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

Abstract

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.

FT-IR spectroscopic investigation of the surface reaction of CH4 with NO x species adsorbed on Pd/WO3–ZrO2 catalyst

“…The pre-adsorbed NO x species are created allowing the samples to stay in contact with NO + O 2 mixture (20 Torr, NO:O 2 = 1:2) for 30 min at room temperature followed by evacuation to 3.0 Â 10 )3 Torr (spectra RT). According to the results of our previous investigation [20], the absorptions in the 1650-1000 cm )1 region detected on both samples at room temperature (spectra RT) are characteristic of various types of surface nitrates. The band at 2134-2140 cm )1 corresponds to the m(NO) stretching mode of NO + species [20].…”

“…According to the results of our previous investigation [20], the absorptions in the 1650-1000 cm )1 region detected on both samples at room temperature (spectra RT) are characteristic of various types of surface nitrates. The band at 2134-2140 cm )1 corresponds to the m(NO) stretching mode of NO + species [20]. The weak bands at 1867 and 1815 cm )1 observed on the Pd/WZ catalyst ( figure 1(B)) belong to the m(NO) stretching vibrations of NO adsorbed on Pd(II) sites in two different environments [20].…”

“…The band at 2134-2140 cm )1 corresponds to the m(NO) stretching mode of NO + species [20]. The weak bands at 1867 and 1815 cm )1 observed on the Pd/WZ catalyst ( figure 1(B)) belong to the m(NO) stretching vibrations of NO adsorbed on Pd(II) sites in two different environments [20]. The NO x species adsorbed on both samples disappear at 400°C.…”

“…Palladium-promoted tungstated zirconia (denoted Pd/WZ) was prepared by the impregnation of the WZ sample with a solution of Pd(NO 3 ) 2 AE2H 2 O (Merck-Schuchardt) ensuring 0.1 wt.% of nominal palladium content. After drying the samples were calcined at 500°C for 5 h. According to XRD, the materials obtained have tetragonal structure and contain randomly distributed mesoporous phase [20].…”

“…This paper describes the results of the interaction of methane with surface NO x complexes obtained by room-temperature adsorption of NO + O 2 mixture on Pd-promoted (0.1 wt.% Pd) and Pd-free tungstated zirconia (18.6 wt.% WO 3 ). The structural characterization of the materials and the results of identification of the NO x species upon adsorption of NO and its coadsorption with oxygen have been reported previously [20].…”