Evidence of the role of non-stoichiometry in Zn-Cr catalysts by thermal investigations

doi:10.1016/0040-6031(88)87152-1

Thermochimica Acta

Volume 133, October 1988, Pages 155-161

https://doi.org/10.1016/0040-6031(88)87152-1 Get rights and content

Abstract

The formation, stability and surface reactivity of monophasic non-stoichiometric spinel-type Zn/Cr catalysts (NSS) were investigated by different thermal techniques. It was found that they form prevalently $via$ chromate intermediates and are non-equilibrium phases, which upon use as catalysts or when heated tend to form ZnO and ZnCr₂O₄. The non-stoichiometry increases the CO adsorption capacity considerably, with a large fraction of high energy sites identifiable as modified chromium atoms present at the NSS surface. Data from the TPD of methanol, show that the deviation from the stoichiometry decreases the oxidizing power of the surface and creates new stable active sites, which may be identified as surface zinc species.

References (13)

J.A. Campbell
Spectrochim. Acta
(1965)
G. Busca et al.
J. Catal.
(1987)
G. Natta, U. Colombo and I. Pasquon, in P.H. Emmet (Ed.), Reinhold, New York, 1953, Ch....
A. Riva et al.
J. Chem. Soc., Faraday Trans. 1
(1987)
G. Del Piero et al.
J. Chem. Soc., Chem. Commun.
(1984)
M. Di Conca et al.
Proc. 8th Int. Congr. Catal., Dechema, Frankfurt
(1984)

There are more references available in the full text version of this article.

Cited by (10)

