Rare Earth/Transition Metal Oxides for Syngas Tar Reforming: A Model Compound Study

Abstract

A major problem in biomass or coal gasification is removal of syngas byproducts such as H2S, NH3, and tars (heavy hydrocarbons) that cause catalyst deactivation and clogging problems downstream. Rare earth oxides (REOs) doped with transition metals (TMs) are promising catalysts for tar reforming. With propane as a model compound, we compared such catalysts to a typical supported Ni catalyst, and also to recent density functional theory (DFT) results modeling these systems. The REO/TM catalysts are active over the range 920–1000 K, with no significant deactivation in non-sulfur containing feeds. In particular, a Mn/CeO2 catalyst showed good reforming activity with low carbon, CO2, and CH4 yields. This catalyst also maintained some activity in the presence of 40 ppm H2S. Kinetics calculations showed that most such catalysts have near zero order kinetics with respect to water, making them usable with a variety of gasifier effluents. Characterization of used catalysts by multiple techniques suggests that the metal-doped REOs do not undergo much (if any) phase separation in extended use under tar reforming conditions, with Mn- and La-doped CeO2 being especially stable.

Engineering heterogenous catalysts for chemical CO2 utilization: Lessons from thermal catalysis and advantages of yolk@shell structured nanoreactors

“…Sulphur poisoning is resistible with the addition of certain components; CeO 2 has been found to help prevent sulphur poisoning through the exploitation of the oxygen mobility that this material is known for. The mobile oxygen is then able to interact with the S present on the catalyst sites to form sulphur oxides that are more easily removed at the high temperatures used for reforming reactions [40][41][42]. Another method to prevent S poisoning is using alkaline materials as sacrificial components to adsorb the S before it can reach the catalyst active sites [39,43].…”

Catalytic Cracking of Biomass-Derived Hydrocarbon Tars or Model Compounds To Form Biobased Benzene, Toluene, and Xylene Isomer Mixtures

“…This behavior was explained by authors as a result of high copper dispersion in the compound oxides, which facilitated the reduction of iron oxides to metallic iron and prevented catalytic deactivation due to a decrease in the surface area of the catalysts during the reaction. The steam reforming of naphthalene with syngas mixtures and H 2 S over rare earth oxides (REOs) mixed with transition metals was reported by Li et al It was found that Fe- and Mn-doped supported REOs are promising tar removal catalysts with higher sulfur tolerance, less coking, and less methanation than conventional Ni-based high-temperature reforming catalysts in the temperature range of 650–800 °C. This phenomenon was related to the increased generation of oxygen vacancies in the metal-doped REOs.…”

“…& Engineering Chemistry Research of naphthalene with syngas mixtures and H 2 S over rare earth oxides mixed with transition metals was reported by Li et al98 It was found that Fe-and Mn-doped supported REOs are promising tar removal catalysts with higher sulfur tolerance, less coking, and less methanation than conventional Ni-based high-temperature reforming catalysts in the temperature range of 650−800 °C. This phenomenon was related to the increased generation of oxygen vacancies in the metal-doped REOs.…”

Mechanochemical Redox: Calcination‐free Synthesis of Ceria‐hybrid Catalyst with Ultra‐High Surface Area

“…[1,10] Doping transition metals into CeO 2 is a promising way to create oxygen vacancy and lattice distortion, thus leading to enhanced catalytic performance. [11][12][13][14][15][16][17][18] Among various doped CeO 2 catalysts, Ce 1-x Mn x O 2-δ is undoubtedly a key class due to the flexible valence of Mn species, [19][20][21][22] and a huge size gap between Ce and Mn ions. [22,23] In principle, divalent Mn 2 + dopants with a lower positive valence would produce more charge compensating vacancies, which favor the diffusion of active oxygen species.…”

“…However, it is well known that the catalytic activity of single CeO 2 is quite limited [1,10] . Doping transition metals into CeO 2 is a promising way to create oxygen vacancy and lattice distortion, thus leading to enhanced catalytic performance [11–18] . Among various doped CeO 2 catalysts, Ce 1‐x Mn x O 2‐δ is undoubtedly a key class due to the flexible valence of Mn species, [19–22] and a huge size gap between Ce and Mn ions [22,23] .…”