H.C. Yao

Ceria in automotive exhaust catalysts: I. Oxygen storage

Abstract Oxygen storage capacities of CeO 2 , CeO 2 Al 2 O 3 , and PM/CeO 2 /Al 2 O 3 , measured by a pulse injection method, are affected by pretreatment temperature, pulsing temperature, partial pressure of CO, presence of precious metals (PM), and the concentration of CeO 2 on alumina. They are lowered by higher pretreatment temperature but increase with the pulsing temperature in the range of application. At a pulsing temperature ≤500 °C, the capacities are not affected by oxygen pressure but increase with partial pressure of CO. The presence of PM lowers the reduction temperature and increases the oxygen storage capacity of CeO 2 . TPR was used to measure the oxygen removal at various temperatures. At 900 °C, the maximum amount of oxygen removed from unsupported or alumina-supported ceria is about 25%. The TPR traces of the unsupported ceria show two peaks at 500 and 750 °C which are associated with the reduction of surface capping oxygen and bulk oxygen anions, respectively. For alumina-supported ceria, the TPR traces show a third peak at 850 °C which is associated with the reduction of the shared oxygen anions at the interface. The presence of PM lowers only the reduction temperature of the capping oxygen anions but not of the other two oxygen species. Both oxygen chemisorption and TPR were used to measure the oxygen anion restoration at various temperatures following the reduction at 500 and 900 °C, respectively. Chemisorption data show that the oxygen uptake per CeO 2 is highest at the lowest CeO 2 concentration. The TPR traces show that a new oxygen species, probably a molecular oxygen anion, is formed at 25 °C which converts slowly at 500 °C to the capping oxygen anion. Complete restoration of all three types of oxygen anions is accomplished at 850 °C in air.

Metal-support interaction in automotive exhaust catalysts: Rh-washcoat interaction

This work compares the effect of three washcoat materials, ..gamma..-Al/sub 2/O/sub 3/, ..cap alpha..-Al/sub 2/O/sub 3/, and ZrO/sub 2/, on the CO chemisorption capacity, thermostability, and catalytic activity for the H/sub 2/-NO reaction of Rh on a monolithic support. The Rh loading was analyzed by the conventional x-ray fluorescence method and by the inductive coupled plasma technique. The ZrO/sub 2/ and ..gamma..-Al/sub 2/O/sub 3/ washcoat show resistance to thermal deactivation. Since the BET area of ZrO/sub 2/ and ..gamma..-Al/sub 2/O/sub 3/ washcoats was five to seven times lower than that of ..gamma..-Al/sub 2/O/sub 3/ washcoat it was necessary to compare the chemical resistance of these catalysts for Pb, P, and S in fuel. Experiments were carried out in a pulse-flame reactor with isooctane fuel containing either 6 mg Pb/gallon (certification fuel) or 60 mg Pb/gallon with 0.8 mg P/gallon and 0.03 wt. % S. The results showed that using a fuel containing a ten times excess of lead over the certification fuel, the Rh/..cap alpha..-Al/sub 2/O/sub 3/ and Rh/ZrO/sub 2/ catalysts show significantly higher deactivation compared with a Rh/..gamma..-Al/sub 2/O/sub 3/ catalyst after 15,000 simulated miles of pulsator aging. These results indicate that the catalysts prepared on thermally stable waschcoatsmorexa0» may be compatible only with fuels of low lead level (Pb < 6 mg/gallon). 1 figure, 2 tables. (DP)«xa0less

Characterization of molybdenum-platinum catalysts supported on γ-alumina by X-ray photoelectron spectroscopy

Abstract X-Ray photoelectron spectroscopy (XPS) has been used to investigate the reduction of MoO 3 and MoO 3 + PtO 2 supported on γ-Al 2 O 3 . Hydrogen reduction of MoO 3 γ -Al 2 O 3 at 500 °C produced a mixture of Mo(VI), Mo(V), and Mo(IV). Hydrogen reduction of dispersed MoO 3 + PtO 2 on γ-Al 2 O 3 showed a similar partial reduction of the Mo(VI) to Mo(V) and Mo(IV) but at significantly lower temperatures than was the case with the supported MoO 3 alone. The extent of the reduction of Mo(VI) increased with the amount of PtO 2 in the samples.

