Walter E. Alvarez

Synergism of Co and Mo in the catalytic production of single-wall carbon nanotubes by decomposition of CO

The catalyst composition and operating conditions for the synthesis of single-wall carbon nanotubes (SWNT) from CO decomposition have been systematically varied in order to maximize the selectivity towards SWNT. A simple quantification method based on the standard Temperature Programmed Oxidation (TPO) technique has allowed us to determine the distribution of the different forms of carbonaceous deposits present on the catalysts after the CO decomposition reaction. A synergistic effect between Co and Mo has been observed. When both metals are simultaneously present, particularly when Mo is in excess, the catalyst is very effective. However, when they are separated they are either inactive (Mo alone) or unselective (Co alone). To understand this synergistic effect, X-ray absorption spectroscopy (EXAFS and XANES) has been used to characterize the state of Co and Mo on the catalysts before and after the production of SWNT.

Correlation between catalytic activity and support reducibility in the CO2 reforming of methane over Pt/CexZr1-xO2 catalysts

We have investigated the relationship between the reducibility of the support and the catalytic activity of supported Pt on a series of catalysts supported on ceria‐zirconia mixed oxides. The supports were prepared by co-precipitation with varying Ce/Zr ratios. X-ray diffraction analysis indicated that, depending on the Ce/Zr ratio, solid solutions of cubic Ce x Zr1 x O2 can be formed. The reducibility of the supports was determined by X-ray photoelectron spectroscopy (XPS). After reduction at 773 K in hydrogen the fraction of reduced cerium (i.e. Ce 3C ) was found to vary with the Ce content, exhibiting a maximum at a composition Ce0:5Zr0:5O2. A good correlation was found between the reducibility and the catalytic activity. It was found that the conversion of methane and CO2 obtained on the different catalysts after 22 h on stream went through a maximum as a function of Ce content in the support and that maximum occurred at the composition that exhibited the maximum reducibility. The H2/CO product ratio was also a function of the support composition, also presenting a maximum for the Pt/Ce0:5Zr0:5O2 catalyst. The amount and nature of carbonaceous deposits were investigated by combining temperature-programmed oxidation (TPO) studies with (XPS). The TPO profiles of all the spent samples revealed two oxidation peaks, one in the low-temperature region, 623‐723 K, and the other in the high-temperature region, 873‐973 K. The peak in the high-temperature region is dominant in the unpromoted catalysts, while the peak at low temperature is more prominent in the Pt/Cex Zr1 x O2 catalysts. XPS exhibits three types of carbon with different binding energies on the spent catalysts, two of them are two forms of coke, and the third one is due to carbonates. However, all of the peaks decreased after an oxidation at intermediate temperatures (i.e. 723 K). Therefore, it appears that the different peaks observed in TPO are not due to different forms of carbon, but rather to different locations on the catalyst surface. The amount of carbonates on the spent catalysts increased with the Ce content, but the correlation between carbonate concentration and activity was not as good as that between reducibility of the support and activity.

A Scalable Process for Production of Single-walled Carbon Nanotubes (SWNTs) by Catalytic Disproportionation of CO on a Solid Catalyst

Existing single-walled carbon nanotube synthesis methods are not easily scalable, operate under severe conditions, and involve high capital and operating costs. The current cost of SWNT is exceedingly high. A catalytic method of synthesis has been developed that has shown potential advantages over the existing methods. This method is based on a catalyst formulation that inhibits the formation of undesired forms of carbon; it can be scaled-up and may result in lower production costs.

State of Pd on H-ZSM-5 and other acidic supports during the selective reduction of NO by CH4 studied by EXAFS/XANES

Abstract The selective reduction of NO by methane has been studied on a series of Pd catalysts. When supported on non-acidic materials, palladium is totally unselective for the conversion of NO and very active for the combustion of methane. By contrast, lowloading Pd catalysts supported on acidic (protonic) materials (H-ZSM-5, H-Mordenite, sulfated zirconia) exhibit much higher selectivity. X-ray absorption studies (XANES and EXAFS) were conducted on several Pd catalysts before and after the reduction of NO in the presence of oxygen. These studies have demonstrated that, upon exposure to the reaction mixture (CH 4 + NO + O 2 ), Pd gets oxidized. The morphology of the oxidized Pd species strongly depends on the metal loading and the acidity of the support. On low Pd loading catalysts over acidic supports, the metallic Pd particles, initially present, are rapidly transformed in Pd 2+ ions by the reaction mixture. By contrast, on the non-acidic materials, the Pd particles are transformed into PdO clusters. These clusters have high catalytic activity for the methane combustion, resulting in low SCR selectivities.

