Catalyst nickel/ceria

In order to improve the resistance of Ni/Al203-based catalysts to sintering and coke formation, some workers have proposed the use of cerium compounds [36]. Ceria, a stable fluorite-type oxide, has been studied for various reactions due to its redox properties [37]. Zhu and Flytzani-Stephanopoulos [38] studied Ni/ceria catalysts for the POX of methane, finding that the presence of ceria, coupled with a high nickel dispersion, allows more stability and resistance to coke deposition. The synergistic effect of the highly dispersed nickel/ceria system is attributed to the facile transfer of oxygen from ceria to the nickel interface with oxidation of any carbon species produced from methane dissociation on nickel. 

A rhodium/nickel/ceria catalyst containing 5wt.% rhodium, lOwt.% nickel and 15 wt.% ceria on alumina performed even better. Full conversion was achieved at 500 ° C with only methane and carbon oxides as carbon products. This catalyst showed full conversion at 650 °C for more than 100 h [24]. 

Similar results were reported by Frusteri et of. for their nickel/magnesia and nickel/ceria catalysts [198]. At a reaction temperature of 650 °C, coke formation was significant under conditions of steam reforming. In the nickel/magnesia catalyst, which contained 15wt.% nickel, less than O.lwt.% carbon was formed within a 20-h test duration when operated under autothermal conditions. Consequently, no deactivation of the catalyst was observed. However, the 0/C ratio that was required to achieve this stable performance was rather high at 1.2, and the S/C ratio of 4.2 was also very high. Besides methane, small amounts of acetaldehyde were formed as a by-product. 

The isooctane feed was pulsed at frequencies of from 0.001 to 0.5 Hz at a pulse duration of 2 ms corresponding to a gaseous isooctane feed of 4.2 cm per pulse. At the maximum pulse frequency of 0.5 Hz, 480cm min isooctane were therefore fed to the reactor along with a nitrogen flow at 200 cm min. The S/C ratio was set to 1 and the O/C ratio to 0.64, corresponding to autothermal conditions. Up to 98% conversion could be achieved over a rhodium/ceria catalyst. Nickel/tungsten and... 

Fuel sulfur is also responsible for a phenomena known as storage and release of sulfur compounds. Sulfur oxides (S02,S02) easily react with ceria, an oxygen storage compound incorporated into most TWC catalysts, and also with alumina. When the air/fuel mixture temporarily goes rich and the catalyst temperature is in a certain range, the stored sulfur is released as H2S yielding a rotten egg odor to the exhaust. A small amount of nickel oxide incorporated into the TWC removes the H2S and releases it later as SO2 (75—79). 

Alloying the nickel of the anode to improve tolerance for fuel contaminants has been explored. Gold and copper alloying decreases the catalytic activity for carbon deposition, while dispersing the anode with a heavy transition metal catalyst like tungsten improves sulfur resistance. Furthermore, ceria cermets seem to have a higher sulfur tolerance than Ni-YSZ cermets [75],... 

A marked effect of the Ce02/Zr02 composition (in samples containing 40 wt.% NiO) on the catalytic activity was noticed. The catalysts with Ce Zr =1 1 (6A) were not only more active (than 7A and 8A) but were also stable during the reaction. Sample 8A containing no zirconia in the support showed a low activity. The NiO crystallite size (Table 11.2) in these compositions varied in the order 7A < 6A < 8A. It may be recalled that on ceria-based catalysts the crystallite size of nickel metal was similar to that of NiO. The higher activity for 6A than 7A indicates that in addition to accessibility of... 

Precipitation-deposition can be used to produce catalysts with a variety of supports, not only those that are formed from coprecipitated precursors. It has been employed to prepare nickel deposited on silica, alumina, magnesia, titania, thoria, ceria, zinc oxide and chromium oxide.36 It has also been used to make supported precious metal catalysts. For example, palladium hydroxide was precipitated onto carbon by the addition of lithium hydroxide to a suspension of... 

E. Poggio-Fraccari, F. Marino, M. Laborde, G. Baronetti, Copper and nickel catalysts supported on praseodymium-doped ceria (PDC) (or the water-gas shift reaction, Appl. Catal. A Gen. 460-461 (2013) 15-20. 

The anodes consisting of a nickel catalyst and of cermet mixed with yttria-doped zirconia electrolyte that are used in conventional solid oxide fuel cells also lose their ability to work at lower temperatures because of a loss of conductivity by the ceramic. This suggests that, for the ceramic in the anode, a material having a higher conductivity at intermediate temperatures should be used. It was in fact shown that an anode made with a nickel/samaria-doped ceria cermet has a much lower polarization than the conventional variant. 

Hoekman et al. [40] studied CO2 methanation reaction over Haldor Topspe commercially available methanation catalysts consisting of Ni and NiO on an alumina substrate with total nickel loading of 20-25% and an operating temperature range of 190-450 C in an extruded ring-shaped catalyst. Approximately 60% conversion of CO2 was observed at r= 300-350°C and stoichiometric CO2/H2 ratio. Aldana et al. [41] found that Ni over ceria-zirconia (prepared by sol-gel synthesis) shows an initial COj activity of almost 80%, with a CH4 selectivity of 97.3%, decreasing down to 84.7% after 90 hours of reaction. By IR operando analysis, they found that for Ni-ceria-zirconia catalysts the main mechanism for CO2 methanation does not require CO as reaction intermediate and the mechanism is based on CO2 adsorption on weak basic sites of the support. 

As a cheap alternative to noble metal and even nickel catalysts, Laosiripojana et al. proposed high surface area ceria [234]. Complete conversion of liquefied petroleum gas (prepared as a sulfur-free 60 wt.% propane/40 wt.% butane mixture) could be achieved above a reaction temperature of 800 °C. The S/C ratio was low at 1.45 and ethylene as a by-product could be suppressed at O/C ratios exceeding 0.6. The weight hourly space velocity was relatively low at 120 L (h gcat). however, catalyst deactivation was moderate within 70-h test duration, which is a promising result. 

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