Interactions of Base and Noble Metals with Insulator Supports

Resume Dans notre laboratoire nous avons etudie des systemes de metaux nobles disperses sur des substrats isolants. Les catalyseurs etaient destines a operer a haute temperature dans des conditions oxydantes. Pour caracteriser ces systemes nous avons employe, pour la plupart, des methodes chimiques, par exemple la chimisorption, la reduction a temperature programmee et la reactivite. En outre, la microscopie optique, la microscopie electronique, et des methodes spectroscopiques (photoelectronique, infra-rouge, Auger et RPE) ont donne des informations utiles. En plusieurs occasions, nous avons trouve que les interactions entre les catalyseurs et les substrats ont ete dimportance pratique et quelles ont pu ětre modifiees par laddition doxydes de metaux bases. Abstract The work on catalyst-support interactions carried out in the authors laboratory dealt with systems of noble metals dispersed on insulator support. The catalysts were destined to operate at high temperatures under oxidizing conditions. The methods employed for characterization were mainly chemical: chemisorption, temperature-programmed reduction and reactivity. Further, spectroscopical methods (XPS, IR, EPR, Auger) and electron microscopy were applied to provide additional information. In several instances, the understanding of the nature of the interactions has led to significant practical results. These were achieved by modifying the interaction in the desired direction by the incorporation of suitable additives.

CO oxidation over Ptγ-Al2O3 under high pressure

The oxidation of CO over a Ptγ-Al2O3 catalyst was studied by cyclic injection of CO and O2 pulses under CO and O2 pressures of 2 and 1 kPa, respectively, at temperatures of 330–650 K. A boundary reaction model yielded kinetic parameters in agreement with the results of published low-pressure studies. For CO2 formation from O2(g) + [CO]ad, CO desorption was the most important step and showed multipeak characteristics and a low activation energy (10 kJ/mol) for the boundary reaction was found. CO2 formation from O2(g) + [CO] as well as from CO(g) + [O]ad resulted from boundary reaction at oxygen island. A reaction mechanism was proposed to account for the kinetics observed and interpret the varying observations noted in low-pressure studies in terms of diffusion-disguised kinetics.

Surface interaction in the Ptγ-Al2O3 system. III. The reaction of neopentane with H2

The reaction of neopentane with hydrogen over a Ptγ-Al2O3 catalyst is examined as a function of Pt-loading in the range of 0.48 to 12.3 μmol/m2 (BET). It is found that the turnover frequency and the activation energy are not significantly different when the Pt changes from a dispersed phase to a paniculate phase but the product distribution is affected. While the dispersed phase catalyzes only a stepwise hydrogenolysis to yield isobutane, propane, ethane, and methane, the particulate phase promotes mainly an isomerization to form isopentane which is then hydrogenolyzed to n-butane, isobutane, propane, ethane, and methane. A large increase of isomerization selectivity has been achieved by sintering the catalyst in H2 at 500 °C or by increasing Pt concentration in the particulate phase.

Infrared Spectra of No Chemisorbed on a Pt/MoO3/Al2O3 Catalyst

Infrared spectra of NO chemisorbed at 298K on prereduced Pt/Al 2 O 3 M0O 3 /Al 2 O 3 , and Pt/MoO 3 /Al 2 O 3 were compared. Only the spectra of NO on MoO 3 /Al 2 O 3 reduced at 773K and that on Pt/MoO 3 /Al 2 O 3 reduced at 573K showed two intense bands at 1800 and 1700cm -1. These bands are assigned to the symmetric (1800cm -l ) and antisymmetric (1700cm -1 ) NO stretching fundamental of a (N0) 2 cis-dimer. The ratio of the relative band intensities leads to an estimate of the angle between the two NO oscillators. These results are applied to interpret the MoO 3 -promoted selective reduction of NO over Pt/MoO 3 /Al 2 O 3 to N 2 O and N 2 instead to NH 3 , as found over a Pt/Al 2 O 3 catalyst.