acid (Ba 2+ ) sites. Thermal aging, nitration, and carbonation have a direct impact on the morphology of the two-/three-dimensional (2D/3D) BaO x aggregates on Pt(111). While thermal aging in vacuum leads to the sintering of the BaO x domains, nitration and carbonation results in redispersion and spreading of the BaO x domains on the Pt(111) substrate.
DFT study of CO adsorption on Pd-SnO2(1 1 0) surfaces
Applied Surface Science, 2015
We studied the effect of Pd on the adsorption of CO on the tin oxide surface SnO 2 (1 1 0) by Density Functional Theory calculations. Molecular CO adsorbs more strongly in the presence of Pd pre-deposited on the surface. The most stable adsorption sites are those bonded to the Pd atom near a Pd top position on a tilted configuration. In this case the C O distance increases, producing a weakening of the bond and the calculated stretching frequency decreases. Analysis of the atomic orbital interactions reveals that Pd CO bonding involves C sO p and p orbitals from CO, with Pd d orbitals. For CO sites bonded to Pd, CO bonds to the surface producing a weakening of the surface Pd-O bond. The electronic configuration analysis shows that in all cases the CO molecule withdraws charge from the surface.