NO_x-TPD as a Tool to Estimate the Accessible Zirconia Surface of ZrO₂-Containing Materials

Abstract

NO x adsorption-desorption experiments were carried out on a series of W-free and W-containing ZrO 2 samples. To the best of our knowledge, this study establishes for the first time NO x -TPD (temperature-programmed desorption of NO x ) as a reliable method to estimate the accessible ZrO 2 surface of complex oxides. The NO x uptake by ZrO 2 was found to be 5.8 ( 0.2 µmol/m 2 . On the tungstated zirconia series of samples, the NO x uptake decreases linearly with W density up to about 5 W/nm 2 , which is in excellent agreement with literature data for which the pseudo monolayer coverage was obtained by spectroscopic methods. The accessible ZrO 2 surface area of ZrO 2 /SBA-15 materials, which cannot be ascertained through common spectroscopic methods, was estimated by WO 3 thermal spreading and the NO x -TPD method. Both values compare very well but NO x -TPD, unlike WO 3 thermal spreading, does not alter the integrity of the investigated sample as the sample is pretreated at lower temperatures (500 instead of 700°C).

Assessing carbon or tungstates coverage of ZrO2 nanoparticles supported on MWCNT via NOx-TPD

“…After each NO x -TPD experiment, the NO x analyzer was calibrated with a NO/He standard mixture containing 933 ppm of NO. As reported earlier [27,52,53], it should be noted that NO x chemisorption does not occur on WO x species. The reactor outflow was also analyzed periodically using a µ-GC (Agilent Technologies, CP4900) equipped with a 5A…”

“…O 2 was removed from these feeds and replaced with He to maintain identical flow rates. It was verified on a bulk ZrO 2 sample tested earlier (ZrO 2 -600 fa [27]), that replacing the O 2 -He pretreatment and desorption feeds with He did not alter the desorption profiles (not shown) or the amount of NO x desorbed (475.5 µmol/g compared to 445.5 µmol/g [27]). Prior to the NO x -TPD experiments, the MWCNT samples (~0.15 g) were annealed in situ in He at 500 °C (3 °C/min) for 2 h with a flow rate of 100 mL NTP /min.…”

“…Reactant gases, used as received, were fed from independent gas cylinders (purchased from Air Liquide) by means of mass flow controllers (Brooks 5850TR) with a total flow rate of 230 mL NTP /min. Due to the presence of the MWCNT support, and its known sensitivity to oxidation at high temperature in the presence of O 2 [9], the pretreatment and desorption feeds were modified compared to those used in our earlier studies [27,28]. O 2 was removed from these feeds and replaced with He to maintain identical flow rates.…”

“…Recently, we have shown that the accessible surface of ZrO 2 in complex materials can be accurately estimated with the use of a novel chemisorption technique, namely the adsorption of nitrogen oxides (NO x = NO + NO 2 ) at RT followed by their temperature-programmed desorption (NO x -TPD) [27,28]. Thus, we have found of particular interest to characterize the size and accessible surface of ZrO 2 nanoparticles supported on MWCNT (ZrO 2 /MWCNT) by this newly-developed technique.…”

Promoting Ag/Al2O3 Performance in Low-Temperature H2-C3H6-SCR by Thermal Pretreatment of γ-Alumina in Water

“…sorption measurements were carried out on a Belsorp max instrument (Bel Japan) at 77 K. It was found that the BET surface of the hydroxylated samples both after drying at 120 °C for 12 h in an oven and calcination at 550 °C for 2 h in a muffle furnace (185 m 2 /g) did not vary to a significant extent from that of the starting Al 2 O 3 support (180 m 2 /g).XRD measurements were carried out from 10 to 80° by step of 0.01°using a thetatheta D8 Advance (Bruker) powder diffractometer with Cu-K radiation (0.154 nm) operated at 30 kV and 30 mA, and equipped with a 1D LynxEye detector set to a 3° opening.The ultraviolet-visible diffuse reflectance spectra (UV-vis DRS) of Ag/Al 2 O 3 and Ag/Al 2 O 3 -OH were collected on a spectrophotometer (Cary 5000, Varian) equipped with a diffuse reflectance cell at RT in ambient air from 200 to 600 nm after calcination at 550 °C for 2 h and using the corresponding calcined Al 2 O 3 supports for the background spectra.The hydroxylated materials were also characterized by the NO x -Temperature-Programmed Desorption (NO x -TPD) method[6,18,19]. Prior to the NO x -TPD experiments, the samples were calcined in situ under a flow rate of 100 mL NTP /min of 20% O 2 in He at 500 °C (3 °C/min) for 2 h. The samples (about 0.2 g) were firstly contacted with a NO-O 2 -He (385 ppm-8%-balance, 230 mL NTP /min) mixture at room temperature (RT) until recovery of the inlet NO x concentration.…”