NO reduction by CH4 in the presence of excess O2 over Pd/sulfated zirconia catalysts

Abstract The selective catalytic reduction (SCR) of NO by methane in the presence of excess oxygen has been studied on a series of Pd catalysts supported on sulfated zirconia (SZ). This support is not as sensitive to structural damage by steaming as the acidic zeolites, such as H-ZSM-5 and H-Mor. In previous studies, it was shown that this type of acidic zeolites are able to stabilize Pd2+ ions and promote high SCR activity and selectivity, which are typically not seen in Pd catalysts. In this contribution, it has been demonstrated that SZ is able to promote the NO reduction activity in a similar way to the acidic zeolites, by stabilizing Pd2+ ions that is selective for NO reduction. As in the case of acidic zeolites, the stabilization of Pd2+ ions can occur through a transfer of Pd species from particle to particle. One of the attractive features of Pd/SZ catalysts is that they are less sensitive to water and SO2 poisoning than Pd/H-ZSM-5 catalyst and exhibit higher reversibility after removal of water or SO2.

Sulfated zirconia and tungstated zirconia as effective supports for Pd-based SCR catalysts

It is now well established that even though Pd supported on acidic zeolites is highly selective for CH 4 -SCR, zeolitic structures are susceptible to dealumination under hydrothermal environments. In this contribution, we have investigated the ability of non-zeolitic acidic materials to promote the SCR selectivity and stability in the presence of H 2 O and SO 2 . The results of catalytic activity measurements and characterization tests indicate that sulfated zirconia and tungstated zirconia are supports as effective as the zeolites for the promotion of SCR activity. The high SCR activity of these catalysts can be ascribed to the formation of isolated Pd 2+ ions on acid sites. It is proposed that the stabilization of Pd 2+ on these supports is similar to the stabilization previously reported for acidic zeolites. The remarkable characteristic of these Pd catalysts supported on sulfated zirconia and tungstated zirconia is that when they were tested over a 40 h reaction period in the presence of H 2 O and SO 2 , they appeared to be significantly more resistant than zeolite-based catalysts.

Characterization of the morphology of Pt clusters incorporated in a KL zeolite by vapor phase and incipient wetness impregnation. Influence of Pt particle morphology on aromatization activity and deactivation

Abstract Two series of Pt/KL catalysts with varying metal loading were synthesized by the methods of incipient wetness impregnation (IWI) and vapor phase impregnation (VPI) to compare the effects of the different morphologies that result when the metal loading and, in particular, the preparation method are varied. Catalysts were characterized by a variety of techniques. TEM and DRIFTS studies indicated that on the low-loading samples the majority of particles were located inside the channels of the L-zeolite. In agreement with recent studies, the DRIFTS results evidenced the formation of Pt carbonyls, which further support the presence of very small particles. EXAFS and TEM showed that the VPI catalysts resulted in smaller particles than the catalysts prepared by the IWI method. In addition, EXAFS demonstrated for this series a higher degree of interaction with the L-zeolite framework oxygen atoms. Pulse testing of the methylcyclopentane ring opening showed that the very small clusters produced by the VPI preparation did not result in collimation of the MCP molecule, implying that the reactants and products can easily diffuse over the Pt cluster. This is in contrast with the particles produced by the IWI method, which clearly displayed a collimation effect. The characteristic morphology produced by the VPI method was found to improve the performance of the catalyst under clean and sulfur-poisoned conditions, enhancing the catalyst’s resistance to the formation of coke and decreasing the particle agglomeration rate.