Direct conversion of syngas to light olefins (C2–C3) over a tandem catalyst CrZn–SAPO-34: Tailoring activity and stability by varying the Cr/Zn ratio and calcination temperature
2020, Journal of Catalysis
Citation Excerpt :
In addition, ZnO has been identified as the only active component, and the promoting role of Cr2O3 has been attributed to the spinel formation, ZnCr2O4, which prevented the sintering of small ZnO crystallites by providing considerable surface area and better distribution of ZnO particles. On the other hand, some studies reported that the nonstoichiometric Zn–Cr spinel was the active phase in methanol and iso-butanol synthesis [12,13,17,22–24]. Nonstoichiometry of the Zn–Cr spinel was obtained by the dissolution of excess ZnO in ZnCr2O4, which results from the solid reaction between ZnO and Cr2O3 during calcination [25].
A series of CrZn oxide catalysts with different Cr/Zn ratios were synthesized by co-precipitation and calcined at different temperatures. The catalysts were tested in a physical mixture with SAPO-34 in the direct conversion of syngas to olefins. X-ray diffraction, x-ray photoelectron and absorption spectroscopy, and electron microscopy were used to evaluate the impact of the Cr/Zn ratio and calcination temperature on the physicochemical properties of the resultant materials. CrZn–SAPO-34 catalysts were able to selectively convert syngas to C₂–C₃ hydrocarbons with a high olefin/paraffin ratio and low methane yield. However, the catalytic performance of these systems exhibited a trade-off between activity and stability in the olefin yield over time. By tuning the Cr/Zn ratio, it is possible to improve the olefin production stability, but with some compromise in the overall activity. The nonstoichiometric and inversed ZnCr₂O₄ spinel phase was considered to be the active phase for CO hydrogenation, while the decay in olefin production over time is correlated with abundance of ZnO on the resultant catalyst. Spectroscopic analysis demonstrated that the inversed spinel evolves toward its normal form under reaction conditions, which is accompanied by the formation of segregated ZnO; this phase may also increase the olefin hydrogenation activity.
The challenge for the rational design of an optimal CrZn–SAPO-34 system is the stabilization of inversed ZnCr₂O₄, while minimizing the presence of the ZnO phase in the catalyst formulation.
Mesoporous oxidic holey nanosheets from Zn-Cr LDH synthesized by soft chemical etching of Cr<sup>3+</sup> and its application as CO<inf>2</inf> hydrogenation catalyst
2017, Journal of CO2 Utilization
Citation Excerpt :
The broad reduction peak obtained after H2 adsorption at temperatures ranging from 300 to 500 °C was due to the presence of excess Zn active site. Since, higher is the reducibility of ZnO to free state Zn better is the catalytic activity[70,71]. Hence, this catalyst showed better catalytic activity which was due to Cr3+ depletion and through that enrichment of free state Zn as well as mesoporosity development.
High surface area mesoporous holey hydroxidic nanosheets were synthesized by reacting Zn-Cr-LDH with β-diketonate ligands. The different β-diketonate ligands having keto-enol tautomeric behaviour and the strength of enolic acids increasing in the order 1-phenyl-1,3-butanedione < 2,4-pentanedione < 1,1,1-trifloro-2,4-pentanedione were reacted with Zn-Cr-LDH. Strong 1,1,1-trifluoro-2,4-pentanedione reacted with Zn-Cr-LDH at room temperature itself and broken the layered structure of LDH forming a segregated Cr³⁺ stable diketo complex. On the other hand, 2,4-pentanedione and 1-phenyl-1,3-butanedione reacted with Zn-Cr-LDH at temperatures 45 and 65 °C respectively. 2,4-Pentanedione preferentially etched out Cr³⁺ ion by leaving behind the LDH like structure with mesoporous holes. 1-Phenyl-1,3-butanedione due to its still weaker acid strength could not etch out Cr³⁺ ion. The pores obtained in the LDH nanosheets from the reaction with 2,4-pentanedione were found within a narrow size range of 2–10 nm. Also, their BET surface area was higher. The calcined form of this mesoporous material was used as a catalyst for the hydrogenation of CO₂. The catalytic reactions carried out at temperature 200–400 °C showed approximately 100% selectivity towards CH₄ with a small amount of H₂O as a by-product without the deposition of coke and formation of additional CO. Thus, by virtue of its improved performance at lower temperatures it is expected to have an advantage over many existing similar CO₂ hydrogenation catalysts.
Iso-butanol direct synthesis from syngas over the alkali metals modified Cr/ZnO catalysts
2015, Applied Catalysis A: General
Citation Excerpt :
After H2 adsorption, chemical compound ZnO has become free state Zn2+; the active species in the studied reaction are free state Zn2+ inside of the non-stoichiometric spinel ZnxCr2/3(1–x)O structure. If the reducibility of non-stoichiometric spinel ZnxCr2/3(1−x)O is higher, there will be more free state Zn2+ formed from chemical compound ZnO, as a result the catalyst activity will be better [22,23]. Cr/ZnO–Li catalyst has the highest reduction temperature of 390 °C among the tested catalysts.
