Does Pelletizing Catalysts Influence the Efficiency Number of Activity Measurements? Spectrochemical Engineering Considerations for an Accurate Operando Study

Abstract

Porosity is a factor affecting catalyst efficiency in pelletized form. This implies that care should be taken with uncritically relating activity measurements from transmission operando FTIR to final catalyst performance. If the pelletizing pressure is excessive, a destruction of the pore structure of, for example, support oxides might take place, which in turn affects the pore size distribution and the porosity of the catalyst, leading to the observation of lower activity values due to decreased catalyst efficiency. This phenomenon can also apply to conventional activity measurements, in the cases that pelletizing and recrushing of samples are performed to obtain adequate particle size fractions for the catalytic bed. A case study of an operando investigation of a V2O5-WO3/TiO2-sepiolite catalyst is used as an example, and simple calculations of the influence of catalyst activity and internal pore diffusion properties are considered in this paper for the evaluation of catalyst performance in, for example, operando reactors. Thus, it is demonstrated that with a pelletizing pressure of <1–2 ton/cm2, the pore structure is only negligibly altered, and small deteriorations of estimated catalyst efficiencies are observed for first-order kinetic constants lower than 100 mL/gs. However, if the operando study deals with highly active catalysts, it is necessary to consider efficiency losses. A simple procedure for evaluating efficiencies based on pellet dimensions and solid phase characteristics is proposed. The Thiele modulus is directly proportional to the thickness of the pellet, and, thus, inversely related to the catalyst efficiency. As a rule of thumb, we found that for catalytic constants below 100 mL/gs, the maximum thickness of the pellet pressed at 2 tons/cm2 has to be as low as 80 μm to exhibit catalyst efficiencies above 90%. For catalysts with k′ = 10 mL/gs, the value is 260 μm. This strongly underlines the importance of taking internal diffusion limitations into account when working with highly active catalysts.

Experimental methods in chemical engineering: Fourier transform infrared spectroscopy—FTIR

“…More recent operando TIR configurations are capable of characterizing shaped industrial catalyst conformed monolithic catalysts. The instrument measures chemisorption profiles along the length of the of the bed, which is a powerful advance to understand reaction mechanisms of commerical catalytic materials …”

Engineering operando methodology: Understanding catalysis in time and space

“…The reactants are then used up within a thin superficial layer of catalyst, while most of the inner catalyst wall remains inactive. The directly proportional relationship between the Thiele modulus and the thickness of the pellet can be used as a simple procedure for evaluating efficiencies on the basis of pellet dimensions and solid phase characteristics [81]. Figure 9 shows the efficiency of a titaniasupported vanadia-tungsta catalyst pressed into a self-supported wafer for NOx selective catalytic reduction in an operando infrared transmission reactor.…”

“…Pore size distribution (line) and cumulative volume (dotted line) for uncompacted and compacted titania-supported vanadia-tungsta catalyst at different pressures[81].…”

“…Effect of pelletizing pressure and wafer thickness on catalyst efficiency and intrinsic activity during NOx selective catalytic reduction on titania-supported vanadiatungsta wafers in an operando transmission IR reactor. Reproduced with permission from[81].…”

Self-healing catalysts: Co3O4 nanorods for Fischer–Tropsch synthesis

“…15 Similarly, water oxidation is more readily catalysed by the (040) surface of BiVO 4 than by (110) surfaces. 19 Another recent report illustrated significant enhancement of surface redox reactions on cobalt oxide nanorods, as compared with cobalt oxide nanoparticles. 20 Cobalt oxide nanorods exhibit {110} facets with both Co 2+ and Co 3+ exposed on the surface.…”