[Show abstract][Hide abstract] ABSTRACT: Binding of AuCl(4)(-) to amine groups tethered to the interior of a 2 nm siloxane nanocage was determined in solutions containing various concentrations of acid. The mode of binding was inferred from EXAFS and UV-vis spectra to be by ligand exchange of amine for chloride, which implies that the amines remain unprotonated. Cyclic voltammetry confirmed that the Au complexes bind to the nanocage interior and established a 1:1 relationship between bound Au complex and amine groups. The results suggested a 5-7 pH unit shift in the protonation constant of the interior amines relative to free amines in solution.
Full-text · Article · Dec 2008 · Journal of the American Chemical Society
[Show abstract][Hide abstract] ABSTRACT: The effect of halide poisoning of Au/TiO2 catalysts in low temperature CO oxidation was investigated using bromide as the poison and a combination of X-ray absorption spectroscopy (XANES and EXAFS), quantitative CO adsorption, and catalytic measurements. It was found that halide prevented full reduction of cationic Au by displacing oxyhydroxy ligands and remaining bound to Au during low temperature reduction, causing a reduction in catalytic activity. On reduced Au samples, bromide (likely as NaBr molecule) was preferentially adsorbed on Au and not on TiO2 , and suppressed both the adsorption of CO and the catalytic activity. At low Br contents, each adsorbed Br suppressed adsorption of three CO, suggesting that Br was adsorbed on three-fold sites but the effect decreased with increasing Br content possibly due to crowding of adsorbed Br. When 5–10% of the Au was bound to Br, the catalytic activity was completely blocked, although ~35% of the original CO adsorption capacity remained. The data suggest that not all CO adsorption sites are catalytic active sites, and are consistent with the perimeter Au atoms at/near the particle-support interface (perimeter) being active sites.
Full-text · Article · Mar 2008 · Applied Catalysis A General
[Show abstract][Hide abstract] ABSTRACT: The adsorption of CO and its reaction with oxygen were investigated using a combination of in situ Fourier transform infrared spectroscopy, step response measurements in a microreactor, (18)O isotopic labeling, and X-ray absorption near edge structure spectroscopy. An as-prepared sample in which Au is present as a surface oxyhydroxy complex does not adsorb CO. On an activated sample in which only metallic Au is detected, 0.18 +/- 0.03 mol CO/(mol Au) are adsorbed on Au at -60 degrees C, which shows an IR band at 2090 cm(-1). When oxygen is present in the gas phase, this species reacts with a turnover rate of 1.4 +/- 0.2 mol CO(mol Au min)(-1), which is close to the steady-state turnover rate. In contrast, there is a very small quantity of adsorbed oxygen on Au. A small IR peak at 1242 cm(-1) appears when an activated sample is exposed to CO. It reacts rapidly with oxygen and is shifted to 1236 cm(-1) if (18)O is used. It is assigned to the possible intermediate hydroxycarbonyl.
No preview · Article · Jun 2006 · The Journal of Physical Chemistry B
[Show abstract][Hide abstract] ABSTRACT: The catalytic performance of Au/Al2O3 catalyst is highly sensitive to preparation procedure. EXAFS and TEM characterization of key steps in the preparation in conjunction with activity measurements result in deeper insights into precautions needed and the complex manner residual chloride impacts catalytic activity. Chloride affects the morphological (Au particle size), chemical (reducibility) as well as the catalytic (poisoning) properties of Au. Alternate preparation procedures to the conventional calcination of a catalyst prepared with deposition–precipitation at neutral pH were explored to increase Au loadings. It was found that low temperature H2 reduction of a catalyst prepared at low pH but washed with NaOH is an effective preparation method.
Preview · Article · Sep 2005 · Applied Catalysis A General
[Show abstract][Hide abstract] ABSTRACT: Changes in a Au/TiO(2) catalyst during the activation process from an as-prepared state, consisting of supported AuO(x)(OH)(4-2x)(-) species, were monitored with X-ray absorption spectroscopy and FTIR spectroscopy, complemented with XPS, microcalorimetry, and TEM characterization. When the catalyst was activated with H(2) pulses at 298 K, there was an induction period when little changes were detected. This was followed by a period of increasing rate of reduction of Au(3+) to Au(0), before the reduction rate decreased until the sample was fully reduced. A similar trend in the activation process was observed if CO pulses at 273 K or a steady flow of CO at about 240 K was used to activate the sample. With both activation procedures, the CO oxidation activity of the catalyst at 195 K increased with the degree of reduction up to 70% reduction, and decreased slightly beyond 80% reduction. The results were consistent with metallic Au being necessary for catalytic activity.