This report demonstrated the iso-butanol direct synthesis from syngas over varied alkali metals modified Cr/ZnO catalyst. The Cr/ZnO catalyst was prepared by a general co-precipitation method and then modified with varied alkali metals: Li, Na, K and Cs. The prepared alkali metals modified catalysts, Cr/ZnO–Li, Cr/ZnO–Na, Cr/ZnO–K and Cr/ZnO–Cs, were evaluated by the reaction of iso-butanal direct synthesis from syngas. Among these catalysts, the K modified catalyst Cr/ZnO–K exhibited excellent catalytic ability on iso-butanol synthesis. The CO conversion obtained on this Cr/ZnO–K catalyst reached up to 27.39%, with the highest iso-butanol selectivity of 15.58%. We also investigated the effect of different Li precursors, Li₂CO₃ and LiOH, on the catalytic performance of Cr/ZnO catalyst, and the reaction results indicated that the LiOH precursor modified catalyst Cr/ZnO–Li–OH had better catalytic performance. All the catalysts were characterized by BET, XRD, H₂-TPR, SEM–EDS and TG–DTA to further disclose the function of alkali metals to Cr/ZnO catalyst in iso-butanol synthesis. We found that the reducibility of the active phase of non-stoichiometric spinel Zn_xCr_2/3(1−x)O, the crucial species for iso-butanol synthesis, was significantly influenced by alkali metals modification, which consequently determined in the diverse catalytic performance of alkali metals modified catalysts for iso-butanol synthesis.
ZnO‒Cr2O3 + ZSM-5 catalyst with very low Zn/Cr ratio for the transformation of synthesis gas to hydrocarbons
1995, Applied Catalysis A, General
Three ZnCr mixed oxide plus ZSM-5 compound catalysts with different compositions covering a wide range of Zn/Cr atomic ratios (0.064 to 1.913), aimed at the direct conversion of synthesis gas into hydrocarbons, were tested under different experimental conditions: temperature ranged from 356 to 410°C, pressure from 3.60 to 4.49 MPa and space velocity from 0.2 to 3.0 mmol reactants/(g_cat min). Reaction products included carbon dioxide, water and hydrocarbons with methanol conversion (through which hydrocarbons are formed) being complete. The catalyst with the least content of zinc gave the highest yields of liquid hydrocarbons (up to 74% of total hydrocarbons). Different crystalline phases (ZnO, ZnCr₂O₄ and Cr₂O₃) were found in the methanol synthesis component as a function of Zn/Cr ratio. The low Zn/Cr catalyst was characterized by X-ray diffraction, differential thermal analysis, nitrogen adsorption, temperature-programmed reduction, Infrared and X-ray photoelectron spectroscopy. The only phases observed in this catalyst were ZnCr₂O₄ and Cr₂O₃. Calcination temperature had an influence in both physical and chemical catalyst properties. After calcination, Cr^VI and Cr^III species could be seen on the catalyst surface, but only Cr^III species were observed after reaction or reduction. The evidences gathered suggest that Cr₂O₃ is mainly responsible for methanol synthesis, while ZnCr₂O₄ contributes to increase the specific surface area of the catalyst and influences gas product distributions. Compound catalysts with the mixed oxide (ZnCr) as the methanol synthesis component showed to be more active than those with the individual (Cr or Zn) oxides.
Hydrotalcite-type anionic clays: Preparation, properties and applications.
1991, Catalysis Today
Non-Stoichiometry, a Key to Modify the Activity and Selectivity of Spinel-type Catalysts for Hydrogenation Reactions
1991, Studies in Surface Science and Catalysis
Possible effects induced by non-stoichiometry in different mixed oxides were examined. Emphasis was placed on how such effects can be successfully controlled and applied to the preparation of catalysts for the hydrogenation of carbon monoxide and/or different organic molecules. Non-stoichiometric phases may be obtained by low-temperature methods, and form by different mechanisms as a function of the nature of the elements present. All these phases are metastable and evolve towards stoichiometric forms with increasing temperature. Binary systems (Zn/Cr, Cu/Cr, and Co/Cr) and ternary and quaternary systems (containing Co, Cu, Zn, and Cr) were examined to show how multicomponent catalysts may be obtained, whose properties can be regulated by proper selection of the component cations and appropriate adjustment of the composition. We report examples of synergic effects, related to the presence of the different cations in the same structure, which result in a considerable increase in the catalytic activity in the hydrogenation of both CO and organic molecules. The poisoning of low-temperature methanol catalysts by small amounts of cobalt also may be attributed to a specific interaction.

View all citing articles on Scopus

View full text

Inorganic materials, superconductors, glasses, ceramics, cetallurgy, catalysis, reactions and syntheses, thermal decompositionsEvidence of the role of non-stoichiometry in Zn-Cr catalysts by thermal investigations

Abstract

Spectrochim. Acta

J. Catal.

J. Chem. Soc., Faraday Trans. 1

J. Chem. Soc., Chem. Commun.

Proc. 8th Int. Congr. Catal., Dechema, Frankfurt

Inorganic materials, superconductors, glasses, ceramics, cetallurgy, catalysis, reactions and syntheses, thermal decompositions
Evidence of the role of non-stoichiometry in Zn-Cr catalysts by thermal investigations