Induction of activity and deactivation of Fe, Mn-promoted sulfated zirconia catalysts

Abstract The isomerization of n-butane on sulfated zirconia catalysts promoted by the addition of Fe and Mn ions has been studied. The evolution of the catalytic activity of these materials as a function of time on stream exhibits a typical shape containing an induction period during which the activity increases. The induction period is followed by a rapid deactivation and then a slower deactivation. The shape of the conversion—time curves contains important information about the way these catalyst operate. Slight changes in the preparation and pretreatment of the catalysts result in changes in the overall activity and shape of the conversion—time curves. The results of this study suggest that the induction period is due to the formation and accumulation of reaction intermediates on the surface. These intermediates participate in the reaction as part of an inter-molecular mechanism. The n-butane molar fraction, the presence of olefins or hydrogen in the feed, and the reaction temperature strongly affect the induction period. A simple mathematical model that successfully describes the behavior of the conversion—time curves has been developed. This model is consistent with the existence of two types of sites with different reactivity and stability. One type is responsible for most of the activity observed during the first few minutes on stream, but it is rapidly deactivated. This type of sites is easily deactivated by simple exposure to hydrogen, so it is consistent with an oxidized species recently proposed. The second type of sites is more resistant to deactivation.

Study of preparation parameters of powder and pelletized Pt/KL catalysts for n-hexane aromatization

Abstract Pt/KL powder and pelletized catalysts were synthesized by different methods including ion exchange (IE), incipient wetness impregnation (IWI), and vapor phase impregnation (VPI) methods. A detailed characterization was conducted on the various samples in order to determine the distributions of Pt particle size and location resulting from each preparation, as well as the relationships between these distributions and the catalytic performance. The catalysts were pretreated at two different reduction temperatures — 400 and 500°C — to investigate the sensitivity of each catalyst to thermal treatment. All catalysts showed high dispersion, with H/Pt ratios greater than unity. However, FTIR of adsorbed CO showed that more important than dispersion, it is the distribution of Pt cluster size and location, which influences the resulting catalytic stability. Standard IE catalysts were found to have a high fraction of Pt particles external to the L-zeolite and were the most sensitive to thermal treatment, displaying Pt migration out of the L-zeolite. These catalysts deactivated rapidly by coke formation. The addition of excess K + ions in the exchange solution and a longer aging period, as suggested in the patent literature, improves the Pt dispersion. IWI and VPI catalysts showed a majority of Pt clusters inside the L-zeolite channels. However, after high temperature reduction treatment, the IWI catalysts displayed particle growth inside the channels, in contrast with the VPI. The IWI catalysts were found to deactivate to about half their initial activity, while the VPI maintained the highest activity and stability. Different VPI methods including a moderate vacuum and a helium flow technique were examined and they showed similar Pt cluster distributions and catalytic performance to the catalysts synthesized using high vacuum. Preparation directly on the pellet for the methods investigated in this work (standard IE, IWI, and VPI) resulted in large fractions of Pt clusters external to the pores and likely on the binder. Thus, rapid deactivation by coke formation was evident. These results indicate that pelletized catalysts prepared by any of these techniques would require preparation on the powder prior to pellet extrusion.

Comparison of the different promoting effects of FeMn, Ni and Pt on the n-butane isomerization activity of sulfated zirconia catalysts

Abstract The n -butane isomerization has been studied on a series of transition metal-promoted sulfated zirconia catalysts. In support of the bimolecular mechanism that involves the participation of olefins and oligomeric carbenium ions on the surface, it was found that the activity of the catalysts under continuous flow operation was much higher than that obtained in the pulse mode. However, when small quantities of 1-butene were added to the pulses, the activity was greatly enhanced. This enhancement significantly varied with the type of metal promoter present in the SZ catalyst. A strong influence of the purity of the n -butane used on activity and sensitivity to hydrogen was observed. When pure n -butane feed was used, the presence of hydrogen had a strong inhibiting effect on activity at low temperatures, but a promoting effect at high temperatures. This promoting effect was particularly prominent on PtSZ and NiSZ catalysts and is primarily due to a decreased deactivation. When n -butane with olefin impurities was used as a feed, the effect of hydrogen was only evident on the PtSZ catalyst. The NiSZ catalyst did not show the inhibition at low temperatures nor the enhanced stability at high temperatures exhibited when n -butane of high purity was used. It was found that the formation of coke has a direct relationship with the low temperature activity. A comparison of the different catalysts investigated under various conditions reflected a trend: the lower the temperature at which n -butane can be converted, the more rapid deactivation and the higher the amount of coke formed.