Full-text · Article · Jun 2005 · The Journal of Physical Chemistry B
[Show abstract][Hide abstract] ABSTRACT: NO reduces HNO3 to HONO and surface nitrates to nitrites on a BaNa–Y zeolite, and it reacts with NO2 to form N2O3. Data are also presented which show that adsorbed NO+ reacts with water to form HONO. In the presence of NH3 and H2O these processes lead to the formation of ammonium nitrite, which efficiently decomposes near 100 °C to N2+H2O, effecting the catalytic reduction of NOx to N2. A criterion for this path is that the optimum yield of N2 is obtained with an equimolar mixture of NO+NO2. Since ammonium nitrate, which can also form on this catalyst, does not significantly decompose at 200 °C, a typical temperature for diesel exhaust, the reduction of nitrate to nitrite serves to regenerate active sites on the BaNa–Y zeolite.
No preview · Article · Apr 2005 · Journal of Catalysis
[Show abstract][Hide abstract] ABSTRACT: Chemical vapor deposition (CVD) of FeCl3 has been used to deposit Fe3+ ions on the surface of sulfated zirconia (SZ) and silica-alumina (SA). Upon exposure to FeCl3 vapor most Brønsted acid sites and silanol groups are replaced by Fe, as evidenced by IR. With SZ the concentration of the acid sites and thus the retention of Fe increase with the sulfate loading up to approximately 45% of a monolayer, followed by an abrupt decrease at higher loadings. This indicates condensation of sulfate groups to polysulfates, which is in line with a lower number of Brønsted sites per sulfate. Release of HCl due to the reaction of Brønsted sites with FeCl3 peaks at 85 degrees C for SZ but only at 345 degrees C for SA. After replacing Cl- by OH- and calcining, the materials were tested as De-NOx catalysts and characterized by temperature-programmed reduction (TPR) with H2 or CO. Mononuclear and dinuclear oxo-ions of Fe coexist with Fe oxide particles in calcined Fe/SA, resulting in a low selectivity for NOx reduction. During reduction of Fe/SA up to 800 degrees C, a significant fraction of the Fe forms a chemical compound with SA, possibly an aluminate. In Fe/SZ the Fe dramatically increases the reducibility of the sulfate groups, from 57% partial reduction to SO2 in the absence of Fe, to 90% deep reduction to S2- ions in its presence. Formation of Fe sulfide is indicated by the enhanced sulfur retention upon reduction. Fe/SZ is active for NOx reduction with isobutane. Catalysts with low Fe content that are prepared by controlled sublimation are superior to those prepared by impregnation. At 450 degrees C and GHSV = 30,000 h(-1), 65% of NOx is reduced to N2 in excess O2.
No preview · Article · Mar 2005 · The Journal of Physical Chemistry B
[Show abstract][Hide abstract] ABSTRACT: A low activation energy pathway for the catalytic reduction of nitrogen oxides to N2, with reductants other than ammonia, consists of two sets of reaction steps. In the first set, part of the NOx is reduced to NH3; in the second set ammonium nitrite, NH4NO2 is formed from this NH3 and NO + NO2. The NH4NO2 thus formed decomposes at ~100 °C to N2 + H2O, even on an inert support, whereas ammonium nitrate, NH4NO3, which is also formed from NH3 and NO2 + O2, (or HNO3), decomposes only at 312 °C yielding mainly N2O. Upon applying Redhead's equations for a first order desorption to the decomposition of ammonium nitrite, an activation energiy of 22.4 is calculated which is consistent with literature data. For the reaction path via ammonium nitrite a consumption ratio of 1/1 for NO and NO2 is predicted and confirmed experimentally by injecting NO into a mixture of NH3 + NO2 flowing over a BaNa/Y catalyst. This leads to a yield increase of one N2 molecule per added molecule of NO. Little N2 is produced from NH3 + NO in the absence of NO2.
No preview · Article · Oct 2004 · Catalysis Letters