Rhodium Dispersion during NO/CO Conversions

Abstract

The catalyzed reaction between NO and CO for NO x removal is a key component of the role of rhodium in automotive exhaust catalysts.[1] Kinetic models for this reaction have been reported [2] with structural inferences based on Rh(111). Surface-science studies have provided elegant descriptions of surface oxidation (by O 2 ).[3] However, the mean nuclearity of the metal on Rh/Al 2 O 3 during the reduction of NO by H 2 varies with gas partial pressures and temperature.[4] Hence, an assumption of an invariant core catalyst may be invalid for highly dispersed catalysts. Indeed, combinations of CO and NO at low temperatures (< 473 K) aggressively corrode nanoparticulate rhodium to release Rh(CO) 2 centers, [5] which are mononuclear [6] and whose generation has also been monitored kinetically. [7] Our previous experiments showed that 5 wt % Rh/Al 2 O 3 , prepared from rhodium trichloride, underwent structural changes during a thermal ramp under a 1:1 mixture of 5 % CO/He and 5 % NO/He.[5] At room temperature, the predominant IR bands for the adsorbates were owing to Rh I (CO) 2 units and the mean Rh-Rh coordination number was found to be approximately 3. By 473 K, the intensity of the n(CO) as IR band (as = asymmetric) owing to Rh(CO) 2 centers was just past its maximum.In this study, the simultaneous monitoring of catalysts by diffuse reflectance and time dependence of any structural changes was monitored at 473 K on a prereduced sample, with the sequence starting under CO/He. The NO and CO streams were switched alternatively every 10 seconds for a total of nine switches; little CO 2 or N 2 O were formed during this sequence (Figure 1). The initial Rh-Rh coordination number (7.3) was reduced to approximately 4 with the second NO pulse and oscillated between values of 4 and 5 thereafter; these small changes were slower than the pulsing interval. The simultaneous monitoring of the catalysts by diffuse-reflectance infrared Fourier transform spectroscopy (DRIFTS) showed that bridging (1866 cm À1 ) and terminal (2059 cm À1 ) CO bands that are typical of sites on metallic rhodium were essentially lost after 10 s of NO exposure; these were replaced by absorptions owing to the Rh(CO) 2 unit (2096 and 2031 cm À1 ) upon exposure to CO. After the second cycle, intensity patterns following the gas switches became established with the two IR bands of the geminal dicarbonyl being formed during CO exposure, and being rapidly lost after switching to NO. A new IR band at 1931 cm À1 then emerged, which can be assigned to a linear rhodium nitrosyl on an oxidized rhodium of some form. [8,9] In the previous temperature-ramp study, [5] significant NO conversion was evident at 573 K, the Rh(CO) 2 unit was not observed, and the steady-state Rh-Rh nuclearity had begun to increase. In the isothermal gas-cycling experiment at 573 K, the reduction in the Rh-Rh coordination number was even greater than at 473 K, falling to approximately 2.8 after the second NO pulse and then tracking the switching sequence oscillating between approximately 2.8 (...

Promotion Effects in the Oxidation of CO over Zeolite-Supported Rh Nanoparticles

“…16,54 It results in two bands in the CO IR spectrum: one in-phase CO stretching vibration between 2118 and 2070 cm -1 and one out-of-phase stretching vibration between 2053 and 2007 cm -1 . 6,[12][13][14][15][16][17][18]30,31,[33][34][35][43][44][45][46][48][49][50][51][52][53][54][55][56] Next to these, linearly bonded CO adsorbed on Rh metallic particles has been reported to give rise to a band between 2075 and 2031 cm -1 , whereas CO linearly adsorbed on oxidized Rh 1+ or Rh 2+ crystallites is attended with a band between 2135 and 2110 cm -1 . 6,12,[15][16][17]19,[32][33][34][43][44][45][46][47][48][49][50][51]53 In order to unambiguously assign...…”

Applying Dynamic and Synchronous DRIFTS/EXAFS to the Structural Reactive Behaviour of Dilute (≤1 wt%) Supported Rh/Al2O3 Catalysts using Quick and Energy Dispersive EXAFS

“…Originally developed at the University of Southampton, UK, [6][7][8][9][10] a resource of this type is now implemented on ID24 (dispersive EXAFS) a the ESRF, Grenoble [11,12]. It is, however, a mobile experiment and can be utilised at other beamlines.…”

“…Synchronously applied combinations of structurally direct X-ray based techniques, such as EXAFS, XRD, and SAXS/WAXS, together with vibrational spectroscopies (such as Raman [2][3][4][5] and infrared spectroscopies [6][7][8][9][10][11][12]), and techniques that give a global overview of catalytic performance (such as mass spectrometry and gas chromatography), are particularly attractive. A number of these types of experiments [2][3][4][5][6][7][8][9][10][11][12] have now been demonstrated, and it seems certain that they will become ever more powerful as the process of refining them, and developing M. A. Newton (&) The European Synchrotron Radiation Facility, 6 Rue Jules Horowitz, BP-220, Grenoble 38043, France e-mail: newton@esrf.fr new ones, continues in response to the ever increasing desires of the catalysis, chemistry, and wider material science communities.…”

Precise Observation of Growth of Surface Oxide Layer for Pd and Cu Nanoparticles During Oxidative/Reductive Gases Cyclic Flow Studied by Real-Time-Resolved XAFS Spectroscopy

“…However, metal nanoparticles are expected to not only provide highencounter fields but show peculiar performance originated in nanometer scale. Many studies about metal nanoparticles have revealed that various and unique structural changes are induced in metal nanoparticles themselves by changing temperature and/or atmosphere [1][2][3][4][5][6][7][8]. These results indicate that observation of metal nanoparticles under only normal atmospheric condition may cause limited understandings of metal nanoparticles especially about the mechanism of surface catalytic reaction.…”