Adsorption and hydrolysis of isocyanic acid on TiO2

In selective catalytic reduction (SCR) systems for diesel vehicles the injected urea solution decomposes to ammonia and isocyanic acid (HNCO), which reacts with water to another ammonia molecule and carbon dioxide over the SCR catalyst or a special urea decomposition catalyst. The second reaction step, i.e. the catalytic hydrolysis of HNCO was studied on the anatase TiO 2 (101) surface and Al 2 O 3 (100) surface with ab initio density functional theory (DFT) calculations using a cluster model as well as with in situ diffuse reflectance infrared Fourier transform spectra (DRIFTS) investigations and kinetic experiments. The following mechanistic pathway has been identified to be most feasible: HNCO dissociatively adsorbs on the metal oxide surface as isocyanates, which are attacked by water, thereby forming carbamic acid at the surface. In a further step this intermediate is transformed to a carbamate complex, which leads to CO 2 desorption and consequently NH 3 formation. The comparison between the sum of the theoretical vibrational spectra of the reaction intermediates with the in situ DRIFT spectra also strongly supports the accuracy of the second reaction pathway. This mechanism holds also for the HNCO hydrolysis over c-Al 2 O 3 and the reactivity compared to TiO 2 was found to be consistent with the heights of the barriers in the energy diagrams. Based on these promising preliminary results a computational screening has been started in order to predict the most active metal oxides and surfaces for this reaction.

show abstract

“…Isocyanic acid was produced by thermal decomposition of cyanuric acid and subsequently mixed with the feed gas, as described in [1].…”

Section: Methodsmentioning

confidence: 99%

“…The selective catalytic reduction (SCR) process with urea is an efficient technology for the reduction of nitrogen oxides emissions of diesel vehicles [1][2][3][4]. The reason for the use of urea is its easy thermal decomposition to ammonia, which is the real reducing agent in the SCR reaction.…”

Section: Introductionmentioning

confidence: 99%

Decomposition of Urea in the SCR Process: Combination of DFT Calculations and Experimental Results on the Catalytic Hydrolysis of Isocyanic Acid on TiO2 and Al2O3

Czekaj

Kröcher

2009

Top Catal

Self Cite

View full text Add to dashboard Cite

show abstract

“…Since HNCO, originating from urea thermolysis, plays a key role in byproduct formation, a catalyst can largely reduce byproduct formation by HNCO hydrolysis [10,11,37]. The best hydrolysis catalyst known for the urea-SCR application is anatase TiO 2 [10,37,[47][48][49]. Also, V 2 O 5 /WO 3 -TiO 2 [13,50] and zeolite-based [10,11,51] SCR catalysts provide high hydrolysis activities.…”

Section: Urea Decomposition Byproducts and Catalyst Deactivationmentioning

confidence: 98%

Ammonia Storage and Release in SCR Systems for Mobile Applications

Peitz

Bernhard

Kröcher

2014

Urea-SCR Technology for deNOx After Treatment of Diesel Exhausts

Self Cite

View full text Add to dashboard Cite

The SCR reaction requires ammonia for the reduction of nitrogen oxides. While compressed or liquefied ammonia is used for the supply of ammonia in stationary applications, the odorous ammonia gas had to be replaced in mobile SCR applications by an ammonia precursor compound for the safe storage and reliable release of ammonia in the right quantities and right dynamics. Thus, significant innovation was needed before mobile SCR systems could be realized, even though commercial operation of ammonia SCR systems in coal power plants ([250 MW) started already around 1980 [1]. After the invention of HC-SCR for mobile NO x sources, to avoid handling NH 3 [2], and the discovery of cyanuric acid as a safe NH 3 storage compound, [3] urea was proposed as a storage material for NH 3 in 1988 [4]. By then, urea was already known to work as an NH 3 precursor for SCR in stationary applications for 3 years [5]. First results on mobile urea-based SCR were publically presented in 1990, already showing NO x conversions above 90 % from 250°C at gas hourly space velocities (GHSV) of 12,900 h -1 [6]. The first patented mobile applications of urea solutions for SCR were registered in 1990 [7].Due to the large-scale production of urea as a bulk commodity, it is readily available in large quantities. Since the involved reactants CO 2 and NH 3 needed for urea synthesis are bulk chemicals that are produced directly from air and natural gas, using the well-known industrial Haber-Bosch process, urea can be produced at a low price [8]. Also, urea is nontoxic, noncorrosive and can easily be handled as aqueous solutions, such as a 32.5 wt % solution with the trade name AdBlue

show abstract

“…Moreover, it is known from previous investigations that the surfaces of the applied metal oxides are easily hydrated in the presence of water. These hydrated particles will move in the fluidized bed and are able to rapidly hydrolyze the very reactive adsorbed isocyanate groups even at the lower end of the bed [12,[22][23][24]. In a previous study we have shown that small water pulses are sufficient to hydrolyze isocyanate species adsorbed on TiO 2 anatase at a temperature of only 150 • C [22].…”

Section: First Tests With -Al 2 O 3 "Uetikon 1" As Fluidized Bed Matementioning

confidence: 99%

“…These hydrated particles will move in the fluidized bed and are able to rapidly hydrolyze the very reactive adsorbed isocyanate groups even at the lower end of the bed [12,[22][23][24]. In a previous study we have shown that small water pulses are sufficient to hydrolyze isocyanate species adsorbed on TiO 2 anatase at a temperature of only 150 • C [22]. On the other side it is important to note that although HNCO is very easily hydrolyzed on the surface of metal oxides, it is also known to be quite stable in the gas phase in the presence of water [7].…”

Section: First Tests With -Al 2 O 3 "Uetikon 1" As Fluidized Bed Matementioning

confidence: 99%